Operational Air Quality Forecasting in a Changing Emissions and Meteorological Environment in the Mid-Atlantic Region

In order to protect public health, state and local agencies across the Mid-Atlantic region have provided daily forecasts of ozone (O₃) and fine particulate matter (PM_2.5) since the late 1990s. When O₃ or PM_2.5 concentrations are forecasted to exceed the National Ambient Air Quality Standard (NAAQS; 75 parts per billion for 8-hour average O₃ and 35 micrograms per cubic meter for 24-hour average PM_2.5), an Air Quality Alert (AQA) is issued. Accurate predictions of AQA days are critical to ensure that citizens can take action to protect their health when the NAAQS is exceeded; it is equally important to limit the number of "false alarms," days when an exceedance of the NAAQS is forecasted but not observed. Historically, O₃ NAAQS exceedances in the Mid-Atlantic occurred in multi-day events and were linked to region-wide heat waves of hot, stagnant, and sunny weather. These meteorological conditions are straightforward to forecast and allowed for relatively high accuracy in predicting O₃ NAAQS exceedances. Since 2012, however, there has been a breakdown in the relationship between hot weather and O₃ exceedances, with most exceedance days now occurring as single day "spikes." The result has been a consecutive 3-year run of record low O₃ concentrations across the region, with an order of magnitude fewer NAAQS exceedance days. In a similar manner, PM_2.5 concentrations have also been steadily decreasing since about 2009, most notably during the summer months, to the point that PM_2.5 NAAQS exceedances are now extremely rare during the summer in the Mid-Atlantic. The main factors responsible for the shifts in ambient O₃ and PM_2.5 levels will be discussed, including continued reductions in emissions of O₃ and PM_2.5 precursors and fewer occurrences of O₃-favorable synoptic conditions (i.e., heat waves), possibly due to climate change. O₃ exceedance days are now much more dependent on mesoscale features, such as weak frontal boundaries, which are difficult for numerical forecast models to resolve and thus, challenging to predict. We will present new and updated forecasting tools that have been developed in response to the changing emissions and meteorological conditions, and describe recent successes and challenges in forecasting O₃ and PM_2.5, with a focus on the Philadelphia metropolitan area. Suggestions for modifying the AQA process to reflect increased occurrence of single day "spikes" in ambient O₃ will also be discussed, as will the implications of the proposed reduction in the O₃ NAAQS.