Summary

A major portion of ACD’s mission is 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. Another important aspect of ACD’s mission is development of new measurement capabilities and methodologies, development and application of numerical models, and the planning and execution of complex field experiments. The Division fulfills its mission through collaborative efforts with colleagues from the university, government, and international communities to measure atmospheric chemical composition and controlling processes from satellite, airborne, and ground-based platforms and to examine those measurements synergistically with laboratory studies and modeling efforts.

I. Regional and Global Air Quality

The main goal for this area of study is to understand and quantify the impact of urban emissions on air quality. Priority is 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.

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

The ACD MIRAGE science team submitted a science plan to NSF and a proposal to OFAP requesting deployment of the C-130 to Mexico City. In anticipation of MIRAGE-Mex, preliminary modeling with WRF-Chem to study the interaction of urban outflow plume with the surrounding region found that this interaction leads to more regional O₃ production than would be expected from three sources (urban, biogenic, and biomass burning) taken in isolation (Figure 15; all figures in this Executive Summary correspond to the same figure numbers found in the "ACD Achievements" section of this 2004 ACD Annual Science Report). Several ACD scientists participated in field campaigns that are complementary to MIRAGE. Preliminary results of ozone, NOx, and peroxyacetyl nitrate (PAN) measurements collected downwind from New York City during the NOAA New England Air Quality Study (NEAQS) in the summer showed a significant degree of structure, likely due to air masses of different photochemical age and different initial pollution levels, and an excellent correlation down to the fine time scale of seconds between ozone and PAN as a result of photochemical activity (Figure 13). These ACD PAN measurements are the first reliable high temporal resolution measurements made from an aircraft and the instrument will be available for MIRAGE-Mex. Emissions from fires as well as urban areas may impact regional as well as global air quality. The MOPITT science team measured an unusual increase in CO levels in late 2002 and early 2003 that affected the entire Northern Hemisphere. Further investigation indicated a Russian source: massive peat fires smoldered near Moscow in the late summer and fall of 2002, and unusually intense Siberian wildfires in the spring and summer of 2003. Finally, an analysis of Total Ozone Mapping Spectrometer (TOMS) satellite data showed a significant increase in tropospheric aerosols over China and India during 1980-2000 (Figure 37). These increases were expected, due to large increases in population and SO₂ emissions (converted to sulfate aerosols) in China and India, but this was the first study to quantify the changes based on satellite observations.

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 NOx in the NOx-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).

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.

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.

Additional achievements include:

1. Continued development and improvements were made to several ACD instruments, including the PAN CIMS instrument, the actinic flux spectroradiometer, the laser spectrometer for measurement of CO2 isotopic ratios, and the TDLAS and DFG instruments for measurement of formaldehyde.

2. The MOPITT science team completed a study on the CO budget over Europe using MOPITT CO measurements, ground based data, and simulations from MOZART. The results showed that, on average, the majority of surface contributions to the anthropogenic CO load were from regional sources with a significant contribution from North America and Asia. With altitude the influence of CO from other continents gained in importance with Asian emission showing the highest contributions during most months and at most altitudes.

3. A steady state photochemical model reproduced observed peroxy radical concentrations during the NASA TRACE-P campaign except during encounters with clouds or air masses with high aerosol surface density and when NO mixing ratios were very high. These results provide indirect evidence for heterogeneous update of radicals by clouds and aerosols and also imply missing or misrepresented processes involving high levels of NO.

B. Reactive Carbon Research

Reactive carbon research involves the study of a number of 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. Collaborative studies that include ACD measurements of isoprene nitrate, suggest that this compound, which has not been considered in the past, may play a role in sequestering atmospheric NOx and may serve as a source of nitrogen for nitrogen-limited forests. The study concludes that isoprene nitrates should be measured as part of future field campaigns to adequately determine their impact on nitrogen cycling.

Additional achievements include:

1. Distributions of oxygenated VOCs in the Pacific during springtime and their relationships to tracer species such as CO, acetonitrile, and methyl chloride, are being investigated with respect to source relationships. These compounds are important because of their potential role in HOX chemistry and in secondary organic aerosol formation.

2. The TDCIMS instrument for characterizing organic compounds in ultrafine aerosol has undergone several improvements over the past year and was successfully tested over a several months at NCAR and at the Marshall Test site. Plots of particle size distributions may be found at http://acd.ucar.edu/~jimsmith/POP/ under Field Activities.

3. Laboratory studies examined oxidation pathways for alkyl iodides, nitrate production from toluene oxidation, and oxidation mechanisms of methyl formate and methyl acetate. Understanding these reactions is a necessary aspect of understanding the details of photochemistry in a given region.

C. Multiphase Processes in the Troposphere

Multiphase processes are an important, but not well understood, part of atmospheric chemical processes. Multiphase processes include aerosol formation and growth, interactions between clouds (aqueous and ice), aerosols, and chemical species, and photochemical processing in snow. In order to determine if ion inducted nucleation has a role in the formation of new atmospheric particles, ACD scientists have developed an instrument for measuring very low concentrations of ion clusters that would be expected to be present in the atmosphere. 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 colleagues at the NOAA Aeronomy Laboratory. ACD and MMM scientists 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. Initial results show redistribution of CO and formaldehyde (CH2O) from convective transport (Figure 22). A nearly total solar eclipse occurred during the NSF sponsored Antarctic Tropospheric Chemistry Investigation (ANTCI), which was designed to study photochemical processing across the polar plateau. Since the sun height normally changed little during the study period, the 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.

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.

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.

Additional achievements include:

1. Differences in depth profiles among various VOCs are being analyzed to characterize consumption and production processes in the snowpack at Niwot Ridge in order to quantify emissions.

2. An intercomparison study for convective cloud chemistry models was conducted as part of the 6th International Cloud Modeling Workshop, Cloud Chemistry Case. The intercomparison case focused on the 10 July 1996 STERAO storm for which observations of carbon monoxide, ozone, and nitrogen oxides (NO_x) were available. Initial results showed passive tracer transport was similar among the models and agreed fairly well with observations. The intercomparison of NO_x produced from lightning and of the soluble species is currently being pursued.

3. The NCAR large-eddy simulation (LES) was coupled with gas and aqueous chemistry by ACD and MMM scientists and used to determine that cloud chemistry has a significant impact on the distribution of OH and isoprene, which are two species involved in ozone production.

II. Chemistry in the Climate System

Atmospheric chemistry and transport are key factors in determining the abundance and distribution of trace gases and aerosols that determine the Earth’s radiation budget and climate. We are evaluating this system through model development and simulations of various climate states. In addition, we are conducting focused studies of biogenic emissions on a broad range of spatial scales from leaf, plant, and canopy scales, through to regional scales. Finally, we are investigating chemical and transport characteristics that influence the distributions of radiatively important trace gases in the UTLS and middle atmosphere.

A. Climate Simulations

Model simulations and data analysis of ozonesondes have shown that the interannual variability of ozone during the Northern Hemisphere springtime is strongly related to the phase of the North Atlantic Oscillation (NAO). Simulated ozone fields from MOZART covering the time period 1890 to 1990 were used in CCSM simulations of the 20^th century climate for the next IPCC Assessment.

Additional achievements include:

1. An offline-transport version of the Community Atmosphere Model (CAM) was successfully developed. This model imports meteorological fields from other sources (e.g., NCEP, ECMWF) into the CAM model, which provides the opportunity to simulate the transport and chemistry of trace constituents using meteorological analysis.

2. Interactive chemistry has been implemented from the MOZART model into CAM3 using the WACCM framework. The radiative coupling is done through the ozone distribution and the aerosol distributions.

3. The MOZART model was updated with improved chemical mechanisms, reaction rates, emissions, and aerosol parameterizations. The new version is MOZART-4 and will be included in the online and offline CAM with chemistry.

B. Biogeochemical Cycles

Biogeochemical studies in ACD are focused on the role of biosphere-atmosphere interactions in the Earth system and predictions of the response to human perturbations. We are conducting multidisciplinary field, laboratory and modeling studies of the processes controlling these interactions on various scales.

Laboratory based biomass burning studies demonstrated that oxygenated volatile organic compounds (VOC) 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. The 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. The aircraft observations generally confirmed the emission profiles measured in the laboratory studies as shown in Figure 32. Results from the Chemical Emissions, Losses, Transformations and Interactions with Canopies (CELTIC) study indicate that isoprene emission increases with elevated ozone and, on a canopy scale, with elevated CO₂. Future changes in ozone and CO₂could thus change regional isoprene emission rates leading to feedbacks. Global modeling studies showed that uncertainties in biogenic VOC emissions are a significant contributor to the total uncertainty associated with estimates of global radiative forcing of aerosols. Also, 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. A biogenic VOC model was integrated into the NCAR Community Climate System Model [CCSM] Land Surface Model [LSM] and simulation results showed that 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.

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.

Additional achievements include:

1. Laboratory studies have shown that isoprene emission recovers relatively quickly from drought, sesquiterpene emission rates from some important tree species are comparable to monoterpene emission rates and exhibit similar temperature dependence, and methanol emission rates were observed to vary across and within plant species by at least an order of magnitude.

2. Measurements of trace gas fluxes over a Colorado grassland showed that the grassland was a significant source of methanol and a sink for acetic acid and methyl acetate.

3. Development of a regional air quality monitoring system and emission scenarios was initiated to study the impact of future climate and land cover on regional air quality in the Pacific Northwest and north central U.S. Biogenic and fire emission models and databases were developed for the scientific and air quality regulatory communities.

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

The upper troposphere and lower stratosphere (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. ACD scientists are involved in studying various aspects of transport and chemistry that influence the distributions of these gases in the UTLS as well as developing models and instruments to support these studies.

Results from high vertical resolution temperature profiles derived from GPS radio occultation measurements demonstrated 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. Chemical tracer distributions and correlations from aircraft measurements and model output were used to quantify mixing of stratosphere and troposphere air near the subtropical jet as shown in Figures 36 and 39. An analysis of Total Ozone Mapping Spectromenter (TOMS) satellite data showed a significant increase in tropospheric aerosols over China and India during 1980-2000. These increases were expected, due to large increases in population and SO₂ emissions (converted to sulfate aerosols) in China and India, but this was the first study to quantify the changes based on satellite observations. Transport characteristics of the Asian monsoon complex and impacts on the upper troposphere and lower stratosphere were examined using observations from aircraft and global chemical and climate models. Modeled fluxes of water vapor indicate that the Asian Monsoon circulation may contribute 75% of the total net upward water vapor flux in the Tropics at tropopause levels from July to September. Some of this air may enter the tropical stratosphere and bypass the tropical tropopause altogether. In situ measurements during the NASA CRYSTAL-FACE campaign revealed an unusual circumstance of near-zero mixing ratios of gas-phase HNO₃, in the absence of ice particles, at and just above the tropical tropopause. This removal was interpreted as HNO₃ uptake on sulfate aerosols in an ice-supersaturated environment, which 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.

Figure 36: Color image shows the NASA Langley LIDAR 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.”

Figure 39: CLaMS simulation of mixing in the vicinity of the subtropical jet (color points), compared with observations (black points, from DA Figure 2). 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.

Additional achievements include:

1. ACD scientists coauthored a SPARC-IGAC white paper describing Chemistry Climate Interactions, which will form the basis for joint scientific activities during the next decade.

2. An idealized model of the TTL was used to simulate the distribution of water isotopes, which were compared against observations from in-situ aircraft measurements. The model reproduced the range of isotopic depletions observed in the data and suggested the importance of detrained ice in the TTL, as hypothesized from previous work.

3. Several instruments will be built by ACD staff for community use on the HIAPER GV aircraft. These include the Trace Organic Gas Analyzer (TOGA), HIAPER Radiation Package (HARP), NO/NOy and O3. Additional ACD instruments that will be built for HIAPER include OH and FTIR instruments.

D. Middle Atmosphere Science

The HIRDLS (HIgh Resolution Dynamics Limb Sounder) instrument is onboard NASA's AURA satellite, which was launched from Vandenberg AFB on a Delta II booster at 3:02 a.m. on Thursday, July 15, 2004 (Figure 54). Quantities to be measured by HIRDLS include temperature, ozone, water vapor, methane, nitrous oxide, nitrogen dioxide, dinitrogen pentoxide, nitric acid, CFC 11 & 12, chlorine nitrate and aerosol properties.