Influence of conducive weather on ozone in the presence of reduced NOx emissions: A case study in Chicago during the 2020 lockdowns

This study analyzes the response of near-surface O3 in Chicago to the COVID-19 lockdowns using observational data at the surface and from satellite. Even though the lockdowns caused NOx emissions to decrease by 18%, Chicago still experienced 17 high-O3 (>70 ppb) days in 2020, and the mean O3 mixing ratio did not show a significant change in 2020 compared with 2015–2019. Ozone production in summer 2020 in Chicago was in the “transitional” regime (HCHO/NO2 column ratio = 2.9), and not sensitive to changes in NOx in either direction. The primary driver for Chicago's O3 exceedances in 2020 was the dry tropical (DT) weather, which was associated with hot, dry, and stagnant meteorological conditions. There were 15 DT days in 2020, which led to more efficient production and greater accumulation of O3. The results suggest that a dramatic one-year 18% NOx reduction can be overcome by conducive meteorology and that NOx and VOC controls need to be more substantial and wide-ranging. This study also highlights the important role of different climate regimes, and not solely temperature, on the formation of O3.