emissions resulting from the solar cell manufacturing process and rocket launch activities. However, the potential exists for atmospheric emissions of gallium, arsenic, hydrogen sulfide, and other compounds during GaAlAs manufacture.1^1 Emissions of potentially hazardous trace metals are discharged into the atmosphere with the waste gases from combustion at coal-fired generating plants. Although the quantities emitted are small, their degree of enrichment in the flyash and their hazardous nature requires that this be considered an area for concern. Other coal-related emissions require attention: coal combustion emits primary SO2 and N0x, which are precursors of secondary sulfate and nitrate particles and have been implicated in acid precipitation and impairment of visibility. Lakes with reduced buffering capacity can acidify and deteriorate the aquatic environment, resulting in commercial and recreational losses. Degraded visibility may impair navigation and result in cancelled or delayed airline flights. Emissions of regulated criteria pollutants from coal combustion (SO2, N0x, TSP, HC, CO), releases of cooling tower drift from coal and nuclear generating stations, and fluoride from nuclear fuel-conversion facilities are areas of moderate concern and are likely to be controlled to acceptable levels. However, these pollutants have been implicated in the physical damage to and reduced useful life of metals, building materials, and stone objects such as sculptures. Pollutants at elevated levels have affected the productivity, appearance, and yield of crops, thereby causing economic damage. 4.4.4 Climatic Changes Due to Air Pollution The impact on global climate that is currently given the most attention is the global warming effect caused by increasing CO2 levels in the atmosphere and the resulting "greenhouse effect." Atmospheric models predict that doubling of CO2 levels above pre-industrial levels will produce a global average warming of 1° to 3°C.15j This could occur as early as 2025, although noticeable warming should not be detected before 2000. Such a temperature increase may significantly affect precipitation patterns, agricultural production, energy use, and ocean levels through melting of polar ice. Combustion of coal releases substantial amounts of CO2 into the atmosphere (5 x 10$ metric tons annually for a 1,000-MW power plant). Although this is not a local problem, coal combustion contributes significantly to the total man-made input of CO2 into the atmosphere. Furthermore, coal combustion also releases amounts of other "greenhouse" gases such as SO2 and H2O, which appear to be of a less severe nature. Another area of concern is the increase in global levels of atmospheric particles, which, depending on their location and optical properties, act to change the radiative properties of the earth-atmosphere system and effectively warm or cool the earth’s surface. It has not been established whether or not the increase in atmospheric particle concentrations over the past century has produced a net warming or cooling effect. The direct emissions of particles from coal combustion, and emissions of gaseous species such as sulfur and nitrogen oxides which are converted to particles in the atmosphere, do not contribute significantly to global levels of atmospheric particles.[154]
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