High ground-level ozone events over the Eastern United States: meteorology and source attribution

Abstract:

To establish effective emission controls to achieve National Ambient Air Quality Standards (NAAQS), state air agencies need to quantify contributions to extreme ozone events from anthropogenic precursor emissions sources from a given state versus from regional and international pollution transport or from natural sources. An understanding of the meteorology and sources contributing to observed high ozone events is a first step towards assessing the impacts on surface ozone from future changes in regional climate. We analyze two Eastern U.S. (EUS) high ozone events that led to exceedances of the ozone NAAQS in multiple states: (1) June 10-18, 2007 and (2) June 24-July 14, 2012. These events are prototypes for determining source contributions and establishing a baseline for comparison with other EUS ozone pollution episodes. We (1) use daily NOAA weather maps, observed surface ozone concentrations, and satellite NO₂ columns to identify regional transport and (2) analyze a series of emission perturbation simulations with the GEOS-Chem chemistry-transport model (2ºx 2.5º) to estimate contributions from several sources. We find that enhancements of the NO₂ column persist for days over multiple states and are examining whether the persistence of this typically short-lived ozone precursor is due to regional transport and its contribution to the pollution event. As in other historical EUS extreme ozone events, high pressure systems facilitate regional ozone build-up during June 10-18, 2007. For example, observed surface ozone over WI is 60-85 ppb. GEOS-Chem captures the main features of the observed ozone enhancements and attributes less than 15% (~4-10 ppb) to WI anthropogenic emissions; over 50% (~32-50 ppb) to other U.S. states’ anthropogenic emissions; and greater than 10% (~10-20 ppb) to Canadian anthropogenic emissions. Similar to the 2007 event, high pressure systems dominate the EUS during the 2012 episode and the model indicates that anthropogenic U.S. sources contribute the most to the high ozone observed on these days. During the first two weeks of July, however, the region is influenced by stationary fronts. We will discuss how the changing meteorological situation influences the relative source contributions to regional ozone accumulation.