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

Rieder, Harald; Fiore, Arlene M.; Polvani, Lorenzo M.; Lamarque, Jean-Francois; Fang, Yuanyuan

In order to quantify the impact of recent efforts to abate surface ozone (O3) pollution, we analyze changes in the frequency and return level of summertime (JJA) high surface O₃ events over the eastern United States (US) from 1988–1998 to 1999–2009. We apply methods from extreme value theory (EVT) to maximum daily 8-hour average ozone (MDA8 O₃) observed by the Clean Air Status and Trends Network (CASTNet) and define O₃ extremes as days on which MDA8 O₃ exceeds a threshold of 75 ppb (MDA8 O₃>75). Over the eastern US, we find that the number of summer days with MDA8 O₃>75 declined on average by about a factor of two from 1988–1998 to 1999–2009. The applied generalized Pareto distribution (GPD) fits the high tail of MDA8 O₃ much better than a Gaussian distribution and enables the derivation of probabilistic return levels (describing the probability of exceeding a value x within a time window T) for high O₃ pollution events. 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.

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Environmental Research Letters

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