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 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.