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Acid deposition and photochemical smog are urban air pollution problems, and they remain localized as long as the sulfur, nitrogen, and hydrocarbon pollutants are confined to the lower troposphere (below about 1-kilometer altitude) where they are short-lived. If, however, the contaminants are rapidly transported to the upper troposphere, then their atmospheric residence times grow and their range of influence expands dramatically. Although this vertical transport ameliorates some of the effects of acid rain by diluting atmospheric acids, it exacerbates global tropospheric ozone production by redistributing the necessary nitrogen catalysts. Results of recent computer simulations suggest that thunderstorms are one means of rapid vertical transport. To test this hypothesis, several research aircraft near a midwestern thunderstrom measured carbon monoxide, hydrocarbons, ozone, and reactive nitrogen compounds. Their concentrations were much greater in the outflow region of the storm, up to 11 kilometers in altitude, than in surrounding air. Trace gas measurements can thus be used to track the motion of air in and around a cloud. Thunderstorms may transform local air pollution problems into regional or global atmospheric chemistry problems.

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Semi-empirical formulae are presented which can be used to estimate precipitation scavenging and dry deposition of particles and gases. The precipitation scavenging formulae are appropriate both for in-and below-cloud scavenging and comparisons with data indicate the importance of accounting for aerosol particle growth by water vapor condensation and attachment of the pollutant to plume or cloud particles. It is suggested that both wet and dry removal of gases is usually dictated by other than atmospheric processes. Dry deposition of particles to a canopy is shown to depend on canopy height, biomass, vegetative type and mean wind. Two large-scale practical problems are addressed dealing with the relative importance of wet and dry deposition and with the sources which contribute to deposition in a specific location.Essentially all air pollution is eventually cleansed from the atmosphere by the natural processes generally referred to as precipitation scavenging and dry deposition. The purpose of this report is to present formulae which can be used to approximately describe these cleansing processes. Within the details of the development of these formulae, some unifying features may not be apparent and it may be worthwhile to mention them here. One is that in all cases, whether the removal is by rain, snow, grass, leaves, water surfaces or whatever, the efficiency with which the pollutant is removed from an air stream by an obstacle must be considered. This collection efficiency is usually written as the product of a collision efficiency, defined in Figure 1, multiplied by a retention efficiency. In most cases, out of ignorance, the retention efficiency will be taken to be unity although some comments will be made about the retention of aerosol particles by vegetation and the desorption of gases from liquids. A second unifying feature is that after a collection efficiency has been obtained, it is necessary to sum over all collecting elements to obtain an overall removal rate and usually these integrals must be rather severely