Ozone Production Efficiencies in the Three Largest United States Cities from Airborne Measurements

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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