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.