Tropospheric NO₂ and O₃ Response to COVID-19 Lockdown Restrictions at the National and Urban Scales in Germany

This study estimates the influence of anthropogenic emission reductions on nitrogen dioxide ( ${NO}_2$ ) and ozone ( $O_3$ ) concentration changes in Germany during the COVID-19 pandemic period using in-situ surface and Sentinel-5 Precursor TROPOspheric Monitoring Instrument (TROPOMI) satellite column measurements and GEOS-Chem model simulations. We show that reductions in anthropogenic emissions in eight German metropolitan areas reduced mean in-situ (& column) ${NO}_2$ concentrations by 23 $\%$ (& 16 $\%$ ) between March 21 and June 30, 2020 after accounting for meteorology, whereas the corresponding mean in-situ $O_3$ concentration increased by 4 $\%$ between March 21 and May 31, 2020, and decreased by 3 $\%$ in June 2020, compared to 2019. In the winter and spring, the degree of ${NO}_X$ saturation of ozone production is stronger than in the summer. This implies that future reductions in ${NO}_X$ emissions in these metropolitan areas are likely to increase ozone pollution during winter and spring if appropriate mitigation measures are not implemented. TROPOMI ${NO}_2$ concentrations decreased nationwide during the stricter lockdown period after accounting for meteorology with the exception of North-West Germany which can be attributed to enhanced ${NO}_X$ emissions from agricultural soils.