The term aerosol in the present context refers to suspended particles in the atmosphere such as sulfate, mineral dust, carbonaceous particles, other organic aerosols and sea salt. The microphysical and chemical effect of aerosols on global climate is emerging as a new frontier in global change studies. While aerosol science is almost as old as the rest of atmospheric sciences, the urgency and the great impetus for studying it in the context of global change arise from several recent developments which demonstrate its significance to global climate and chemistry. Potential anthropogenic radiative forcing of climate.
Direct effect refers to to the increase in clear-sky albedo.
The indirect effect is the increase in cloud albedo. From IPCC , 1994. Increase in sulfate aerosols can lead to a radiative cooling directly by enhancing clear sky albedo and indirectly through modification of the short-wave cloud forcing. The experimental discovery of the dominant role of heterogeneous chemistry in the Antarctic ozone hole has forced us to think about such heterogeneous chemistry effects in other regions as well. In particular, the role of heterogeneous chemistry within the cold tropical upper troposphere cirrus clouds, where temperatures equal minimum temperatures over the Antarctic, in determining the chemical composition, and thereby radiative properties of clouds is just one of many emerging issues.