Cars and clean air: A reappraisal

Concentrations of most commonplace air pollutants recorded in US urban areas have declined sharply in the past two decades, resulting in dramatic reductions in violations of national air quality standards. Much of this progress is attributable to reductions in pollutant emissions by motor vehicles, which have occurred despite sustained increases in car ownership and use. Lower emissions have resulted from tighter federal tailpipe emission standards for new cars and from federally-mandated changes in gasoline formulation. Declines in automobiles' emissions during on-road driving, however, have not kept pace with reductions in federal emissions standards for new cars; on-road emissions average four to ten times those of new cars under test conditions. The main causes of this difference are normal deterioration in new automobiles' emissions control systems as they age, differences between the carefully-controlled conditions used to measure new automobiles' emissions and those encountered in ‘real-world driving’, malfunctioning emissions control systems in a small fraction of cars, and evaporation of gasoline during refueling and from parked cars. Although most of the current debate over strategies to further reduce motor vehicle emissions remains focused on technologies to make new cars cleaner under test conditions, measures that reduce the gap between test and on-road emissions promise to be more effective. Pollution control efforts should be broadened to focus on other sources that have not been as extensively regulated as automobiles.