6. Criteria Pollutants

This chapter presents emissions estimates for the criteria pollutants carbon monoxide (CO), nitrogen oxides (NO_x), and nonmethane volatile organic compounds (NMVOCs).⁽¹⁰⁶⁾ The Clean Air Act of 1970 required that air quality standards be established for pollutants with adverse effects on public health or welfare. They are termed "criteria pollutants" because the EPA based each National Ambient Air Quality Standard (NAAQS) on health-based criteria from scientific studies. Although these gases are not considered to be greenhouse gases themselves, estimates of their emissions are presented here because of their indirect impacts on atmospheric concentrations of greenhouse gases, including carbon dioxide, methane, and ozone.

Ozone is produced largely from atmospheric chemical reactions involving these criteria pollutants. While stratospheric ozone absorbs harmful ultraviolet solar radiation, ozone in the troposphere exhibits the greenhouse properties that warm the Earth's surface. The temperature differential between the radiation absorbedby stratospheric ozone and the radiation redirected to the Earth's surface by tropospheric ozone determines the magnitude of ozone's greenhouse effect. Ozone is highly reactive with other atmospheric gases, and its concentration is influenced by meteorological conditions. As a result, it remains in the troposphere for only hours or days. Hence, concentrations of tropospheric ozone tend to be centered around cities where high levels of criteria pollutants are found. Ozone concentrations are measured at individual urban sites throughout the United States. The EPA reported that the composite average ozone concentration for its 573 U.S. testing sites has declined by 6 percent since 1986.⁽¹⁰⁷⁾

The EPA Office of Air Quality Planning and Standards has compiled emissions data for the various criteria pollutants in the document National Air Pollutant Emission Trends, 1900-1995.⁽¹⁰⁸⁾ The emissions estimates in this report are based on that document. The EPA continues to modify emissions data with improved estimation methods and updated information.

Since the passage of the Clean Air Act of 1970 and subsequent amendments, implementation of pollution control measures and replacement of older, less fuel-efficient vehicles have restrained potential growth in criteria pollutant emissions that otherwise would have been expected from growth in the economy, increased driving, and expansion of industrial output. In short, emissions of both carbon monoxide and NMVOCs are currently significantly below peak levels seen in the early 1970s despite year-to-year fluctuations. Although emissions of nitrogen oxides are now higher than in 1970, the level of emissions has been relatively stable in the 1990s (Table 34).

Most emissions of carbon monoxide result from incomplete oxidation during combustion of fuels used for transportation (Table 35). Transportation emissions, primarily from highway vehicles, accounted for about 79 percent of 1995 emissions. Total emissions in 1995 amounted to about 83.5 million metric tons, an amount considerably below the peak of roughly 116.2 million metric tons seen in the early 1970s. Between 1994 and 1995, carbon monoxide emissions decreased by 6.8 percent, due largely to decreases in emissions of 3 million metric tons each from highway vehicles (down by 5.2 percent) and forest fires (down by 68.3 percent). Otherwise, 1995 emissions were generally the same or slightly below 1990 levels. One exception was emissions from "other off-highway vehicles," which were not as well controlled as other sources. Emissions from this source during 1995 were 0.8 million metric tons (6.5 percent) higher than in 1990. Carbon monoxide emissions are expected to decrease through the year 2000 as a result of more stringent tailpipe standards and other factors.

Nitrogen oxide emissions are related to air-fuel mixes and combustion temperatures during the burning of fuels. Emissions are reduced by the deployment of pollution control equipment, such as catalytic converters. Since 1989, total U.S. emissions of nitrogen oxides have hovered around 21 million metric tons per year (Table 36). Although this does not represent a decline (as seen with the other criteria pollutants), it is much lower than the rate of growth in fuel consumption (such as consumption of gasoline by motorists and coal by electric utilities). Emissions are split between transportation and stationary sources. Although total emissions of nitrogen oxide during 1995 (19.7 million metric tons) were about 5.8 percent lower than their 1990 levels, they were approximately 5.6 percent greater than the 1970 levels. In the future, emissions are expected to decline with implementation of various additional emissions control measures.

NMVOCs are a principal component in the chemical and physical atmospheric reactions that form ozone and other photochemical oxidants. Nearly half (47.9 percent) of the 20.7 million metric tons of NMVOC emissions during 1995 came from industrial processes (Table 37), of which solvent use was the largest source. Most (70.1 percent) of the remaining 10.8 million metric tons of emissions were from combustion of transportation fuels. Emissions of NMVOCs declined by some 7.8 million metric tons (25.4 percent) from 1970 through the 1990s, while fuel consumption in the transportation sector increased and activity in the industrial sector expanded.⁽¹⁰⁹⁾ This improvement was accomplished by some reformulation of petroleum products, implementation of pollution abatement measures, and changes in industrial processes. Emissions from solvent utilization declined as a result of the substitution of water-based emulsified asphalt for asphalt liquefied with petroleum distillates.

Emissions of NMVOCs amounted to about 20.7 million metric tons in 1995, down by 1.8 percent from their level in 1994. Although NMVOC emissions from both solid waste disposal and industrial sources were higher in 1995 than in 1990, total emissions were modestlylower (0.7 million metric tons or 3.1 percent). Emissions are expected to decline through the year 2000 (although the rate of decline will be limited), as fleet turnover progressively results in a greater share of vehicles being operated with emission controls in place, and as lower emitting industrial processes are more widely used. As for the criteria pollutants as a group, these forecasts are based on existing laws and regulations scheduled to come into effect. They do not take into account the possible outcome of more stringent measures that have been proposed and are not yet law, but which will likely affect future emissions.

TO:
Criteria Pollutant Data Tables

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Chapter 7. Land Use Issues

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