because they are physically separated from the immediate influence of precipitation processes. 2.2.2 Potential Climatic Response Mesoscale Response The role of particles in cloud and precipitation processes may be significant to weather and climate over a mesoscale (50 km) area where pollutant emissions have created an abundance of condensation and freezing nuclei. Numerous studies have shown an increase in precipitation in the vicinity of large urban areas such as St. Louis42 and Chicago^ where there is an abundance of particles. The cause of precipitation anomalies around cities has not been established. Particles may play an important or a negligible role in comparison to waste heat and mechanical turbulence caused by cities. At this time, it is impossible to link an excess of particles with any local or mesoscale precipitation increase. Certainly this would apply as well to any energy facility or energy park. Particles can substantially alter radiation processes over a mesoscale area. Measurements have indicated that many cities have experienced an attenuation of 10 to 30% in surface solar radiation,21 and a large portion of this attenuation is caused by particles in the atmospheric boundary layer. The attenuation is made up of a scattering component and an absorption component. Calculations have shown that aerosol absorption can cause warming at the surface of up to a few degrees centigrade in a polluted atmosphere.44,45 Global Response Currently, the effect of changes in atmospheric particle concentration cannot be clearly identified. The impact of volcanic activity on tropospheric and stratospheric temperatures appears to exist in the measured records but cannot be interpreted as a causal relation. The backscatter and absorption components of the total attenuation by particles have competing effects on atmospheric temperature. The back- scattered radiation is unavailable for heating of the earth's surface and atmosphere and is lost to space. On the other hand, the solar radiation absorbed by particles is not available to heat the surface but does heat the atmosphere.
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