6. Criteria Pollutants
6. Criteria PollutantsCarbon monoxide, nitrogen oxides , and nonmethane volatile organic compounds (NMVOCs) do not contribute directly to global warming, but they may do so indirectly insofar as they react chemically in the atmosphere in ways that increase greenhouse gas concentrations, most notably, concentrations of ozone . These substances are referred to as "criteria pollutants ," because their presence creates health problems in urban airsheds. Under the 1970 Clean Air Act and subsequent amendments, national air quality standards are set for each of them. Regulation of criteria pollutants has been effective, particularly in the case of carbon monoxide and NMVOCs. Emissions of carbon monoxide have decreased by approximately 12 percent, from 90 million metric tons in 1987 to 79 million metric tons in 1992, and NMVOC emissions have decreased from 22 million metric tons in 1987 to 21 million metric tons in 1992 (Table 34). Emissions of nitrogen oxides have remained relatively stable over the same period.
Criteria pollutants have limited direct radiative effects on global climate. They are significant, however, for their indirect effects, caused by their reactions with other atmospheric chemical compounds. The most important of these reactions results in the formation of tropospheric ozone, a greenhouse gas that may increase radiative forcing and alter the atmospheric lifetimes of other greenhouse gases. This report does not provide detailed estimates of tropospheric ozone emissions or concentrations, because tropospheric ozone formation is the result of complex chemical interactions which vary in a particular airshed under particular weather conditions-rather than direct anthropogenic emissions.
It is difficult to discern trends in tropospheric ozone concentrations by current methods. Large volumes of tropospheric ozone tend to form in and downwind of large urban areas. The EPA monitors ozone levels in urban areas to gauge compliance with the National Ambient Air Quality Standards (NAAQS). Trends in ozone levels are assessed on the basis of the second-highest daily maximum 1-hour concentration during a given year. Since ozone levels are highly sensitive to local weather conditions (changes in sunlight, rainfall, wind, etc.), however, it is difficult to generalize available data on ozone concentrations beyond the local airshed. The urban "heat island" effect, which warms metropolitan areas compared with the surrounding countryside, also promotes local ozone formation.
The EPA has reported downward trends in national ozone levels, based on the annual second-highest daily maximum 1-hour ozone concentration measured at 509 sites across the country, (149) but it is difficult to be certain that this is the case. A report by the National Academy of Sciences suggests that the method used to assess ozone trends (the second-highest daily maximum concentration in a given year) is too sensitive to meteorlogical fluctuations to be a reliable means of measuring progress in reducing ozone levels in a particular area. (150) In 1987, there were 60 areas that did not meet the NAAQS for ozone, but by September 1993 the number had risen to 94. (151) Also, scientific evidence suggests that nitrogen oxides are the most important ozone precursors of all the criteria pollutants, and emissions of nitrogen oxides have risen slightly since 1987.
Emissions of criteria pollutants are reported in EPA's National Air Pollutant Emission Trends, 1900-1992. (152) It should be noted that the EPA has changed the methods used to estimate criteria pollutant emissions, and estimates published in last year's report differ from the most recently published figures. According to the EPA, the revisions between last year's estimates and this year's are designed to achieve consistency with State emissions inventories. Also, EPA's emissions factor model for vehicles, MOBILE4, has been revised and is now called MOBILE5. Differences in the published figures illustrate the uncertainty associated with estimating these emissions. For instance, recently published carbon monoxide emissions estimates for 1990 are roughly 24 percent higher than previously reported; nitrogen oxide emissions are 10 percent higher; and NMVOC emissions are 22 percent higher.
Carbon monoxide is generated by the incomplete combustion of fossil fuels (complete combustion produces carbon dioxide). Carbon monoxide indirectly affects global climate in several ways-most notably through its interaction with the hydroxyl radical (OH), an important chemical scavenger of many trace gases, including methane . Hydroxyl is the primary agent in the oxidation of both carbon monoxide and methane. As atmospheric carbon monoxide levels increase, less hydroxyl is available to oxidize methane, and methane concentrations consequently rise.
Carbon monoxide also contributes to global warming as a precursor to tropospheric ozone . In addition, carbon monoxide is ultimately oxidized to carbon dioxide, thus adding to the global carbon dioxide budget. However, because of the relatively high efficiency of most fossil fuel combustion processes, the contribution to carbon dioxide levels is relatively small.
Highway vehicles are the largest contributor to carbon monoxide levels, representing 63 percent of total emissions (Table 35). However, despite annual increases in vehicle miles traveled, carbon monoxide emissions have declined steadily since 1985, due largely to the implementation of pollution controls and the retirement of older vehicles with uncontrolled emissions. In 1992, U.S. emissions of carbon monoxide were 79 million metric tons, down 12 percent from 1987 levels.
The oxides of nitrogen (NO and NO2, referred to collectively as NOx ) are radiatively interactive gases, but in current concentrations they do not make a significant direct contribution to global warming. Nitrogen oxides do, however, affect global climate in their role as catalysts in the formation of tropospheric ozone and through interaction in methane oxidation.
The major sources of nitrogen oxide emissions are fossil fuel combustion in electric generating stations and in highway vehicles. Emissions depend on the nitrogen content of the fuel, air fuel mix, combustion temperature, and pollution control measures. Nitrogen oxide emissions totaled 21 million metric tons in 1992, an increase of 1.4 percent from 1987 levels (Table 36).
As with the other criteria pollutants , NMVOCs contribute to greenhouse warming as tropospheric ozone precursors. Increases in NMVOC emissions may also lower concentrations of the hydroxyl radical , reducing its oxidizing capacity and resulting in higher levels of atmospheric methane .
The major contributors to NMVOC emissions are highway vehicles and industrial solvent utilization, each accounting for 27 percent of total 1992 NMVOC emissions (Table 37). Industrial solvent use is included in "Industrial Processes" in Table 37. Since 1987, NMVOC emissions have fallen by 8 percent due to pollution control measures and changes in industrial processes.
