POLLUTANT TRANSPORT DURING A REGIONAL O-3 EPISODE IN THE MID-ATLANTICSTATES

Ozone (O-3) concentrations in the Baltimore-Washington (B-W) metropoli tan area frequently exceed the National Ambient Air Quality Standard ( NAAQS) in the summer months. The most extreme O-3 events occur in mult i-day high O-3 episodes.(1) These events can be regional in scale, wit h O-3 concentrations exceeding the NAAQS at numerous locations along t he eastern U.S. seaboard, and are typically associated with slow-movin g or stagnant high pressure systems.(2-5) In the B-W region, the most extreme events typically occur with surface high pressure overhead or just west of the region and an upper air high-pressure area (ridge) to the west or northwest.(1) Besides providing conditions conducive to l ocal O-3 production (subsidence and strong low-level inversions, weak horizontal winds, lit-tie cloud cover), this weather pattern may also result in transport of O-3 and its precursors from heavily industriali zed areas west and north of the B-W region. In this paper, observation s and back trajectories made during the severe regional O-3 event of J uly 12-15, 1995, are used to confirm the hypothesis that significant r egional-scale transport of O-3 and its precursors occur during extreme O-3 events of the standard type in the B-W area.