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 involving volatile organic compounds (VOCs) and nitrogen oxides (NOx) precursors [1]. Furthermore, meteorological stagnation, high solar radiation, high temperatures and low precipitation favor the formation of tropospheric O3 at surface levels exceeding target regulatory values [2]. Due to the complex and poorly constrained physico-chemical O3 formation and removal pathways, no straightforward strategies currently exist for reducing O3. 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 [2]. In this study, we provide a quantitative estimation of the foreign and domestic contributions to surface ozone on Spain, relative to European countries and the contribution of hemispheric background ozone. For that, we use the CMAQISAM within the CALIOPE air quality modelling system to simulate the O3 dynamics over Europe quantify national contributions for the ozone season from May to October in 2015. 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
