ABSTRACT Ozone (O
3) concentrations in the Baltimore-Washington (B-W) metropolitan area frequently exceed the National Ambient Air Quality Standard (NAAQS) in the summer months. The most extreme O
3 events occur in multi-day high O
3 episodes.
1 These events can be regional in scale, with O
3 concentrations exceeding the NAAQS at numerous locations along the eastern U.S. seaboard, and are typically associated with slow-moving 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 local O
3 production (subsidence and strong low-level inversions, weak horizontal winds, little cloud cover), this weather pattern may also result in transport of O
3 and its precursors from heavily industrialized areas west and north of the B-W region. In this paper, observations and back trajectories made during the severe regional O
3 event of July 12-15, 1995, are used to confirm the hypothesis that significant regional-scale transport of O
3 and its precursors occur during extreme O
3 events of the standard type in the B-W area.
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