Urban core-downwind differences and relationships related to ozone production in a major urban area in Texas

Abstract

San Antonio, the second-most populous city in Texas and the seventh-most populous city in the United States (US), has been designated a marginal non-attainment area by the US Environmental Protection Agency with respect to the 2015 ozone (O₃) National Ambient Air Quality Standard. While stationary air quality monitoring sites are operated in the region by the Texas Commission on Environmental Quality (TCEQ), there are limited in situ field measurements for O₃ and its precursors in the urban core. To better understand O₃ dynamics in San Antonio, a suite of meteorological and gas instruments was deployed during May 2017. We incorporate field measurements from two campaign sites and one TCEQ stationary monitoring site into a zero-dimensional O₃ model to characterize the local formation and destruction rates of O₃, hydroxyl radical (OH) reactivity of volatile organic compounds (VOCs), O₃ production efficiency, and O₃ formation regime in the urban core and directly downwind of San Antonio. Upwind/downwind differences indicate the importance of photochemical processing of VOCs with carbon-carbon double bonds. San Antonio was mostly in a nitrogen oxide (NO_X)-sensitive regime throughout the daytime during the campaign period, with O₃ formation peaking at noon in the city center and early afternoon at the downwind region. Formaldehyde (HCHO), isoprene, and alkenes dominated VOC reactivity, with alkenes and isoprene from San Antonio's core (upwind) likely contributing to the downwind formation of HCHO and enhancing its OH reactivity. However, their direct impact on downwind O₃ production was not observed. Model results suggest further strengthening NO_X emission controls to decrease O₃ formation in San Antonio.

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