"INTRODUCTION It is a popular misconception that almost everything present in the earth's atmosphere@ except nitrogen@ oxygen@ and a few inert rare gases@ is an air pollutant or the remnant of pollution activities. This is not the case. There are sources in the natural environment of a wide variety of gaseous and particulate materials which are commonly classed as air pollutants when they are emitted by man-made sources. In addition@ the atmosphere possesses a number of mechanisms which act to remove@ sometimes at a quite rapid rate@ most if not all of the materials emitted into it. It is@ of course@ also obvious that while these atmospheric scavenging mechanisms are quite effective they can be overburdened by excessive emissions occurring in relatively short periods of time or in limited geographical areas. The gradual accumulation of carbon dioxide in the atmosphere is a case where the available environmental scavenging mechanisms are significantly lagging the pollutant emission rate. On a much smaller scale@ the air pollution problems in major urban areas are examples of situations where the pollutant emission rate seriously overburdens the scavenging processes for significant but limited periods of time within the confines of the urban area. The topic of this report@ the atmospheric cycles of trace chemicals including but not being restricted to air pollutants@ is one that has rarely received detailed consideration. Instead it often seems to be tacitly assumed that pollutants once released into the atmosphere will disappear as soon as the air mass moves across the city boundary. It is also often assumed@ apparently@ that commonly identified pollutants are uniquely related to man's activity and that pollutant emissions constitute 1 a permanent and ever-worsening burden for our atmospheric environment. These assumptions are drastic oversimplifications and can lead their proponents far from reality. It is the purpose of this report to examine our present state of knowledge of the atmospheric cycles followed by a variety of common gaseous materials. This will be done by analyzing the major natural and urban pollution sources@ estimating the effectiveness of applicable atmospheric reaction processes@ and determining the effectiveness of the processes by which the material is finally removed from the atmosphere. It will be apparent that our knowledge of these atmospheric processes is@ in many cases@ fragmentary. Where this is the case@ further research is called for and will be pointed out. Sources of the various materials have been estimated from the available literature@ but a detailed inter-comparison of source data has not been undertaken because a detailed ""emissions inventory"" was not within the scope of this program. In general@ the magnitudes of the various pollutant sources can be estimated reasonably well while the possible natural sources can only be crudely defined. We have in a general sense divorced our considerations from a detailed reporting and analysis of conditions within specific urban areas and from studies which report the periodic ""highest-ever"" type of statistics. The total atmospheric system is so much greater than the polluted envelope of any one city that the transient effects noted in a given area are important to our study only to the degree that they indicate sources of pollutants that can affect the atmospheric system. In many ways this study is a first effort at solving the problem of the integrated effects of natural and urban emanations in the atmosphere. Progress has been made here@ but more needs to be done."