Changes in the frequency and return level of high ozone pollution events over the eastern United States following emission controls

This new approach confirms the significant decline in both frequency and magnitude of high O₃ pollution events over the eastern US during recent years reported in prior studies. Our analysis of 1-yr and 5-yr return levels at each station demonstrates the strong impact of changes in air quality regulations and subsequent control measures (e.g., the 'NOx SIP Call'), as the 5-yr return levels of the period 1999-2009 correspond roughly to the 1-yr return levels of the earlier time period (1988-1998).

Regionally, the return levels dropped between 1988-1998 and 1999-2009 by about 8 ppb in the Mid-Atlantic (MA) and Great Lakes (GL) regions, while the strongest decline, about 13 ppb, is observed in the Northeast (NE) region. Nearly all stations (21 out of 23) have 1-yr return levels well below 100 ppb and 5-yr return levels well below 110 ppb in 1999-2009. Decreases in eastern US O₃ pollution are largest after full implementation of the nitrogen oxide (NOx) reductions under the 'NOx SIP Call'.

We conclude that the application of EVT methods provides a useful approach for quantifying return levels of high O₃ pollution in probabilistic terms, which may help to guide long-term air quality planning.