Foreign and domestic contributions to surface ozone among European countries

Tropospheric ozone (O₃) exerts strong adverse impacts on human health, climate, vegetation, biodiversity, agricultural crop yields and thus food security. O₃ is formed in the atmosphere through non-linear photochemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx) precursors. Currently, there are no observational methods that differentiate the origin of O₃. Despite their inherent uncertainties, chemical transport models (CTMs) allow for the apportionment of the contribution of any source to O₃ concentrations. The mass-transfer source apportionment method is an optimal approach to study the contribution of different sources to ozone levels.

In this study, we provide a quantitative estimation of the foreign and domestic contributions to ozone among European countries relative to the contribution of hemispheric imported ozone. We use CMAQ-ISAM within the CALIOPE air quality modelling system to simulate the O₃ dynamics over Europe at a 18 x 18 km² horizontal resolution and quantify national contributions for the ozone season from May to October during the years 2015, 2016 and 2017. We tag both O₃ and its precursors, NOx and VOCs, from the different European countries, all the way through their lifetime, from emission to deposition. We discuss the results for 35 European countries, including their ozone contribution to other countries, the role of hemispheric background ozone concentrations in each country, and the changes from one year to another.