Converging evidence for reduced global atmospheric oxidation in 2020.

The hydroxyl radical (OH) plays a central role in the oxidation of methane and many other reduced gases in the Earth's atmosphere. The inter-annual changes of global mean OH concentrations are poorly constrained, posing challenges to understanding annual methane budgets. Here, we use observations of multiple atmospheric species to investigate the anomaly of global OH concentration in 2020. We develop a new method to infer the zonal distribution of OH concentrations from satellite carbon monoxide (CO) observations. This method finds that the global OH concentration reduced by 4.0% ± 0.9% in 2020 relative to 2018-2019 and that the reduction occurred in both Northern (2.4% ± 1.2%) and Southern (5.7% ± 1.2%) Hemispheres. This result is also consistent with our analysis of methane and methyl chloroform observations. Atmospheric chemistry simulations suggest that the OH reduction in the Northern Hemisphere is primarily explained by reduced reactive nitrogen emissions during COVID-19 lockdowns and in the Southern Hemisphere partly by enhanced reactive carbon emissions from extreme Australian fires, hence resulting in distinct chemical mechanisms for OH reduction in the two hemispheres. This contrast is further supported by opposite anomalies in hemispheric tropospheric ozone observed by satellite. Our results highlight the critical role of both anthropogenic and natural perturbations in reduced atmospheric oxidation and rapid methane increases in 2020. This has important implications for future scenarios, given the projected decrease in anthropogenic emissions and increase in fire emissions.