Source attribution of European surface O 3 using a tagged O 3 mechanism

Abstract. Tropospheric ozone (O 3 ) is an important air pollutant that affects human health, ecosystems and climate. The contributions of O 3 precursor emissions from different geographical source regions to the O 3 concentration can help to quantify the effects of local versus remote transported precursors on the O 3 concentration in a certain area. This study presents a “tagging” approach within the WRF-Chem model that attributes O 3 concentration in several European receptor regions to nitrogen oxides (NO x ) emissions from within and outside of Europe during April–September 2010.We also examine the contribution of these different precursor sources to various O 3 metrics, and their exceedance events. Firstly, we show that the spatial distributions of simulated monthly mean MDA8 from tagged O 3 sources regions and types for late spring, summer and early autumn 2010 varies with season. For summer conditions O 3 production is dominated by national and intra-European sources, while in the late spring and early autumn intercontinental transported O 3 is an important contributor to the total O 3 concentration. We have also identified shipping activities in the Mediterranean Sea as an important source of O 3 for the Mediterranean countries, as well as the main contributor to high MDA8 O 3 concentration, modelled in the Mediterranean basin itself. Secondly, to have a better understanding of the origin of MDA8 O 3 exceedances, we compare modelled and observed values of MDA8 O 3 concentration in the “Alps” and “Germany-Benelux” receptor regions, revealing that the contribution from local sources is about 45 % and 38 % of modeled MDA8 O 3 during the exceedances days respectively. By examining the relative contributions of remote NO x sources to modelled and observed O 3 exceedance events, we determine that model underrepresentation of long-range O 3 transport could be contributing to a general underestimation of modelled O 3 exceedance events in the Germany-Benelux receptor region. Thirdly, we quantify the impact of local vs. non local NO x precursors on O 3 production for each European receptor region using different O 3 metrics. The comparison between mean, MDA8 and 95th percentile O 3 metrics accentuate the importance of large contributions from locally-emitted NO x precursors to the high-end of the O 3 distribution. When we compare the vegetation and health metrics, we notice that the SOMO35 and AOT40 indexes exhibit a rather similar behaviour, while the W126 index accentuates the importance of local emissions. Overall, this study highlights the importance of a tagging approach to quantify the contribution of local and remote sources to the MDA8 O 3 concentration during several periods as well to different O 3 metrics. Moreover, this method could be applied to assess different mitigation options.