Forecasting Air Quality in a Changing Atmosphere during Recent Historically Low Ozone Years in Delaware

In the Mid-Atlantic region, daily forecasts are issued for ambient ozone (O3) during the summer months because O3 causes acute health impacts. When the maximum daily 8-hour average O3 is expected to exceed the National Ambient Air Quality Standard (NAAQS) of 75 ppbv, an Air Quality Action (AQA) health alert is issued to the public. Historically, Delaware had about 20 AQA days each summer. Since 2013, however, there have not been more than three per year. This recent and dramatic decrease in AQA days had made forecasting air quality for Delaware much more challenging. Identification of a few key synoptic weather conditions used to be sufficient to confidently issue an AQA forecast for multiple successive days, but NAAQS exceedances are now occurring in isolated spikes (in regards to both location and time frame), and they are more dependent on mesoscale weather features. These factors increase the likelihood of a false alarm exceedance forecast or an unforecasted exceedance event. In order to adapt to recent O3 trends in Delaware, new forecasting tools are necessary, such as updated statistical models and revised local climatologies. Additionally, the EPA is considering lowering the NAAQS for O3 to 60-70 ppbv, which will also affect the number of O3 exceedance days in Delaware. The likely causes of the recent unprecedented decline in O3 exceedance days in Delaware will be discussed, along with the implications of a lower O3 NAAQS, and options for new tools to increase forecasting skill will be presented.