Aerosols are now recognised to play a central role in such diverse environmental problems as respiratory diseases, climate change, visibility impairment and eutrification of remote areas. Aerosols are also the sites on which heterogeneous reactions of gaseous trace constituents occur. In order to regulate and reduce their detrimental effects, the sources of each of the major chemical constituents of the aerosols must be known and their role in atmospheric processes must be elucidated. Models must become available to describe the sources, transformation, transport and sinks of aerosols.