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.  The Division fulfills this 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.  Our emphasis is on characterizing the distributions of chemically relevant atmospheric constituents, focusing on specific spatial and temporal regimes to understand photochemical processes, examining emission and exchange processes between the biosphere and the atmosphere, studying aerosol composition and formation processes, and examining the role of chemistry in the global climate system on a variety of temporal and spatial scales.

These collaborative efforts have lead to significant advancements this year in understanding ozone variability in the tropical Atlantic and the mid-high latitudes of North America, in evaluations of global carbon monoxide (CO) emissions, in understanding VOC emissions from a number of regions, in characterization of aerosol formation, in understanding distributions of water vapor, chemical tracers, and cirrus clouds near the tropopause region, and in characterizations of middle atmosphere transport processes. 

A.  Atmospheric Trace Gases

ACD has been a leader in the measurement of a variety of atmospheric trace gases for over thirty years.  Characterization of the distributions of chemically relevant atmospheric constituents, spatially and temporally, is crucial in the process of identifying potential controlling mechanisms as well as impacts of those distributions on chemistry, transport, and climate.  Global and regional distributions of CO from the Measurement Of Pollution In The Troposphere (MOPITT) satellite instrument have been used in conjunction with other satellite and airborne measurements and model simulations to explain the influence of biomass burning, interhemispheric transport, and lightning produced NO_x on the distribution of ozone in the tropical Atlantic.  This work represents the first effort to use multiple satellite, aircraft, and ground-based measurements synergistically to address an outstanding issue in atmospheric chemistry.  Modeling simulations based on measurements of vertical profiles of volatile organic compounds (VOCs) from Transport and Chemistry Evolution over the Pacific (TRACE-P) suggest that acetone, and potentially other VOCs, plays an extremely important role in tropospheric oxidation mechanisms above eight kilometers (Figure 1).  Trace gas measurements from whole air samples collected from a German-led cruise in the tropical Atlantic showed a complex distribution reflecting interhemispheric transport, biomass burning effluents, and transport of industrial emissions from Europe.  Measurements from the High Resolution Dynamics Limb Sounder (HIRDLS) satellite instrument, to be launched in 2004, are expected to significantly enhance our knowledge base of the chemical composition of the atmosphere.

Additional Accomplishments include:

1.  We have continued improvement of the data reduction algorithms and data processing for MOPITT CO.  Version 3 MOPITT Level 2 data product was comprehensively evaluated relative to in-situ measurements and as a result was upgraded from provisional to validated.  Corrections are being devised to allow CH₄ column retrievals from MOPITT.  (http://eos-atm.eos.ucar.edu/mopitt/index.html)

2.  MOPITT CO measurements and Model for Ozone and Related Chemical Tracers (MOZART)-2 were used in the first effort to constrain global surface fluxes and assess the accuracy of emission inventories using satellite data.  As a result of this work, a new set of monthly mean surface CO emissions was produced for a full year and is available to the community for use in numerical simulations (Figure 2). 

3.  HIRDLS was successfully integrated, mechanically and electrically, with the Aura spacecraft and has undergone vibration and acoustic testing and will complete thermal vacuum testing soon. Significant progress was made on development of HIRDLS retrieval algorithms and operational code.  Simulations from MOZART-3 and Whole Atmosphere Community Climate Model (WACCM) were used to test the algorithms and as pathfinders for problems to be addressed using HIRDLS data.  (http://www.eos.ucar.edu/hirdls/)

4.  Successful test flights of a newly designed and constructed cryogenic whole air sampler were conducted on a balloon platform with 25 samples collected between 10 and 33 km altitude.  Improvements were made on the Fast Gas Chromatography/Mass spectrometry (FGCMS) system to allow measurement of more than 40 compounds and on the tunable diode laser/difference frequency generation (TDL/DFG) instrument with an eye towards modifications that can be applied to a new instrument for High-performance Instrumented Airborne Platform for Environmental Research (HIAPER).  Advances were also made in the development of a new laser spectrometer for measurement of CO₂ isotopic ratios.

B.  Photo-Oxidants

Photochemically produced oxidants are a critical part of the atmospheric chemical system.  Ozone is of particular interest because of its role in tropospheric air quality, as greenhouse gas, and as an absorber of stratospheric UV radiation.  The hydroxyl radical (OH) is also extremely important as a result of its involvement in ozone chemistry and its role in the oxidation of many compounds emitted to the atmosphere.  Our emphasis in this area is on evaluation of photochemical processes in both urban and remote regions and the resulting impacts on regional and global scales.  Megacity Impacts on Regional and Global Environments (MIRAGE) is an ACD-led multidisciplinary study of urban pollutants and their impact on regional air chemistry.  A planning workshop was held at NCAR and a potential field campaign in Mexico City was discussed.  Preliminary modeling work to characterize the outflow from Mexico City using the NCAR Master Mechanism indicated the reactivity of the outflow remained high for several days after emission which implies that urban pollution can have major regional effects.  NCAR’s Weather Research Forecast model, developed with on-line chemistry (WRF-Chem) was modified to include emissions for Mexico City and used to confirm that, under the conditions specified in the model, ozone continued to be produced far down-wind of the city.  Results of this type will be exceptionally useful in providing a framework for designing the field campaigns.  Our priority region for examining photochemistry in the remote atmosphere will be the Upper Troposphere Lower Stratosphere (UTLS) region.  ACD led the community-wide development of a white paper describing research priorities in the UTLS and potential field campaigns using the NSF HIAPER aircraft to address these issues.  A community workshop was held at NCAR to discuss the white paper and initiate working groups in the identified areas of interest.  Our evaluation of photochemical processes in both urban and remote regions relies on high quality results from laboratory kinetics experiments.  These results are required in photochemical models and recent laboratory results were used to update the TUV and MOZART models.

Additional Accomplishments include:

1.  Laboratory studies identified an array of previously unknown products from the oxidation of cyclopentane and cyclohexane, characterized the temperature dependence of butane oxidation, measured the yield of tertiary alkyl nitrate species from the photo-oxidation of simple branched alkanes, and identified previously unknown products in the reaction of an organic peroxy radical with HO₂, which will lead to re-evaluation of both the reaction mechanism and the kinetics of this reaction.

2.  Analysis of the ozone budget for Tropospheric Ozone Production about the Spring Equinox (TOPSE) using MOZART-2 and ACD’s regional-scale chemistry-transport model, HANK, found that chemical production and destruction were the dominant drivers of the observed spring maximum (Figure 3). 

3.  Significant progress was made on the design, development, calibration, and testing of several instruments, including a new NO/NO_y instrument for HIAPER, spectroradiometers, Per CIMS (Chemical Ionization Mass Spectrometer), a Fast- Gas Chromatograph/Mass Spectrometer (FGC/MS), fast peroxyacyl nitrate-gas chromatograph (PAN-GC), Thermal Dissociation (TD)-CIMS for PAN measurements, and CIMS instrument for ammonia measurements.

C. Biogeochemistry

ACD research in this area is focused on investigating exchange between the biosphere and atmosphere of organic carbon and nitrogen-containing compounds and the impacts of those compounds on atmospheric chemistry. Measurements of nitrogen deposition from various studies and sites were compiled and mapped as part of the NCAR Biogeosciences Initiative to be used as input to terrestrial models of biogeochemistry and regional and global models of atmospheric chemistry and transport.  An initial study found that estimated nitrogen emissions exceeded deposition in the U.S. and were in balance in Europe, suggesting significant export of nitrogen from the U.S. but not Europe.  A number of field campaigns were conducted to examine leaf-level to canopy-level fluxes of VOCs.  Strong seasonal variations in VOC emissions were observed in China, Brazil, and Costa Rica and the measurements suggest that the prevalent land-use changes in tropical regions are likely to result in substantial changes in the chemical composition of the atmosphere (Figure 4).  The Chemical Emission, Loss, Transformation and Interactions within Canopies (CELTIC) campaign in Duke Forest included an unprecedented array of enclosure and whole canopy trace gas and aerosol measurement systems.  Initial results suggest isoprene emission increases with elevated ozone, canopy isoprene emission increases with elevated CO₂, and soil and leaf litter are a net sink of oxygenated VOC.  Final results from this study will be used to improve models, which are currently unable to accurately simulate observed biosphere-atmosphere exchange of trace gases.  Additional work on biogeochemistry in ACD was conducted as part of the Wildfire Initiative.  Laboratory studies indicated that VOC emissions from fires influence fire dynamics and ignitability of the vegetation.  Modeling tools are being developed to investigate the impact of fires on emissions and regional air quality.

Additional Accomplishments include:

1.  Modeling studies showed that insect herbivory may play an important role in affecting carbon and nitrogen cycling under conditions of high nitrogen deposition.

2.  Laboratory studies showed monoterpene emissions increased substantially in response to mild drought.

3.  Development has continued on an international biogenic VOC measurement database.  (http://www.acd.ucar.edu:8080/bvoc/vocIndex.jsp)

D.  Aerosols

Aerosol research in ACD includes studies of aerosol distributions in the atmosphere, particle nucleation, chemical composition of ultrafine particles, and uptake and reaction of gas phase species on aerosol surfaces.  HIRDLS is expected to make significant contributions to our characterization of the distributions of aerosols and clouds, including sub-visible cirrus and polar stratospheric clouds.  Initial measurements of ultrafine aerosol particles from a study in Mexico City allowed the first characterization of aerosol nucleation events in this region (Figure 5).  Studies such as this will help determine future instrument needs and the aerosol study location to be used during MIRAGE. 

Additional Accomplishments include:

1.  Significant progress has been made in laboratory studies of the rates and mechanisms of nucleation and particle chemistry involving sulfuric acid, water, and ammonia.

2.  A new instrument, the Direct beam Irradiance Airborne Spectroradiometer (DIAS), was deployed during the NASA Stratospheric Aerosol and Gas Experiment (SAGE) Ozone Loss Validation Experiment (SOLVE) II campaign to measure overhead ozone column and aerosol optical depths as a function of wavelength.  Final data reduction and analysis is in progress.

3.  Preliminary design work and component acquisition for a tandem differential mobility analyzer has been completed.  The new instrument will complement the TDCIMS system by providing hygroscopicity and volatility measurements that can be combined with the TCDIMS chemistry results.

4.  Continuous measurements of the chemical composition of ultrafine aerosols were collected using the TDCIMS instrument during several one- to two-month long sampling periods in each season.  Several nucleation events were observed, as was a large source of nitrate ions which did not appear to originate from ammonium nitrate.  These experiments provided information required to continue improvement of the TDCIMS system.

E. Chemistry-Climate Interactions

Chemistry-climate studies in ACD primarily involves the use of satellite, aircraft, and ground-based data and our modeling capabilities to study both troposphere and middle atmosphere chemistry-climate interactions.  A combination of MOZART simulations and ozonesonde data allowed the first identification of the Arctic Oscillation (AO) signature in the interannual variability of tropospheric ozone.  Variability in the AO explains up to 50% of the tropospheric ozone variability in the lower troposphere over North America (Figure 6).  Measurements of water vapor and temperature anomalies in the UTLS region showed strong correlations near the tropical tropopause, which may explain the observed global-scale water vapor changes.  Seasonal variations of methane, water vapor, and nitrogen oxides in the UTLS, based on satellite measurements and MOZART-2 simulations, highlight the importance of the Northern Hemisphere summer monsoons as regions for transport into the lowermost stratosphere (Figure 7).  In addition, aircraft measurements and model simulations show a transition layer around the thermal tropopause that is related to stratosphere/troposphere exchange processes.  Simulations from the ROSE model were used to show that the sources of quasi-stationary planetary waves in the upper mesosphere are due to upward propagation of Rossby waves below 80 km and the in situ generation of planetary scale variations by gravity wave dissipation above 80 km. 

Additional Accomplishments include:

1.  Initial simulations using an updated version of MOZART-2 suggest that clouds have a significant impact on photolysis frequencies and, hence, photochemistry, and that MOZART aerosol distributions compared favorably in most regions with satellite measurements.

2.  An updated community assessment of climatological winds and temperatures in middle atmosphere data sets was completed under the auspices of Stratospheric Processes and their Role in Climate (SPARC).

3.  The area of cirrus clouds observed from satellites was substantially smaller following significant volcanic events.  This result is not readily explained by cloud microphysics or temperature variations and remains an outstanding question.

4.  Thermospheric nitric oxide changes may lead to changes in stratospheric ozone, so a three-dimensional empirical model of nitric oxide in the lower thermosphere was developed that can be utilized in climate simulations without the need to incorporate additional thermospheric processes.