downwash effects. Increases in relative humidity and decreases in sunshine duration have also been measured within a few kilometers of mechanical-draft towers.38 Currently, the sun shading produced by cooling tower plumes is of great concern in Europe. Over a period of several years, changes in solar radiation can cause changes in natural vegetational cover.30 Fog frequently occurs over and near cooling ponds, especially during meteorological conditions that normally produce fog, such as cool, calm mornings.39 in the winter, such fog can produce light icing on vertical objects; however, both the fog and icing are limited to within a few hundred meters of the cooling pond. In general, it appears that although the atmospheric impacts of waste heat rejection at an individual site are often noticeable, these impacts will be confined to an area close to the site. Thus, site selection and the selection of the appropriate cooling technology will be important in minimizing the adverse impacts of these local atmospheric perturbations. 2.2 ATMOSPHERIC PARTICLES Suspended particles, chiefly in the size range of 0.01-10 pm, are abundant in the atmosphere. The properties of these particles affect weather and climate in two ways. First, particles act as condensation and freezing nuclei for the formation of cloud droplets and ice crystals in the atmosphere. Thus, particles play an important role in cloud and precipitation processes. Second, because of their optical properties, particles interact with both solar and terrestrial radiation. Atmospheric aerosols (i.e., the suspension of particles in the atmosphere) can scatter and absorb incoming solar radiation and absorb and re-emit terrestrial infrared radiation. The proportion of the anthropogenic contribution to the total amount of atmospheric particles has increased over the past century. The particles produced by man may be currently influencing weather and climate and may play an important role in future weather and climatic change. 2.2.1 Sources and Sinks of Atmospheric Particles Sources Atmospheric particles are generated by both natural and anthropogenic sources. They can be injected into the atmosphere as primary particles or be
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