The Air Pollution Problem In Urban Centers

In spite of the advances made in automobile emission control, air pollution is still a problem in urban centers. The daily ambient air levels of CO were exceeded periodically in 42 cities in 1991, and the corresponding ozone levels were exceeded frequently in 9 cities. Los Angeles continues to have high levels of photochemical smog and ozone, air pollution problems that are more severe than almost any other U.S. city. Currently the American city with the greatest air pollution is Houston, Texas. This continued urban pollution is initially surprising, considering that tailpipe emissions from new cars have been cut more than 10-fold over the 1968-1993 time period. Although the automobile emission standards for new cars have been lowered markedly, air sampling studies in highway tunnels, where other sources of polluted air are minimal, have shown that the average automobile emissions have not decreased as much as the mandated tailpipe emissions. For example, there has been only a three-to fourfold decrease in hydrocarbon and carbon monoxide and only a two- to threefold decrease in NO* in the 1968-1993 time period (Table 6-3).

Tests on individual automobiles show that about 50% of the emissions come from 10% of the automobiles. Older automobiles have higher emissions. These cars were made when emission standards were more lenient, and they have catalytic converters with diminished or no ability to effect a decrease in tail pipe emissions. The higher emission rate of these older cars is partially offset by the lower number of miles they are driven annually in comparison to new cars. Surprisingly, about 20% of the new cars in every model year are high emitters. About 30% of these have had their emission controls intentionally disabled, and the emissions controls in the other 70% of such cars are not operating because they have not been properly serviced. It should be possible to solve these problems by providing incentives to replace old cars with newer models and mandating periodic checks of emission systems.

Others factors also contribute to the continued urban air pollution. One is that the miles driven in urban environments have doubled in the past 25 years. Another is emissions from stationary sources. For example, oil refineries, power plants, dry cleaning businesses, and auto repair shops emit hydrocarbon solvents, while restaurants and homes give off emissions from stoves, furnaces, fireplaces, lawn mowers, and grills. These sources, plus the natural emissions of terpenes from citrus and other plants, are becoming more important components of the photochemical smog in Los Angeles now that contributions from automobiles are decreasing.

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