ATMOSPHERIC CHEMISTRY

Working Group Participation


Guy P. Brasseur, Chairman

Ralph J. Cicerone

Richard M. Goody

Pamela A. Matson

S. Ichtiaque Rasool

A. R. Ravishankara

Michael J. Prather

Karl K. Turekian

Designated Federal Liaison: Daniel Albritton

Rapporteur: Anne Linn

WORKING GROUP SUMMARY

Guy P. Brasseur, Chairman

Changes in the chemical composition of the atmosphere on the global scale are not hypothetical. They have been occurring rapidly over the last hundred years. Increases in carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs), and decreases in stratospheric ozone are well documented. Volcanic dust has been observed to rise to the stratosphere and impact the global climate for periods of months to years. Surface ozone abundances in industrialized regions have changed dramatically as a result of surface input of NOx and volatile organic carbon compounds (VOCs), but changes in midtropospheric ozone on the global scale are less certain. The release of anthropogenic nitrogen and sulfur compounds has led to an increase in the acidity of precipitation and has increased the deposition of critical nutrients and toxins in many regions of the Northern Hemisphere.

The observed changes in the chemical composition of the troposphere and stratosphere are having adverse affects on human enterprises, including agriculture and human health; they also affect the productivity of natural ecosystems and have increased the radiative forcing of climate.

In the last decades, global change research has been successful in leading to a scientific understanding of a number of these changes. For example, the well-documented year-by-year increases in CO2 have led us to recognize the ability of humans to perturb the global Earth system through combustion of fossil fuel and deforestation. In addition, the Antarctic ozone hole was discovered and diagnosed, and its cause is now largely understood to be the emission of halocarbons. These advances occurred because of the existence of a strong research capability in observations, theory, and laboratory studies that could be focused rapidly on these problems. Nevertheless, major scientific problems involving changes in atmospheric composition remain to be resolved. For example, the role of marine versus terrestrial systems in the uptake of anthropogenic CO2 is not yet understood. Understanding of the balance between the two is required to project future CO2 abundances in the atmosphere. Similarly, the understanding of ozone changes in the lower stratosphere and troposphere is incomplete and yet is essential to comprehend the relative importance of the various causes of climate change.

Among the key scientific questions are the following:

  1. Although the processes responsible for the formation of the Antarctic ozone hole are largely identified, we need to understand why the observed ozone depletion at midlatitudes in the lower stratosphere is greater than that derived from chemical models. A better understanding is important to predict future changes in the level of ultraviolet-B (UV-B) radiation at the Earth's surface over the next 10 years during which the maximum ozone losses will occur.

  2. Although the global increases of trace gases such as CO2 and CH4 are well documented, we must assess the relative role of fossil fuels, land cover change, and natural ecosystems in controlling those patterns in order to accurately project trends into the future.

  3. Although we understand the reason for the high levels of ozone over several regions of the world, we need to better establish the distribution of ozone in the troposphere in order to document and understand the changes in the abundance of global tropospheric ozone. This information is needed to quantify the contribution of ozone to the Earth's radiative balance and to understand potential impacts on the health of the biosphere.

  4. Having recognized the importance of particles in the chemistry of the stratosphere, we must determine how aerosols and clouds affect the chemical processes in the troposphere. This understanding is essential to predict the chemical composition of the atmosphere and to assess the resulting radiative forcing effects in the climate system.

  5. Finally, we must determine if the self-cleansing chemistry of the atmosphere is changing as a result of human activities. This information is required to predict the rate at which pollutants are removed from the atmosphere.
To address these questions, the coordinated research strategy based on observations, laboratory studies, and modeling needs to be sustained and judiciously focused. Surface-based observations of chemical concentrations are the key to long-term monitoring of chemical changes in the atmosphere. Similarly, measurements of exchanges among the terrestrial ecosystems, oceans, and the atmosphere are critical for understanding the inputs to and removal of chemical species from the atmosphere. Airborne measurements provide insights into the specific processes occurring at various levels of the atmosphere. Observations from space are the only practical way to provide global coverage of the atmosphere. Laboratory studies provide the fundamental information on the chemical reactivities of atmospheric species. Modeling provides a comprehensive statement of our understanding and is needed for the interpretation of global observations and the prediction of future changes.

Satellites have been essential for the global observation of ozone and other chemical species in the stratosphere and for our assessment of ozone trends, particularly in the Southern Hemisphere, where ground-based stations are sparse. Satellite observations of terrestrial ecosystems and the ocean have also been used to characterize their interactions with the atmosphere and hence their influence on its chemistry. Likewise, meteorological observations have been essential for developing chemical transport models. Space-borne observations will continue to be a necessary component of the observational program.

This coordinated research strategy is supported by contributions from several federal agencies, and the research is carried out in universities, federal laboratories, and the private sector. Maintenance of these capabilities is the most cost-effective strategy for addressing both the recognized and the unforeseen problems of the future related to the chemistry of the atmosphere.

These capabilities and research strategy have been built into the plans of the U.S. Global Change Research Program (USGCRP) and also those of the international scientific community as represented by the International Global Atmospheric Chemistry Program (IGAC) of the International Geosphere-Biosphere Program (IGBP) and the Stratospheric Processes and Their Role in Climate (SPARC) Project of the World Climate Research Program (WCRP). Activities are being carried out to support international conventions and assessments of ozone and greenhouse gases.

The Earth Observing System (EOS) space program will provide important measurements to address global change issues related to atmospheric chemistry (e.g., lower-stratospheric composition). Not all key information, however, can be gathered from space (e.g., reactive nitrogen budget in the troposphere), and are required observations from other types of platform. Both components are necessary.

Observing Strategy

In addition to maintaining the above research strategy of field and laboratory process studies, monitoring, and modeling investigations, we conclude that the following specific foci are needed in an observing strategy:

Stratospheric Ozone and Other Chemical Compounds

The continued operation of TOMS-like and SBUV-like instruments is needed to determine future trends in the total ozone column abundance. It would be useful, however, to coordinate efforts at the international level, since similar measurements will be performed in Europe (e.g., GOME and later OMI) and in Japan. In order to address the most pressing scientific questions (e.g., processes affecting the evolution of ozone in the lower stratosphere), it is also important that SAGE, MLS, HIRDLS, and TES be implemented and launched as soon as possible. Among several important observed quantities, SAGE will provide information on the global distribution of aerosols and their size distribution (key to our understanding of heterogeneous chemical processes) and their variation resulting from potential future volcanic eruptions. MLS will provide global coverage of the abundance of reactive chlorine (key to assessing ozone depletion, especially in polar regions). HIRDLS will observe at high spatial resolution the distribution of ozone, several other molecules, and aerosols in the lower stratosphere and upper troposphere. This will be key to verifying chemical transport models and providing for the first time global observations of chemical and radiatively active compounds in the upper troposphere and lower stratosphere. TES will measure tropospheric ozone and provide information on its precursors.

The continued operations of field campaigns using aircraft such as the ER-2 and DC-8 National Aeronautics and Space Administration (NASA), the P-3 National Oceanic and Atmospheric Administration (NOAA), and the WB-57 National Science Foundation; ground-based observations using a variety of techniques; and balloon-borne instruments are essential to ensure a solid base of observational data in the next decade. In addition, it is essential that the observations be integrated into theoretical modeling studies.

Tropospheric Ozone and Other Chemical Compounds

To obtain essential information on the global distribution of ozone and to understand the processes responsible for changes in its abundance, the recommended strategy should involve the following simultaneous actions:

  1. Extend the existing (but very limited) ozone network, which ideally should include on the order of 50 stations judiciously distributed worldwide, and provide ozone sounding and lidar observations on a regular basis.

  2. Develop a TES instrument focusing on tropospheric ozone and other species that affect tropospheric ozone concentrations to work in conjunction with the international ozone network.

  3. Conduct a number of in situ airborne campaigns designed to investigate the chemical and physical processes that affect ozone in the global troposphere. Several ongoing and planned regional studies can contribute to this global effort.

  4. Integrate the above observations into complementary laboratory studies and theoretical modeling and interpretation.

As currently planned, MOPITT on EOS AM-1, which measures the global distribution of carbon monoxide, and hence provides information on tropospheric intercontinental transport and on biosphere-atmosphere interactions, is the only space experiment in the U.S. program addressing questions of atmospheric chemistry that will be launched before the next century.

Tropospheric Aerosols

Although it has been suggested that aerosols in the troposphere play a significant role in climate forcing, the quantification of this forcing has been hampered by a large number of uncertainties (e.g., aerosol mass scattering efficiencies, chemical and optical properties, formation processes). These questions will best be addressed through field campaigns, augmented by laboratory and modeling studies, and by "closure" studies conducted from aircraft or balloons and from surface stations.

Space observations will provide aerosol climatologies needed to calculate the radiative forcing, using a combination of AVHRR and Seawifs, augmented with data from POLDER (a French instrument flying on a Japanese satellite) and GOME (on ERS-2). Lidars on free- flyers will be very useful to gather information over both land and oceans.

Conclusion

In the scientific subject areas described in this appendix, information should be provided through appropriate international scientific assessments that describe and evaluate research results. The research and assessment plan delineated here would provide end-to-end service to the nation on key issues relating to atmospheric chemistry and must involve all scientific stakeholders. Just as atmospheric chemistry has provided timely information to decision makers in industry, government, and the public on stratospheric ozone change, so too can this research program continue to serve the nation's current and future information needs in this area.