Urban, Suburban, and Rural Ozone Trends Across the United States over a Period of Decreasing NOx Emissions

Abstract:

We have completed a comprehensive evaluation of ambient ozone trends at urban, suburban, and rural monitoring sites across the United States over a period of decreasing NOx emissions (1998-2011). We find that at sites with statistically significant trends, 5^th percentile ozone concentrations have generally increased by 0.5-1 ppb/year and 95^th percentile ozone concentrations have generally decreased by 1-2 ppb/year. Both increasing and decreasing trends are observed in mid-range ozone (median and mean concentrations), with urban sites being more likely to show increasing trends and suburban/rural sites being more likely to show decreasing trends. Regional and seasonal differences in these trends are also explored. In general, these results indicate that as anthropogenic NOx emissions have decreased, the ozone distribution has been compressed, leading to less spatial and temporal variability in high and low percentile concentrations.