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7 Mobile Sources and Control Approaches While the sulfurous fogs crept across London in early December 1952, the townspeople were not fully aware of what was happening. But many townspeople were pleased that the conversion of the city’s trolleys from electricity to diesel powered buses was now complete. London, December 1952 Probably the most important change in sources of U.S. air contaminant emissions from the early 20th to the early 21st century was the shift from stationary sources to predominantly mobile sources. Technologies and approaches to reducing mobile source emissions lagged behind approaches applied to stationary sources because of the lag in recognition. | 7 Mobile Sources and Control Approaches While the sulfurous fogs crept across London in early December 1952 the townspeople were not fully aware of what was happening. But many townspeople were pleased that the conversion of the city s trolleys from electricity to diesel powered buses was now complete. London December 1952 Probably the most important change in sources of U.S. air contaminant emissions from the early 20th to the early 21st century was the shift from stationary sources to predominantly mobile sources. Technologies and approaches to reducing mobile source emissions lagged behind approaches applied to stationary sources because of the lag in recognition of the mobile sources contribution to the problem. Motor vehicles make up the largest number of sources of air contaminant emissions in the United States. These vehicles include passenger vehicles light-duty trucks SUVs LDTs and medium- to heavy-duty trucks M HDTs . As seen in Table 7.1 passenger and light-duty trucks form the majority over 95 of the motor vehicles on the road. The significance of these mobile sources is that they are powered primarily by gasoline-burning combustion systems. Gasoline-powered vehicles are responsible for 78 and 92 respectively of the gallons of fuel consumed and miles driven. Diesel fuel oil accounts for the balance. Individual emissions from each source are small however because of the large number of sources involved the aggregate emissions are significant. As a consequence we need to understand how the major types of engines that drive our mobile sources work how they can be modified and the most efficient ways available of controlling the emissions that are ultimately released. Equally important is the effect of the fuels they burn on their respective criteria and toxic haz-ardous air emissions. Finally a word needs to be said about alternatives to traditional engines and fuels and some new approaches. ENGINES AND AIR POLLUTANT EMISSIONS On a pollutant-specific basis
