Global tropospheric ozone responses to reduced NOx emissions linked to the COVID-19 world-wide lockdowns

Efforts to slow the transmission of COVID-19 led to rapid, global ancillary reductions in air pollutant emissions. Here, we quantify the impact of these reductions on tropospheric ozone using a multi-constituent data assimilation system. Total anthropogenic NOx emissions dropped by at least 15% globally and 18-25% regionally in April and May 2020, which led to up to a 5 ppb decrease in free tropospheric ozone, consistent with independent satellite observations. A decline in the global total tropospheric ozone burden of 6 TgO3 (2 %) in May-June 2020 was largely due to emission reductions in Asia and the Americas, the effects of which were amplified by regional ozone production efficiencies up to 4 TgO3/TgN. Our results show that COVID-19 mitigation led to a clear and global atmospheric signature that altered atmospheric oxidative 1 capacity and climate radiative forcing and can be used to inform policies that co-benefit air quality and climate.