coal combustion and emission of N0x and SO2 from fossil fuel combustion. The uncertainties in these estimates are fairly substantial; probably, the estimate of natural emissions in Table 2.6 is high. However, the natural component is definitely the dominant one. Worldwide particulate loading produced by human activity has increased significantly in this century.40 Increasing population and energy consumption in the future portend increased atmospheric levels of particulates. However, in recent decades many nations have taken steps to control pollutant emissions to protect public health: for example, air quality measurements in the United States indicate that in the past decade urban SO2 and total suspended particulate levels have decreased.41 Therefore, it is conceivable that in some locations the anthropogenic contribution to global particle loading could remain fairly constant or even decrease in the future despite increasing population and energy use. However, the potential exists for several-fold increases in world anthropogenic particle emissions in the next 50 to 100 years.10,40 Sinks Despite the extremely large input of particles into the atmosphere, they are removed quite efficiently, which prevents a large, cumulative buildup of particles in the global atmosphere. The mean residence time of a particle in the troposphere is of the order of a few days to a few weeks. There are two major mechanisms by which atmospheric particles are removed. The first is the gravitational fallout of particles, which are eventually deposited on a surface (dry deposition). The second is referred to as wet deposition, which occurs when water vapor condenses on a particle, forming a cloud droplet, which eventually falls as precipitation (rainout). Wet deposition also occurs when a particle is captured by falling rain or snow and is carried to the ground (washout). The residence time of any given particle in the atmosphere is dependent on the characteristics of the particle and its location in the atmosphere. Particles capable of acting as cloud condensation nuclei or freezing nuclei are much more likely to be removed by cloud and precipitation processes. Particles emitted in or injected into the stratosphere, such as those from aircraft or volcanoes, may remain in the atmosphere for considerable periods
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