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FIGURE 2.1 The atmospheric air pollution system.

auspices of the International Global Atmospheric Chemistry Project (IGAC), and various available inventories are described by Graedel et al. (1993). Data for some of the major pollutants follow.

1. Oxides of Nitrogen

We shall follow here the convention in current use that defines NOx as the sum of (NO + NOz) and NOy as the sum of all reactive nitrogen-containing species, e.g., NO>( = (NO + N02 + HN03 + PAN + HONO + N03 + N2Os + organic nitrates etc.). By far the most significant species emitted by anthropogenic processes is nitric oxide, produced when N2 and 02 in air react during high-temperature combustion processes. In addition, some NOx is formed from nitrogen in the fuel. Smaller amounts of N02 are produced by the further oxidation of NO; trace amounts of other nitrogenous species such as HN03 are also formed.

The fraction of the total that is emitted as NO clearly depends on the conditions associated with the specific combustion process. While most (typically > 90%) of the NOx emitted is believed to be in the form of NO, the fraction of NOz can vary from less than 1% to more than 30% (e.g., Lenner, 1987).

Figure 2.2 shows the contribution of various sources to the total anthropogenic NOx emissions, 23 X 106 short tons, or 21 Tg (expressed as N02), in the United States in 1996 (1 Tg = 1 teragram = 1012 g and one short ton = 0.907 X 106 g). This can be compared to total global anthropogenic emissions of approximately 72 Tg of NOx (expressed as N02) (Miiller, 1992).

Figure 2.3 shows the trend in NOx emissions for North America, Europe, the USSR, and Asia from 1970 to 1986 (Hameed and Dignon, 1992). While those

Fuel Combustion Electrical Utility 28%

Non-Road Engines and Vehicles 19%

Fuel Combustion Electrical Utility 28%

Non-Road Engines and Vehicles 19%

On-Road Vehicles 30%

Fuel Combustion -Industrial 13%

FIGURE 2.2 Contribution of various sources to total anthropogenic NO^ emissions in the United States in 1996 (from EPA, 1999).

On-Road Vehicles 30%

All Other 5% Fuel Combustion - Other 5%

Fuel Combustion -Industrial 13%

FIGURE 2.2 Contribution of various sources to total anthropogenic NO^ emissions in the United States in 1996 (from EPA, 1999).

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