Easily-observed manifestations of atmospheric aerosol interactions with solar and terrestrial radiation include regional visibility degradation in the eastern U.S. and colorful sunsets observed after the Mt. Pinatubo eruption. Liu et al. [1991] suggested that anthropogenic aerosols have served to screen ultraviolet radiation from the lowest levels of the atmosphere, possibly masking the effects of mid-latitude stratospheric ozone depletion.
However, as discussed briefly in the Introduction, the global radiative forcing attributable to aerosols has been a primary research focus. An examination of the key uncertainties in estimating direct climate effects of aerosols ( Penner et al. [1993a]) reveals that fundamental and applied aerosol research in a diversity of areas has a direct bearing on the development of improved understanding of aerosol / climate issues. For example, for both sulfate and biomass burning aerosols, uncertainties include terms in the global budgets (gaseous precursor and direct emissions source strengths, oxidation pathways and particle formation processes, atmospheric aerosol lifetimes) and aerosol thermodynamic and optical properties (hygroscopicity and mass scattering and/or absorption efficiencies).
Natural sources emit sulfur gases which are ultimately oxidized to
sulfate aerosol in the atmosphere. Volcanoes, wild fires, and
terrestrial and oceanic biogenic sources contribute a variety of
gases, and may dominate the sulfur cycle in remote regions in the
Southern Hemisphere. However, on the global scale the contribution of
anthropogenic SO
from fossil fuel combustion comprises more than
half of the estimated annual S emissions, and a larger percentage in
the Northern Hemisphere. Thus the perturbation of the S cycle by
anthropogenic activity, and the climatic consequences thereof, are a
primary focus. Carbonaceous aerosols are also known to contribute to
optical effects and human activity has certainly affected the
atmospheric cycling of C compounds.
Other species of importance include ammonium and nitrate, both of which affect aerosol hygroscopicity and thus optical properties, and mineral dust.