Among the major sources of air pollutants, the burning of fossil fuels-primarily coal, oil, and natural gas-tops the list (Figure 18.1). Of the users, the transportation sector (such as automobiles and trucks) is the largest contributor of pollutants. In the 1970s, for every 1,000 gal (3,785 L) of gasoline consumed, the automobile emissions into the atmosphere were enormous: 1,434 kg of carbon monoxide, 90-180 kg of organic vapors, 9-34 kg of nitrogen oxides, 8 kg of aldehydes, 7.5 kg of sulfur compounds, 0.9 kg of organic acids, 0.9 kg of ammonia, and 0.13 kg of such solids as zinc and other metal oxides. Local vehicular emissions contribute 12% of the global anthropogenic

gas emissions (Faiz 1993). During construction, use, and maintenance of roads, the transportation sector, at present, is responsible for about 20 and 40% of the global carbon dioxide (CO2) and oxides of nitrogen (NOx) emissions, respectively (Hahn and Simonis 1991). In city centers, road traffic accounts for 90-95% of carbon monoxide (CO) and lead (Pb), 60-70% of nitrogen oxides (NOx) and hydrocarbons (HC),

and a major share of particulates (Johnstone and Karousakis 1999) (Table 18.2).