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Overview

The ACD (Atmospheric Chemistry Division) mission is (1) to understand the chemical composition of the atmosphere, the processes that modify and control the composition, and potential changes that may result from natural and human induced forcings; (2) to provide relevant, reliable, accessible, unbiased, and timely information on atmospheric chemistry to government and society; and (3) to act as an intellectual resource and enabler to the wider atmospheric sciences community through the development of new measurement capabilities and methodologies, development and application of numerical models that reliably simulate present and future atmospheric conditions, and the planning and execution of field experiments to address specific scientific questions of regional and global significance. 

Over the next few years, ACD staff will direct their research efforts at two grand challenges that confront society and where atmospheric chemical processes play a crucial role, namely Regional and Global Air Quality and Chemistry in the Climate System.  The directions and priorities of the division’s research efforts have been chosen for their societal relevance and scientific merit.  These plans were developed and formalized in the ACD Science Plan and were developed with the recognition that the accomplishment of any of the major goals will require significant collaboration with other NCAR divisions and with the university community.  The goals were determined as a result of an iterative process involving the division director, the division internal management and external advisory committees.

The main goal for Regional and Global Air Quality is to understand and quantify the impact of urban emissions on air quality. Priority will be given to studying the large-scale impacts of intense emissions originating from megacities, and the multiphase (gas-aerosol-cloud) processes that transform pollutants in the atmosphere. The goals for Chemistry in the Climate System are to understand the interactions between the physical climate system, the chemical climate system, and the biosphere. Priority will be given to the simulation of the recent past and future chemical climate states based on current climate simulations and to the study of the crucial role of the upper troposphere/lower stratosphere (UTLS) in the physical and chemical climate system.

The two primary research areas, Regional and Global Air Quality and Chemistry in the Climate System, encompass the research areas outlined in the 2003 ACD program plan.  The table below shows the relationship between the 2003 ACD Program Plan, the ACD Science Plan, and the 2004 ACD Program Plan.

Regional and Global Air Quality

Characterization of air quality is an important issue, not only in urban areas, but also on regional and global scales.  Satellite, aircraft, and ground-based observations throughout the global atmosphere are confirming that air pollution can be transported over large distances, e.g., from eastern Asia to the western U.S., from North America to Europe, and from mid-latitudes to the Arctic.

Discussions of air quality generally center on issues related to atmospheric oxidants and particulates.  Increases in ozone are of particular concern as this compound may impact humans and natural ecosystems, and is currently ranked as the third most important “greenhouse” gas.  Thus, future trends in its concentration have strong links to both global air quality issues and to climate change.  Potential global changes in OH radical concentrations are another major concern.  OH is responsible for initiating the oxidation of many compounds emitted to the atmosphere into more soluble species that are removed from the atmosphere by wet and dry deposition or adsorbed into aerosols in multiphase processes.  Critical challenges within the context of global change will be to determine if the capacity of the atmosphere to remove these gaseous species will be reduced or if climate change will affect the oxidation or emission processes and lead to significant change in the chemical composition of the troposphere. 

There is ample evidence for increases in oxidants and other pollutants in urban and regional environments, where air quality is a major issue.  Furthermore, transport of ozone and some ozone precursors from densely populated areas has led to increases on the global scale (estimated at 30% since the preindustrial era), and larger increases are expected with the demands of population growth.   One of the major research themes within ACD is to investigate the influence of the export of urban pollution on regional and global air quality.

Reactive carbon species, also referred to as volatile organic compounds (VOC), play a central role in determining tropospheric air quality.  Primary VOC emissions are extremely diverse chemically, with both natural and anthropogenic sources, and range in complexity from simple gases such as CO and methane to more complex chemical forms like the terpenes (naturally-occurring isomeric compounds of general formula C₁₀H₁₆).  Once in the atmosphere, these gases are oxidized to more soluble forms (carbonyl compounds, organic nitrates, organic acids, multi-functional species, etc.) that are more readily removed from the atmosphere by deposition, or that can couple back into the HO_x and NO_x chemical cycles - often at significant distances from the original source regions.  The cycle of VOC emission, transformation, and heterogeneous loss is a major topic of research within ACD.

Multiphase processes (gas-aerosol-cloud interactions) are increasingly recognized as important to tropospheric air quality.  High particulate concentrations, now common in and around urban centers but increasingly present on continental scales, pose a significant health risk.  Many aerosols serve as cloud condensation nuclei and therefore influence cloud particle properties (e.g., albedo) and precipitation.  They also scatter or absorb light, thus altering the atmospheric radiation field and reducing visibility.  In addition, particles can act as sinks for some atmospheric oxidation products including sulfuric and nitric acid, ammonia, and many products of reactive carbon oxidation.  Clouds also influence both tropospheric air quality and climate through their impact on the hydrologic cycle, as transporters, processors, and scavengers of pollutants, and via their effects on the tropospheric radiation field.  Convective clouds play a particularly important role as transporters of trace species from the polluted boundary layer to the upper troposphere, which enhances ozone production in the upper troposphere where absolute changes in ozone have their largest radiative impact.  Elucidation of the many complex physical/chemical processes involved in the multiphase chemistry of the troposphere - from particle nucleation and growth, to impacts of aerosols on gas phase composition, and the impacts of clouds as transporters and processors of trace gases is a major focus of ACD research activity.

 

Influence of Urban Pollution on Regional and Global Air Quality (Influence of Urban Emissions on Atmospheric Composition)

ACD efforts in this area include planning for the MIRAGE-Mex (Megacity Impacts on Regional and Global Environments, 2005 Mexico City campaign) field campaign, development of instruments to be used for tropospheric chemistry studies, development and deployment of community instruments and continued measurement support, participation in field campaigns that are complementary to MIRAGE, improvements to and use of MOPITT (Measurements of Pollution In The Troposphere) retrievals, photochemical research, and development and use of regional chemical transport models.

 

Community Involvement and C-130 Deployment (Megacity Impacts on Regional and Global Environments [MIRAGE] Planning)

While the importance of urban pollution to regional and global environments is widely acknowledged, it remains poorly quantified because of the complexity of physical and chemical processes that must be understood over a broad span of spatial and temporal scales. 

The MIRAGE program is a new activity under development by scientists at NCAR and the university community, based on the recognized need for an integrated, multidisciplinary approach to this complex environmental issue.  Planning is underway for a major field campaign in Mexico City in 2005 (MIRAGE-Mex).  Sasha Madronich, Frank Flocke, and John Orlando were involved in preparing and submitting a MIRAGE-Mex proposal to NSF.  The proposal will be reviewed by the National Science Foundation’s Observing Facilities Advisory Panel (OFAP) in April 2005.  Additional information on MIRAGE-Mex logistical and scientific planning is found below under the Strategic Initiatives – MIRAGE section.

 

Instrument Development

A number of instruments have been developed or improved by ACD staff for use in tropospheric chemistry studies.  These include the Peroxy Radical Chemical Ionization Mass Spectrometer (PeRCIMS), peroxyacyl nitrates (PAN), radiation, isotopes, and formaldehyde instruments.

 

Improvements to PeRCIMS Instrument

Chris Cantrell of the Atmospheric Radical Studies (ARS) Group has developed an instrument to measure peroxy radicals in the atmosphere.  The PeRCIMS instrument operates on the principle of chemical conversion of ambient or calibrator-generated peroxy radicals to gas-phase sulfuric acid, followed by chemical ionization by reaction with negatively charged nitrate ions, and separation and detection of the ions via mass spectrometry.  Signals for the total peroxy radical concentration [HO₂+RO₂] and for the hydroperoxy radical concentration [HO₂] have been acquired by greatly changing the concentration of the reagent gases, nitric oxide (NO) and sulfur dioxide (SO₂).  For HO₂+RO₂ measurements, the reagent concentrations in the inlet were in the tens of ppmv range.  For HO₂ measurements, they were in the thousands of ppmv range.  This necessitated the use of pure NO and pure SO₂ for the HO₂ quantification.  Recently, we have investigated the use of oxygen or nitrogen dilution as an alternative to changing the reagent gas concentrations.  The separation of HO₂ and RO₂ exploits the competition between the reaction of alkoxy radicals (RO), which are generated in the reaction of RO₂ with NO, with oxygen and NO.  When RO radicals react with oxygen, HO₂ is usually a product (depending somewhat on the structure of RO), which leads to measurement of the RO₂.  When RO radicals react with NO, alkyl nitrate stable products (RONO) are formed, which leads to RO₂ not being measured.  Thus, the parameter which affects the sensitivity to RO₂ is the NO/O₂ ratio.  At low NO/O₂ values, RO₂ radicals are measured efficiently, and at high values they are not.

Dilution of ambient air in a ratio of 1:10 with either oxygen or nitrogen leads to modulation of the oxygen concentration by a factor of 49, allowing RO₂ measurements with high efficiency (>80%) in the HO₂+RO₂ mode or fairly low efficiency (<30%) in the HO₂ mode.  The non-ideal responses (100% and 0% in the two modes, respectively) can be corrected using laboratory measurements of RO₂/HO₂ response factors and model estimates of the makeup of RO₂ species.  Comparisons of the reagent gas concentration change versus the oxygen modulation method were performed in the laboratory, and representative results are shown in Figure 1, along with a numerical model of the expected results.  These preliminary results are promising, and suggest that the oxygen modulation technique may be nearly as efficient at separating HO₂ and RO₂, with the advantage that change between the two modes of operation can be accomplished quickly.  Development will continue in FY05, with a design goal to quantify atmospheric HO₂ and HO₂+RO₂ within one minute.  This will require modifications to the inlet and the computer control software.

Figure 1: Measured and modeled response of PeRCIMS to RO₂ radicals versus the ratio of the concentrations of NO to O₂.  The large colored triangles show normal operating conditions using the method of changing reagent concentrations.  The large colored squares show potential operating conditions using oxygen dilution modulation.  Open circles show laboratory measurements of sensitivity for other operating conditions, using changing reagent concentrations.  The lines represent models of the sensitivity of RO₂ when changing reagent concentrations (blue shades) and oxygen dilution modulation (green shades).  The different curves correspond to different SO₂/NO ratios, in which the chain length (CL) is the SO₂/NO ratio divided by five.  Implicit in the model is an assumption about the rate of the reaction of RO with SO₂ to form HO₂ (inferred from laboratory studies).

 

Peroxyacyl Nitrates (PAN) instrument

Frank Flocke and Aaron Swanson (University of Colorado, Boulder (CU); NOAA Aeronomy Laboratory) flew a new instrument for fast (1-2 sec), continuous measurements of PAN and related species on the NOAA P-3B aircraft as part of the NOAA-led International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) and New England Air Quality Study (NEAQS) programs.  The instrument, called PAN-CIGAR (PAN-CIMS [Chemical Ionization Mass Spectrometer] Instrument by Georgia Institute of Technology [GIT] and NCAR), has been a joint development with Greg Huey and David Tanner (GIT).  Prior to deployment, much of the year was spent on redesigning a prototype instrument to make it ready for unattended aircraft operation.  The pump system was redesigned and gas loads were lightened.  The operation and data acquisition software was improved as well as some electronics to allow unattended operation and controlled recovery from power outages, etc.  Additional instrument characterization (investigation of the sensitivity dependence on humidity, flow, and pressure) was performed.  Other instrument improvements were made with help from David Hanson (ACD).  From October 2003 to early April 2004, Sandra Lopes, a graduate student from the University of Wiesbaden, Germany worked with Swanson and Flocke.  She successfully completed her Master’s degree in Chemical Engineering with them by participating in the development and construction of a new in situ calibration source for the instrument, a unit that is 50% smaller than the old unit and has augmented control capabilities for use with PAN-CIGAR.

 

Actinic Flux Spectroradiometer

Members of ACD’s Atmospheric Radiation Investigations and Measurement (ARIM) Group, Rick Shetter, Sam Hall, Barry Lefer, and Edward Riedel, developed a new generation actinic flux spectroradiometer.  The ARIM Group has deployed ground-based and airborne Scanning Actinic Flux Spectroradiometer (SAFS) instruments for the determination of actinic fluxes and photolysis frequencies for the last seven to eight years.  These have operated well, but have limited time resolution (~10 seconds for a spectrum) and require frequent calibration and maintenance.  A new instrument based on a monolithic monochromator with a UV-enhanced windowless CCD detector has been developed. This instrument is smaller and lighter with a new PC-104+ data acquisition and control system. The instrument has been quite reliable and has the ability to collect actinic flux spectra from 280 to 680 nm at 1-10 Hz, allowing for fast photochemistry investigations. The smaller design will allow deployment on aircraft platforms with limited space for instruments and operators including the NSF HIAPER (High-performance Instrumented Airborne Platform for Environmental Research) aircraft and future Unmanned Aerial Vehicles (UAV) platforms. One of these new generation instruments was deployed on the NASA DC-8 for the INTEX-NA experiment.

 

Analytical Photonics & Optoelectronics Laboratory (APOL) Technology Development in Airborne Observing Systems and Chemical Sensor Systems

Alan Fried, James Walega, and Dirk Richter of the joint Atmospheric Technology Division (ATD)/ACD APOL have developed and improved several instruments.

 

Continued Development of a New Laser Spectrometer for High Precision Measurements of ¹³CO₂/¹²CO₂ Isotopic Ratios

The Annual Scientific Report (ASR) last year described this project in detail along with progress achieved in designing and constructing the new instrument (APOL_(ACD/ASR03)).  Figure 2, which illustrates a modified version of the optical setup shown last year, has been constructed and a number of tests have been carried out. This new setup allows the sample and reference beams to make two traversals of the absorption cells for improved measurement precision. As discussed in our previous ASR, the absorption signals from the two cells containing the sample gas and an isotopic reference standard are rapidly compared using online fitting analysis. The fitting procedures have been developed and wait further testing on CO₂ absorption features, which should take place within the next month or two.

Figure 2: Optical schematic of modified Differential Frequency Generator (DFG)-based system for high precision carbon dioxide isotopic ratio measurements. 

 

Figure 3 shows the design of the inlet system, which represents the second major task associated with high precision CO₂ measurements. This system, which is in the testing phase, allows various combinations of reference standards and ambient samples to be introduced to the absorption cells under closely matched conditions of pressure and flow. Other studies frequently do not take such precautions.

   

 

Figure 4: Schematic of optical system with cutaway view of temperature-stabilized enclosure employed during INTEX-A. Two separate laser channels with separate beam collection optics are shown. However, during INTEX-A only one laser channel was employed. The enclosure dimensions are approximately 33” long x 24” wide x 12” high.

 

 

Figure 5: Photograph of the APOL Group’s DFG stage mounted to a new, more stable Herriott cell. The size of the DFG stage with enclosure is 7”long x 9.5” wide x 3.5” high. For comparative purpose, the cell here is the same size as the cell in Figure 4. 

 

 

 

Figure 6:  Power spectral distribution of  two-minute ambient CO₂ measurements of marine boundary layer air during the 2004 Gulf of Tehuantepec Experiment.  Observations were made at 380 msl off the coast of Salina Cruz, Mexico.

 

 

Figure 7: The power spectral distribution of a two-minute time series during which CO₂ calibration gas was sampled in-flight at 380 msl in the marine boundary layer.  The measurement represents a quantification of the spectral dependence of the air motion sensitivity of the CO₂ sensor.

 

A collaboration between the Atmospheric Technology Division, the Atmospheric Chemistry Division, and the Biogeosciences Initiative has been in place for several years to fund development and field support of airborne facility in situ trace gas instruments.  Supported developments include water vapor and fast-response ozone measurements in addition to carbon dioxide and carbon monoxide.  The activities have been successful on several fronts in maintaining and improving the quality of requestable instrumentation and data sets. 

The airborne CO₂ instrument was modified to enable flux measurements by the eddy covariance method.  Assessment of measurement capabilities was assessed by using the instrumentation on the GOTEX experiment (Kenneth Melville [Scripps Institution of Oceanography], Carl Friehe [University of California, Irvine]) and the Airborne Carbon in the Mountains (ACME) experiment (Russell Monson [CU], David Schimel, Jielun Sun, and Britton Stephens, [NCAR’s Climate and Global Dynamics [CGD] Division).  Spectral analysis of GOTEX marine boundary layer data imply an instrument frequency response of 4-Hz, and time series analysis implied a mixing ratio measurement precision of 0.2 - 0.3 ppmv (1-s for a 1-second average).  The flux artifact of this sensor due to air motion sensitivity was quantified in both marine and terrestrial boundary layer environments during both missions by introduction of calibration gas into the sample cell during a portion of a boundary layer transect.  A small artifact due to correlated air motion sensitivity was observed in GOTEX data at frequencies lower than 2 Hz.  However, the amplitude of these fluctuations is a factor of 50 smaller than the equivalent power spectral content of ambient data obtained in the marine boundary layer at the same altitude.  Preliminary ACME results from intercomparison to analyses of flask samples by Heather Graham and Ralph Keeling (Scripps Institute of Oceanography), imply an accuracy of ± 0.2 ppmv.  This result has been reproduced by intercomparison to CME ground network working standards provided and analyzed by the CO₂ calibration facility by Britton Stephens (UCAR/RAF).

The commercial NCAR open-path diode laser absorption hygrometer was modified to improve accuracy of lower tropospheric water vapor mixing ratio observations and to increase the time response of the instrument from an 8-Hz sampling rate to 18-Hz.   The modified laser hygrometer performed well as an experimental measurement on the second Alliance Icing Research Study (AIRS-II) and GOTEX experiments.  Results from the AIRS-II mission established confidence in the accuracy of clear sky water vapor measurements to be +/- 5%, relative to chilled mirror sensors on the same platform.  Preliminary results from the GOTEX data have confirmed the capability of this sensor to measure vertical fluxes by the eddy covariance method.  Spectral analysis of 18-Hz data from boundary layer transects agree well with lyman alpha hygrometer data.

To increase confidence in the absolute accuracy of all NCAR facility water vapor measurements, an accurate, large dynamic-range water vapor calibration system was purchased.  This commercial system includes a humidity generation system and a certified reference sensor with traceability to both the U.S. and U.K. standards organizations.  Both cover a dew point range of -80º to +20º C, and the reference sensor accuracy is ± 0.1º C in the -60º to +20º C range and ± 0.2º C in the -80º to -60º C region.  This facility is a crucial component of ensuring accuracy of water vapor measurements both in the troposphere and UTLS regions.

 

MSI Group Measurement Support

For the past decade Eric Apel of the Measurements, Standards, and Intercomparisons (MSI) Group has played a leadership role in community measurement support activities. These include supplying the community with primary calibration standards, conducting informal intercalibration exercises, and organizing and conducting formal intercomparisons of research instrumentation.

As part of the MSI Group’s continuing effort in community measurement support activities this year, Apel prepared and provided a primary standard which was used by all research groups involved with the measurement of volatile organic compounds during the ICARTT campaign. The standard contained the following compounds: Methane, CO, ethane, acetylene, ethane, propene, propane, butane, 134A, benzene, toluene, acetone, acetonitrile, carbon tetrachloride, chlorofluorocarbon (CFC) 113, and iso-propyl nitrate. The MSI Group received assistance from Steve Montzka and Paul Novelli at NOAA’s Climate Monitoring and Diagnostics Laboratory (CMDL) and Elliot Atlas of the University of Miami in the calibration of specific compounds in the standard.

In addition, Eric Apel, along with Alan Fried of ACD/ATD, organized and will convene a special session on “Tropospheric Photochemistry” at the annual AGU meeting in December.

This year the MSI Group also contributed to an ACD document outlining plans for a community facility to be housed in the new ACD building. This community facility would focus on chemical instrument calibration and validation, which are clearly-perceived needs in the community and a natural outgrowth of this Group’s past and ongoing community support activities. It is necessary to quantify measurement uncertainties for instrumentation involved in key atmospheric experiments in which results have implications for local and national policy decisions. Kuster et al., 2004, documents an intercomparison study that the MSI Group participated in during Texas Air Quality Study (TEXAQS) 2000, a study that clearly has policy implications. 

Managing the National Science Foundation Atmospheric Chemistry Program

Chris Cantrell accepted a temporary assignment with the National Science Foundation to help manage the Atmospheric Chemistry Program.  This was implemented under the Intergovernmental Personnel Act (IPA), in which persons “rotate” from their home institution for a period of time (usually one to three years) to perform services needed by the Federal government.  The Atmospheric Chemistry Program at NSF considers ad hoc proposals from the community to investigate problems related to atmospheric composition (gas phase and condensed phase), transport and transformation, interaction between ecosystems and the atmosphere, and connections between chemistry and climate through theoretical, observation, and laboratory investigations.  The program considers 150-200 proposals per year, which are each reviewed by four to six experts in the particular field of study, and the most meritorious are awarded funding.  Many projects are at the boundaries of atmospheric chemistry and other disciplines (e.g., climate, meteorology, Arctic or Antarctic science, oceanography, international), and as such are reviewed by two or more programs.  The assignment allowed a unique look at the federal process of funding scientific research, in particular the NSF process, and enabled Cantrell to become more familiar with the persons and activities in the atmospheric chemistry field.  The goal of the IPA program is for rotators to bring back to their home institution the information gained in order to improve the relationship with the agency.

There were also many opportunities to interact with representatives of other domestic and foreign agencies.  These processes can lead to enhanced interactions, and leveraging of limited funds that could enhance the outcomes of our investment in scientific research.

NSF also places a considerable emphasis on the “broader impacts” of scientific research.  Thus, given a project with high priority scientific goals, and reasonable chances of success, the principal investigator would be expected to contribute to the wider aspects of science including education and training activities that are integrated with the research, broader participation of underrepresented groups, enhance infrastructure, wide dissemination, and provide benefits to society.  Program officers at NSF examine the balance between the intellectual merit of quality, high priority scientific projects, and their contribution to these broader issues. 

Field Campaigns Complementary to MIRAGE

ACD scientists from the ARIM, APOL, AON (Atmospheric Odd Nitrogen), and STM (Stratospheric/Tropospheric Measurement) Groups participated in two field campaigns in the summer of 2004, which were under the auspices of the ICARTT field campaign.  These campaigns were clearly complementary to the objectives of MIRAGE (see Strategic Initiatives-MIRAGE section below) and the results will provide an improved understanding of pollution transport across the U.S. and the northern Atlantic to Europe.

 

International Consortium for Atmospheric Research on Transport and Transformation (ICARTT)

Several groups in North America and Europe independently developed plans for field experiments in the summer of 2004, designed to provide improved understanding of the factors that shape air quality in their respective countries and the remote regions of the North Atlantic.  NASA and NOAA conducted experiments under the INTEX-NA and the NEAQS - ITCT (Intercontinental Transport and Chemical Transformation) 2004 programs respectively.  The Europeans (U.K., Germany, and France) organized coordinated studies under Intercontinental Transport of Pollution (ITOP).  While each of these programs has regionally focused goals and deployments they share many of the same goals and objectives and the proposed study areas overlap significantly.  ICARTT was formed to take advantage of this synergy by planning and executing a series of coordinated experiments to study the emissions of aerosol and ozone precursors their chemical transformations and removal during transport to and over the North Atlantic.  The capabilities represented by the consortium will allow an unprecedented characterization of the key atmospheric processes.  Several ACD groups participated in INTEX-NA and NEAQS-ITCT 2004 as described below.

 

NASA Intercontinental Chemical Transport Experiment (INTEX-NA)

INTEX-NA was a major NASA science campaign to understand the transport and transformation of gases and aerosols on transcontinental and intercontinental scales and their impact on air quality and climate.  A particular focus of INTEX-NA was to characterize and quantify the inflow and outflow of pollution over North America.  Measurements from INTEX-NA will also provide important validation of satellite observations with ongoing satellite measurement programs, such as Terra, Aura, and the European Space Agency’s (ESA) Envisat (Environmental Satellite). The experiment was conducted over the continental United States during the summer of 2004 using a variety of science aircraft. 

The ARIM Group deployed two SAFS systems on the NASA DC-8 aircraft to measure the in situ total spectral actinic flux as a function of wavelength and calculate the in situ photolysis frequencies of 23 important photochemical reactions.  Results are discussed in the Photochemistry/Radiation section below. 

The APOL Group deployed the modified TDLAS system on the NASA DC-8 to measure formaldehyde (CH₂O). The instrument improvements worked extremely well in all respects, and this allowed the Group to successfully acquire ambient CH₂O measurements on 19 out of the 20 mission flights.  Preliminary results are discussed in the Photochemistry/Formaldehyde section below.

 

NOAA New England Air Quality Study – Intercontinental Transport and Chemical Transformation (NEAQS-ITCT 2004) (formerly Northeast-North Atlantic [NENA] 2004)

NOAA's Health of the Atmosphere research is focused on the atmospheric science that underlies regional and continental air quality, with the goal of enhancing the ability to predict and monitor future changes. Under this program NOAA organized the NEAQS-ITCT 2004 campaign with the P-3B aircraft and NOAA research vessel Ron Brown operating out of New Hampshire.  The study focused on air quality along the Eastern Seaboard and transport of North American emissions into the North Atlantic.

The AON Group deployed the PAN-CIGAR to measure PAN and related compounds.  These measurements are a crucial part of evaluating photochemical processing and the influence of natural and anthropogenic hydrocarbons.  Initial results are discussed in the Photochemistry/PAN section below.

Also, as a contribution to the NEAQS program, a sensitive NO instrument that the AON Group had previously flown on aircraft was modified and lab-tested to loan to Alex Pzenny (University of New Hampshire) and Eric Williams (NOAA Aeronomy Laboratory) for use at one of the ground sites.

The STM Group deployed a Whole Air Sampler on the P-3B to collect air samples for analysis of hydrocarbons, halocarbons, alkyl nitrates, and CO.  These measurements are required to adequately assess photochemistry and transport.  Sample analysis was recently completed and data quality control and analysis is currently underway.

 

Measurement of Pollution In The Troposphere (MOPITT)

The MOPITT experiment has provided atmospheric scientists with the first multi-year, global view of carbon monoxide (CO) profile concentrations in the troposphere. The experiment is approaching the five-year anniversary of its launch on the Terra satellite in December, 1999. With this, the project at NCAR will pass a milestone, with the completion of the initial phase that began more than 10 years ago. During this period the data processing software was developed and the first years of orbital data processed, validated, archived, and provided to the atmospheric chemistry community, as well as being used for scientific research. The mission is a joint Canadian-U.S. effort, and ACD is responsible to NASA for the continued development of the data reduction algorithms and for operational data processing at every stage from instrument counts to calibrated radiances, through to globally retrieved CO vertical profiles and total columns.  MOPITT Version 3 validated tropospheric CO profiles are currently being processed and delivered to NASA for use by the international community. This is the first dataset of its kind, and represents a significant advance in satellite remote sensing of the troposphere. The next phase of the core effort will concentrate on some additional improvements to the retrieval algorithms, as well as continuing with the scientific applications of the data.

The MOPITT Group has been active in 3 main areas this year: (1) continued improvement of the data reduction algorithms in preparation for Version Four, data processing, and delivery of the final products to NASA for later distribution to the community; (2) support of aircraft field campaigns; and (3) use of the MOPITT data in tropospheric chemistry and transport studies. More information about the MOPITT project can be found at http://www.eos.ucar.edu/mopitt/ and http://www.atmosp.physics.utoronto.ca/MOPITT/home.html.

 

MOPITT Accomplishments

 

Operational Data Production

The NCAR MOPITT team, led by John Gille, has been engaged in the ongoing data reduction process to produce geophysical quantities for scientific use by the community from the instrument count data. The elements of this processing capability and the people responsible, are the data handling interfaces and protocols between NCAR and the NASA centers which receive and archive the satellite data and the ancillary meteorological data (Daniel Ziskin, Jarmei Chen); the Level 0-1 processor which calibrates the instrument counts to produce geolocated radiances (Ziskin, Debbie Mao, Chen); and the Level 1-2 processor which comprises a forward model which provides a full simulation of the MOPITT measurement, a cloud detection algorithm, and a retrieval algorithm (David Edwards, Gene Francis, Merritt Deeter, Ben Ho, and Gille). The retrieval combines information from the measurements, the forward model, and previous measurements which define the current understanding of the atmosphere, to obtain the most likely CO profile consistent with the measured MOPITT signal. The Moderate-resolution Imaging Spectroradiometer (MODIS) cloud mask is also used operationally in the current data processing to provide added information about cloud contamination in the MOPITT field of view.

The algorithms continue to be developed. The data for the mission Phase 1, before the instrument cooler failure in April 2001, have been designated as a validated product. The data for the post-April 2001 period, Phase Two, are currently provisionally validated and will be upgraded to fully validated in the near future. Work has continued to characterize the instrument performance in the Phase Two period, and to develop the retrieval algorithms to ensure maximum utilization of the instrument signal to retrieve CO vertical structure.

The algorithms are now being prepared for the next data Version Four. Enhancements will include: (1) a new forward model with improved description of the MOPITT gas correlation cells; (2) an new description of the retrieval a priori surface emissivity; and (3) a variable CO retrieval a priori. The validation of MOPITT retrieved surface skin temperature using collocated MODIS measurements has been a major effort led by Ho. Surface skin temperature information retrieved from MODIS measurements has been used to generate a more accurate global monthly surface a priori emissivity (mean and variance) for use in the MOPITT retrieval algorithm to improve retrievals of surface temperature and CO profile. Monthly 1 degree grid mean 4.7 micron surface emissivity maps have been generated using an iterative algorithm taking as inputs MODIS skin temperature and surface characterization index in combination with MOPITT A-signals, which are most sensitive to the background scene. Analysis of the impact of the new a priori emissivity on the retrievals of tropospheric CO profiles indicates that more vertical information can be obtained, especially at night. This has the effect of reducing the apparent diurnal change in the MOPITT CO retrieval. A manuscript detailing this work is now in preparation, and this will be used in the new Version Four algorithm. The adoption of a new seasonally- and latitudinally-variable CO retrieval a priori (based on both in situ data and current operational MOPITT retrievals) is also planned for version 4. This will hopefully reduce retrieval biases that have been characterized by validation studies. This work is being led by Deeter.

 

MOPITT Data Validation

The on-going activity of validation of the MOPITT CO retrievals (Louisa Emmons) shows the continued accuracy of the data.  In situ measurements from small aircraft, provided by Paul Novelli (NOAA CMDL), continue to form the basis for the validation comparisons.  Figure 8 shows the high correlation between MOPITT retrievals and the in situ data (transformed with the MOPITT averaging kernels) for the Phase Two data through 2003. 

 

Figure 8: Validation of MOPITT CO retrievals for August 2001 to December 2003. Correlations of MOPITT CO with aircraft in situ data are shown for each retrieval level and the total column.  Aircraft CO profiles, provided by Paul Novelli (NOAA CMDL) for five sites, have been transformed with the MOPITT averaging kernels.

The vertical resolution of the MOPITT CO retrievals is a key indicator of retrieval performance and is of interest to many end users. However, vertical resolution is highly variable, and depends on retrieval configuration, surface temperature and emissivity, atmospheric temperature profile, etc. One index for quantifying vertical resolution is termed Degrees of Freedom for Signal (DFS).  DFS approximately represents the number of layers in the retrieved profile which are retrieved independently. Analysis of the MOPITT averaging kernels indicates that current MOPITT retrievals contain up to two pieces of information corresponding to the mid- and upper-troposphere. Figure 9 shows the latitude dependence of the zonal-mean DFS for both daytime and nighttime retrievals, and for both Phase One and Phase 2 periods. Most importantly, the figure shows that retrieval information content decreased only marginally as a result of the loss of half of the MOPITT channels in 2001. This work was recently published in Geophysical Research Letters.

Figure 9: Latitude dependence of the zonal-mean DFS for both daytime and nighttime retrievals, and for both Phase One and Phase 2 periods.

Studies were completed using the 2001 flight data taken by the MOPITT Algorithm Test Radiometer (MATR) aircraft instrument (Alan Hills, Janguo Nui, Deeter) for (1) during April and May over the Oklahoma Cloud and Radiation Testbed (CART) site, and (2) during November over three western U.S. cities (Los Angeles, Las Vegas, and Denver).  These flights were supported with in situ measurements by Paul Novelli (NOAA CMDL). MATR measurements over the western cities show plumes of CO, the direction and distribution of which appear to agree well with the prevailing meteorology. This work has recently been published in Applied Optics.

 

MOPITT Education and Outreach

The MOPITT website continues to be developed. This is regarded as an important means of community outreach and education. The website currently includes details of how to obtain the data and how to use it correctly, along with other MOPITT references.

The MOPITT team makes regular story contributions to the NASA Earth Observatory website. These often highlight intense burning events around the world that are of general interest. The team was also involved in the production of an animation of MOPITT data with accompanying story that was produced by the American Museum of Natural History for distribution as an educational tool. 

 

MOPITT Science Studies

MOPITT Participation in INTEX-A

Remote sensing of CO directly addresses many of the scientific questions motivating aircraft field campaigns, and measurement of this gas is usually rated as being mission critical. CO is an important trace gas in tropospheric chemistry due to its role in determining the atmospheric oxidizing capacity, as an ozone precursor, and as an indicator and tracer of both natural and anthropogenic pollution arising from incomplete combustion. The satellite perspective provides the more general temporal and spatial context to the aircraft and ground-based measurements. During the summer 2004 NASA ICARTT/INTEX campaign, MOPITT CO retrievals were provided in near-real-time using an expedited data processing stream (managed by Ziskin, Chen, and Mao) for use in field analysis and flight planning (Edwards, Emmons and Gabrielle Pfister). Maps of the data were created and posted on the MOPITT website, and Emmons and Pfister were in the field to present and discuss the MOPITT data during daily planning meetings. The large wildfires in Alaska during the summer of 2004 had a strong impact on the CO concentrations in the Midwest and eastern U.S. at times, and the MOPITT data were valuable for tracking the high CO values.  This is shown in Figure 10.

Figure 10: MOPITT 700 hPa CO mixing ratio for the period 15-23 July, 2004, showing the plumes produced by fires in Alaska. The emissions from these fires can be traced across North America and The Atlantic Ocean.

 

INTEX-A also provided MOPITT validation opportunities with in situ CO measurements on the DC-8 from G. Sachse, NASA Langley. Edwards was also Co-Investigator on the French component (ITOP) of the overall ICARTT mission, and on-going collaboration with Jean-Luc Attie at the Laboratoire d'Aerologie in Toulouse, France, resulted in a near-real time assimilation of the MOPITT data into the MOCAGE model for forecasting air quality in the area of flight operations. Measurements taken during the ICARTT campaign are now the subject of several scientific investigations into North American air quality modeling and inverse modeling to improve source emission estimates.

 

Long-range Transport of CO Emissions from Russian Fires

Studying satellite data on global air pollution levels, Edwards and other MOPITT team members at NCAR and other institutions noticed an unusual spike in CO levels in late 2002 and early 2003 that affected the entire Northern Hemisphere. Analysis pointed to Russian source: massive peat fires smoldered near Moscow in the late summer and fall of 2002, and unusually intense Siberian wildfires broke out in the spring and summer of 2003. During the peat fires, Russian colleagues working with spectrometer data from the Obukhov Institute of Atmospheric Physics in Moscow (Leonid Yurganov and Evgeny I. Grechko) took air quality measurements from Moscow and a nearby site at Zvenigorod. The research combined the ground-based measurements with observations of atmospheric pollutants from two instruments on the Terra satellite: CO levels from MOPITT, and aerosol optical depth from MODIS. The results showed that smoke and pollutants from the fires had been carried over much of the Northern Hemisphere, affecting CO levels as far away as North America.

Edwards and colleagues also looked at the impact of emissions from Siberia and other parts of eastern Russia. Massive fires in the giant boreal forests and on the tundra, often started by farmers, typically rage throughout the spring and summer. The 2003 fire season was particularly intense. Satellite data showed plumes from Asia following the jet stream, with high levels of CO crossing the Pacific Ocean, reaching western Canada, and traveling down to the U.S. East Coast—evidence of the global impacts of large-scale fires (Figure 11). The research will be published in the JGR. 

Figure 11: In the spring of 2003, the MODIS instrument on the Terra satellite instrument detected a large number of fires in Siberia, especially in the Baikal region (top). These fires produced large amounts of fine carbon aerosol, also detected by MODIS, that spread over the Pacific Ocean but lasted only a few days (center). They also produced CO, which was detected by the MOPITT (bottom). CO can last over a month, which allowed it to cross the Pacific Ocean and reduce air quality over North America before continuing on around the globe.

  

The CO Budget over Europe

Pfister completed a study on the CO budget for Europe. For this work, CO retrieved from MOPITT was used in conjunction with ground-based data provided by the Austrian Federal Environment Agency and CMDL and aircraft instruments (data provided by the German Aerospace Center [DLR] and National Center for Scientific Research, Paris [CNRS]), and compared to simulations from the MOZART CTM. The model simulations were provided by Gabrielle Petron (NCAR’s Advanced Study Program [ASP]). The results showed that, on average, the largest contribution to the anthropogenic CO load over Europe at the surface comes from regional sources. Further significant impact is evident from sources in North America and Asia. With increasing altitude the influence of European sources weakens, and the impact of CO from other continents gains in importance with Asian emission showing the highest contributions during most months and at most heights. Studies where the CO was tagged in the model according to the time of emission showed that the transport time needed for pollution from North America to reach Europe might be as short as one week. Asian emissions can be seen in Europe less than two weeks after being emitted when transported towards the northwest. The results of these studies have been published in JGR.

 

Pollution from Mega-Cities

Work has begun to see if the MOPITT instrument could be used to track CO pollution originating from cities (Cathy Clerbaux). Though MOPITT was not optimized to detect pollution plumes from cities due to poor measurement sensitivity in the boundary layer, initial results look promising. By averaging the data over four years to reduce noise from transient plumes, signatures from cities have been distinguished from transported emissions. This pollution from distant sources, clouds and the instrument's low boundary layer sensitivity bias the results. This initial investigation focused on cities such as Mexico City, Kuala Lampur, Singapore, Jakarta and Surabaya, Indonesia. MOPITT shows how El Niño-related wildfires in Kalimantan on Borneo Island in Indonesia, contaminated the air in 2002 above Jakarta and Surabaya. The instrument also shows how pollution gets dispersed from cities. Mexico City and Jakarta are both surrounded by mountains. Due to topography, the satellite reveals that pollution can only escape upward or through openings---to the north for Mexico City and east for Jakarta. This work will provide the basis for future efforts in support of the MIRAGE field campaign in Mexico City.

 

Involvement with Other Satellite Missions

Collaborations are underway with Larrabee Strow and Wallace McMillan (University of Maryland, Baltimore County [UMBC]) who are investigating the potential for retrieving CO from the Aqua/AIRS (Atmospheric Infrared Sounder) instrument. Data are also being made available to the National Institute for Space Research in The Netherlands (Ilse Aben) for use in CO validation studies for the Envisat/SCIAMACHY (SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY) instrument. MOPITT data will also provide key validation of the CO retrievals from the recently launched NASA Aura satellite, carrying the TES (Tropospheric Emission Spectrometer) instrument, and Edwards is the Aura Validation Team Liaison for MOPITT.

 

Seasonal and Inter-Annual CO Variability

Now that there is an almost five-year record of MOPITT data, the seasonal and inter-annual CO variability may be examined. Edwards has led a project to investigate the extent to which seasonal variability is explained by the CO photochemical sink in conjunction with sporadic perturbations caused by wildfires.  Gille and Yudin have also studied interannual variations by using MODIS Fire counts to identify fire sources and have compared with year-to-year CO variations.  Using inverse modeling tools in conjunction with the MOZART CTM and surface network measurements, they estimated the potential inter-annual variations of the biomass burning over North Asia.  This work is being prepared for submission to GRL.

 

Data Assimilation and Inverse Modeling

Work continues on the assimilation of MOPITT retrievals in chemistry transport models and on inverse modeling of CO surface emissions using different schemes and techniques. Two articles on  the inverse modeling of CO surface fluxes  (Petron et al., 2004) and assimilation of  MOPITT CO retrievals with optimized fluxes (Yudin et al., 2004) will be published in the next issues of GRL. Jean-Francoise Lamarque is leading an effort to evaluate biomass burning emission inventories using the assimilation of MOPITT data.

 

Comparison of CO and Aerosol Emissions from Biomass Burning

Edwards is leading a study to investigate emissions from biomass burning in Africa and South America. This is a major source of pollution in the tropical troposphere and a major forcing of tropospheric chemistry. The outflow of both aerosol and CO from the widespread fires in southern Africa and South America during September-November 2000, is being studied using observation from the Terra satellite, CO profiles from MOPITT and aerosol optical depth from MODIS. Recent developments in aerosol retrieval allow the distinction to be made between fine and coarse mode particles. The fine mode particles are produced by the same anthropogenic combustion processes that emit CO, and comparison of the fine mode aerosol and CO distributions provide information about biomass plume aging and the associated advection and convection of the emissions. The long range transport of these pollutants and the impact on atmospheric air quality in remote oceanic regions is also being studied, along with the seasonal changes in the pollutant distributions by comparing with the observations of CO and fine mode aerosol made during the December 2000-February 2001 period. A paper describing this work will soon be submitted to JGR.

  

Photochemistry

Urban areas are strong sources of pollutant gases and aerosols that can influence both regional and global environments.  These pollutants include primary pollutants released into the urban atmosphere (primary pollutants, e.g., hydrocarbons and NO_x), as well as secondary pollutants produced in the atmosphere by photochemical processing (secondary pollutants, e.g., ozone, inorganic and organic nitrates, acids, peroxides, carbonyls, and other partly oxidized organics).  Many of these secondary pollutants play an important role in the atmosphere far from source regions, for example as sources of HO_x and NO_x in the remote atmosphere.  Understanding the details of this photochemical processing is a primary focus of ACD.

 

Peroxy Radical Studies

Chris Cantrell of the ARS Group investigates the behavior of odd hydrogen and organic radical species in the troposphere through observations using state of the art instrumentation.  Over the last few years, the ARS Group has developed a chemical ionization mass spectrometric-based method for the quantification of hydroperoxy and organic peroxy radicals, which has been deployed in several aircraft- and ground-based campaigns.  Work this year involved publication of results from recent studies and further laboratory work to improve the performance of the instrument during field deployments.  Progress was slowed by the fact that Cantrell took a temporary assignment as Associate Program Director for Atmospheric Chemistry at the National Science Foundation in Arlington, Virginia, from August 2003 to August 2004.

In 2001, observations of peroxy radicals were made from the NASA P-3B aircraft during the Transport and Atmospheric Chemistry near the Equator – Pacific (TRACE-P) campaign.  This experiment examined the emissions and evolution of trace gases from eastern Asia.  Funding for participation in the campaign, analysis of the observations, and preparation of a manuscript (Cantrell et al., 2003) were provided by the NASA Tropospheric Chemistry Program.  The paper presents methods for approximating the concentration of free radical species and ratios of species.  A steady state model was employed that satisfactorily reproduces the observed concentrations except during encounters with clouds or air masses with high aerosol surface area densities, and when the NO concentrations were very high (> 0.5 ppbv) or very low (<5 pptv).  The observations provided indirect evidence of the heterogeneous uptake of radicals by cloud droplets and aerosols.  The failure of the model and low and high NO concentrations implies missing or misrepresented processes that will be investigated further.  Cantrell contributed to several other papers related to the TRACE-P campaign.

The TRACE-P campaign involved significant collaboration with scientists from universities and national laboratories.  The members of the Science Team are shown in Table 1.

ARS Table 1: Science Team members for TRACE-P mission. 

 

Bruce E. Anderson	NASA Langley
Eric C. Apel	NCAR
Elliot L. Atlas	NCAR
Melody A. Avery and Stephanie A. Vay	NASA Langley
Alan R. Bandy	Drexel University
Donald R. Blake	University of California – Irvine
Edward V. Browell	NASA Langley
William H. Brune	Pennsylvania State University
Christopher A. Cantrell	NCAR
Gregory R. Carmichael	University of Iowa
Antony D. Clarke	University of Hawaii
James H. Crawford	NASA Langley
Douglas D. Davis	Georgia Institute of Technology
Fred L. Eisele	Georgia Institute of Technology
Johann Feichter	Max-Planke-Institut fur Meteorologie
Frank Flocke	NCAR
Alan Fried	NCAR
Henry E. Fuelberg	Florida State University
Brian G. Heikes	University of Rhode Island
Daniel J. Jacob	Harvard University
Makoto Koike	Nagoya University
Yutaka Kondo	Nagoya University
Reginald E. Newell	MIT
Samuel J. Oltmans	NOAA
Michael J. Prather	University of California – Irvine
Daniel D. Riemer	University of Miami
Glen W. Sachse	NASA Langley
Scott T. Sandholm	Georgia Institute of Technology
Richard E. Shetter	NCAR
Hanwant B. Singh	NASA Ames
Robert W. Talbot	University of New Hampshire
Anne M. Thompson	NASA Goddard
Charles R. Trepte	NASA Langley
Rodney J. Weber	Georgia Institute of Technology

 

In May and June of 2002, a ground-based comparison of the NCAR peroxy radical CIMS instrument (PeRCIMS) and Bill Brune’s (Pennsylvania State University) Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS) was carried out.  It involved comparing ambient concentrations of HO₂ and HO₂+RO₂ radicals and comparing signals from two synthetic radical sources.  The results were very good and indicated the ability to independently generate known peroxy radical levels (see Figure 12), and the ability to quantify ambient radical levels.  The results were published this report year (Ren et al., 2003).  The outcomes were encouraging enough that a second comparison may take place in the spring or summer of 2005.  This project involved collaboration with NCAR and Pennsylvania State University faculty and staff (see Ren et al. [2003] author list) that led to collection of valuable data and publication of two papers (Ren et al., 2003; Ren et al., 2004).

 

 

 

Figure 12: Comparison of GTHOS measured HO₂ with that generated by the NCAR calibrator (top panel), comparison of PeRCIMS measured HO₂ with that generated by the Penn State calibrator (middle panel) and comparison of PeRCIMS and GTHOS measurements on the same calibrator, either NCAR (squares) or Penn State (circle) (bottom panel).

 

Two final papers were published this year on the ACD-led TOPSE (Tropospheric Ozone Production about the Spring Equinox) campaign that involved measurements of a number of species aboard the NCAR/NSF C-130 aircraft during the winter to spring seasonal transition in 2000.  These deal with the behavior of reactive nitrogen species (Murphy et al., 2004) (in collaboration with Ronald Cohen and students at the University of California, Berkeley) and photochemical ozone production (Stroud et al., 2004) (in collaboration with internal and external TOPSE participants) and help to complete the interpretation of the TOPSE observations.

One final paper on the ACD-led International Photolysis frequency Measurement and Modeling Intercomparison (IPMMI) exercise was published this year related to the observations and radiative transfer modeling of the UV-B photolysis of ozone, j(O(¹D)) (Hofzumahaus et al., 2004).  The IPMMI campaign and subsequent analysis involved collaboration with a number of scientists from the U.S. and Europe, and summarized the ozone photolysis results that solidifies the current understanding of the ozone molecular parameters and ability of the community to accurately measure and model this quantity.

Additional laboratory work on peroxy radicals was done by members of the Laboratory Kinetics Group, Geoff Tyndall and John Orlando, who, in collaboration with Alam Hasson (California State University, Fresno), conducted detailed modeling studies of previously reported laboratory experiments which revealed substantial yields of hydroxyl radicals in the reactions of HO₂ with both acetyl peroxy and acetonyl peroxy radicals.  In the former reaction, an OH yield of 40% was found, while in the latter case a yield of 67% was obtained.  Prior to the beginning of this work, no evidence had been reported for the direct production of OH.  Modeling studies indicated that the effects on tropospheric ozone and OH would probably be small (<5%); however, the implications for the stable oxygenated products would be greater.  The reaction of HO₂ with acetyl peroxy has previously been thought to produce only acetic and peracetic acids.  The yields of these compounds (and hence their source from this reaction) will now have to be reduced by almost a factor of 2.  If other peroxy radicals from oxygenated species also undergo a similar reaction, the predicted yields of hydroperoxides will be substantially reduced.  (Hasson et al., J. Phys. Chem. A., 2004).

 

Peroxyacyl Nitrates (PANS)

As mentioned above, Flocke and Swanson flew a new instrument, the PAN-CIGAR (PAN-CIMS Instrument by GIT and NCAR), for fast (1-2 sec), continuous measurements of peroxy acetyl nitrate (PAN) and related species on the NOAA P-3B aircraft as part of the NOAA-led ICARTT and NEAQS programs.

The instrument was flown during ICARTT missions without an on-board operator and produced better than 85% data coverage for the entire mission.  PAN, PPN, MPAN, APAN, PiBN, PBzN and MethoxyPAN were measured simultaneously.  The performance is demonstrated in Figure 13, which shows some results from the July 20 flight while near 1 km altitude and while crossing the polluted outflow from New York City at a downwind distance of about 120 km. 

Figure 13: Preliminary data from the 7/20/2004 flight through the NY City Plume, ~120 km downwind, ~1 km altitude, during the ICARTT program. 

 

While this outflow event shows a large amount of structure, probably caused by air masses of different photochemical age and different initial pollution levels, an excellent correlation down to the fine time scale of seconds is observed between ozone and PAN as a result of photochemical activity.  These PAN data are the first reliable high-time resolution (2 sec) continuous measurements made from an aircraft.  Similar transects of biomass burning plumes encountered during ICARTT will allow unprecedented analysis of the partitioning of reactive nitrogen species in aged plumes.  The accuracy of the instrument could be verified through comparison with other measurements of reactive nitrogen and the sum of all reactive nitrogen on board the NOAA P-3 aircraft, and comparisons with instruments on-board the NASA DC-8, that also flew in conjunction with some of the ICARTT missions.  Two abstracts analyzing results from the flights have been submitted for the AGU Fall Meeting. 

ICARTT/NEAQS was an international multi-aircraft, ship and ground-based program including instrumented aircraft from England and Germany.  Principal collaborators with the PAN Group were James Roberts, Andy Neuman, John Nowak, and Fred Fehsenfeld, all of the NOAA Aeronomy Laboratory.

 

Formaldehyde (CH₂O)

The APOL Group (Alan Fried, Jim Walega, and Dirk Richter) has been actively involved in efforts to advance understanding of tropospheric photochemistry based upon high quality ambient CH₂O measurements acquired during various field campaigns. Last year’s ASR describes one aspect of this activity involving the APOL Group’s TRACE-P data. The Group is continuing with this endeavor this year with INTEX-NA data. Although it is far too early to formulate a comprehensive story, a preliminary first look at this data reveals a very important finding: namely, the persistence of elevated CH₂O mixing ratios at high altitudes during the summer time over the continental United States. Persistent continental thunderstorm activity during the summer months vertically convects polluted boundary layer air to high altitudes where it eventually mixes with cleaner background air in the outflow of clouds. Although this process has been well established, vertical convection of soluble and reactive gases, like CH₂O, has not been well characterized during such events. The flight tracks during the INTEX-NA study, which passed over natural and anthropogenic source regions of the south and mid-west United States, provided an excellent opportunity to study cloud outflow and cloud processing events.

The histogram plot of Figure 14 displays the APOL Group’s one-minute CH₂O measurement distributions acquired during four early flights in July on NASA’s DC-8 aircraft during the INTEX-NA campaign. These measurements were all acquired at pressure altitudes > 30,000 feet where one typically expects to observe CH₂O mixing ratios around 50 to 100 pptv. Although the observed spread is fairly large, the median value of 192 pptv is more than a factor of 2 higher than typical expected background levels. Such elevated CH₂O levels in the upper troposphere can play a significant role in the HO_x radical budget since the primary source of these radicals, O¹(D) + H₂O, decreases as the available H₂O vapor decreases with altitude. Despite the fact that this observation is based on very limited data, the Group observed a similar persistence of elevated CH₂O at high altitudes during many other flights.

 

Figure 14: Histogram of the CH₂O distribution observed Palt > 30 kft during four selected INTEX-NA flights in early July 2004.

 

Radiation

Photochemical reactions provide the driving force for much of the chemistry in the atmosphere.  The in situ rates of these photolysis reactions are important in understanding production and loss terms for the key atmospheric species odd hydrogen radicals and ozone.  The objective of this research by the ARIM Group (Rick Shetter, Barry Lefer, and Sam Hall) was to deploy two SAFS systems on the NASA DC-8 aircraft platform during the INTEX-A mission in the summer of 2004 to measure the in situ total spectral actinic flux as a function of wavelength and calculate the in situ photolysis frequencies of 23 important photochemical reactions.  Photolysis frequencies for photodissociation reactions involving O₃, NO₂, CH₂O, HONO, HNO₃, N₂O₅, HO₂NO₂, PAN, H₂O₂, CH₃OOH, CH₃ONO₂, CH₃CH₂ONO₂, CH₃COCH₃ , CH₃CHO, CH₃CH₂CHO, CHOCHO, CH₃COCHO, CH₃CH₂CH₂CHO, and CH₃COCH₂CH₃ will be calculated from the measured UV-VIS spectral actinic flux. The instrumentation, which has flown on the NASA DC-8 during the Pacific Exploratory Measurements (PEM)-Tropics A , Subsonic Assessment, Ozone and Nitrogen Oxide Experiment (SONEX), PEM-Tropics B, Stratospheric Ozone Loss Validation Experiment (SOLVE), and the TRACE P missions, had an approximated detection limit of less than 0.01 W/m2/nm dependent on wavelength resulting in photolysis frequency detection limits of 2 X 10^-7 sec^-1 for jO(¹D) and 1 X 10^-7 sec^-1 for jNO₂, all with a time response of 10 seconds.

In addition, the ARIM Group deployed a newly developed actinic flux system equipped with a monolithic monochromator with a cooled back thinned windowless CCD detector and a PC014+ based data acquisition and control system. This new system will have a time response of 0.1 to 10 HZ.

The instruments performed quite well on all of the aircraft flights with 100% data return. Final calibrations are currently being performed and data will be submitted to the INTEX/ICARTT archive in the next few months for use by the INTEX and ICARTT science team members.

 

Regional and Global Chemical Transport Models and Air Pollution Studies

Many of the chemical and physical processes that occur in the atmosphere are non-linear, so it is not possible to apply simple scaling relationships (e.g., simple dilution) to estimate the impacts on chemistry and climate of current emissions and future urban growth.  Therefore, detailed numerical model representations of the primary chemical and physical processes are required.

 

Weather Research Forecasting with Chemistry (WRF-Chem)

ACD scientists are playing a major role in the development and testing of a new, state-of-the-art regional chemistry transport model, the Weather Research Forecasting with Chemistry (WRF-Chem) model.  Xuexi Tie, Sasha Madronich, and visiting graduate students, Zhuming Ying (York University) and GuoHui Li (Texas A&M University [TAMU]), are collaborating with George Grell of NOAA’s Forecast Systems Laboratory (FSL) to implement chemical models developed at NCAR into WRF-Chem and to apply the model to the planning of a major international field campaign, MIRAGE-Mex, planned for early 2006.

The WRF-Chem model was successfully ported over the central Mexico region, with emissions data for Mexico City and the surrounding region and was used to perform simulations at various resolutions (4 km, 6 km, and 12 km).  Mexico City emissions were implemented through a collaboration with Aron Jazcilievich of the Universidad Nacional Autonoma de Mexico (UNAM).  Preliminary results show that the modeled temperatures and winds reproduce well the observed diurnal cycles, but with a cold bias indicating that the “heat island” of the city is not yet well represented by the model.  The diurnal cycles and magnitudes of surface concentrations of CO, NO_x (NO +NO₂), and O₃ within Mexico City compare well with observations, and show that the model is able to capture high pollution events.  In anticipation of the MIRAGE-Mex field campaign, the interaction of urban outflow plume with the surrounding region was studied.  For this study, Alex Guenther and Christine Wiedinmyer provided current estimates of regional biogenic (vegetative) emissions, and Jean-Francois Lamarque provided biomass burning emissions.  Preliminary results show (Figure 15) that this interaction leads to more regional O₃ production than would be expected from the three sources (urban, biogenic, and biomass burning) taken in isolation.  These preliminary results have been presented at conferences (Tie, and Madronich, Modeling of Mexico City Air Pollution and Outflow with WRF-Chem, 5^th WRF/14^th MM5 User’s Workshop, June 22-25, 2004, Boulder, CO; Tie, Madronich, Effect of mobile, biomass burning, and biogenic emission on Mexico City’s Air Pollution and Outflow, 13^th International Scientific Symposium of Transport and Air Pollution, Boulder, September 13-15, 2004, Boulder, CO.) 

Figure 15: Ozone (O₃) concentrations simulated with the WRF-Chem model, showing the interaction between the Mexico City pollution plume with regional biomass burning and biogenic emissions. Upper panel shows the O₃ resulting from urban emission only. Middle panel includes effects adding biomass burning emissions, which contribute NO_x in the NO_x-limited regime (far-field, ca. 200-300 km).  Lower panel adds biogenic emissions, which contribute hydrocarbons in the hydrocarbons-limited regime (near-field, 50-150 km).

 

The WRF-Chem model is also being used to determine the most favorable location for ground-based measurements during MIRAGE-Mex.  This heavily instrumented “supersite” will be a link between the urban measurements and the mostly aircraft-based, regional-scale measurements, and therefore should be located outside the city in the direction of prevailing outflow.  Initial WRF-Chem simulations for late February 2004 indicate that the outflow was predominantly towards the north-east (Figure 16), but additional simulations are underway to better establish the frequency distributions.  This information, together with logistic considerations, will be used to select the location of the measurement site.

 

HANK Regional Transport Model

As the development and testing of the WRF-Chem model continues, some regional studies have been carried out with ACD’s more mature HANK model. HANK is an off-line regional model based on MM5 meteorology and is in use by the broader community.  A collaborative study by Peter Hess, Tie, and Wenfang Lei (TAMU) examined the air quality in the Houston-Galveston area, and, in particular, the role of industrial emissions, with two publications describing the results (Lei et al., J. Geophys. Res., 2004; Zhang et al., Proc. Natl. Acad. Sci., 2004). The HANK model was also used by Hess and collaborators, Moti L. Mital (Ohio Supercomputing Center) and C. Sharman, (National Physical Laboratory, New Delhi, India) on first-ever regional air quality simulations in India, where pollutant concentrations are exceptionally high and therefore of regional and even global interest.

 

Global Chemical Transport Models and Air Pollution Studies

The MOZART-2 model, developed by ACD scientists in collaboration with researchers from universities and other institutions, has been used to elucidate a variety of global-scale issues. Hess, in collaboration with David Parrish (NOAA/Aeronomy Laboratory) used MOZART-2 to examine decadal trends of pollutants on the west coast of the U.S. between 1985 (measurements made at Punta Arenas) and 2002 (measurements made during the ITCT-2K2 experiment). The model results suggest that much of the measured differences during these two time periods are due to the sampling of different meteorological regimes, rather than pollution trends. A paper describing this work has been accepted.

Hess and post-doctoral fellow, Kazuyo Murazaki, used MOZART-2 to examine for the first time the effect of climate change on U.S. air quality, by comparing simulations for 1990-2000 and 2090-2100. The main conclusion is that while climate change increases the local ozone production over the eastern U.S, it decreases the import of ozone from Asia. Significant changes were also found in photolysis rates over the U.S. as well as in chemical inflow, outflow, and transport. Overall, these results suggest that climate change and air quality will be fundamentally linked over the next century.

Tie, Madronich, and collaborators, Daniel Lack, Neville Bofinger, Aaron Wiegand  (Queensland University of Technology) and Bernard Aumont (University of Paris) implemented a parameterization for the formation of secondary organic aerosols (SOA) in MOZART-2, and carried out simulations to assess the effect of the SOA on global atmospheric oxidants. These results (Lack et al., 2004.) are part of a wider assessment of the role of aerosols in global tropospheric chemistry (Tie et al., in preparation).

The MOZART-2 model was also used by Tie, Guenther, and Wiedinmyer to examine how regional and global tropospheric chemistry might be affected by changes in biogenic volatile organic emissions that occur as a result of land cover change.  Tie, with collaborators Renyi Zhang and David Bond (TAMU) examined the relative impacts of anthropogenic and natural NO_x sources over the U.S., and with collaborators Sushil Chandra and Jerrold R. Ziemke (NASA) found evidence for elevated tropospheric O₃ over the oceans in the Northern Hemisphere. (Zhang et al., Proceedings of National Academic Science, 2003; Wiedinmyer et al., submitted 2004; and Chandra et al., submitted 2004.)

Louisa Emmons, Hess, and Lamarque, in collaboration with John Mak (State University of New York [SUNY], Stony Brook), have been studying the signatures of ¹³C in CO using observations and model simulations (Figure 17).  Since different sources of CO have different isotope ratios, ¹³CO will vary depending on the relative contributions of the sources.  Tracers of the ¹³C isotopes of all of the carbon compounds have been added to the MOZART mechanism.  Using the biomass burning emissions developed by James Randerson (University of California, Irvine), that include the C3/C4 plant type fraction, they have been able to include the appropriate ¹³C emissions for forest versus savanna burning.  The model simulation of ¹³CO has helped explain the observed seasonal and interannual variations in the time series at several stations.   The differences between modeled and measured amounts of both CO and the isotope fraction are being used to estimate improvements to the CO emission inventories. 

Figure 17: Comparison of measurements and MOZART results for CO and d¹³CO at Barbados (13N, 59W) for 1997 through 1999.  The top  panel shows CO measurements from J. Mak, SUNY Stony Brook (blue dots) and the NOAA/CMDL network (black stars), along with the simulated CO.  The various source contributions to CO (CH₄ and NMHC oxidation, and fossil fuel, biofuel, biomass burning, biogenic and ocean emissions) are also shown.  The second panel shows measured and modeled d¹³CO.  The third panel shows the CO source contributions as fraction of the total CO (in %).

 

Research Groups

Analytical Photonics and Optoelectronics Laboratory (APOL)

Atmospheric Odd Nitrogen (AON)

Atmospheric Radiation Investigation and Measurement (ARIM)

Biosphere-Atmosphere Interactions (BAI)

Laboratory Kinetics (LK)

Measurement of Pollution in the Troposphere (MOPITT)

Measurements, Standards, and Intercomparisons (MSI)

Regional and Process Studies (RPS)

Stratospheric/Tropospheric Measurements (STM)

 

 

Reactive Carbon Research

One of the major challenges in tropospheric chemistry over the next decade will be to understand and predict the fate of reactive carbon.  Achieving this goal requires the study of the many complex chemical, physical, and biological processes that control the surface emissions of these species, their chemical transformations in the atmosphere (in both gas and condensed phases), and their eventual removal from the atmosphere via deposition.  A number of activities in ACD, including field and laboratory observations, focused on the study of reactive carbon over the past year.

 

Emissions of Primary Organic Compounds

This year the Eric Apel of the MSI Group collaborated with university and government researchers on a number of important data interpretation projects resulting in two publications.

In Grossenbacher et al., 2004, two data sets are compared in which isoprene nitrate concentrations are measured along with (among others) isoprene, NO_x, and first generation reaction products with isoprene: methacrolein and methyl vinyl ketone.  A conclusion of the study is that it is important that isoprene nitrates be quantified as part of future atmospheric field studies, along with other biogenic nitrates, to assess biogenic organic nitrates’ relative role in sequestering atmospheric NO_x, and the extent to which the biogenic organic nitrates represent a significant fraction of the alkyl nitrates detected in previous studies.  This is particularly important in light of suggestions that atmospheric deposition of nitrogen may be an important source of nitrogen to nitrogen-limited forests, and that gas-phase nitrogen compounds can undergo direct uptake by leaves, thus potentially impacting the carbon cycle.

Jobson et al., in press, discusses that Houston, Texas, has one of the worst air pollution problems in the country.  The 2000 TEXAQS was initiated to help ascertain the causes of the poor air quality.  The MSI Group’s instrument was one of four in situ instruments making measurements of C₁-C₁₀ hydrocarbons at the La Porte supersite during the TEXAQS Experiment.  This paper explores the sources of VOCs in the Houston area and finds that industrial emissions have a strong impact on the atmospheric mixing ratios of light alkanes and light alkenes; the light alkenes at times dominated the airmass reactivity.  These species react very quickly and in the presence of NO_x can rapidly form ozone, one of the major components of poor air quality.  

 

Reactive Carbon Species as Sources and Reservoirs of Radical Species

Eric Apel of the MSI Group has also collaborated with university and government researchers on studies of hydrocarbons as sources of radical species.

Measurements of the Penn State Total OH Loss Measurement (TOHLM) instrument (DiCarlo et al., 2004) with OH losses calculated from the sum of the measurements of individual species thought to be the largest contributors to OH loss in the forested environment. A major discrepancy was found which leads to the conclusion that there are unmeasured (unknown) reactive species in the forested environment that are the cause. Potential candidate reactive species are the C₁₅ sesquiterpenes. It is suggested that significant work remains in order to understand the chemical environment in the forest.

Oxygenated Volatile Organic Carbon (OVOC) contribution to the HO_x cycle

There is much uncertainty in our understanding of regional and global distributions of oxygenated VOCs (OVOCs), their contribution to the HO_x cycle and to secondary organic aerosol formation (Apel et al., in preparation). If recent measurements are accurate, there is a suggestion that the OVOCs may in fact be more abundant than non-methane hydrocarbons (NMHCs) (Singh et al., 2004).  Recent experiments have begun to show a convergence between formaldehyde measurements and model results (Fried et al., 2003). However, this is not the case with higher molecular weight carbonyl compounds (Singh et al., 2004); most measurements in the free troposphere for the higher aldehydes (C₂ and up) show significantly higher mixing ratios than models predict. This paper in progress explores the distribution of acetone, methanol, acetaldehyde and methyl ethyl ketone in the Pacific during springtime. Relationships to tracer species such as CO, acetonitrile and methyl chloride are investigated with respect to source relationships. The Model for Ozone and Related chemical Tracers (MOZART) model is used to calculate expected mixing ratios and arithmetic ratios of species of interest in the TRACE-P regime and these are compared with measurements.

 

Gas-aerosol Partitioning of Organic Species

Jim Smith and Fred Eisele of the Photochemical Oxidation and Products Group (POP) have developed an instrument, the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS), for characterizing organic compounds in ultrafine aerosol.  Historically, the identification of organic compounds has been one of the most difficult challenges in aerosol measurement, as organic compounds in aerosol (1) have very low vapor pressures and are not easily detected, (2) are usually large compounds and easily decompose during analysis, and (3) often have many structural isomers, and are thus difficult to identify.  The TDCIMS technique, which has undergone several improvements over the past year, is well suited for overcoming many of these challenges.  Since it relies on chemical ionization mass spectrometry to ionize and detect compounds in the aerosol, it is both sensitive and imparts less fragmentation to the parent ion compared to techniques that rely on electron impact ionization or laser ablation and ionization.  An additional feature is its use of a triple quadrupole mass spectrometer, which allows for the identification of complex ions and structural isomers by collision-induced dissociation of ions of a pre-selected mass.  In addition to these inherent advantages, a number of additional improvements have been incorporated in the TDCIMS instrument specifically targeted at organics.  New data acquisition and control software now allows for rapid mass spectrum acquisition of up to 50 amu s^-1, thus allowing large mass ranges to be scanned in the time that the aerosol is undergoing thermal desorption.  Temperature programming has also been added to the thermal desorption procedure allowing this to be ramped from room temperature to ~300 °C.  An octopole ion guide has also been installed in the ion beam path, resulting in improvements in sensitivity of up to an order in magnitude.  Evaluation and calibration of the instrument is on-going, as are measurements of ambient aerosol performed outside the POP laboratory and at the Marshall Test Site.  This work is being carried out by a second graduate student, also supported by the ASP graduate fellowship program and co-advised by J. Smith (ACD) and Jose Jimenez (CU).

 

Laboratory Kinetics

The Laboratory Kinetics (LK) Group (Geoffrey Tyndall, John Orlando, and David Hanson) has continued its studies of tropospheric oxidation processes, maintaining an emphasis on the chemistry of volatile organic compounds.

 

Oxidation Pathways for Alkyl Iodides

Alkyl iodides (e.g., ethyl iodide, CH₃CH₂I) are emitted from the oceans to the atmosphere. Once in the marine boundary layer these compounds are rapidly oxidized by photolysis and by reaction with OH and chlorine atoms, leading to the release of free iodine atoms.  The subsequent chemistry of these iodine atoms is believed to result in the destruction of ozone and the formation of new particles in the boundary layer.  The LK Group, along with colleagues from the GIT and Auburn University, has been collaborating on a study of the mechanism of the oxidation of these alkyl iodides.  The reactions of ethyl iodide and 2-propyl iodide with chlorine atoms have been shown by both experimental and theoretical means to proceed in part via the formation of a bound complex and in part via more conventional hydrogen abstraction pathways.  Figure 18 shows product yields from the oxidation of 2-propyl iodide. The propene and chloroacetone are formed following elimination of iodine from the initially formed primary alkyl radical, while acetone comes from the secondary oxy radical.  The 2-chloropropane is produced from an exchange reaction via the adduct.  Product yield data are in qualitative disagreement with a previous study, but are in keeping with expectations based on this Group’s previous studies of the analogous bromine compounds and with general halocarbon oxidation structure/reactivity rules.

 

Figure 18:  Products observed in the Cl-atom initiated oxidation of 2-iodopropane, for [CH₃CHICH₃]_o = 14 x 10¹⁴ molecule cm^-3, in 720 Torr air diluent:  Acetone, solid circles and solid line (upper); propene, open circles and solid curve (upper); 2-chloropropane, open squares and solid line (lower); chloroacetone, solid squares and solid line (lower).

 

Nitrate Production from Toluene Oxidation

The rate-limiting step in the tropospheric production of ozone is the reaction of peroxy radicals (HO₂ and organic peroxy radicals, designated RO₂) with NO:

RO₂ + NO ® RO + NO₂

In competition with this radical cycling pathway is the formation of organic nitrates,

RO₂ + NO + M ® RONO₂ + M

a process that removes reactive radicals from the atmosphere and limits the extent of ozone production.  There is currently a great deal of activity centered on achieving a quantitative understanding of organic nitrate production over the range of temperature and pressure conditions encountered in the troposphere.  Working with scientists at NCAR and at the Ford Motor Company, Cherelle Blazer (SOARS) has been examining the extent of nitrate production from the oxidation of toluene, a major anthropogenic emission.  Chlorine atoms are reacted with toluene in an environmental chamber to provide a clean source of benzyl peroxy radicals in the presence of NO, and the yield of benzyl nitrate is then quantified as a function of temperature and pressure.  The data indicate a strong positive dependence of the yield of benzyl nitrate on pressure as expected.  However, preliminary indications suggest that the nitrate yield does not increase significantly with decreasing temperature.  This latter result conflicts with current understanding, and may lead to substantial changes in predicted nitrate formation rates in the troposphere.

 

Oxidation Mechanisms of Methyl Formate and Methyl Acetate

Methyl formate and methyl acetate are important members of the ester family, oxygenated organic molecules that are present in the atmosphere. While both may be emitted from vegetation, their major sources are thought to be from their use as solvents and flavoring agents in the food industry. They can also be produced directly in the atmosphere from the oxidation of ethers. While esters are undoubtedly present in urban atmospheres, their measurement has proven difficult. ACD scientists and post-doc Andre Pimentel (NASA Goddard) have studied the oxidation of both these esters by OH radicals, their primary mode of destruction in the atmosphere.

            OH + CH₃C(O)OCH₃ ® H₂O + CH₃C(O)OCH₂                 (1a)

            OH + CH₃C(O)OCH₃ ® H₂O + CH₂C(O)OCH₃                 (1b)

k₁ = 3.5x10^-13 cm³ molecule^-1 s^-1

            OH + HC(O)OCH₃ ® HC(O)OCH₂                         (2a)

            OH + HC(O)OCH₃ ® C(O)OCH₃                                        (2b)

k₂ = 1.8x10^-13 cm³ molecule^-1 s^-1

 

Both reactions are very slow, which makes quantification of the product yields difficult. However, theoretical prediction of the branching ratios is also difficult for slow reactions, making measurement essential. Detailed studies of both molecules revealed evidence for OH attack at both carbon atoms in each molecule. Pathways (1a) and (2a) both lead to the formation of organic acids and anhydrides (which will probably be taken up by droplets and hydrolyzed to the corresponding acids). Pathways (1b) and (2b), on the other hand, produce simpler products such as CO and CO₂. Figure 19 shows formation of acetic acid, acetic formic anhydride, and CO₂ from the oxidation of methyl acetate by OH radicals. The CO₂ yield is curved upwards, indicating that it is formed from the rapid decomposition of another product.

 

Figure 19: Product yields from the reaction of OH with methyl acetate (MeAc) at 700 Torr total pressure in the presence of 300 Torr O₂. The data for CO₂ have been offset vertically by 0.3 mTorr, and the line through the CO₂ data is a quadratic fit, for visual purposes. The data clearly indicate OH attack at both possible sites in the molecule.

 

In a separate set of experiments, chlorine atoms were used to oxidize methyl acetate at a variety of temperatures. The resulting oxy radical, CH₃C(O)CH₂O∙ can either react with O₂, forming formic acetic anhydride (AFAN), or decompose by the so-called alpha-ester rearrangement to give acetic acid and the formyl radical.

            CH₃C(O)OCH₂O® CH₃C(O)OH + HCO

The latter step is temperature dependent, and so a strong variation of the product yields with both oxygen and temperature is anticipated. Figure 20 shows the yields of AFAN and acetic acid from the combined set of experiments.  The lines represent a global fit to all the data using an activation energy of 10.1±1.5 kcal/mole. The ester rearrangement is a reaction unique to oxy radicals derived from esters, involving a five-membered Hydrogen atom transfer to the carbonyl group, and this is the first measurement of its activation energy.

The results of both sets of experiments were combined with experiments of the reaction mechanisms initiated using atomic chlorine (Cl) to provide a complete picture of the oxidation of methyl acetate and methyl formate under atmospheric conditions (Tyndall et al., J. Phys. Chem. A., 2004).

Figure 20: Plot of product yields from the oxidation of methyl acetate by chlorine atoms as a function of oxygen pressure at different temperatures. Circles, 253 K; Triangles, 273 K; Squares, 296 K; Diamonds, 324 K. Acetic formic anhydride is given by closed symbols, acetic acid by open ones. The lines are a simultaneous fit to all the data using an activation energy of 10.1 kcal/mole for the alpha-ester rearrangement.

 

Research Groups

Photochemical Oxidation and Products (POP)

Atmospheric Radical Studies (ARS)

Analytical Photonics and Optoelectronics Laboratory (APOL)

Laboratory Kinetics (LK)

Measurement, Standards, and Intercomparisons (MSI)

 

Multiphase Processes in the Troposphere

Multiphase processes are an important part of atmospheric chemical processes and additional studies are required to quantify the chemical and physical gas- and condensed-phase processes that control the number density, size distribution, chemical composition, and physical and optical properties of aerosols.  ACD scientists are addressing these needs through instrument development and field campaigns.

Interactions of clouds, aerosols, and chemistry are complex and varied.  Cloud effects on chemical species include modification of the radiative properties, which directly influence photochemistry and emissions of biogenic organic compounds.  Cloud chemistry, which includes aqueous and/or ice chemistry and modified gas chemistry, affects photochemistry and aerosol composition.

ACD scientists are using cloud-chemistry modeling to evaluate the impact of boundary layer processes and convective transport on chemical species distributions and interactions.

Recent advances in atmospheric chemistry research have focused on the central role of photochemical processing within snow pack.  ACD scientists participated in several field campaigns to evaluate these photochemical processes in mid latitudes and Polar regions.

 

Aerosols

Jim Smith, Lee Mauldin, Ed Kosciuch, and Fred Eisele of the POP Group conducted field measurements of atmospheric ion clusters and testing of a linear ion trap.  If ion inducted nucleation is to play a role in the formation of new atmospheric particles, certain ion clusters such as (H₂SO₄) _n would be expected to be present in the atmosphere at concentrations on the order of 10¹ to 10² ions cm^-3.  Measurements of these clusters is difficult because of their low concentrations, but by using techniques developed in the early 1980s along with some recent improvements, these ions were observed and studied this past summer at the NCAR Marshall field site.  The results are still being analyzed but appear to be supportive of an ion-inducted nucleation model recently developed by the NOAA Aeronomy Laboratory.

Linear ion traps offer several advantages over conventional mass spectrometers for the rapid measurement of large numbers of organics. As is the case for conventional ion traps, linear traps provide near simultaneous measurements of multiple compounds by acquiring spectra in a quasi-parallel manner by trapping and storing a wide range of ion masses and then analyzing them in a time short compared to their collection.  The overall trapping and analysis efficiency of a standard ion trap, however, is quite low while linear traps offers both high ion throughput and high trapping efficiency. Last year a quadrupole mass spectrometer was converted into a linear ion trap mass spectrometer and tested in the laboratory. This year the linear ion trap was interfaced with an ion source capable of atmospheric sampling and taken into the field. The instrument is presently beginning some preliminary measurements at the NCAR Marshall field site.

The POP Group has also designed and built new aerosol instruments and improvements have also been made to the TDCIMS.  The first new instrument is a Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA).  The HTDMA exposes size-selected particles to high humidity to characterize their ability to take in water vapor (their hygroscopicity).  The motivation of this research is to understand the relationship between aerosol composition, hygroscopicity and cloud condensation nucleus potential of carbonaceous aerosol.  It is often assumed that based on the hygroscopic growth of an aerosol one can extrapolate and determine the Cloud Condensation Nuclei (CCN) activity of the aerosol.  This may not always be possible, particularly for ambient multi-component aerosols.  An organic species may have a very low hygroscopicity (which would yield low hygroscopic growth at sub-saturated conditions), but still act efficiently as a CCN.  The POP Group’s research will investigate the conditions for which hygroscopicity can be correlated with CCN activity.  With these results, scientists may then be able to formulate an analytic relationship between hygroscopic growth and CCN activation for use in climate models.  This work is carried out in collaboration with Jim Smith and Athanasios Nenes (GIT).  A graduate student from GIT, advised by J. Smith and Nenes, is carrying out this work in the POP laboratory with support from the Advanced Study Program’s Graduate Research Fellowship program.

Another set of instruments, called Radial Scanning Mobility Particle Sizers (RSMPS), were built to perform field measurements of particle size distributions in the diameter range from 3-100 nm.  This work was performed, in part, by Damian Mattis (SOARS).  These instruments are compact and rugged, allowing them to be deployed to remote locations to perform continuous measurements of particle size.  Such surveys will provide a better understanding of the location, meteorology, and chemical environment in which new particle formation and subsequent growth take place.  In some cases these measurements will provide crucial information to allow TDCIMS measurements at a later time.

David Hanson of the Laboratory Kinetics Group (LK) measured the uptake of methane sulfonic acid by dilute sulfuric acid droplets in the laboratory using an aerosol laminar flow reactor (ALFR). This is important for understanding the growth rates of atmospheric aerosols when significant levels of methane sulfonic acid are present. Results indicate that uptake of methane sulfonic acid is very efficient. The results and this conclusion were presented as an invited talk at the 2004 fall meeting of the American Chemical Society. These results are not in agreement with those from a different technique: the droplet train apparatus (DTA) that is in use in several laboratories. Application of these laboratory results to the atmosphere lead to vastly different predictions of particle growth rates which can affect climate processes. Thus it is important to understand the reasons for this descrepancy. Hanson and colleagues seek to understand all the kinetic processes in both the DTA and the ALFR to provide a physically based accounting of them. In this vein, two papers were published in the past year, and another is in preparation, on the physical basis of the emperical relationships that currently support the analysis of the DTA results. These publications are the result of collaborations with Akihiro Morita, Inst. of Molecular Science, Okazaki, Japan, and Masakazu Sugiyama, Dept. of Elect. Eng., Univ. of Tokoy, Japan.

 

Clouds - Cloud Chemistry Modeling

Effect of Boundary Layer Processes on Chemical Species’ Distributions - Segregation of Chemical Reactants in a Shallow Cumulus Boundary Layer

Shallow cumulus clouds provide locations for aqueous chemical reactions in addition to enhancing transport and mixing and scattering radiation.  Mary Barth (ACD’s Regional and Process Studies [RPS] Group, joint appointment with NCAR’s Mesoscale and Microscale Meteorology [MMM] Division), Si-Wan Kim (MMM visitor, Seoul National University), and Chin-Hoh Moeng (MMM) used the NCAR large-eddy simulation (LES) coupled with gas and aqueous chemistry (over 50 species) to investigate the segregation between hydroxyl radical (OH) and isoprene, species which react to eventually produce ozone.  Knowing the degree of segregation between reactants will help improve regional and global-scale chemistry simulations that assume the reactants are well-mixed; it will also improve interpretation of chemical species measurements in the atmospheric boundary layer.  Results of a simulation with only gas-phase chemistry show that segregation of OH and isoprene reaches 30% in the cloud layer (Figure 21).  Inside the cloud layer, the turbulent flux of isoprene is positive and the vertical gradient of the mean OH concentration is also positive.  This leads to a major sink of the covariance between these species, and hence a larger segregation. When aqueous-phase chemistry is also included, the segregation between the two species reaches 40% in the cloud layer, further indicating the importance of cloud chemistry on the distribution of OH and isoprene.  These LES results will continue to be analyzed to determine which chemical reactions are contributing to the covariance of isoprene and OH.

Figure 21:  Intensity of segregation between isoprene and OH at (a) 1200 LT, (b) 1300 LT, (c) 1400 LT and (d) 1500 LT.  Red lines with aqueous chemistry, blue lines with only gas chemistry.  Gray lines are liquid water content (scaled at top of figure).

 

WRF-AqChem: A Coupled Meteorology and Multi-Phase Chemistry Model - Development and Initial Results of Model

Barth, William Skamarock (MMM), and Kim implemented a simple gas and aqueous-phase chemistry mechanism into the WRF model to begin investigations on the effect of convection on the chemical environment.  Convection plays an important role in transporting pollutants from the surface to the upper troposphere, scavenging soluble species and depositing much of the soluble species to the surface, and converting species chemically in the cloud drops. Simulations of the 10 July 1996 STERAO storm were performed to provide a way to evaluate the modeled chemistry results with observations (Figure 22). Sensitivities of the chemical species distributions to the cloud microphysics representation are currently being investigated.  This WRF-AqChem model will continue to be developed by incorporating the aqueous chemistry modules into the community WRF-Chem model, by incorporating lightning production of NO_x and interactions between gas and ice phase species.  Barth and Skamarock also plan to continue collaborations with John Worden and Kevin Bowman (NASA Jet Propulsion Laboratory) to examine the feasibility of using convective-scale cloud chemistry modeling with satellite data retrieval of ozone and carbon monoxide.

Figure 22:  Cross-section of the total (gas + cloud water + rain + ice + snow + hail) mixing ratio of carbon monoxide (CO) and of formaldehyde (CH₂O).  Both species are found in high concentration near the surface and lower concentration above the boundary layer. Carbon monoxide, an insoluble species, is primarily transported to the upper troposphere, while CH₂O, a soluble and reactive species, has a fraction reacted or precipitated to the ground.

 

Intercomparison of Convective Cloud Chemistry Models

Barth designed and led an intercomparison study for convective cloud chemistry models as part of the 6th International Cloud Modeling Workshop, Cloud Chemistry Case (http://box.mmm.ucar.edu/individual/barth/TracerTransportDeepConvection.html). An intercomparison of these models will show how well-reputed models perform when simulating the same storm, giving a range of results that agree reasonably with observations.  The intercomparison case focused on the 10 July 1996 STERAO storm for which observations of carbon monoxide, ozone, and nitrogen oxides (NO_x) are available.  Barth and Kim contributed results from the WRF-AqChem model. These results are being compared to those from Chien Wang (Massachusetts Institute of Technology), Jean-Pierre Pinty and Celine Mari (Laboratoire d'Aerologie, Toulouse, France), Ann Fridlind (NASA, Ames Research Center), Vlado Spiridonov (Hydrometeorological Institute, Skopje, Macedonia), Maud Leriche and Sylvie Cautenet (Laboratoire de Météorologie Physique, Clermont-Ferrand, France), and Ken Pickering, Lesley Ott (University of Maryland) and Georgiy Stenchikov (Rutgers University). Passive tracer transport show similar results among the models and agree fairly well with observations.  The intercomparison of NO_x produced from lightning and of the soluble species, nitric acid, hydrogen peroxide and formaldehyde, is currently being pursued. 

 

Cloud Chemistry Process Studies - The Importance of the Cloud Drop Representation on Cloud Chemistry

Previous studies have shown that the representation of cloud drop microphysics affects the amount of sulfate produced by aqueous chemistry because aqueous sulfur chemistry depends on pH, and pH varies with drop size.  Because other aqueous chemical reactions (e.g., those involving hydroperoxy radical and formic acid) also depend on pH, Barth is using a cloud parcel model coupled with gas and aqueous chemistry to investigate the importance of the model's cloud drop representation on cloud photochemistry.  Barth's study focused on ozone, hydrogen peroxide, formaldehyde and formic acid under remote and moderately-polluted chemical and aerosol conditions.  Results of the study showed that formaldehyde and formic acid are sensitive to the cloud drop representation, while ozone and hydrogen peroxide are not.

 

Snow/Ice

Mid-latitude Snow Chemistry Experiment

The MSI Group (Eric Apel) was selected to receive funds from the NCAR Director’s Opportunity Fund for a project entitled: “Investigation of snow-mediated photochemical processes at mid-latitudes”. This is a two-year project that is being conducted at the CU Alpine Research station at Niwot Ridge.  Eric Apel, Elliot Atlas (now at University of Miami), Aaron Swanson, and Barry Lefer (now at University of Houston) are collaborating on this project with Russell Monson and Mark Williams (CU) and a number of colleagues from other universities.  Two students, from the Louis Pasteur Institute in France, played an active role, under the direction of Apel, in the set-up of the experiment last winter and the initial data analysis.

Objectives of the study included increasing our understanding of key aspects of photochemical processes in snow that may be occurring at mid-latitudes. The combination of biological and photochemical processes within a snow covered terrain could have significant impacts on the seasonal budgets of many trace gas species, along with important effects on the chemistry of the boundary layer atmosphere.  Figure 23 shows the students, Maxime Jaeger and Vincent Schell, participating in the hard work required to set up the experiment.  Figure 24 shows a view of the sampler that was constructed and placed in the snow. The sampler was designed to obtain depth profiles of targeted VOCs in the interstitial air throughout the snow pack and above the snow pack.  Figure 25 shows Swanson obtaining snow samples to be analyzed for physical and chemical makeup in a recently excavated snow pit.

 Figure 23: Students transporting equipment to the site.

Figure 24: Sampler constructed and placed at the site for sampling the interstitial air in the snow pack.

 

 

Figure 25: After digging a snowpit, Swanson takes snow samples.

 

A significant fraction of the trace-gas emissions are thought to be products of biomass growth in microbial and fungal communities brought on by thermal insulation of the soil by the snow (Swanson et al., 2004, and references therein). The MSI Group is investigating whether the emissions from wintertime activity are an important source of VOCs to the atmospheric background mixing ratios and on what scales, local or global. Figure 26 shows some results that were obtained during one of the experiments conducted in spring 2004.

Figure 26: Depth profiles of select VOC species during spring 2004 experiment.

 

The Group is continuing to work up the data collected throughout spring 2004, in anticipation of next year’s experiment. From Figure 26, it is clear that the terpenes show large concentration gradients and high relative concentrations near the soil surface. There are some clear differences in depth profile shapes for a number of species. With the data, species that increase significantly with depth and those which show losses to the soil surface can begin to be categorized to help us understand consumption and production processes for a variety of species.  This data will be compared to data previously collected at this site by Swanson and co-workers, as well as data collected at other sites.

 

Deployment of a Snow Pack UV Radiation Profiler to Summit, Greenland

Sunlit snow has been shown to be one of the most photochemically active, and strongly oxidizing, regions of the entire troposphere, rather than simply a passive sink for the products of tropospheric chemical processing.  Photolysis of nitrate initiates very active chemistry that leads to the release of a number of important trace gases.  Initial measurements suggest that just above sunlit snow the production of HO_x from photolysis of HCHO, HOOH, CH₃CHO and HONO are all significant, and collectively dominate over photolysis of O₃.  The net result is a large enhancement of OH and HO₂ in air just above the snow.  Oxidation by OH is the main sink for a number of gases important for climate change and stratospheric O₃ depletion, so this enhancement may perturb chemistry in much of the free troposphere and also modify the chemical records of atmospheric composition ultimately preserved in glacial ice.  While recent work has shown that photochemical and physical processes in the snow pack can impact the chemistry and composition of both the atmosphere and snow pack, these processes are, in general, poorly understood.  This is especially true for the processes that produce and consume OH and HO₂.

In an effort to better understand this photochemical environment, Barry Lefer and Rick Shetter of the ARIM Group deployed a custom-profiling spectroradiometer to determine the intensity of solar UV radiation at five different depths in the surface snow pack.  This instrument serially samples the radiation field from 290 to 560 nm from five different fiber optic cables inserted into the snow pack at different depths between 1 and 150 cm.  These snow pack profiles of solar irradiance were used to determine the attenuation rate of UV in the snow pack and how this varied with changes in environmental variables such as snow pack density, soot content, solar zenith angle (SZA) and overhead ozone column.  Given the isotropic nature of the radiation field in the snow pack, the measured irradiance spectra were converted to actinic flux spectra and the photolysis frequencies of several important snow pack photochemical reactions were calculated for the different sampling depths.  This data has helped determine that the photic zone (or photochemically active region) of the snow pack is only the top 15 cm of the Greenland snow pack.  This instrument was deployed for the months of March, April and May 2004 at the Greenland Summit Ice Camp (Figure 27) as part of the NSF-funded Collaborative Research Grant titled: “Impact of snow photochemistry on atmospheric radical concentrations at Summit, Greenland.”

 

Instrument Description

The NCAR Snow Profiling spectroradiometer (NSPS) instrument consists of a small cosine-corrected irradiance probe (Ocean Optics), 5m fiber-optic with high UV throughput (Ocean Optics), a precision computer controlled dovetail slide (Velmex), a double monochromator (CVI Digikrom CM 112), a photomultiplier tube with a bialkali photocathode (Electron Tubes, LTD), a custom four-stage current-to-voltage amplifier and a PC computer for fully automated data acquisition and system control.  Spectra from 280 to 560 nm are scanned every 30 seconds.  The spectral band pass full width at half maximum (FWHM) for this optical system is 1.0 nm.  Absolute spectral calibrations are performed with a National Institute of Standards and Technology (NIST) traceable 1000 Watt QTH irradiance standard in the NCAR laboratory before and after instrument field deployment.  Field calibrations are performed with 250 W secondary QTH lamps every four-to-five days to assess the relative stability of the instrument sensitivity.  Wavelength calibrations of the monochromator are performed in conjunction with each sensitivity calibration using the emission lines from a mercury lamp to track any drift in the monochromator wavelength assignment.

Figure 27: Summit, Greenland snow photochemistry camp.

 

A Scanning Actinic Flux Spectroradiometer was also deployed to Greenland to measure the UV radiation incident to the snow pack surface.  The quartz optical collector for this system was mounted on the roof of the mobile sampling laboratory and had a 30 cm artificial horizon to limit the field of view to 2pi steradians to minimize the influence of reflected light from the other sampling inlets.

It is quite difficult to measure snow pack radiation levels due to issues of self-shading by the optical sampling inlet.  To this end, the ARIM lab has selected miniaturized Teflon sampling inlets (approximately 1 cm in diameter and less than 2.5 cm long), which attach directly to the end of a fiber optic using an SMA connector. These miniature-sampling inlets were inserted into the snow pack at a location near the gas sampling activities at five different depths between the surface and one and a half meters.  Each sampling inlet was attached to a 5m fiber optic cable that was connected to the same spectroradiometer.  The snow spectroradiometer serially samples each of these five depths, thus collecting a complete snow profile every two and a half minutes. The above-snow spectroradiometer continuously measured the actinic flux reaching the snow surface, which can account for any changes in the light environment (e.g., due to clouds) while a specific snow profile is being collected.

Antarctic Polar Plateau

One of the more fascinating recent advances in atmospheric research has been the recognition of the central role of photochemical processing within the snow pack on the chemistry of polar plateau boundary layer air.  The most dramatic effects of the snow pack emissions appear to exist in Antarctica where, as part of the Antarctic Tropospheric Chemistry Investigation (ANTCI) field experiments, Fred Eisele, Lee Mauldin, Bruce Henry, and Edward Kosciuch (all members of the ACD Photochemical Oxidation and Products [POP] Group), in collaboration with colleagues from GIT and several other universities, have gained major new insights into the vertical and horizontal distribution of NO and NO_y.  Very high levels of NO have been previously reported at the South Pole ground site with concentrations in the hundreds of pptv (parts per trillion by volume).  These concentrations were shown in recent Twin Otter flights to be maintained well beyond the near vicinity of the South Pole.  These high levels persisted in near surface air samples for ~ 300 km both upwind and downwind of the pole.  It now seems likely that somewhat similar levels will be found over much of the plateau region.  The influence of these snow surface emissions of NO_x was also observed to extend into the free troposphere.  NO concentrations on the order of tens of pptv were still observed up to a half a km above the snow surface, suggesting that OH concentrations, and, hence, oxidation rates are very likely to be enhanced above those previously expected for this region.  This observation is quite significant because it now demonstrates that a much larger fraction of the troposphere has an enhanced oxidation capacity and it is through this region that sulfur and other reactive compounds must be transported in order to reach the Pole and other plateau locations.  Thus, reactive compounds such as DMS and SO₂ can be oxidized during transport far more quickly than previously calculated in model simulations.  This decreased DMS lifetime is consistent with past and present observations showing very low concentrations of DMS, SO₂, and gas phase H₂SO₄, and MSA at the South Pole.  Additional Twin Otter flights near the coast addressed another previous mystery: Why have past observations of NO concentrations at the coast been so much lower than those observed at the Pole?  Several flights along the Trans-Antarctic Mountains, particularly across the base of large glacial flows such as Byrd Glacier, at times revealed very high concentrations of NO and NO_y  (hundreds of pptv).  These enhanced concentrations are presumably the result of NO-rich boundary layer air, which, due to the katabatic flow, is transported from the plateau down these glacial valleys resulting in the transport of this NO_x enriched air to a few select outflow regions at the coast.  The potential importance of these results for understanding ice core MS/sulfate ratios is the suggestion that the ratios of these compounds deposited to the snow surface may well depend on the distance from the coastal source of sulfur and the proximity of glacial outflow relative to sulfur inflow.

Some preliminary results arising from the in situ ground-based South Pole measurements have also been obtained.  For example, PAN should have an atmospheric lifetime of months over Antarctica, and therefore could provide a mechanism for transporting reactive nitrogen from mid latitudes to the polar region.  The results of PAN measurements at the Atmospheric Research Observatory (ARO) building at the South Pole, however, suggest that there are insufficient PAN concentrations to significantly affect the nitrogen chemistry over the plateau.

Early in the study, several instruments were operated during a nearly complete solar (~85% occlusion) eclipse.  Since the sun height normally changes little during our study period, this eclipse was a rather unique opportunity to observe changes in photochemically-generated compounds with nearly an order of magnitude change in UV light as shown in Figure 28.  Several interesting relations are demonstrated by this figure.

Figure 28:  Some interesting South Pole boundary layer photochemistry observed during the near total solar eclipse that occurred on November 23, 2003. Since the sun only sets and rises once a year at the South Pole, this unique and rapid photochemical transient will be the basis for a modeling case study. 

 

Prior to the eclipse, the anti-correlation between NO and OH can be seen.

·        Above ~175 pptv of NO, NO_x levels are large enough that the reaction of OH with NO₂ becomes a significant sink for OH, thus a decrease in NO (and hence NO_x), causes an increase in OH.

During the eclipse, the percentage change in OH is larger than the percentage change in UV radiation (J(O¹D)).

·        This observation reflects the fact that primary production of OH, via O(1D) production, has been decreased, while at the same time the steady state partitioning between NO and NO₂ is shifted to primarily NO_2, an OH sink. Thus during the eclipse, the production of OH is decreased while the sinks for OH are increased, causing the larger observed percentage change.

There is a time delay between UV and the response of NO.

·        The time delay in the response of NO to the decrease in UV reflects the lifetime of NO in the atmosphere. The delay in response to the increase in UV reflects the photolysis lifetime of ambient NO₂ and nitrate in the snow pack (the source of much of the NO_x in the South Pole boundary layer).

Also note the small OH concentrations (~4x10⁴ molecule cm^-3, or ~2 parts per quadrillion by volume) that the instrument is capable of measuring.

A new, compact mass spectrometer instrument has been designed and built this past year in order to allow OH to be measured on a Twin Otter aircraft during the upcoming ANTCI 2005 study.  This instrument, when complete, should be four to five times smaller and lighter than the existing four channel system but still provide similar sensitivity for measuring OH, H₂SO₄ and MSA.  The new system makes use of advances in mass spectrometer and octopole lens design and will serve as a prototype for a future HIAPER OH instrument.

 

Research Groups

Atmospheric Radiation Investigation and Measurement (ARIM)

Laboratory Kinetics Group (LK)

Measurement, Standards, and Intercomparisons (MSI)

Photochemical Oxidation and Products (POP)

Regional and Process Studies (RPS)

 

Chemistry in the Climate System

The global climate system is a composite of local, regional, hemispheric, and interhemispheric processes that occur from the surface to the upper atmosphere on time scales of minutes to decades.  The role of chemistry in this complex system involves multiple interconnected components with a range of temporal and spatial scales.

A primary focus of the chemistry-climate modeling studies in ACD is to evaluate direct anthropogenic and biogenic emissions both spatially and temporally, to determine the influence of  transport processes on the distribution of those emissions, and to examine the impacts of the emissions on atmospheric chemistry.  In addition, the modeling studies also examine aerosol distributions, formation processes, influence on chemical species, and radiative properties and impacts including indirect aerosol and cloud effects.  These studies, based on the current climate system, additionally provide a foundation for model simulations designed to study the influence and feedbacks of climate and global change on chemistry of past and, most importantly, future climate regimes.

Focused studies of biogenic emissions are a key aspect of ACD research on biogeochemical cycles.  These studies include measurements of emissions and vertical flux of many species, including VOCs, NO_y, O₃, and CO.  These measurements have been collected on a broad range of spatial scales from leaf, plant, and canopy scales, through to regional scales.  The implications of these flux determinations on the local, regional, and global atmosphere are evaluated through the incorporation of empirically-based flux parameterizations in numerical model simulations.

The upper troposphere/lower stratosphere region of the atmosphere plays a crucial role in the climate system because of the radiative impacts of water vapor, ozone, and aerosols in that region.  ACD research in the UTLS region is focused on understanding the distributions of ozone and water vapor and how photochemistry, multiphase chemistry, convection, and stratosphere-troposphere exchange influence those distributions.

The role of the middle atmosphere (the region between the lower stratosphere and the lower thermosphere) in the climate system is not well characterized.  ACD research in this area is focused on understanding the broad interactions between chemistry, radiation, and transport using a combination of models and measurements/calculations of chemical constituents and energy budgets.

 

Climate Simulations

The distribution and abundance of radiatively active gases and aerosols are important determinants in the Earth's radiation budget and the state of the climate system.  Atmospheric chemistry and transport are key factors in determining the abundance and distribution of these trace gases and aerosols.  Changes in the chemistry-climate-biogeochemistry system are expected to be strongly coupled with important, but as yet not well quantified, feedbacks.  ACD scientists are evaluating this system through model development and simulations of various climate states.

 

Present-Day Climate

Tropospheric Ozone Interannual Variability

Using the available ozonesondes data, Lamarque has studied the interannual variability of tropospheric ozone focusing on the Northern Hemisphere poleward of 40°N. This analysis shows that the spring ozone interannual variability is strongly related to the phase of the NAO (Lamarque et al., 2004).  Lamarque, Hess, and Mahowald are in the process of extending this analysis for a further understanding of the ozone budget in the troposphere. 

 

Studies of the Arctic Oscillation

The study of the Arctic oscillation in tropospheric ozone by Hess and Lamarque was one of the first to show the influence of the Arctic oscillation on tropospheric ozone. It is shown in both a data analysis of ozonesondes and in model simulations. The NAO explains a significant amount of the interannual variability of springtime ozone over North America (Lamarque and Hess, 2004).

 

Role of Convective Versus Synoptic Transport

Hess examined the relative importance of convective versus synoptic transport and the seasonal variation and the atmospheric regions affected by these modes of transport. This study is important for understanding the different processes which influence the UTLS (and was summarized during the UTLS workshop). These modes of transport affect the chemistry in different ways (Hess, submitted).

 

Santa Fe Project

As an outcome of the Chemistry-Climate Interactions workshop organized by Lamarque and Jeffrey Kiehl (CGD), a project for the study of nitrogen deposition on the biosphere and its impact on the carbon cycle was designed. Lamarque, in collaboration with various groups (GISS, UKMO, Lawrence Livermore, UCI, Max-Planck Mainz, Max-Planck Hamburg, IPSL Paris) has combined and analyzed the results of nitrogen deposition. These results have been presented as a poster at the International Global Atmospheric Chemistry (IGAC) meeting. In addition, this model intercomparison will also be used for a paper on the ozone radiative forcing in the 21st century.

 

Past and Future Climates

Intergovernmental Panel on Climate Change (IPCC) Simulations

Lamarque, in collaboration with Hess, Emmons, Lawrence Buja (CGD), Warren Washington (CGD), and Claire Granier (NOAA/IPSL), has performed MOZART simulations from 1890 to 1990 in 20-year increments.  The simulated ozone field has been used by the CCSM group in their simulations of the 20th century climate for the next IPCC Assessment.

 

Model Development

Off-line Community Atmosphere Model (CAM)

Phil Rasch (CGD), Hess, Stacy Walters, and Francis Vitt have successfully created an off-line-transport version of CAM, the transport model embedded within the CCSM framework. This model imports meteorological fields from other sources (e.g., NCEP, ECMWF) into the CAM model instead of using internally generated fields. This provides the opportunity to simulate the transport and chemistry of trace constituents (reactive and passive species and aerosols) using meteorological analysis. These distributions can then be compared with measurements on an episodic basis and used in process studies. This model introduces a new capability into the CCSM framework and reduces the software engineering effort required for maintaining multiple chemical transport models within the same institution. It also allows improvements to the consistency of representations of physical processes within the model and allows all components of the climate system to influence the chemical simulations. We anticipate that this model will replace the MATCH and MOZART CTMs.  

Simple trace constituents have been successfully simulated in the off-line model using meteorological fields from NCEP, ECMWF and CAM. These tracers include an ozone-like tracer and other diagnostic species. The trace constituent distributions have been compared against those produced in MOZART, online CAM and MATCH with good results. This model is now coded to the computational standards of the CCSM. Chemistry is currently being implemented into the off-line-CAM model.

 

Interactive Community Atmosphere Model (CAM3)

Lamarque has completed the implementation of interactive chemistry (including aerosols) from the MOZART model into CAM3, using the WACCM (Whole Atmosphere Community Climate Model) framework.  The radiative coupling is done through the ozone distribution and the aerosol distributions.  This work has been presented at the CCSM workshop in July and will be the basis for the released version of CAM3 with interactive chemistry. This work is in collaboration with CGD (Rasch and Mahowald) and Walters (WACCM).

Lamarque has expanded the aerosols package developed by Tie to have a better representation of the formation of ammonium nitrate.  This package is now running in the newest version of MOZART and in CAM3.  In addition, several modifications were made to the modeling of dry deposition in MOZART in order to accommodate aerosols.  Finally, interactive biogenic emissions of isoprene and soil NO_x emissions are now part of MOZART.

 

Model for Ozone and Related Chemical Tracers (MOZART)-4

MOZART-4 is an update to the MOZART model published by Horowitz et al. in 2003. This update has been necessary to transform MOZART into a state-of-the-art chemical transport model and to make it available to the community at large. The updates to MOZART will be included in the online and off-line CAM with chemistry. Numerous updates have been implemented by Lamarque, Emmons, John Orlando, Geoff Tyndall, Tie, Walters, and Hess.  The improved chemical mechanism now considers three classes of large anthropogenic hydrocarbons: a large alkane (butanes and larger), a large alkene (butenes and larger) and an aromatic species (toluene). Reaction rates have been updated. Updated emissions have been constructed consistent with the new chemical mechanism. The ability to input multi-year emission datasets has been added. Lower boundary conditions can now be fixed for long-lived species (e.g., H₂ and CH₄). Interactive emissions are implemented, e.g., for isoprene and soil NO_x, as these depend respectively on the model temperature and photosynethic radiation and on the calculated soil wetness.  The aerosol parameterizations have been updated.  Photolysis frequencies are calculated with the fast TUV code, allowing online calculations consistent with model clouds and aerosols. Dry deposition is interactive for all species and with an additional parameterization of the effect of microbes on the dry deposition of CO and H₂. The lightning parameterization has been updated.  New output fields have been enabled. A synthetic ozone field can be used in the stratosphere so as to constrain the stratospheric-tropospheric flux of ozone to reasonable values when running MOZART with analyzed wind fields.  Interactive water fields, instead of diagnosed water, have been implemented.  Finally, a new website documenting the model, related publications, and ongoing research in global tropospheric modeling has been developed which allows downloading of the source code.

 

Tracer Version of MOZART-4

A tracer version of MOZART-4 has been developed by Hess, Vitt, and Gabrielle Pfister.  This tracer version allows reading a number of chemical species into the model and specifying their chemical sources. For example, it allows the input of an OH field and the source of CO from the oxidation of hydrocarbons. The tracer version of MOZART-4 will be used for inverse modeling studies at considerably less expense than running the full version.

 

Adjoint Version of MOZART

Hess, Gabrielle Petron, and Baker developed most of the components for an adjoint version of the MOZART code during this year. The basic framework for the MOZART adjoint (i.e., running the model backwards) and the appropriate check pointing has been implemented. An adjoint of the diffusion code and convective code has been developed. In addition, an optimization code to solve for emissions has been developed.

 

Research Group

Global Modeling

 

Biogeochemical Cycles

The mission of the Biosphere-Atmosphere Interactions (BAI) Group is to advance understanding of the role of biosphere-atmosphere interactions in the Earth system and predict its response to human perturbations. This is being accomplished through multidisciplinary field, laboratory and modeling studies of the processes controlling these interactions on various scales (e.g., leaf to canopy to regional/global).  BAI members during FY04 included Alex Guenther, Jim Greenberg, Peter Harley, Thomas Karl, Ryan Schnell, Andrew Turnipseed, and Christine Wiedinmyer. 

 

Laboratory Studies

BAI scientists (Guenther, Greenberg, Harley, and Karl) are using laboratory facilities to characterize the processes that control biomass burning and biogenic emissions. Biomass burning studies were conducted in the ACD laboratory roasting/burning chamber facility in collaboration with Hans Friedli (ASP) and in the United States Department of Agriculture Missoula fire laboratory burn facility in collaboration with Robert Yokelson and Theodore Christian (both of University of Montana). Biogenic emission studies were conducted in the ACD phytotron and growth chamber facilities in collaboration with Emiliano Pegoraro (Edinburgh University, U.K.), Allison Steiner (University of California, Berkeley) and Teresa Nunes (University of Aviero, Portugal).

The laboratory biomass burning studies demonstrated that oxygenated volatile organic compounds (VOCs) and nitrogen compound emissions from fires are higher than most previous estimates and likely have an important role in fire dynamics (i.e., fuel ignitability) and regional air quality. This research also suggests that emissions of some compounds from smoldering fires (including charcoal production and post flaming phase pyrolysis) are underestimated in existing fire emission inventories.   Three manuscripts (Karl et al., Greenberg et al., Christian et al.) describing these results are in preparation for submission to peer-reviewed journals.

The laboratory biogenic emission studies have shown that isoprene emission is more resistant and recovers more quickly from drought than does photosynthesis and stomatal conductance. Isoprene emission rates show a strong correlation with leaf water potential, providing a useful parameterization for including the effects of drought stress in isoprene emission models. Other experiments have demonstrated that sesquiterpene emission rates from some important tree species are comparable to monoterpene emission rates and exhibit similar temperature dependence. Biogenic methanol emission rates were observed to vary across and within plant species by at least an order of magnitude. Leaf age was identified as the major source of within species variation in methanol emission.  Observed light dependent monoterpene emissions varied as a function of growth temperature and certain monoterpenes (e.g., ocimene) were released at elevated levels in response to temperature stress.  This work is described by Pegoraro et al., Harley et al., and Nunes et al., which are in preparation for submission to peer-reviewed journals in late 2004.

 

U.S. Field Studies

BAI scientists investigated biosphere-atmosphere interactions with six U.S. field studies during FY04 and analyzed data from an additional three U.S. field studies. The six FY04 field studies supported the research of investigators from five universities. A study of the contribution of biogenic isoprene emission to regional aerosol and oxidants was conducted at a poplar plantation in northern Oregon and was conducted in collaboration with Brian Lamb, Halvor Westberg and Adam Hill (Washington State University).  The potential allelopathic role of biogenic VOC emissions from invasive Artemesia shrubs in New York was conducted in collaboration with Jed Sparks and Jacob Barney (Cornell University). An aircraft investigation of cicada outbreaks in Indiana was conducted in collaboration with Paul Shepson (Purdue University) and showed that the emissions from decaying cicadas did not have a significant impact on atmospheric chemical composition. Trace gas fluxes over a Colorado grassland were measured in collaboration with Matt Dunn (ASP/CU) and James Smith (ACD).  The observations show that the grassland was a significant source of methanol and a sink for acetic acid and methyl acetate. Ozone deposition was measured at the Niwot Ridge, Colorado AMERIFLUX site in collaboration with Russell Monson (CU).  The negative impact of high O₃ events on the subalpine forests downwind of Denver, Colorado are minimized by their occurrence during late afternoon-early evening periods when plant activity is low due to both water stress and low light levels. Studies of ammonia fluxes from soybean fields in North Carolina were conducted in collaboration with John Walker (North Carolina State University and United States Environmental Protection Agency [USEPA]). Significant fluxes were observed and the results demonstrated the utility of the ammonia CIMS system designed and built by ACD.

FY04 efforts also included a workshop, database development, data analysis and manuscript preparation associated with the Chemical Emissions, Losses, Transformations and Interactions with Canopies (CELTIC) study in Duke Forest, North Carolina led by BAI in collaboration with Chris Geron, Bob Arnts (USEPA) , Walker, Jose Jimenez, Darin Toohey, Alice Delia (CU), Sparks (Cornell), Mark Potosnak (Desert Research Institute), Greg Huey, David Tanner, Darlene Slusher and Robert Stickel (GIT), Kolby Jardine and Bradly Baker (South Dakota Institute of Mining and Technology), Jose Fuentes (University of Virginia) Will Vizuete (University of Texas), Fred Mowry and Jeff Herrick (Duke University), Francesca Rapparini (Istituto di Biometeorologia, Italy), Rei Rasmussen (Oregon Graduate Institute).  The CELTIC results indicate that isoprene emission increases with elevated ozone and, on a canopy scale, with elevated CO₂.

Future changes in ozone and CO₂ could thus change regional isoprene emission rates leading to feedbacks. The pine plantation forest floor (soil and leaf litter) was a significant sink of methanol, acetone, acetaldehyde and isoprene. This is not well represented in existing models and could lead to overestimated ambient concentrations. Substantial sesquiterpene emissions observed from pine trees could be the dominant source of biogenic secondary organic aerosol mass under certain environmental conditions. PAN (and PAN-like compounds) were deposited at a much faster rate than is predicted by current models by a factor of 3-5.  This has significant implications for the lifetime of PAN in the atmosphere and its role as a transporter of reactive nitrogen in the atmosphere. Simultaneous observations of biogenic VOC and total organic aerosol during CELTIC allowed new constraints on aerosol yields and OH formation due to ozonolysis. The average aerosol yield for alpha-pinene, beta-pinene and d-carene are higher than calculated, supporting recent laboratory studies showing that polymerization reactions could enhance aerosol partitioning (Figure 29). Based on observed isoprene decay within the canopy, the upper limit of nighttime OH mixing ratios is less than reported measurements for other forests.  Estimates of biogenic contributions to secondary organic aerosol mass at Duke Forest suggest that biogenic sources dominate which is somewhat surprising for this moderately polluted site.

Figure 29: Comparison of SOA growth between different modeling approaches compared to measured gradients (red circles).

 

Analysis and publication of two other previous U.S. studies were completed in FY04.  A tower flux study of isoprene and oxygenated VOC emissions from a northern Michigan deciduous forest was conducted in collaboration with Raymond Fall (CU) and Armin Hansel (University Innsbruck, Austria). The differences can be explained by the different physiological and phenological processes controlling these emissions. The multiscale (enclosures, tower, tethered balloon, aircraft) Ozarks Isoprene Experiment (OZIE) study was conducted in collaboration with Brian Lamb and Halvor Westberg (Washington State University), Jay Turner (Washington University), Paul Palmer (Harvard University), Geron and Thomas Pierce (USEPA) and demonstrated that central U.S. oak forests and woodlands have very high isoprene emission rates resulting in elevated regional formaldehyde concentrations.  This formaldehyde is often transported northward and eastward towards urban centers such as St. Louis and Chicago, and participates in the production of ozone (Figure 30). Current photochemical models underpredict the magnitude of formaldehyde produced by isoprene oxidation which leads to incorrect chemistry simulations within the urban centers.

Figure 30: Hourly-averaged formaldehyde concentrations for 06:00 CDT on July 21, 1998, as modeled by CAMx. Winds on this day were predominantly from the south-southwest. A plume of elevated formaldehyde can be seen being transported from the forests of the Ozarks towards the northeast and into the Chicago urban area. The majority of this formaldehyde has been formed from the photooxidation of isoprene emitted from the Oak forests of the Ozarks on the July 20.

 

International Field Studies

BAI scientists participated in international investigations of biosphere-atmosphere interactions in Brazil, Ireland, and Japan during FY04 and analyzed results from previous studies in China, Costa Rica, Brazil, and Finland.  The FY04 Chemistry And Production Of Smoke (CAPOS) study in the Brazilian Amazon used airborne and ground measurements to characterize the primary emission composition of VOCs and other gases from tropical fires and within aging plumes in collaboration with Robert Yokelson and Theodore Christian (University of Montana), Don Blake (University of California, Irvine), Paulo Artaxo (University Sao Paulo, Brazil), and Joao Caravalho (Center for Weather Forecasts and Climate Studies [CPTEC] of the National Institute for Space Research [INPE], Brazil). Additional background measurements were made at a primary tropical forest flux tower near Manaus, Brazil in collaboration with Potosnak, Artaxo, Julio Tota and Antonio Manzi (Brazilian National Institute of Amazon Research [INPA], Brazil).  Significant flux of monoterpenes was estimated from the vertical profile shown in Figure 31. The aircraft observations generally confirmed the emission profiles measured in the laboratory studies described above (Figure 32). The contribution of semi-volatile organic gases to secondary organic aerosol mass was studied in Japan in collaboration with Yoshizumi Kajii (Tokyo Metropolitan University).

Figure 31: Altitude profile of monoterpenes above the Z34 tower close to Manaus.

 

 

Figure 32: Comparison of emission profiles of identified compounds obtained by the PTRMS instrument between laboratory experiments (average of 30 experiments) conducted at the USFS fire lab in October 2003 and airborne measurements (35 samples from 10 fires) in 2004. All data are normalized to the biomass burning marker acetonitrile.

The results indicate that humidity plays a significant role in determining the uptake of some semi-volatile compounds. BAI scientists participating in the BIOFLUX field study on the Irish coast investigated biogenic fluxes of organic iodide compounds in collaboration with Colin O’Dowd (National University of Ireland), Liisa Pirjola (University of Helsinki, Finland) and Thorsten Hoffman (Institute of Spectrochemistry and Applied Spectroscopy [ISAS], Dortmund, Germany).  A tethered balloon system was used to characterize vertical gradients of several alkyl halides including di-iodomethane, ethyl bromide and ethyl iodide. Concentrations decreased with height indicating a surface source of these compounds which are believed to be involved in new particle formation.

Two manuscripts were published in FY04 (Spirig et al., and Boy et al.) describing BAI results from the Origins of Secondary Organic Aerosol (OSOA) study that was conducted at a boreal forest site in Finland in collaboration with Christoph Spirig (Swiss Federal Institute of Technology [ETH] Zurich, Switzerland), Michael Boy and Markku Kulmala (University Helsinki, Finland). Total monoterpene oxidation rates during the OSOA study were not correlated with particle production events, but there was a strong correlation with ozone indicating that more ozone reactive biogenic compounds, such as sesquiterpenes, may make an important contribution. Vertical gradients of particles above the forest suggest that particle growth occurs throughout the boundary layer.

Tropical forest biogenic emissions were studied in the eastern Amazon (Brazil) in collaboration with Luciana Vanni Gatti and Carla Trostdorf (Energy and Nuclear Research Institute [IPEN], Brazil) and in Costa Rica in collaboration with Deborah Clark (University of Missouri), Sparks, and Potosnak. Seasonal isoprene variations in the Amazon were much higher than expected (more than a factor of 3 higher in the dry season).  Tropical forest ecosystems in both Brazil and Costa Rica were both a significant source (daytime) and sink (nighttime) of methanol, acetone, and acetaldehyde.  Dry deposition velocities of these and other (e.g., methyl vinyl ketone, methacrolein, and acetonitrile) compounds were a factor of 10-20 higher than estimated from existing deposition models which has important implications for regional and global chemistry and transport models. This work is described in Greenberg et al., 2004; Harley et al., 2004, Trostdorf et al., 2004, and Karl et al., 2004.

Baker et al., 2004, and two manuscripts in preparation describe studies of the impact of land cover change on biogenic emissions from Chinese landscapes that were conducted in collaboration with Baker, Rassmussen, Bai Jianhui (Institute of Atmospheric Physics [IAP] Chinese Academy of Sciences), Susan Owen (Lancaster University), and Geron. Chinese rubber tree and oil palm plantations have much higher terpenoid emissions than native tropical forests. This suggests that there has been and continues to be significant increases in biogenic VOC emissions in some tropical regions of China.  Severe drought results in a dramatic reduction in these emissions demonstrating the equally important role of climate change.

 

Regional Modeling Studies: U.S.

Wiedinmyer and Guenther investigated biosphere-atmosphere interactions in the U.S. using regional chemistry and transport models. FY04 studies included an investigation of the impact of future climate and land cover on regional air quality in the Pacific Northwest and north central U.S. in collaboration with Brian Lamb (Washington State University), Cliff Mass (University of Washington) and Susan Fergusen (U.S. Forest Service) and a study of the contribution of sesquiterpenes and other biogenic VOC emissions to secondary organic aerosols in the eastern U.S. in collaboration with Jana Milford (CU). Both of these projects were initiated in FY04, and efforts were directed at setting up the regional air quality modeling system and developing the emission scenarios.

 

Regional Modeling Studies: International

Wiedinmyer, Sreela Nandi (ASP/ACD Postdoctoral Fellow), and Guenther investigated biosphere-atmosphere interactions in China, Brazil, and Mexico using regional chemistry and transport models in FY04.  The contribution of biogenic VOC emissions to oxidant and particle production in China was studied in collaboration with Xiaoping Wang and Denise Mauzerall (Princeton University). The results indicate that future growth in China’s fossil fuel emissions will lead to costly decreases in public health and crop productivity. Much of this region is VOC-limited so changes in biogenic VOC emissions will have important consequences for air quality.  The BAI Group, along with Hsiao-ming Hsu (MMM and NCAR’s Research Applications Program [RAP]), investigated the influence of biomass burning on regional air quality in the Brazilian Amazon along with a detailed sensitivity study of emission inputs which demonstrated that there are large difference in the fire emissions predicted using different methods recommended in the literature. The BAI Group also supported regional simulations of air quality in Mexico in preparation for the ACD Megacity Impacts on Regional and Global Environments [MIRAGE] study (Xuexie Tie, ACD).

 

Global Modeling Studies

Wiedinmyer and Guenther investigated global scale biosphere-atmosphere interactions in FY04 using global chemistry and transport models. The contribution of biogenic VOC emissions to global organic aerosol was examined in collaboration with Joyce Penner (University of Michigan). The study showed that uncertainties in biogenic VOC emissions are a significant contributor to the total uncertainty associated with estimates of global radiative forcing of aerosols. A biogenic VOC model was integrated into the NCAR Community Climate System Model [CCSM] Land Surface Model [LSM] and used to investigate the sensitivity of emissions to climate and land cover in collaboration with Sam Levis and Gordon Bonan (CGD). Biogenic VOC emissions were sensitive to climate, and the estimated interannual variability exceeded 10% of the estimated annual anthropogenic emission estimates used for the IPCC emission scenarios. Finally, a study examining interactions among global change (climate and land cover), biogenic isoprene emissions, and the chemical composition of the atmosphere was completed in collaboration with Tie, Ronald Neilson (U.S. Forest Service), and Claire Granier (CNRS).  Climate and land cover-driven changes in biogenic VOC emissions impacted simulated regional surface ozone concentrations by as much as -30% to 50% under certain emission and climate scenarios. The ozone production chemistry was shown to change under different emission scenarios (i.e., NO_x versus VOC) and has implications for unhealthy ozone concentrations and future ozone abatement strategies (see Figure 33).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 33: Change in L_N/Q for the month of July for [(FUTVEG-CURCLIM) – (CURVEG-CURCLIM)] (top) and [(FUTVEG-FUTCLIM) – (CURVEG-CURCLIM)] (bottom).

Model and Database Development

Guenther and Wiedinmyer also developed biogenic and fire emission models and databases for the scientific and air quality regulatory communities. FY04 accomplishments include the initial release (user’s guide and files are available on the NCAR community data portal, https://cdp.ucar.edu/) of the Model of Emissions of Gases and Aerosols from Nature (MEGAN) which predicts biogenic emissions on a global scale at 1 km resolution (Figure 34). MEGAN was also integrated with the NCAR global models, MOZART and the Community Land Model (CLM), and an effort was initiated to integrate MEGAN with regional air quality models. In addition, a modeling framework was developed to predict daily emissions from fires for all of North America at a 1 km resolution for easy use in regional air quality model simulations. Fire emission inventories developed by this model will allow air quality modelers to include fire emissions with their model simulations and determine if fire emissions have a large impact on their air quality (Figure 35).

Figure 34: Global annual isoprene emission distribution for the year 2000 estimated by the MEGAN model.

Figure 35: Estimates of fire emissions of PM 2.5 (particulate matter less than 2.5 mm) for Louisiana.

 

Instrument Development

Turnipseed, Guenther, Greenberg, and Karl developed instruments for investigating biosphere-atmosphere exchange in FY04. This included a Disjunct Eddy Accumulation (DEA) system that captures and stores air based on the vertical wind direction and speed.  The concentrations in these stored samples (along with wind statistics) can then be used to determine fluxes of chemical trace species which are difficult to determine by other flux techniques which often need fast (< 1 s) measurement times.  This project was in collaboration with Lamb and Mount, Shepson, and Steve Shertz (ATD).  Two separate systems developed and evaluated in FY04 allow for DEA flux measurements of volatile organic compounds (VOCs) from both a tower and an aircraft platform. 

 

Research Group

Biosphere-Atmosphere Interactions

 

Integrated Study of Dynamics, Chemistry, Clouds, and Radiation of the Upper Troposphere and Lower Stratosphere (UTLS)

The UTLS region is of critical importance for understanding long term global or climate change.  The UTLS is a region where ozone is an effective greenhouse gas, and where water vapor, cirrus clouds, and aerosols each make a significant contribution to the radiation budget.  The UTLS is also a region where transport processes that couple the stratosphere and troposphere occur on a multitude of scales, which when combined with the strong vertical gradients in many chemical constituents, present a challenge to observational techniques and numerical models.  Chemically, the UTLS region represents a transition in the nature of the mechanisms of ozone production and loss.  ACD scientists are involved in studying various aspects of transport and chemistry in the UTLS as well as developing models and instruments to support these studies.

 

Satellite Data Analysis

The Satellite Data Analysis (SDA) Group (William Randel, Laura Pan, Bruce Henry, Steven Massie, and Fei Wu) focuses on studies of global scale chemical behavior using satellite measurements, meteorological data sets, and model simulations. Recent work has focused on chemical and dynamical behavior of the tropopause region, both in the tropics and extratropics, aiming to understand and quantify the processes which contribute to coupling in the UTLS.

The tropical tropopause layer (TTL, ~12-17 km) behaves as a transition between the convectively controlled troposphere and radiatively balanced stratosphere. Because air enters the stratosphere primarily in the tropics, the TTL serves as a boundary condition for the global stratosphere. Key science issues include understanding mechanisms of dehydration and cirrus cloud formation, and fast transport into and through the TTL (with relevance to short-lived chemical species).  Many features of the TTL are poorly observed by conventional data sets, and there are important uncertainties in the relationships between temperatures, winds, clouds and water vapor on large and small scales.  Current work in this Group is aimed at improved understanding of the TTL using new satellite observations, plus global model simulations.

William Randel has used high vertical resolution temperature profiles derived from Global Positioning System (GPS) radio occultation measurements to study temperature variability within the TTL, and links to large-scale tropical convection. Much of the TTL temperature variability is observed to occur in the form of eastward-propagating, planetary-scale Kelvin waves.  These waves have typical vertical wavelengths of ~4-8 km, which are well-resolved by the GPS measurements (but not by other satellite measurements).  The GPS results demonstrate that the global-scale Kelvin waves are directly forced by transient deep convection over Indonesia.  Furthermore, the Kelvin waves show strong coupling to the background winds, in particular, showing enhanced amplitudes coincident with the descending westerly shear phase of the quasi-biennial oscillation (QBO). 

Randel also leads a World Climate Research Program (WCRP) Stratospheric Processes and their Role in Climate (SPARC) initiative on detection, attribution and prediction of stratospheric changes.  This is an international effort aimed at quantifying past changes in stratospheric climate, understanding the responsible processes through model studies, and predicting stratospheric changes in the future.  As part of this work, Randel co-organized a SPARC workshop on Stratospheric Temperature Trends in Silver Springs, MD, in November 2003.  Randel and Andrew Gettelman were co-authors on a joint SPARC-IGAC white paper describing Chemistry Climate Interactions, which will form the basis for joint scientific activities during the next decade.  Randel is also involved as a lead author of an ongoing IPCC Report on Safeguarding the Ozone Layer and the Global Climate System, to be completed in 2005.

Gettelman and Randel are using tropical temperature and water vapor measurements from the AIRS instrument on the NASA Aqua satellite to characterize large- and small-scale variability.  This work is in collaboration with William Irion and Annmarie Eldering (NASA Jet Propulsion Laboratory [JPL]).  Gettelman has made detailed comparisons between AIRS data and aircraft measurements during pre-AVE (Atmospheric Variability Experiment), demonstrating high quality for the AIRS data.  Ongoing work includes quantifying the relationships between temperatures, water vapor and clouds, and understanding the effects of large-scale (satellite) vs. small-scale (aircraft) sampling.

The extratropical UTLS is a dynamically active region characterized by large variability, strong constituent gradients, and significant mass fluxes between the troposphere and stratosphere.  Pan has led several studies aimed at understanding constituent structure and variability in this region, using aircraft and satellite measurements together with model simulations. One project focuses on using tracer correlations to quantify the mixing of stratosphere and troposphere air near the subtropical jet.  Figure 36 shows an example of a stratospheric intrusion event observed during SONEX. The relationships between chemical tracers identifying stratospheric and tropospheric air (O₃ and CO, respectively) are used to understand mixing and exchange and to compare to model calculations.

Figure 36: Color image shows the NASA Langley LIDAR (LIght Detecting And Ranging) measurements of ozone onboard NASA DC-8 research aircraft, October 29, 1997, during the SONEX campaign. The black dots are the thermal tropopause as measured by the Microwave Temperature Profiler. The ozone measurements show an intrusion of stratospheric air into the troposphere near the right edge of the image. The inset shows a scatter plot of CO-O₃ measurements near the intrusion, provided along the aircraft flight track. The letters A, B, C, and D indicate the measurement locations for the “mixing line.”

 

Massie has focused on using satellite measurements to quantify tropospheric aerosols and pollution and their variability over time.  An analysis of Total Ozone Mapping Spectrometer (TOMS) satellite data shows a significant increase in tropospheric aerosols over China and India during 1980-2000 (Figure 37).  These increases are expected, due to large increases in population and SO₂ emissions (converted to sulfate aerosols) in China and India, but this is the first study to quantify the changes based on satellite observations.  This analysis was in collaboration with Omar Torres (Joint Center for Earth Systems Technology, University of Maryland Baltimore County and the NASA Goddard Space Flight Center) and Steven Smith (Pacific Northwest National Laboratory, Joint Global Change Research Institute).

 

Figure 37: Winter averages of TOMS total aerosol optical depths, averaged for consecutive months from November through February for the China coastal plain and the Gobi desert. Aerosols over the Gobi desert are mainly due to desert dust, while aerosols over the coastal plain have a large contribution from sulfate.  Linear fits and decadal trends are indicated.

Mijeong Park (Seoul National University) is working with Randel and Rolando Garcia to study the structure and variability of the South Asian monsoon in the UTLS region, and its effect on trace constituent transport.  She is using satellite measurements of UTLS ozone and water vapor from AIRS, along with output from the Whole Atmosphere Community Climate Model, second version (WACCM2), to understand stratosphere-troposphere coupling associated with the monsoon circulations.  Results from WACCM2 and AIRS reveal consistent patterns of relatively high water vapor and low ozone within the monsoon anti-cyclone during summer. Variations in monsoon convective activity with 10-20 day periods are associated with fluctuations in anti-cyclone intensity and structure, and tracer variability. Analyses of observations and model results will be aimed at quantification of monsoon effects on stratosphere-troposphere exchange.

Additional UTLS related research is described in the Middle Atmosphere Studies section under WACCM results.

 

Model Development

Gettelman is leading a collaborative effort with Chris Webster (JPL), David Noone (CU), Andrew Dessler, Sun Wong (University of Maryland), and Natalie Mahowald (CGD), modeling isotopes of water in the atmosphere. Stable isotopes of water provide information about transport and budgets within the atmospheric hydrologic cycle. Heavier isotopes preferentially partition into condensed phases of water, and so the level of isotopes relative to a standard provides an integrated history of condensation. Thus heavy isotopes of water (chiefly HDO and H2-18O) can be used to understand the different pathways of transport in the upper troposphere. Specifically, air which contains significant quantities of evaporated ice will have a different set of isotopic ratios than air from which all the condensed phase has been removed.

Gettelman used an idealized model of the TTL to simulate the distribution of water isotopes, and compared against observations from in situ aircraft measurements. The model is able to reproduce the range of isotopic depletions observed in the data, and also reproduce individual episodes that mirror or depart from Rayleigh fractionation processes. The observations and the model simulation of HDO for a field campaign in the tropical upper troposphere are indicated in Figure 38, which also contains a theoretical curve of the isotopic ratio of a parcel lifted from the surface with all the condensate removed (a “Rayleigh distillation” curve).

Figure 38: The observations and the model simulation of HDO for a field campaign in the tropical upper troposphere are indicated in Figure 38, which also contains a theoretical curve of the isotopic ratio of a parcel lifted from the surface with all the condensate removed (a “Rayleigh distillation” curve).

The observations clearly indicate that water substance in the upper troposphere does not follow a Rayleigh distillation model. The fact that this simple model reproduces the observed wide distribution of isotopic composition suggests the importance of detrained ice in the TTL, as hypothesized from previous work. The results of the model and the observations also illustrate that stratospheric abundances of stable isotopes of water can be understood based on known isotopic physics, convective detrainment of ice and gradual dehydration. 

Gettelman has also been working as part of a team with university collaborators to put isotopes in the NCAR Community Climate System Model. As part of this work, Gettelman and Mahowald coordinated a workshop on “Isotopes in the Earth System” held at NCAR in January 2004.

In collaboration with Paul Konopka of Research Center of Jülich, Germany, Pan has evaluated the application of the Chemical Lagrangian Model of the Stratosphere (CLaMS) model in the region of the extratropical tropopause. Using a simple initialization of O₃ and CO, they have demonstrated the importance of mixing in reproducing observed tracer variability.  Figure 39 demonstrates the successful simulation of mixing for the event shown in Figure 36.  This shows that the observed O₃-CO relationship is reproduced when mixing is included in the calculations, whereas under pure advection the tracer relationships cannot be obtained.  A successful simulation of the observed “mixing line” will allow quantification of irreversible UTLS transport.

 

 Figure 39: CLaMS simulation of mixing in the vicinity of the subtropical jet (color points), compared with observations (black points, from Figure 36).  The colors denote the fraction of stratospheric air in each parcel. The left panel shows a simulation with mixing, and the right panel is a simulation with no mixing.

 

 

UTLS Chemistry and Aerosols

Brian Ridley and the AON Group have completed analyses covering two subjects relating to the NO and NO_y measurements (made in conjunction with a variety of other gaseous and particulate/ice constituents) obtained from flights of the high altitude NASA WB-57F aircraft during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) project during July 2002. 

The first subject considers the uptake of HNO₃ by sulfate aerosols (Figure 40).  In situ measurements revealed near-zero mixing ratios of gas-phase HNO₃, in the absence of ice particles, at and just above the tropical tropopause for extended periods of one flight south from Key West, Florida.  These data are unique within CRYSTAL-FACE for having depletion of gas-phase HNO₃ not in association with ice particles.  In a collaboration with David Fahey, Ru-Shan Gao, Peter Popp, Tim Marcy (NOAA Aeronomy Lab), Robert Hermann (NASA JPL), Elliot Weinstock (Harvard University), and Darrel Baumgardner (UNAM), the AON Group has interpreted this removal as HNO₃ uptake on sulfate aerosols in an ice-supersaturated environment.  Calculations for ternary (H₂O, H₂SO₄, HNO₃) liquid solutions can approximately account for the low HNO₃ mixing ratios by demonstrating that near complete removal from the gas phase is expected for the observed conditions.  It is possible that a modest amount of NH₃ may contribute to the HNO₃ uptake.  Nitric acid trihydrate (NAT) equilibrium calculations also yield significant uptake, but the nucleation barrier for NAT makes uptake by liquid solutions more likely, especially in the presence of the large aerosol volume for this case.  This phenomenon may have significance for the subsequent evolution of the reactive nitrogen balance in the air mass, as well as for the potential role of these aerosols as sites for ice nucleation in subsequent cirrus cloud formation.  A manuscript is nearly complete.

Figure 40: Plot of measured gas-phase HNO₃ (courtesy of David Fahey et al., NOAA Aeronomy Lab), with near-zero values in periods four and five, along with the calculated uptake of HNO₃ by ternary liquid solutions (STS), binary liquid solutions (BS, H₂O/HNO₃), and NAT.  Ambient temperature is plotted (black), along with the equilibrium threshold temperatures for NAT (red) and NAD (blue).  Although both the STS and NAT equilibrium calculations yield significant uptake, the nucleation barrier for NAT makes uptake by liquid solutions more likely, especially in the presence of the large aerosol volume for this case.  Also, it is possible that a modest amount of NH₃ in solution may enhance the HNO₃ uptake beyond that calculated here.

The second subject considers the production of NO by lightning as investigated by the aircraft during penetration of the anvils of a large number of different thunderstorms that formed over Florida.  A manuscript has just been published.  Exceptionally high mixing ratios (6-10 ppbv) over large scales (20-50 km) were observed as a result of lightning production, which will have a substantial impact on the photochemical generation of ozone in the downwind outflow altitude range.  Larger scale peak mixing ratios of NO were higher on average than found during an earlier study of thunderstorms over eastern Colorado.  Injection of NO from the storms was overwhelmingly into the upper troposphere and very little, if any, into the stratosphere.  The observations were also compared with the lightning parameterization scheme that is used in the MOZART global chemistry-transport model.  Lesley Ott and Ken Pickering (University of Maryland), Lihua Li, Gerry Heymsfield, and Matt McGill (NASA Goddard), Paul Kucera (University of North Dakota), Louisa Emmons (ACD), and Martin Schultz and Guy Brasseur (Max Planck, Hamburg) contributed greatly to the storm analyses and model comparison.

Mike Coffey and Jim Hannigan of the Optical Techniques (OT) Group, as part of the international Network for the Detection of Stratospheric Change (NDSC), operate an infrared Fourier transform spectrometer (FTS) at Thule, Greenland (76.53°N). The NDSC is a network of high quality ground based observing stations for early measurement of changes in the composition and state of the stratosphere and determination of their causes.   Operation of the spectrometer at Thule is automatic, with monitoring from Boulder, whenever the weather is suitable and the sun is above the horizon.  Observations were made on 75 days of the possible 225 sunlit days of 2004.  Missed days usually are due to stormy weather.  Those data were analyzed for column amounts of gases, including both stratospheric gases important in ozone chemistry and tropospheric gases related to climate change.

In conjunction with other observations from the network, composed mostly of instruments from nations other than the U.S., the OT Thule measurements were used in the validation activities of recently launched satellite-borne instruments. Collaborations are ongoing with instruments aboard the Envisat (ESA) platform; plans and collaborations have been established for validation of the EOS-Aura (U.S., U.K., Netherlands, Finland, Launched July, 2004) satellite-borne instruments.  During March 2004, observations were made in conjunction with measurements by the Atmospheric Chemistry Experiment (ACE) instrument aboard the Canadian SCISAT (Scientific Satellite)-1 satellite.  This was the first opportunity for coincident observations by the ACE FTS and the NCAR FTS since the launch of SCISAT-1 in August 2003.  Coincident observations are critical to the validation of space-borne sensors.  Other coordinated observations will be undertaken as the irregular orbit of SCISAT allows.

Results from observations by the airborne Fourier transform spectrometer (0.06 cm-1 resolution), that span more than 20 years, have been reprocessed to produce a unique archive of observations, from the base of the stratosphere, covering a wide range of latitude and season.  This archive of infrared spectra has been used along with recent advances in least squares fitting in multiple wavelength regions to study the long-term trends in water and its isotopes, OCS and CO.  Water transfer across the tropopause and redistribution in the lower stratosphere are important factors to the chemistry, radiation and dynamics of that important atmospheric transition region.  Variations in the behavior of the water isotopes can provide insight into the sources and distribution of water.

Measurements of organic trace gases and other tracers, such as CO₂, N₂O, CH₄ and water vapor, are among those required to adequately characterize the chemistry of any given stratospheric region and to provide important information to diagnose stratospheric transport processes.  Four complementary observational systems are currently employed to measure these trace gases in the stratosphere and each have their own unique set of advantages.  The four include satellite observations, in situ measurements, remote soundings, and measurements from whole air samples.  Elliot Atlas, Sue Schauffler, Verity Stroud, Rich Lueb, and Amy Lueb of the STM Group have collected and analyzed whole air samples from high altitude aircraft, the NASA ER-2 and WB-57, for a number of years.  The maximum altitude for these aircraft is about 65,000 feet.  Recently the STM Group designed and built a whole air sampler to be flown on a high altitude balloon up to 110,000 feet.  Two test flights were flown, one in October, 2003 and the second in September, 2004, from Ft. Sumner, New Mexico.  Figure 41 shows a vertical profile of methane and nitrous oxide (N₂O) from the 2004 flight.  The structure of the profile above about 70,000 feet is similar to that seen in the 2003 flight and the STM Group is currently evaluating potential transport characteristics that my influence the distributions.

Figure 41: Vertical profiles of N₂O and CH₄ from samples collected by the Cryongenic WAS (CWAS) during the balloon flight from Ft. Sumner, NM in 2004. 

 

An important use of whole air sampler trace gas measurements is to determine temporal trends of mixing ratios in the stratosphere to accurately assess the impact of changes in these gases on stratospheric ozone.  Figure 42 shows a correlation between hydrofluorocarbon (HFC) 134a, which is a replacement for CFC-12, and N₂O from the balloon flight in 2004 and a series of ER-2 flights out of northern Sweden in 2000.  The higher 134a mixing ratios in 2004 are a result of increased emissions over time, which leads to increased mixing ratios over time.  The STM Group is continuing analysis of these results for all the major halocarbons. 

Figure 42:  HFC 134a mixing ratios relative to N₂O mixing ratios from WAS samples collected on the NASA ER-2 during the SOLVE campaign in 2000 and from CWAS balloon samples collected from Fort Sumner, NM in 2004.  The higher HFC 134a mixing ratios during 2004 relative to 2000 are a result of increasing tropospheric mixing ratios over time due to increasing emissions.  HFC 134a is a replacement compound for CFC-12 in refrigerant applications.

 

Atlas and Schauffler of the STM Group also flew a Whole Air Sampler (WAS) on the WB-57 during the NASA Pre-AVE campaign conducted out of Houston and Costa Rica in January and February 2004.  Pre-AVE was an initial test experiment leading to AVE, scheduled to begin in FY 2005.  The Pre-AVE campaign was designed to test aircraft instruments to