The role of an elevated reservoir in a six-day pollution event over the eastern US

A heat wave hit the Washington/Baltimore area during July 20 to 23, 2011, leading to historic high temperatures and poor air quality. On July 22, the Baltimore Washington International Airport set daily record high of 41.1 °C (106 °F), and Metropolitan Baltimore region reached "code red" air quality (97.2 ppb 8-hour average O3). This episode was thoroughly investigated by the 2011 NASA DISCOVER-AQ campaign when a UMD research aircraft (Cessna 402B) and the NASA P3B aircraft, and ozone sondes were used to observe the ambient trace gas pollutants and aerosols. An elevated reservoir, part of residual layer from the mixed PBL of previous day, was observed under the near-stagnant winds on July 21, the day before the most severe code red day. O3 altitude profiles were applied to track this reservoir. In an upwind morning flight, spirals over Cumberland MD (39.66N, 78.76W) and Luray VA (38.67N, 78.50W) indicated low O3 (~60 ppbv) near the surface and high O3 (~80 ppbv) between 1000 and 2500 m. At noon, we found the elevated O3 layer (~80 ppbv) existed at 2000 m over Fredrick MD (39.42N, 77.38W). During afternoon flights downwind of Baltimore, an elevated O3 layer (~120 ppbv was detected over Churchville MD (39.57N, 76.20W) at 800 m with 80 ppbv surface O3. Later, ~85 ppbv O3 was observed at 2000 m over Massey MD (39.31N, 75.83W) with ~70 ppbv O3 near the surface. The ozone and precursors were mixed downward high concentrations of ground-level ozone were also observed over NJ, downwind of the Baltimore/Washington area during this episode. We studied ratios of trace gases (CO, SO2, NO2) in the reservoir to show the relative roles of coal combustion and traffic in ozone generation. Both back and forward trajectories are calculated by NOAA HYSPLIT model to investigate the regional effects of pollutant transport. This elevated reservoir was studied during an epic episode, and the results will improve the understanding on the effects of boundary layer dynamics and pollutant transport on regional air quality and help formulate effective pollution abatement policies.