Ozone Production Efficiencies in the Three Largest United States Cities from Airborne Measurements
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Chace, Wyndom S., Caroline Womack, Katherine Ball, Kelvin H. Bates, Birger Bohn, Matthew Coggon, John D. Crounse, et al. “Ozone Production Efficiencies in the Three Largest United States Cities from Airborne Measurements.” Environmental Science & Technology, June 24, 2025. https://doi.org/10.1021/acs.est.5c02073.
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract
Despite ongoing reductions in emissions of ozone (O₃) precursors, nitrogen oxides (NOₓ = NO + NO₂) and volatile organic compounds (VOCs), the three largest urban areas in the United States - New York City (NYC), Chicago, and Los Angeles (LA) - continue to exceed national air quality standards for O₃. Airborne measurements during the 2023 Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) campaign investigated nonlinear O₃ photochemistry in these cities. We report mean ozone production efficiency (OPE), the enhancement ratio of Oₓ (= O₃ + NO₂) to NOₓ oxidation products, of 9 ± 4 (1σ), 6 ± 3, and 6 ± 3 ppbv ppbv-¹ in NYC, Chicago, and LA, respectively. Compared to historical values, OPE has increased in NYC but remains constant in LA. We find that OPE during AEROMMA has a nonlinear, inverse relationship with total reactive nitrogen (NO*, a proxy for initial NOx) and a positive correlation with the nonmethane VOC to NO* enhancement ratio. A zero-dimensional photochemical model supports these observed OPE dependences on NOₓ and VOCs and shows that OPE is a distinct metric from total O₃ production that is informative to the development of O₃ pollution control strategies. We find that OPE values have higher variability, and a larger increase with NOₓ emissions reductions, in areas that experience NOₓ-sensitive rather than NOₓ-saturated O₃ photochemistry; nonetheless, NOₓ reductions under NOₓ-sensitive conditions still reduce total O₃ production despite the corresponding increase in OPE.
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