"INTRODUCTION Air pollution effects have traditionally been considered to be local or urban area problems. Now@ however@ pollutant impacts on a global scale are frequently discussed and seem to be generally accepted. In 1968 and 1969 we published two reports@ ""Sources@ Abundance@ and Fate of Gaseous Atmospheric Pollutants@"" and a ""Supplement"" (Robinson and Robbins^ 1968@ 1969)@ that examined the present state of knowledge of atmospheric emissions and the cycles of a number of common gaseous materials which are emitted by both manmade and natural processes. Obviously@ gases are only part of the story of atmospheric trace constituents@ and in this report we present an analysis of the sources and scavenging processes for atmospheric aerosols. In many respects@ however@ it has not been possible to separate gaseous and particulate materials because@ for major fractions of the gaseous emissions@ one step in the scavenging process in the atmosphere is the formation of an atmospheric aerosol. Thus sources of gaseous emissions are one of the major contributors of atmospheric aerosols@ and we are concerned here with the problem of atmospheric aerosols whether they were originally emitted as particulate material or subsequently formed in the atmosphere by chemical reactions involving gases. This report follows the pattern established in the two previous gaseous contaminant reports in that the total atmospheric burden of particulate materials has been considered@ whether the source of the aerosol was originally pollutant emissions or the result of a natural environmental process. This procedure permits us to assess the total global atmospheric environment and to attempt to determine the effects that pollutant emissions may have on this total environment. In the following discussion we will show that the emissions of pollutant materials can account for only a relatively small amount of the total atmospheric burden of aerosol materials@ and that scavenging mechanisms exist in the atmosphere that remove all particles at a fairly rapid rate. These factors are important when considering the global atmosphere or some major fraction of it. However@ these factors are not necessarily applicable to the same degree to air pollution problems within a major source area. The scavenging mechanisms described in this report do not operate fast enough to provide solutions for local urban air pollution problems@ nor in most cases@ at least@ are natural emission processes substantial enough within an urban area to make a major contribution to local air pollution situations. In fact@ the existence of an air pollution problem in an urban area is evidence that the available scavenging mechanisms have already been overburdened by the local emission rates. The secondary importance of natural emissions to the quality of urban air masses may not continue@ however@ as progress toward meeting very strict air quality standards proceeds and pollutant emissions are significantly reduced. In most cases this report does not deal with emissions from specific local areas@ and in a general sense we have divorced our considerations from a detailed reporting and analysis of conditions within specific urban areas. We have also avoided detailed presentations of urban air quality data and reports of maximum air concentration values. It is readily apparent that the total atmospheric system with which we are dealing is so much greater than the polluted envelope of any one city that the transient concentration effects noted in a given area are important to our analysis only to the degree that they indicate sources of pollutants that can affect the atmospheric system. Our analysis is necessarily simplified@ but we believe that it provides a useful contribution to understanding our global atmospheric environment@"