The COVID-19 pandemic led to widespread reductions in mobility and induced observable changes in the atmosphere. Recent work has employed novel mobility datasets as a proxy for trace gas emissions from traffic, yet there has been little work evaluating these emission numbers. Here, we systematically compare these mobility datasets to traffic data from local governments in seven diverse urban and rural regions to characterize the magnitude of errors in emissions that result from using the mobility data. We observe differences in excess of 60% between these mobility datasets and local traffic data, which result in large errors in emission estimates. We could not find a general functional relationship between mobility data and traffic flow over all regions. Future work should be cautious when using these mobility metrics for emission estimates. Further, we use the local government data to identify emission reductions from traffic in the range of 7-22% in 2020 compared to 2019.

This study estimates the influence of anthropogenic emission reductions on nitrogen dioxide (NO_2) and ozone (O_3) concentration changes during the COVID-19 pandemic period using in-situ surface and Sentinel-5p (TROPOMI) satellite column measurements and GEOS-Chem model simulations. We show that, as a result of reductions in anthropogenic emission in eight German metropolitan cities, meteorology corrected mean in-situ (\& column) NO_2(2020,corr) concentrations decreased by 23 ± 4.7 % (& 16.4 ± 7.2 %) between March 21 and June 30, 2020, whereas meteorology corrected mean in-situ O_3(2020,corr) concentration increased by 4 ± 8.8 % between March 21 and May 31, 2020, and decreased by 3 ± 8.7 % in June 2020, compared to 2019 (uncertainty represents the 1 σ of mean changes of eight metropolitan cities). The impacts of meteorology on in-situ and TROPOMI NO_2 concentration changes during the lockdown compared to 2019 are relatively small (+0.4 % and -0.6 %, respectively), while those on in-situ O_3 concentration changes are more significant (+3.6 % and -13.5 % for March 21 to May 31, 2020 and June 2020, respectively). A NO_X saturated ozone production regime in German metropolitan cities in March to May explains the increased O_3(2020,corr) concentration in response to the decreased NO_2(2020,corr) concentration. This implies that future reductions in NO_X emissions are likely to increase ozone pollution in these cities if appropriate mitigation measures are not implemented. TROPOMI NO_2(2020,corr) concentrations decreased nationwide during the stricter lockdown period, except for North-West Germany, which can be attributed to enhanced NO_X emissions from agricultural soils.