Sources of Surface O<sub>3</sub> in the UK: Tagging O<sub>3</sub> within WRF-Chem

Abstract. Tropospheric ozone (O3) concentrations are known to depend on a combination of hemispheric, regional, and local-scale processes. Estimates of how much O3 is produced locally vs. transported from further afield are essential in air quality management and regulatory policies. Here, a tagged-ozone mechanism within the WRF-Chem model is used to quantify the contributions to surface O3 in the UK from anthropogenic nitrogen oxide (NOx) emissions from inside and outside the UK during May–August 2015. The contribution of the different source regions to three regulatory O3 metrics is also examined. It is shown that model simulations predict the concentration and spatial distribution of domain-wide surface O3 with a mean bias of -3.7 ppbv. Anthropogenic NOx emissions from the UK and Europe account respectively for 13 % and 16 % of the monthly mean surface O3 in the UK, as the majority (71 %) of O3 comes from the hemispheric background. The north and the west of the UK experience the largest contributions from hemispheric O3 with peaks in May, whereas European and UK contributions are most significant in the east and south-east, intensifying towards June and July. It is demonstrated that more stringent emission controls over continental Europe, particularly in western Europe, would be necessary to improve health-related metrics, such as MDA8 O3 above 50 and 60 ppbv. Emission controls over larger areas, e.g., the northern hemisphere, are instead required to lessen the impacts on ecosystems as quantified by metrics such as the AOT40.