in Figure 4-3 [62]. Despite this progress, about 160 million short tons of pollution are emitted into the air each year in the United States, and approximately 146 million people live in counties where air monitored in 2002 was unhealthy because of high levels of at least one of the six criteria air pollutants [62]. Most of the areas that experienced the unhealthy air did so because of particulate matter and/or ozone. This section summarizes the air quality and emissions trends in the United States for the criteria pollutants: NO2, ozone, SO2, particulate matter, carbon monoxide, and lead, along with acid rain, trace elements (specifically, mercury), and CO2. Air quality is based on actual measurements of pollutant concentrations in the ambient air at monitoring sites. Trends are derived by averaging direct measurements from these monitoring stations on a yearly basis. Emissions of ambient pollutants and their precursors are estimated based on actual monitored readings or engineering calculations of the amounts and types of pollutants emitted by vehicles, factories, stationary combustion, and other sources.

Six Principal Pollutants

As previously discussed, under the Clean Air Act, the EPA established air quality standards to protect human health and public welfare. The EPA has set national air quality standards for six principal or criteria air pollutants, which include nitrogen dioxide, ozone, sulfur dioxide, particulate matter, carbon monoxide, and lead. Four of these pollutants—NO2, SO2, CO, and lead—result primarily from direct emissions from a variety of sources. Par-ticulate matter results from direct emissions but is also commonly formed when emissions of nitrogen oxides, sulfur oxides, ammonia, organic compounds, and other gases react in the atmosphere. Ozone is not directly emitted but is formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight.

Nitrogen Dioxide (NO2)

Nitrogen oxides (NOx), the term used to describe the sum of NO, NO2, and other oxides of nitrogen, contribute to the formation of ozone, particulate matter, haze, and acid rain. While the EPA traces national emissions of NOx, the national monitoring network measures ambient concentrations of NO2 for comparison to national air quality standards. The major sources of anthropogenic NOx emissions are high-temperature combustion processes, such as those that occur in vehicles and power plants. Over the past 20 years, monitored levels of NO2 have decreased 21% [62]. All areas of the country that once violated the NAAQS for NO2 now meet the standard. National emissions of NOx have declined by almost 15% overthepast 20years. While overall NOx emissions are declining, emissions from some sources such as nonroad engines have actually increased since 1983. Figures 4-4 and 4-5

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