INTRODUCTION Sulfur compounds are widely considered to be significant factors in the atmospheres of both urban and remote or natural areas. In urban areas the presence of sulfur compounds in the atmosphere results from the prevalence of sulfur in a wide variety of pollutant emissions@ but especially in emissions from the combustion of fossil fuel where SO2 results from the sulfur present as a minor component in the fuel. In the natural atmosphere in remote areas@ sulfur is one of the more significant chemical compounds present in the background aerosol@ and sulfate particles apparently play a role in the natural condensation processes in the atmosphere and in cloud physics phenomena related to precipitation and rainfall. Background levels of both gaseous and particulate sulfur compounds can be traced to sources related to a variety of geochemical and biological processes. In urban areas a number of impacts are currently attributed to the presence of the various possible sulfur compounds. These include effects on health@ plants and vegetation@ atmospheric visibility@ and corrosion of materials. Because of the importance over the years of sulfur compounds@ both as pollutants and as natural components of the atmosphere@ numerous books@ papers@ and monographs have dealt with this topic. In this present report it is our intent to examine the nature of the sulfur compounds in the urban atmosphere with special emphasis on the particulate phases that are present. This task seems to be a useful exercise at this time because of the increased consideration that is now being given to these materials in contrast to the almost exclusive consideration of gaseous sulfur dioxide previously. The change of emphasis has resulted@ in part at least@ from new epidemiological findings and also from the necessity of considering a possible new particulate sulfur compound source@ namely automobile exhaust where the emission control systems employ catalytic convertors. These result in the emission of oxidized sulfur aerosols@ generally assumed to be sulfuric acid. It is the purpose of this report to consider our present knowledge about urban atmospheric sulfur compounds@ and what impact a new vehicle emission source of sulfur compound particles might have on urban and rural atmospheres. We will consider primarily the atmosphere and atmospheric-related effects. We will not discuss health and other related environmental impacts@ although we believe that our report can provide a useful reference for such an analysis by other authors.