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

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 (NOy, a proxy for initial NOx) and a positive correlation with the nonmethane VOC to NOy 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.