Transportation Perspective of Ozone Trends in Clean Air Act Nonattainment Areas

There has been considerable renewal of interest over the seriousness and nature of the air quality problems in this country. The need for transportation and other forms of mobile source controls is dependent on the magnitude of the current problem and the rate at which air quality is improving. Trends in monitored ozone levels for 20 serious and severe nonattainment areas over the 14-year period, 1980–1993, are examined. Trends are examined using ambient air quality data contained in the Aerometric Information Retrieval System data base maintained by the Environmental Protection Agency (EPA). Trends in two measures are analyzed: the maximum hourly ozone value for the second highest violation day and the number of violation days per year. Tests of the statistical significance of air quality trends were conducted, including consideration of the effects of geographic scale, meteorological variables, and economic growth. The results show that, on a nationwide basis, there is a significant downward trend in ozone levels. However, not all the year-to-year variations can be explained by a simple trend, and there also are other factors that have an important influence on peak ozone concentrations. The analyses showed statistically significant results in different urban areas and geographic regions; it also showed that omitting meteorological variables results in an overestimation of the downward trend for the particular urban areas and time period analyzed.