Assessing the Impact of COVID-19 Lockdowns on Ground-Level Ozone Formation in Four Cities

To prevent the spread of COVID-19, cities around the world entered prolonged periods of restricted personal mobility and decreased economic activity. Ground-level ozone, a secondary pollutant formed through chemical reactions in the troposphere, was initially observed to increase in response to these lockdowns. Here, data from two satellite instruments, the Ozone Monitoring Instrument (OMI) on AURA, and the Tropospheric Monitoring Instrument (TROPOMI) on Sentinel-5P, are used to monitor column densities of ozone (O3) and its precursor, nitrogen dioxide (NO2), in four cities. While substantial declines in NO2 were observed in all cities, a significant increase in O3 was observed in only one. These results illustrate the non-linear relationship of ozone to its precursors, as well as the need for air pollution mitigation strategies that address the complex chemistry of secondary pollutants in urban areas.