Assessment of interannual ozone variation in urban areas from a climatological perspective

The frequency distribution of ambient ozone measured in many urban areas of the U.S. varies considerably from year to year, which makes assessment of underlying ozone trends difficult and imprecise. Moreover, these variations can lead to instability in statistical measures used to assess the status of ozone air quality and can create difficulties in designing robust model-based strategies to attain the National Ambient Air Quality Standard (NAAQS). Much of the interannual variation in ozone can be attributed to variations in meteorological conditions that control ozone accumulation. A statistical model that relates daily ozone levels to variations in daily meteorological variables has been proved to be an effective tool for reducing bias and imprecision in ozone trend assessments. By combining the statistical model with climatological data, a method has been developed for calculating the ozone formation potential for those years when ozone measurements were not available. Calculated distributions of annual ozone potential are useful for contrasting the relative severity of recent ozone seasons with ozone potential over the past four decades. The results have implications for the return frequency of the more severe ozone seasons observed in the past decade and for placing recent trends in ozone in historical perspective.