Investigating sources of surface ozone in central Europe during the hot summer in 2018: High temperatures, but not so high ozone

The changing emissions and climate have largely influenced the contribution of different sources on surface ozone, while the relations between the emissions, meteorological conditions and ozone sources are highly uncertain. In this study we modeled the sources of surface ozone in central Europe in the summer (June–July–August) of 2018, one of the warmest summers in Europe, with the regional chemical transport model CAMx and its source apportionment tool OSAT. Additional simulations with the anthropogenic emissions in 2000 were performed as high-emission scenarios to investigate the effects of reduced emissions on ozone sources, and a specific analysis on the coupled effects from meteorology was conducted based on measurements and backward trajectory calculations at three Swiss sites. The model results indicate that the highest contribution to summer afternoon surface ozone in Europe in 2018 comes from the boundary import (65%), followed by off-road traffic (11%) and road traffic (8%). Compared to the case using 2000 emissions, the lower emissions in 2018 lead to lower ozone in most of the areas in Europe except for some urban areas and shipping routes, and the highest decrease is from the road traffic for both the whole domain and the Swiss sites. The backward trajectory analysis at Swiss sites in 2000 (base year), 2003 (the warmest year) and 2018 (the 3^rd warmest year) show that 2018 is the year with the highest fraction of air mass from the high ozone areas (from east in southern Switzerland and from north in northern Switzerland). The results indicate that the reduced anthropogenic emissions are the main reason responsible for the reduced high level ozone in the hot summer in central Europe.