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. The lockdown measures taken to prevent a rapid spreadingof the coronavirus in Europe in spring 2020 led to large emissionreductions, particularly in road traffic and aviation. Atmosphericconcentrations of NO2 and PM2.5 were mostly reduced whencompared to observations taken for the same time period in previous years;however, concentration reductions may not only be caused by emissionreductions but also by specific weather situations. In order to identify the role of emission reductions and the meteorologicalsituation for air quality improvements in central Europe, the meteorologychemistry transport model system COSMO-CLM/CMAQ was applied to Europe forthe period 1 January to 30 June 2020. Emission data for 2020 wereextrapolated from most recent reported emission data, and lockdown adjustmentfactors were computed from reported activity data changes, e.g. Googlemobility reports. Meteorological factors were investigated throughadditional simulations with meteorological data from previous years. The results showed that lockdown effects varied significantly amongcountries and were most prominent for NO2 concentrations in urban areas with 2-week-average reductions up to 55 % in the second half of March. Ozone concentrations were less strongly influenced (up to ±15 %) andshowed both increasing and decreasing concentrations due to lockdownmeasures. This depended strongly on the meteorological situation and on theNOx / VOC emission ratio. PM2.5 revealed 2 %–12 % reductions of 2-week-average concentrations in March and April, which is much less thana different weather situation could cause. Unusually low PM2.5concentrations as observed in northern central Europe were only marginallycaused by lockdown effects. The lockdown can be seen as a big experiment about air quality improvementsthat can be achieved through drastic traffic emission reductions. From thisinvestigation, it can be concluded that NO2 concentrations can belargely reduced, but effects on annual average values are small when themeasures last only a few weeks. Secondary pollutants like ozone andPM2.5 depend more strongly on weather conditions and show a limitedresponse to emission changes in single sectors.

The role of emission reductions and the meteorological situation for air quality improvements during the COVID-19 lockdown period in central Europe