Greenhouse Gases 1987-1994, Chapter 6

 

 

6. Criteria Pollutants

 

 

 

Overview

 

The Clean Air Act of 1970 requires that air quality standards be established for pollutants that have adverse effects on public health or welfare, termed "criteria pollutants" because the U.S. Environmental Protection Agency (EPA) based each National Ambient Air Quality Standard (NAAQS) on health-based criteria from scientific studies. This report includes emissions estimates for carbon monoxide (CO), nitrogen oxides (NOx) and nonmethane volatile organic compounds (NMVOCs), which are criteria pollutants that influence the 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 absorbed by 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 reports that the composite average ozone concentration for its 532 U.S. testing sites has declined by 12 percent since 1984.(Note 1)

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, 1900-1993.(Note 2) 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. Therefore, estimates in this report differ from those in last year's report. This year's modifications include a change in the model used to estimate emissions from highway vehicles and distinct emissions factors for California because of the unique legal and environmental conditions in that State.

Emissions of the three criteria pollutants included in this report increased slightly in 1993 (Table 35). However, emissions of both carbon monoxide and NMVOCs have declined since 1987. Emissions of nitrogen oxides have risen by approximately 4 percent over the same period.

 

Table 35. U.S. Emissions of Criteria Pollutants, 1987-1994
(Million Metric Tons of Gas)

 

 
Source 1987 1988 1989 1990 1991 1992 1993 1994
Carbon Monoxide 95.3 96.2 91.4 94.1 90.6 87.4 88.1 NA Nitrogen Oxides 20.3 21.1 21.1 21.0 20.8 20.8 21.2 NA Nonmethane VOCs 22.1 22.6 21.5 22.0 21.3 20.9 21.1 NA 
 
NA = not available.

Note: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994).

Source: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Pollutant Emission Trends, 1900-1993, EPA-454/R-94-027 (Research Triangle Park, NC, October 1994), pp. A-2 - A-16.

 

The EPA predicts that emissions of criteria pollutants will decrease by the end of the decade:(Note 3)

 

 

 

 

Carbon Monoxide

 

In the atmosphere, carbon monoxide is relatively short-lived, lasting only 2 to 3 months before being oxidized into carbon dioxide. Carbon monoxide also functions as the primary sink for atmospheric hydroxyl radicals (OH--). Through the process of oxidation, hydroxyl radicals chemically break apart methane molecules. Therefore, by reducing the concentration of hydroxyl radicals, carbon monoxide indirectly increases the amount of methane in the atmosphere. In the presence of nitrogen oxides, carbon monoxide also plays a role in the production of tropospheric ozone.

Most carbon monoxide is produced from incomplete combustion of fuels. Transportation sources account for most of the carbon monoxide: consistently 75 to 80 percent of total emissions (Table 36). In 1993, 88 million metric tons of carbon monoxide were emitted to the atmosphere, representing an overall decline of 16 percent from 1980 levels. This reduction can be attributed to pollution controls and replacement of older, less fuel-efficient vehicles.

 

 

Nitrogen Oxides

 

The molecules NO and NO2 are commonly referred to as nitrogen oxides. These gases are important for their role as catalysts in the chemical reactions that create tropospheric ozone. Nitrogen oxides may also reduce stratospheric ozone if they are emitted directly from aircraft at high altitudes.

Fossil fuel combustion is the main point of origin for nitrogen oxide emissions, particularly from electric power generation and transportation sources. In addition, the chemical industry produces a minor amount of noncombustion emissions, included under "Industrial Processes" in Table 37. Nitrogen oxide emissions in 1993 totaled 21 million metric tons. Since 1980, this figure has remained relatively stable.

 

 

 

Nonmethane Volatile Organic Compounds

 

A "volatile" organic compound is one that plays an active role in atmospheric photochemical reactions. NMVOCs include such gases as propane, butane, and ethane, as well as many more complex chemicals. NMVOCs affect climate change in two ways: first, as with carbon monoxide, they may indirectly increase atmospheric methane concentrations through reduction of hydroxyl; second, they lead to production of tropospheric ozone.

In 1993, NMVOC emissions were estimated at 21 million metric tons (Table 38). This is an overall reduction of 10 percent from 1980 levels. Industrial processes continue to be the primary source of NMVOC emissions, due in part to solvent use. Emissions from highway vehicles have declined by approximately 32 percent since 1980 as a result of tailpipe emissions controls mandated by the Clean Air Act.

 

Table 36. U.S. Carbon Monoxide Emissions, 1987-1994
(Million Metric Tons)

 

 
Source 1987 1988 1989 1990 1991 1992 1993 1994
Transportation Highway Vehicles 64.08 64.47 59.91 57.01 56.30 54.29 54.41 NA Aircraft 0.80 0.84 0.87 0.88 0.87 0.89 0.92 NA Railroads 0.10 0.11 0.11 0.11 0.11 0.11 0.11 NA Vessels 0.05 0.05 0.05 0.05 0.05 0.05 0.06 NA Other Off-Highway 11.87 12.15 12.14 12.24 12.23 12.46 12.76 NA Total 76.89 77.62 73.08 70.29 69.56 67.81 68.26 NA Stationary Source Fuel Combustion Electric Utilities 0.27 0.28 0.29 0.28 0.28 0.28 0.29 NA Industrial 0.59 0.61 0.61 0.61 0.62 0.61 0.60 NA Commercial-Institutional 0.07 0.07 0.07 0.07 0.07 0.07 0.08 NA Residential 5.63 5.47 5.27 5.07 4.94 4.44 3.91 NA Total 6.56 6.44 6.24 6.05 5.92 5.41 4.88 NA Total Energy-Related Emissions 83.45 84.06 79.31 76.34 75.48 73.22 73.14 NA Industrial Processes 4.54 4.74 4.78 4.74 4.67 4.71 4.79 NA Solid Waste Disposal Incineration 0.83 0.82 0.79 0.77 0.75 0.78 0.79 NA Open Burning 0.84 0.82 0.79 0.76 0.74 0.77 0.78 NA Total 1.68 1.64 1.58 1.53 1.49 1.56 1.57 NA Miscellaneous Forest Fires 1.07 1.07 1.07 6.83 4.35 3.25 3.98 NA Other Burning 4.56 4.67 4.64 4.62 4.56 4.63 4.64 NA Total 5.63 5.74 5.71 11.45 8.91 7.87 8.62 NA Total All Sources 95.29 96.18 91.38 94.06 90.56 87.36 88.12 NA Carbon Content of Carbon Monoxide 40.84 41.22 39.16 40.31 38.81 37.44 37.77 NA Carbon Content of Energy-Related Carbon Monoxide 35.77 36.03 34.00 32.72 32.36 31.39 31.35 NA 
 
NA = not available.

Note: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994).

Source: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Pollutant Emission Trends, 1900-1993, EPA-454/R-94-027 (Research Triangle Park, NC, October 1994), Table A-1, pp. A-2 - A- 5.

 

Table 37. U.S. Nitrogen Oxide Emissions, 1987-1994
(Million Metric Tons)

 

 
Source 1987 1988 1989 1990 1991 1992 1993 1994
Transportation Highway Vehicles 6.95 6.95 6.97 6.79 6.69 6.75 6.75 NA Aircraft 0.12 0.12 0.13 0.13 0.13 0.13 0.13 NA Railroads 0.77 0.81 0.84 0.84 0.84 0.86 0.86 NA Vessels 0.14 0.15 0.16 0.16 0.16 0.16 0.17 NA Other Off-Highway 1.39 1.56 1.46 1.45 1.41 1.47 1.55 NA Total 9.37 9.59 9.55 9.37 9.22 9.36 9.45 NA Stationary Source Fuel Combustion Electric Utilities 6.47 6.83 6.90 6.82 6.79 6.78 7.06 NA Industrial 2.78 2.89 2.91 2.95 3.00 2.91 2.88 NA Commercial-Institutional 0.27 0.28 0.27 0.27 0.28 0.28 0.28 NA Residential 0.36 0.38 0.38 0.38 0.39 0.38 0.37 NA Total 9.88 10.38 10.46 10.42 10.45 10.34 10.59 NA Total Energy-Related Emissions 19.24 19.97 20.01 19.79 19.68 19.71 20.05 NA Industrial Processes 0.79 0.82 0.81 0.81 0.80 0.81 0.83 NA Solid Waste Disposal Incineration 0.03 0.03 0.03 0.03 0.03 0.03 0.03 NA Open Burning 0.05 0.05 0.05 0.05 0.04 0.05 0.05 NA Total 0.08 0.08 0.08 0.07 0.07 0.08 0.08 NA Miscellaneous Forest Fires * * * * * * * NA Other Burning 0.18 0.19 0.19 0.35 0.28 0.25 0.27 NA Total 0.18 0.19 0.19 0.35 0.28 0.25 0.27 NA Total All Sources 20.30 21.05 21.08 21.02 20.83 20.84 21.22 NA 
 
*Less than 5,000 metric tons.

NA = not available.

Note: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994).

Source: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Pollutant Emission Trends 1900-1993, EPA-454/R-94-027 (Research Triangle Park, NC, October 1994), Tables A-2, pp. A-6 - A-9.

 

Table 38. U.S. Emissions of Nonmethane Volatile Organic Compounds, 1987-1994
(Million Metric Tons)

 

 
Source 1987 1988 1989 1990 1991 1992 1993 1994
Transportation Highway Vehicles 7.44 7.52 6.52 6.22 5.89 5.51 5.53 NA Aircraft 0.16 0.17 0.17 0.17 0.17 0.18 0.18 NA Railroads 0.04 0.04 0.04 0.04 0.04 0.04 0.04 NA Vessels 0.03 0.03 0.04 0.04 0.04 0.04 0.04 NA Other Off-Highway 1.62 1.67 1.66 1.67 1.68 1.71 1.74 NA Total 9.29 9.43 8.43 8.14 7.82 7.47 7.53 NA Stationary Source Fuel Combustion Electric Utilities 0.03 0.03 0.03 0.03 0.03 0.03 0.03 NA Industrial 0.23 0.25 0.24 0.24 0.24 0.25 0.25 NA Commercial-Institutional 0.01 0.01 0.01 0.01 0.01 0.01 0.01 NA Residential 0.42 0.41 0.40 0.38 0.37 0.33 0.29 NA Total 0.69 0.70 0.68 0.66 0.66 0.62 0.58 NA Total Energy-Related Emissions 9.98 10.13 9.11 8.80 8.48 8.09 8.11 NA Industrial Processes 9.65 10.00 9.92 9.96 9.98 10.02 10.16 NA Solid Waste Disposal Incineration 0.06 0.05 0.05 0.05 0.05 0.05 0.05 NA Open Burning 0.26 0.26 0.25 0.24 0.23 0.24 0.24 NA Other 1.73 1.78 1.78 1.76 1.73 1.76 1.76 NA Total 2.05 2.10 2.08 2.05 2.01 2.06 2.06 NA Miscellaneous Forest Fires 0.15 0.15 0.15 0.94 0.60 0.44 0.55 NA Other Burning 0.25 0.27 0.26 0.26 0.25 0.26 0.26 NA Total 0.40 0.42 0.41 1.20 0.85 0.71 0.81 NA Total All Sources 22.07 22.64 21.52 22.01 21.32 20.88 21.14 NA 
 
NA = not available.

Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Assumes a carbon coefficient of 0.83 metric tons of carbon per metric ton of NMVOC. Totals may not equal sum of components due to independent rounding.

Source: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Pollutant Emission Trends 1900-1993, EPA-454/R-94-027 (Research Triangle Park, NC, October 1994), Table A-3, pp. A-10 - A-16.