Ozone Photochemistry in New York City and Baltimore based on Aircraft Observations

Aircraft observations of ozone, ozone precursors, and meteorological parameters were made over the New York City (NYC) and Baltimore areas during ozone exceedance events in summer 2018-2020.  Despite the continued reduction in anthropogenic emissions, ozone exceedance events still frequently occurred in the NYC area.  Ozone production efficiency, defined as the ratio of the ozone production rate to the NO_x oxidation rate, calculated using these observations,  was about 14 ppb ozone produced per ppb NOx oxidized. This high ozone production efficiency likely contributes to the persistent ozone exceedance problem over the Long Island Sound and Connecticut coastal area, downwind of NYC under prevailing southwesterly winds.  There is some evidence for a decreasing trend although COVID-19 restrictions had an impact on 2020 emissions.  A box model, constrained by observations, was used to examine atmospheric photochemical oxidation processes.  Ozone production rates and their sensitivity to nitrogen oxides (NO_x) and volatile organic compounds (VOCs) were calculated based on the model results. In general ozone production is VOC sensitive near emission sources and NOx sensitive away from source regions. While the Baltimore area is predominantly in the NOx sensitive region, the NYC area is transitioning from VOC sensitive to NOx sensitive.  Preliminary results show that controlling both NOx and VOCs reduces ozone production in the NYC area. Reducing VOCs can reduce ozone production in emission source regions and reducing NOx can reduce ozone production farther away from the source regions. The results from this work strengthen our understanding of ozone production and provide scientific information for emission control strategies to reduce air pollution in ozone non-attainment areas.