An ozone episode in the Philadelphia metropolitan area

[1] In July and August 1999 a Northeast Oxidant and Particle Study field campaign was conducted in the Philadelphia metropolitan area to determine causes for episodically high levels of O-3 and particulate matter less than or equal to 2.5 mum in aerodynamic diameter. We report emission estimates, weather information, surface O-3 monitoring data, and aircraft observations, with a focus on 31 July, the last day of an O-3 episode in which concentrations in Philadelphia reached 165 ppb, the highest level observed there in the past 11 years. As is common in the northeastern states, this O-3 episode started with the development of a broad ridge over the central United States and ended with the northeast corridor under the influence of an Appalachian lee trough with airflow from the SW in an along-corridor direction. For a portion of the morning of 31 July, winds were nearly stagnant, allowing local emissions to accumulate. In contrast to typical O-3 episodes in the northeast, transport on 31 July was limited, and O-3 hot spots occurred close to NOx and volatile organic compound (VOC) emission sources. High O-3 was observed downwind of Baltimore and Philadelphia, both major urban areas. High O-3 was also observed in a less likely region near the Delaware-Pennsylvania border, downwind of Wilmington, Delaware, but near utility and industrial emission sources. Surface O-3 monitoring data and morning aircraft observations show that the residual layer aloft contained 80 - 100 ppb of O-3 but almost no O-3 precursors. Same-day photochemistry on 31 July caused surface O-3 concentrations to increase by 60 - 80 ppb. Photochemical model calculations indicate O-3 production rates in excess of 20 ppb h(-1) in regions with NOx > 5 ppb. High NOx concentrations are a consequence of poor ventilation. Peroxide observations and calculations indicate that O-3 production is VOC limited in the high-NOx portions of the Philadelphia urban plume. Our results are contrasted against a severe O-3 episode that occurred in 1995. While the 1999 episode had stagnation conditions leading to local O-3 hot spots, there were mesoscale meteorological features in 1995 that favored interregional transport.