An ozone episode in the Philadelphia metropolitan area

In July and August 1999 a Northeast Oxidant and Particle Smdy field campaign was codducted in the Philadelphia metropolitan area to determine causes for episodically high levels of O₃ and particulate matter ≤2.5 μm in aerodynamic diameter. We report emission estimates, weather information, surface O₃ monitoring data, and aircraft observations, with a focus on 31 July, the last day of an O₃ 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 O₃ episodes in the northeast, transport on 31 July was limited, and O₃ hot spots occurred close to NO_x and volatile organic compound (VOC) emission sources. High O₃ was observed downwind of Baltimore and Philadelphia, both major urban areas. High O₃ 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 O₃ monitoring data and morning aircraft observations show that the residual layer aloft contained 80-100 ppb of O₃ but almost no O₃ precursors. Same-day photochemistry on 31 July caused surface O₃ concentrations to increase by 60-80 ppb. Photochemical model calculations indicate O₃ production rates in excess of 20 ppb h^-1 in regions with NO_x > 5 ppb. High NO _x concentrations are a consequence of poor ventilation. Peroxide observations and calculations indicate that O₃ production is VOC limited in the high-NO_x 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 O₃ hot spots, there were mesoscale meteorological features in 1995 that favored interregional transport.