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FIGURE 4-5. NOZ emissions from 1983 to 2002. (From EPA, Latest Findings on National Air Quality 2002 Status and Trends, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, U.S. Government Printing Office, Washington, D.C., August 2003.)

illustrate the trends of NO2 air quality and NOZ emissions, respectively, for the last 20 years [62]. Of the approximately 10 million short tons of NOZ emitted from fuel combustion (Figure 4-5), power plants contributed less than 4.5 million short tons [62].

Ozone

Ground-level ozone, which is the primary constituent of smog, continues to be a pollution problem throughout many areas of the United States. Ozone is not emitted directly into the air but is formed by the reaction of volatile organic compounds (VOCs) and NOZ in the presence of heat and sunlight. The trends of VOC emissions and their sources for the past 20 years are shown in Figure 4-6 [62]. Fuel combustion contributes approximately 5% of the VOC emissions, with power stations comprising less than half of the 5% [4]. Over the past 20 years, national ambient ozone levels decreased 22 and 14% based on 1-hour and 8-hour data, respectively [62]. During this period, emissions of VOCs decreased 40% (excluding wildfires and prescribed burning). Ozone air quality trends are illustrated in Figures 4-7 and 4-8 [62].

^ Fuel Combustion Q Transportation

I Industrial Processes I Miscellaneous

Fires

^ Fuel Combustion Q Transportation

I Industrial Processes I Miscellaneous

Fires

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