Case Study of the Late July 2005 Ground-Level Ozone Episode in North Carolina

A meteorological scenario conducive for high ground-level ozone occurred in North Carolina from July 25-28, 2005. 8-hour averaged ozone concentrations peaked above 110 parts per billion (ppb) in the Charlotte metropolitan region, and ozone readings above 85 ppb were observed in the Triangle (Raleigh-Durham-Chapel Hill) and Fayetteville metropolitan areas. Meanwhile, the Triad metropolitan region (Winston Salem-Greensboro-High Point) and the North Carolina Foothills and Mountains did not experience 8-hour averaged ozone concentrations in excess of the National Ambient Air Quality Standard (NAAQS) of 85 ppb throughout the entire episode. During this episode, ozone, carbon monoxide, sulfur dioxide, fine and coarse particles, and standard meteorological variables were measured aboard a University of Maryland air quality research aircraft over the Charlotte metropolitan area. An in depth analysis of these aircraft measurements, lower-tropospheric radar profiler data, back trajectories, and surface meteorological and air quality data is made to determine why high ozone was restricted to certain portions of North Carolina. Preliminary analysis shows that boundary-layer mixing heights were in excess of 7000 feet, with ozone approaching 120 ppb throughout the mixed boundary-layer. The elevated mixing heights appear to have played a major role in causing ozone to exhibit a local response in certain metropolitan areas, rather than a more widespread regional response. Comparisons with ground-level ozone events and episodes in 2002, the last active ozone season in North Carolina, are made to determine if recently implemented air quality regulations helped to prevent exceedances of the 8-hour ozone NAAQS in the Triad metropolitan region. Additionally, an analysis is made of the real-time ozone forecast guidance from the Baron Advanced Meteorological Systems' MAQSIP model and from the NOAA-EPA National Air Quality Forecast System.