Laboratory for Atmospheres 2003 Technical Highlights: Section 5 Highlights of Activities

This section highlights the Laboratory's accomplishments for 2003 with summaries by the Branch Heads giving examples of the research carried out by Branch scientists and engineers. The Branch highlights are supplemented by NASA press releases on our work in Appendix A1 and by abstracts of highlighted papers in Appendix A2. A complete list of papers published in 2003 is found in Appendix B7 on the Laboratory Web site. For more details on Branch science activities, the Branch Web sites can be accessed from the Laboratory for Atmospheres home page at

5.1 Mesoscale Atmospheric Processes Branch, Code 912

The Mesoscale Atmospheric Processes Branch seeks to understand the contributions of mesoscale atmospheric processes to the global climate system. Research is conducted on the physical and dynamical properties, structure and evolution of meteorological phenomena ranging from synoptic scale down to microscales, with a strong focus on the initiation, development, and effects of cloud systems. A major emphasis is placed on understanding energy exchange and conversion mechanisms, especially cloud microphysical development and latent heat release associated with atmospheric motions. The research is inherently focused on defining the atmospheric component of the global hydrologic cycle, especially precipitation, and its interaction with other components of the Earth system. Branch members participate in satellite missions and develop advanced remote sensing technology with strengths in the active remote sensing of aerosols, water vapor, winds, and convective and cirrus clouds. There are also strong research activities in cloud system modeling, and in the analysis, application, and visualization of a great variety of data.

1) Branch scientists develop retrieval techniques to estimate precipitation using satellite observations from TRMM and other satellites such as GOES and the new AMSR-E sensor on EOS Aqua. Notable accomplishments in 2003 were: publication of an overview of the 20-year Global Precipitation Climatology Product (GPCP) merged data set, release of a new data product of 3-hourly near-global rainfall fields in near-real time, publication of a review of TRMM monthly rainfall estimates, development of a new effective El Niño predictor, application of TRMM data to study of the urban heat island effect, and publication of a global surface flux data set. The TRMM Ground Validation team processes and applies data from rain gauge networks, and ground-based radars. TRMM and other precipitation data are used within the Branch for a wide spectrum of studies on precipitating cloud systems and the global water cycle. Increasingly, these activities integrate global or regional data sets with modeling. Research is conducted on the assimilation of TRMM observations into models to explore the potential benefits to weather forecasting, such as for hurricanes, and to improve understanding of precipitating cloud systems. Branch scientists are also an integral part of the developing Global Precipitation Measurement (GPM) mission, presently in the formulation phase. GPM seeks to establish an international calibrated satellite network for high-resolution (space and time) global precipitation measurements.

2) Development of lidar technology and application of lidar data for atmospheric measurements are also key areas of research. Systems have been developed to characterize the vertical profile structure of cloud systems (CPL), atmospheric aerosols (MPL), water vapor (SRL), and winds (GLOW) at fine temporal and/or spatial resolution from ground-based, airborne and satellite platforms. In addition, the Cloud Radar System (CRS), a millimeter-wavelength radar for profiling cloud systems has been developed and integrated on NASA's high altitude ER-2 research aircraft for use in sensing the microphysical properties of cirrus and other cloud types, and complements the existing ER-2 Doppler (EDOP) radar that has been extensively used to study precipitating cloud systems.

Of particular note in 2003, seven papers appeared (Welton as coauthor) that used MPL data from various field experiments including Aerosol Recirculation and Rainfall Experiment (ARREX), Southern African Fire-Atmosphere Research Institute (SAFARI)-2K, Puerto Rico Dust Experiment (PRiDE), and ACE-Asia indicating the growing value of investments made in this project and the associated scientific contribution to aerosol science. Also of note, was publication of a major review article on lidar remote sensing and development of an American Meteorological Society (AMS) Short Course for nonspecialists. Analysis of extensive observations obtained during IHOP in 2002 (SRL, GLOW, and HARLIE systems) and the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) in 2002 (CPL, CRS, and EDOP) continued in 2003. Significant advances were also made in the area of Raman lidar techniques.

Branch scientists developed atmosphere-sensing capabilities of the Geoscience Laser Altimeter System (GLAS) that was launched on ICESat early in 2003. GLAS is used to profile the vertical distributions of cloud and aerosol layers. The GLAS data quality exceeds expectations and analysis of the early data from this exciting new mission is ongoing. Branch scientists also serve as Project Scientists for the Earth System Science Pathfinder (ESSP), CALIPSO (lidar), and CloudSat (millimeter-radar) missions that are planned for launch in 2004.

3) Cloud-resolving (Goddard Cumulus Ensemble, GCE) and mesoscale (MM5) models are used in investigations of the dynamic and thermodynamic processes associated with tropical and extratropical cyclones and rainbands, and tropical and midlatitude deep convective systems. The models are also used to research cloud-chemistry interactions, stratospheric-tropospheric interactions, and the effects of the ocean surface (sea surface temperature) and land surface (vegetation and soil moisture) on atmospheric convection and weather systems. Other important applications include assessment of the potential benefits of assimilating satellite-derived water vapor and precipitation fields on simulations and forecasting of tropical and extra-tropical regional-scale weather systems (i.e., hurricanes and cyclones). Long-term integrations of the models are used to investigate climate feedback mechanisms, such as cloud-radiation interactions. The simulations provide a basis for integrated system-wide assessment of important factors, such as surface energy and radiative exchange processes, and diabatic heating and water budgets associated with tropical and midlatitude weather systems. The models are also used to develop retrieval algorithms. For example, the GCE model is providing TRMM investigators with four-dimensional data sets for developing and improving TRMM rainfall and latent heating retrieval algorithms. The scientific output of the modeling activities was again exceptional in 2003 with publication of eight new papers, five major review articles, model documentation, and workshop review articles. Branch scientists have actively participated in, and hold leadership roles in, various international model comparison and evaluation activities of the GEWEX Cloud System Study for the purpose of increasing confidence in the cloud-resolving models and facilitating research on the development and testing of cloud parameterizations used in large-scale climate and forecast models (GCMs).

4) The Branch has developed a world-class visualization lab that is being increasingly used in high profile settings to reach out to scientists and, very importantly, to citizens and government organizations to stimulate understanding and support of NASA's Earth Science Enterprise and its missions. The TRMM Outreach Office, the EOS Project Science Office, Earth Sciences Directorate, and the NASA Earth Science Enterprise (HQ) heavily utilize these capabilities in bringing the value of NASA missions and science accomplishments to the forefront of U.S. Global Change Research.

Highlighted papers appearing in Appendix A2 include Curtis and Adler 2003, Shepherd and Burian 2003, McGill et al. 2003, Whiteman 2003a and b, and Tao et al. 2003.

5.2 Climate and Radiation Branch, Code 913

One of the most pressing issues we face is to understand the Earth's climate system and how it is affected by human activities now and in the future. This has been the driving force behind many of the activities in the Climate and Radiation Branch. We have made major scientific contributions in five key areas: hydrologic processes and climate, aerosol-climate interaction, clouds and radiation, model physics improvement, and technology development. Examples of these contributions may be found in the workshops, seminars, and lists of refereed papers in the Annual Report's B appendices on the Laboratory Web site,, and in the material on the Code 913 Branch Web site.

Besides scientific achievements, we have made great strides in many areas of science leadership, as well as science enabling, education, and outreach. Thanks to the organizational efforts of Yoram Kaufman and Lorraine Remer, the AeroCenter seminar series is running well and is very well attended. The biweekly seminars overflow the meeting room and attract aerosol researchers from NOAA and the University of Maryland on a regular basis. Collaborative papers between AeroCenter members from different disciplines are now commonplace. Previous AeroCenter visitors are now in the process of submitting papers based on the work done during their visits to Goddard. MODIS data were used for quantitative assessment of the emission, transport, and fate of dust from Africa. The MODIS data shows, in agreement with chemical transport models, that 120 Tg of dust are deposited annually into the oceans. It also resolves an old paradox about the need of Saharan dust as the main fertilizer of the Amazon basin and the amount of dust that was calculated to arrive in the Amazon region. We found evidence that heavy smoke in the Amazon significantly reduces formation of boundary layer cumulus clouds and can change the smoke forcing from net cooling to net warming for which a paper has been written and submitted to Science. A strong collaboration has been established with the Environmental Protection Agency (EPA) and with NASA/Langley Research Center for the purpose of air quality monitoring and forecasting. As part of NASA's Applications effort (Code YO) the potential of using the MODIS aerosol products as a Decision Support Tool within the EPA's Air Quality Decision Support System has been demonstrated. The availability of MODIS cloud and aerosol products has opened many new pathways of research in climate modeling and data assimilation in the Laboratory.

We continued to serve in key leadership positions on international programs, panels and committees. Si-Chee Tsay leads a group of scientists from NASA and universities in initiating a new project - Biomass-burning Aerosols in South East-Asia: Smoke Impact Assessment (BASE-ASIA), to study the effects of smoke aerosol on tropospheric chemistry, water and carbon cycles, and their interactions in the Southeast Asia monsoon region, using multiplatform observations from satellites, aircraft, networks of ground-based instruments and dedicated field experiments. Robert Cahalan has served as project scientist of SOlar Radiation and Climate Experiment (SORCE), which was launched in December 2002, and is measuring both Total Solar Irradiance (TSI, formerly "solar constant") and Spectral Solar Irradiance (SSI) with unprecedented accuracy and spectral coverage (1°2000 nm for SSI, 1°100,000 nm for TSI) during a 5-year nominal mission lifetime. Cahalan is also chairing the Observations Working Group of the Climate Change Science Program Office, tasked to evaluate and coordinate multiagency contributions to the U.S. Government climate observing system. He also chairs the 3-Dimensional Radiative Transfer Working Group of the International Radiation Commission and directs the International Intercomparison of 3-dimensional Radiation Codes. During the past year, Warren Wiscombe was appointed as Science Lead for the Earth Science Vision 2025, an activity commissioned by NASA Headquarters. This involves forming science workgroups drawn from NASA Centers and the community at large to decide on specific science questions for NASA's far future in Earth Science.

The new Climate and Radiation Branch Web site ( has a new organization, with a front page that changes almost daily. It provides the latest news in climate research and automatically updates the calendars of users who subscribe. It also has an "Image of the Week" provided by Branch members, and a search page to easily access archived news, images, publications, and other climate information and data. The Branch Web site also has an extensive glossary of Earth science acronyms, and a list of links to related sites. The Earth Observatory Web site ( also continues to provide the science community with direct communication gateways to the latest breaking news on NASA Earth Sciences. It provides the new media and other communications outlets with a "one-stop shopping" resource for publication quality images and data visualizations from NASA Earth Science satellite missions such as Terra, Aqua, and many others. The Earth Observatory Web site now boasts over 27,000 subscribers, with roughly 1 million page views per month worldwide. The contents of the Web site are increasingly syndicated by NASA Headquarters and other public sites.

Alexander Marshak is an editor (together with A. Davis from Los Alamos) of the "Three-Dimensional Radiative Transfer for Cloudy Atmospheres" monograph prepared for publication in Springer-Verlag. He also has authored and coauthored three chapters in the book. Two additional chapters were authored by R. Cahalan and W. Wiscombe.

A new method to measure aerosol absorption from space has been developed. The method measures aerosol attenuation of sun glint over the ocean to derive aerosol absorption. The method will be best applied to future satellites that can measure the same spot over the ocean at an angle at glint and at an angle off glint.

A method to simultaneously analyze measurements from a two-wavelength lidar and a passive Spectroradiometer, such as MODIS, has been introduced. The MODIS data are used to constrain the lidar inversion, thus decreasing the weight of assumptions in retrieving the aerosol profiles. The method was applied to Saharan dust and smoke from Africa in two field experiments.

Highlighted papers appearing in Appendix A2 include: Bell and Kundu 2003, Gao et al. 2003, Ichoku et al. 2003, Kaufman et al. 2003, Levy et al. 2003, Li et al. 2003, Platnick et al. 2003, Sud et al. 2003, and Varnai and Marshak 2003.

5.3 Atmospheric Experiment Branch, Code 915

The Atmospheric Experiment Branch conducts experimental studies to increase our understanding of the chemical environment in our solar system during its formation and to study the physical processes that have continued to shape solar system bodies through time. To achieve this goal, the Branch has a comprehensive program of experimental research, developing instruments to make detailed measurements of the chemical composition of solar system bodies such as comets, planets, and planetary satellites that can be reached by space probes or satellites.

1) The Branch continued providing postlaunch support for several key planetary missions.

a) A Gas Chromatograph Mass Spectrometer on the Cassini-Huygens Probe mission to explore the atmosphere of the Saturn moon, Titan.

Extensive preparations have been made, and are still in progress, for the upcoming Saturn-Titan encounter. The Saturn orbit insertion is scheduled for July 2004. The Probe release to Titan is scheduled for December 25, 2004, and Probe entry into the atmosphere of Titan will occur on January 14, 2005. Existing instrument calibration facilities were prepared for pre- and postflight calibration of the Flight Spare instrument, which is available in our laboratory to simulate flight environments and to assist in the interpretation of the expected flight data. The effort is substantial because of the high complexity of the instrument and the difficulties involved in conducting in situ measurements from a fast descending probe into an essentially unknown atmosphere.

It was discovered last year by the Huygens Probe engineering team that, because of a design error in the Huygens Probe communication system, the relay link to be established between the Cassini Orbiter and the Huygens Probe during the probe descent would not have worked as expected. Fortunately, the problem was discovered in time to allow corrections to be designed, i.e., change the Cassini Orbiter Trajectory to reduce the Doppler shift of the probe signals to lower values that were needed for proper reception. To further increase the communication margin, the probe temperature needs to be raised at the time of entry to take advantage of the probe clock drift, which also reduces the Doppler shift by a small amount. This probe preheating effort required a modification for our instrument flight software, and a large amount of time went into software redesigning or patching and the associated testing and verification. The present status is that everything has been implemented, tested, and verified multiple times in our laboratory test setup and also on the Huygens Probe engineering unit at the European Space Operations Centre (ESOC) in Darmstadt, Germany and finally on the spacecraft in flight.

b) An Ion and Neutral Mass Spectrometer on the Cassini Orbiter to explore the upper atmosphere of both Saturn and Titan.

The Ion and Neutral Mass Spectrometer (INMS) is a Facility instrument that was designed, built, and calibrated by the Branch in-house for the Cassini Mission. Past and current activities focus on participation by Branch personnel as science team members in the science planning and preparations for the Saturn encounter in 2004. The Branch engineering team is also participating in the flight instrument health assessment and in preparations for pre- and postencounter laboratory calibration of an identical spare instrument. Considerable effort went into the design and construction of the molecular beam calibration system with which the environmental conditions of a Titan flyby of the Cassini orbiter can be simulated.

2) Branch members continued to advance development, and participated in the preparation of NASA proposals for measurements on future planetary missions. These include (1) a probe of the deep atmosphere of Venus to carry out precision measurements of isotopes designed to resolve questions of the origin and processing of this atmosphere; (2) a detailed in situ rendezvous mission with the nucleus of a comet to better understand the complexity of organic molecules that might have been delivered to Earth over the course of its history; (3) a comet fly by mission; (4) a comet sample return mission; and (5) a landed experiment on Mars to sample isotopes and molecules from its atmosphere and below its surface that can address studies of past climate and the possibility of past life on this planet. A program of analysis of organics in Mars analog materials was initiated to test and validate the analytical approach.

3) The Branch leads one of four themes of the recently selected Goddard node of the NASA Astrobiology Institute. As part of this activity, Branch members participate in a collaborative astrobiology investigation with the Johns Hopkins University Applied Research Laboratory (JHU/APL) to develop analytical protocols for in situ instruments that will aid in the evaluation of prebiotic chemistry on primitive solar system bodies, which might have contributed directly or indirectly to the emergence of life on early Earth. The approach includes analysis with a miniaturized time-of-flight mass spectrometer combined with a gas chromatograph that will allow both simple and complex organic molecules to be resolved. Direct ionization of solid samples using laser ablation or energetic ions combined with electron ionization of gases thermally released from the same samples will allow a wide range of highly volatile-to-highly refractory components to be analyzed. This powerful technique will also enable in situ characterization of organics contained in solid phase material from the Jovian moons, or Mars.

4) Branch members developed a mission concept in collaboration with scientists and engineers at GSFC and JPL to carry out measurements for 30 days in the polar region of Mars from a Montgolfiere balloon platform. This mission - with the astrobiology focus of a search for pointers to past or present life in the atmosphere, surface, and subsurface - was submitted to NASA Headquarters in response to their announcement for this mission.

5) Branch members participated in several national and international workshops focused on a Comet Nucleus Sample Return mission, a Jupiter deep-entry atmospheric probe mission, and an international workshop on Calibration Techniques for In situ Particle Instruments.

6) Branch members continued the collaborative effort with GSFC's Engineering Directorate in a comprehensive program to achieve a significant reduction in the size and weight of present-day mass spectrometer systems. This includes reduction of the electronics system by using Application Specific Integrated Circuits (ASICS) and other advanced packaging techniques as well as reductions to the mass spectrometer itself by using Micro-Electrical Mechanical Systems (MEMS) techniques.

The highlighted journal article on this topic appears in Appendix A2: Niemann et al. 2003.

5.4 Atmospheric Chemistry and Dynamics Branch, Code 916

The Atmospheric Chemistry and Dynamics Branch develops computer models and remote sensing instruments and techniques, as aids in studies of aerosol, ozone, and other trace gases that affect chemistry, climate, and air quality on Earth. The Branch is a world-class center of research in stratospheric chemistry. Using satellite, aircraft, balloon, and ground-based measurements coupled with data analysis and modeling, Branch scientists have played a key role in improving our understanding of how human-made chemicals affect the stratospheric ozone layer.

Branch scientists have been active participants in satellite research projects. In the late 1960s, our scientists pioneered development of the backscattered ultraviolet (BUV) satellite remote sensing technique. Applying this technique to data taken from NASA and NOAA satellites, Branch scientists have produced a unique long-term record of the Earth's ozone shield. The data record now spans more than three decades, and provides to scientists worldwide, valuable information about the complex influences of Sun, climate, and weather on ozone and ultraviolet radiation reaching the ground. Branch scientists expect to maintain this venerable record using data from a series of BUV instruments that are planned for use on U.S. and international satellites in the next two decades. Branch scientists were also instrumental in developing the Upper Atmosphere Research Satellite (UARS) project, which generated data used by researchers to produce a highly detailed view of the chemistry and dynamics of the stratosphere. Currently, Branch scientists are providing scientific leadership in the development of the EOS Aura satellite, scheduled for launch in June 2004, which is designed to provide data to aid in studies of the chemistry and dynamics of Earth's atmosphere. Branch scientists are also involved in the design of instruments to measure tropospheric air quality and chemical species from orbital spacecraft and small unpiloted vehicles. In addition, they operate a suite of advanced lidar instruments to study the stratosphere from ground and aircraft.

The measurement activities of the Branch are highly coupled with modeling and data analysis activities. The Branch maintains state-of-the-art 2-D and 3-D chemistry models that use meteorological data, produced by the GMAO, to interpret global satellite and aircraft measurements of trace gases. Results of these studies are used to produce congressionally mandated periodic international assessments of the state of the ozone layer, as well as to provide a strategic plan for guidance in developing the next generation of satellite and aircraft missions. A major new thrust of the Branch is to apply the unique synergy between Branch modeling and measurement groups, which proved very successful for the study of stratospheric chemistry, in addition to studies of chemically and radiatively active tropospheric species - including aerosol, CO2, O3, CO, NOx, and SO2 - which affect climate, air quality, and human health.

The following provides more detailed descriptions of some of the current Branch activities:

1) New Instrument Development

The Shuttle Ozone Limb Sounding Experiment (SOLSE-2) flew on STS-107 in January 2003. Twelve orbits of data were taken with two instruments - an imaging spectrometer (SOLSE) and a filter radiometer (LORE) - that measure light scattered from the Earth's limb. The purpose of the flight was to demonstrate that limb scattering is a viable approach for measuring the ozone vertical distribution in preparation for similar instruments to be flown under the NPOESS program. Because of the unfortunate loss of the shuttle Columbia, only a limited amount of data, downloaded during the mission, are available for analysis - about 1 scan in 8 at a limited number of wavelengths for SOLSE, and about 70% of the data from LORE. Initial analysis shows that enough good data were obtained to fulfill most of the scientific goals. Profile comparisons are being made with ground-based measurements from 24 locations. The biggest challenge is proving to be pointing determination, i.e., determining the correct altitudes for the profile retrieval. An approach based on the falloff of Rayleigh scattering has been very successful, and is accurate to about 0.8 km. A vertical resolution of 2-3 km is being achieved for the profile retrievals. Good progress is being made in instrument characterization and algorithm improvement.

Two new instruments are being developed under the IIP, the Solar Viewing Interferometer Prototype (SVIP) and the GeoSpec (Geostationary Spectrograph). The SVIP is a prototype of an instrument that will make measurements at 1-2µ to determine the amounts of CO2, H2O, and CH4 in the Earth's atmosphere from a position at L2. The SVIP is designed for testing in the laboratory, outside at Goddard, and on a mountain top. The GeoSpec is a dual spectrograph operating in the UV/VIS and VIS/NIR wavelength regions to measure trace gas concentrations, profiles of O3, NO2, CH2O and SO2, coastal and ocean pollution events, tidal effects, and aerosol plumes. GeoSpec is intended to support future missions in the combined fields of atmospheres, oceans, and land. GeoSpec is a collaboration of our Laboratory, Pennsylvania State University, Washington State University, and Research Support Instruments.

2) Measurement and Modeling of Atmospheric Carbon Dioxide

Recent research activity in Carbon Cycle Science has come in the areas of atmospheric transport modeling, CO2 radiative transfer studies, and instrument construction and testing. The atmospheric chemistry and transport model, used for calculating global CO2 transport, is being coupled to a land biosphere model to produce biospheric CO2 fluxes and mixing ratios tied to actual meteorology. These distributions will be compared with real-time observations to improve our knowledge of the response of carbon cycle processes to climate change and to constrain the so-called "missing sink" for atmospheric carbon. Radiative transfer calculations and analysis have shown that methods for detection of CO2 from space using reflected sunlight (e.g., the Orbiting Carbon Observatory) must carefully account for the effects of cloud and aerosol scattering on the sunlight in order to achieve the high precision required for carbon studies. Ground tests and calculations for the Fabry-Perot Interferometer for Column CO2 indicate that the instrument has sufficient sensitivity to measure CO2 at high precision. Companion channels for measuring pressure and temperature, required to complement the CO2 column abundances, are currently being tested. The whole instrument is being packaged into an aircraft configuration for test flights on the NASA DC-8.

3) Global Transport of Aerosol-Comparison of Model with Measurements

Aerosol radiative forcing is one of the largest uncertainties in assessing global climate change. To understand the various processes that control the aerosol properties and to understand the role of aerosol in atmospheric chemistry and climate, the Branch scientists have developed the Global Ozone Chemistry Aerosols Radiation and Transport (GOCART) model. In the past few years, the GOCART model has been used to study tropospheric aerosol and its effect on air quality and climate forcing. Major types of aerosol particles are simulated, including sulfate, dust, black carbon, organic carbon, and sea salt. Among these, sulfate, black- and organic carbon mainly originated from human activities, such as fossil fuel combustion and biomass burning. Dust and sea salt are mainly generated by natural processes such as uplift of dust from deserts by strong winds.

The modeling activities have been strongly connected to the satellite and aircraft observations. Our recent research involves studies of intercontinental transport of dust and pollutants using a combination of models and data. The data is from satellite observations (MODIS and TOMS), ground-based network (AERONET), and in situ measurements (ACE-Asia). The aerosol absorption in the atmosphere is based on the GOCART model and AERONET data, and the aerosol radiative forcing is based on assimilated products of the model and MODIS data. In addition, the model results are used by many groups worldwide for studies of air pollution, radiation budget, tropospheric chemistry, hydrological cycles, and climate change.

4) 3-D Stratospheric Chemistry Model Studies

Several Branch scientists are analyzing a recently completed 50-year chemical transport model simulation of stratospheric ozone chemistry. They are focusing on its comparison to long-term data records from satellites and ground-based instruments. The goal is to use the model results, combined with data, to draw inferences about long-term ozone trends and about the expectation of a decrease in stratospheric chlorine accompanied by a recovery of ozone while the Montreal Protocol is being implemented. Our Branch has further initiated collaborative work with GMAO to examine the sensitivity of the finite volume general circulation model dynamics to changes of the ozone climatology in the radiation code of the GCM. This work is being combined with the effort to put the full-chemistry transport model online in the GCM. This is the first step in coupling the chemistry to the dynamics for chemistry-climate studies.

5) Research in Nonlinear Chemistry

Previous papers published by a Branch scientist, Richard Stewart, have shown that the nonlinear equations describing steady-state tropospheric photochemistry can exhibit multiple solutions as background odd-nitrogen levels change. It is not known whether the multiple solutions correspond to observable phenomena in the time-dependent Earth's atmosphere. In a just completed study, Stewart showed that these steady-state solutions have an analogue in time-dependent calculations which model photochemical behavior. The time-dependent calculations show rapid changes in oxidant levels as the chemical state of the background troposphere undergoes seasonal variation. Hydrogen peroxide (the focus of the study) is especially sensitive to such change and its annual cycle might, under some circumstances, represent an observable consequence of the nonlinear behavior shown in the models. Observations show that such rapid transitions do not occur on a global scale, in a relatively clean environment. The importance of the rapid changes lies in the fact that should they occur on (what are increasingly polluted) urban-to-regional scales, they would involve large changes in oxidant levels and pollutants over periods of several days.

Highlighted papers appearing in Appendix A2 include Douglass et al. 2003, Mayr et al. 2003, Chin et al. 2003, Ziemke and Chandra 2003, Hsu et al. 2003, Weaver et al. 2003, and Olsen et al. 2003.