A common theme underlying many aspects of recent atmospheric aerosol research is the role of particulate matter in climate and climate change. Aerosols influence climate directly, by scattering and absorbing solar radiation, and indirectly via their role as cloud condensation nuclei (CCN), by modifying optical properties and lifetimes of clouds. The atmospheric cycles of two radiatively-important species---sulfate and soot---have been significantly perturbed by anthropogenic activities, and thus it is expected that anthropogenic aerosols will have significant effects on global mean radiative forcing.
There is general agreement that, despite uncertainty in estimates, the averaged global direct effects of aerosols are not negligible and are probably comparable in magnitude but opposite in sign to anthropogenic greenhouse gas forcing. However, all estimates are subject to significant uncertainty, in part because of gaps in understanding of critical factors and relationships. To minimize these uncertainties, a combination of activities must be undertaken ( NRC (National Research Council) [1993] and Penner et al. [1993a]): long-term observations at fixed sites, to establish background conditions and trends; short-term intensive observations, suface-based and airborne; laboratory analyses and process studies; model development; and satellite observations to connect local observations to global measurements. There is also a need for new techniques for automated and real-time measurements of aerosol chemical and physical properties. Components of IGAC (International Global Atmospheric Chemistry Program) and its U.S. component, GTCP (Global Tropospheric Chemistry Program) are focused on obtaining the data needed for quantifying the effects of aerosols on climate ( NRC (National Research Council) [1993]), and indeed much of the research reported here reflects this priority.
During the period covered by this report, two events occurred which
offered unique opportunities for the observation of aerosols and their
chemical and radiative effects on the atmosphere. The eruption of
Mount Pinatubo in the Philippines in June 1991 injected large amounts
of sulfur dioxide into the stratosphere, which led to the formation of
an estimated 20 to 30 megatons of sulfuric acid aerosol (
McCormick and Veiga [1992]). The Kuwait oil fires, ignited in the
Spring of 1991, emitted large amounts of soot and SO
that had
severe local effects. A large number of noteworthy papers have been
published on observations from these two events alone (see, for
example, the Special Issue of Geophysical Research Letters on
Pinatubo ( McCormick [1992]) and the Special Isssue of
Journal of Geophysical Research on Kuwait ( Hobbs [1992]). It is
possible to mention only a very few of these here.
The scope of recent U.S. aerosol research encompasses a broad range of areas that reflect the interdisciplinary nature of the science. Submicron particles may be directly emitted (primary sources) or derived from chemical reactions occurring in the atmosphere (secondary sources), and undergo further chemical modifications as they age; thus emissions, chemical kinetics, and atmospheric transport of trace gases that serve as particulate precursors are of interest. Reliable sampling and analysis techniques are a challenge for the numerous semivolatile species, including water, in particulate matter. Urban areas have unique emissions and particulate matter characteristics and pollution abatement research programs. Much fundamental and applied atmospheric aerosol research in the U.S. is performed in support of environmental standards, including health effects and Environmental Protection Agency (EPA) criteria documents ( Friedlander and Lippmann [1994]). A full discussion of these many aspects is clearly intractable. In this report, global and regional scale research efforts will be focused on, particularly those relating to climate and climate change issues. A limited reporting of advances in fundamental aerosol microphysics, instrumentation, and measurement techniques is also included, to convey a sense of where current capabilities lie and where research opportunities exist.