USGCRP
Program Elements

Atmospheric Composition

Ecosystems

Global Carbon Cycle

Decision-Support Resources Development and Related Research on Human Contributions and Responses

Climate Variability and Change

The Global
Water Cycle

Observing and Monitoring the Climate System

Communications

International Research and Cooperation

The following are some of the USGCRP's major accomplishments related to the Composition and Chemistry of the Atmosphere during Fiscal Year 2002:

Aerosol Characterization of Asian Dust Storms and Pollution Plumes

Reducing uncertainty about the role of atmospheric aerosol particles in the Earth's climate system is one of the highest priorities for research in the atmospheric composition area. The Aerosol Characterization Experiment-Asia (ACE-Asia) is making substantial progress toward its goal of characterizing the aerosol particles leaving Asia and moving out over the the North Pacific Ocean, and evaluating their impact on the Earth's radiative budget. Several large dust storms, as well as pollution plumes and regional dust layers, were studied during an intensive field campaign in spring 2001, sponsored primarily by NSF, DOD, and NOAA, with significant assistance from NASA. Significant aggregation of soot and/or sulfate with mineral dust is common when dust from the Chinese interior mixes with pollution plumes from major industrial cities. This aggregation alters the optical properties, and hence the climatic impact, of these aerosols. Some notable differences between Asian and North American aerosols have been identified. The extensive suite of surface-based, airborne, and space-based observations from ACE-Asia is improving the accuracy of modeling the distributions and radiative impacts of Asian aerosols. Further, the Intercontinental Transport and Chemical Transformation (ITCT) study in the eastern Pacific has directly observed the types and abundances of gases and aerosols that can make their way across the Pacific, as well as how well current models can simulate such processes. This program, planned and being implemented by NOAA, NSF, and NASA, is also providing insight into the degree that trans-Pacific transport and transformation could influence the air quality of the western United States.  

Figure 2.1. 
Spring dust storm
smothers Beijing
 

Effects of Sulfate Aerosols on Cloud Properties

Atmospheric scientists from two USGCRP agencies have analyzed satellite observations of cloud reflectivity in the context of modeled sulfate aerosols to examine the effects of aerosols on cloud properties. It has been recognized for some time that the addition of microscopic aerosol particles from industrial sources leads to increased numbers of cloud drops. This phenomenon is expected to result in increased scattering of sunlight by these altered clouds, thereby exerting a cooling influence on the Earth (though quite likely not as large as the warming influence of the greenhouse gases). Satellite measurements in combination with chemical transport modeling have now observed this effect and have begun to demonstrate and quantify this relationship on scales of days and thousands of kilometers.  

Antarctic Ozone Hole

Satellite observations indicate that the Antarctic ozone thinning in the 2001 Austral (Southern Hemisphere) spring began earlier than usual and did not break up until the first two weeks of December. This depletion of stratospheric ozone produced an extremely large and persistent ozone hole covering almost 10 million square miles (25 million km2) for 20 days. The ozone level fell to 99 Dobson units on September 26, 2001, comparable to the extremely low values observed during the 1990s.  

Arctic Ozone Depletion and Polar Stratospheric Clouds

In one of the Arctic stratosphere's coldest winters on record, scientists measured depletion of the ozone layer as great as 60 percent during February and March 2000. The findings may be an indication that future cold winters in the Arctic could prolong the depletion of ozone by industrial chlorine compounds, despite the fact that the amount of chlorine in the atmosphere is now decreasing in response to international agreements. In addition, an unusual class of large Polar Stratospheric Cloud particles was observed for the first time. The newly discovered class of particles has given scientists a better understanding of the processes that "set the stage" for chlorine-caused ozone depletion in the Arctic stratosphere, and will enable scientists to make better predictions of ozone loss in the Northern Hemisphere in the future. These observations resulted from the SAGE III Ozone Loss and Validation Experiment (SOLVE), conducted from November 1999 to March 2000, using a suite of space-based, ground-based, balloon and airborne observations. Scientists from NASA, NOAA, other agencies, and academia collaborated in the experiment, which produced a rich data set that scientists are mining to enhance understanding of complex issues associated with chemical reactions occurring on cloud and aerosol particles that play a significant role in high-latitude ozone depletion.  

Midlatitude Ozone Layer Change

Trends of stratospheric water vapor over the past half-century were newly characterized in an international study, with the results showing that increases in water vapor have likely made significant contributions to the decline in midlatitude ozone observed over the past 20 years. The increases in water vapor have also played a substantial role in stratospheric cooling. Other analyses have shown that the North Atlantic Oscillation influences ozone changes observed in midlatitudes. The research shows that a combination of long-term dynamical changes and chemical processes will provide a better understanding of the observed ozone behavior and hence a better predictive capability for future changes in the ozone layer. A better understanding of such processes will help identify and evaluate the beginning of the recovery of the ozone layer that is expected over coming decades, as the abundance of ozone-depleting chemicals continues its decline as a result of the implementation of international agreements.  

Exposure of Plants to Elevated Ultraviolet Radiation

Using newly developed ground-based radiometers that measure ultraviolet radiation (UV) at the biologically sensitive wavelength of 300 nm at 31 sites across the United States, the USDA UVB Monitoring and Research Program (UVMRP) has measured threefold differences in noontime UV irradiances over two successive days under clear skies.Transport of lowozone equatorial air to midlatitudes, as monitored by the Total Ozone Mapping Spectrometer (TOMS) remote-sensing instrument, is responsible for these episodes of sudden high levels of UV. The USDA Agricultural Research Service, in cooperation with UVMRP, the NOAA UV Index Forecast, and the NASA TOMS group, will study the effects of large UV exposure events on plants in their early stages of growth that have not yet developed UV screening material.  

Transcontinental Air Pollution Transport

Theoretical models have indicated for a long time that intercontinental transport of pollution is a persistent feature of the atmosphere. Assimilation of data from the Measurements of Pollution in the Troposphere (MOPITT) instrument, onboard the Earth Observing System Terra satellite, into the models is now leading to characterization of this effect. MOPITT is now producing global maps of carbon monoxide (CO), an important tropospheric pollutant. Experimental assimilation and inverse modeling of MOPITT CO data in chemical transport models are providing new insights about global transport of CO and the variability and strengths of its surface sources.  

Figure 2.2.  Air pollution plume from Southeast Asia moving over the Pacific Ocean and Reaching North America (March 2000).
 

Forest Fires and Carbon Monoxide Abundances in North America

Studies and analyses using airborne instruments have shown that emissions of carbon monoxide from large forest fires can influence the air quality in regions far from the areas burned. For example, the 1995 fires in northwestern Canada were found to be the origin of the previously unexplained episodes of high levels of carbon monoxide measured in the southeastern United States. CO is a local and regional pollutant whose sources were commonly thought to be limited to the surrounding region. Yet the fires in Canada were responsible for more than one-third of total carbon monoxide in the United States for the entire year. These findings have significance for understanding the global CO budget, and indicate that fires are an example of the coupling of local and global phenomena.  

Methane Trends and Anomalies

Methane is an important greenhouse gas, as well as playing a role in atmospheric chemistry and contributing to the increase in stratospheric water vapor. Global measurements of the atmospheric concentration of methane have shown that there has been a slowdown in its rate of growth over the last two decades, but USGCRP-supported observations recently indicated that there was a 5 percent increase in growth rate in 1998, compared to the preceding three years of the monitoring record. Global process-based models of methane sources suggest that emission anomalies occur as a result of varying conditions in both the northern wetlands and southern tropical wetlands. Such "event-structure" in decadal trends provides the opportunity for characterization of underlying emission mechanisms, such as correlations between surface temperature and biogenic emissions.