Attributing land transport emissions to ozone and ozone precursors
in Europe and Germany
Authors: Mertens Mariano , Kerkweg Astrid , Grewe Volker , Jöckel Patrick , Sausen Robert 
Published: September 2019

Abstract. Land transport is an important emission source of nitrogen oxides, carbon monoxide and volatile organic compounds, which serves as precursors for tropospheric ozone. Besides the direct negative impact of nitrogen oxides, air quality is also affected by these enhanced ozone tropospheric ozone concentrations. As ozone is radiativly active, its increase contributes to climate change. Due to the strong non-linearity of the ozone chemistry, the contribution of land transport emissions to tropospheric ozone cannot be calculated or measured directly, instead atmospheric-chemistry models equipped with specific source apportionment methods (called tagging) are required. In this study we investigate the contributions of land transport emissions to ozone and ozone precursors using the MECO(n) model system, coupling a global and a regional chemistry climate model, which are equipped with a tagging diagnostic. For the first time the effects of long range transport and regional effects of regional emissions are investigated. This is only possible by applying a tagging method simultaneously and consistently on the global and regional scale. We performed two three-year simulations with different anthropogenic emission inventories for Europe by applying our global model with two regional refinements, i.e. a European nest (50 km resolution) in the global model and a German nest (12 km resolution) in the European nest. We find contributions of land transport emissions to reactive nitrogen (NOy) near ground-level in the range of 5 to 10 nmol mol−1, corresponding to 50 to 70 % of the ground level NOy values. The largest contributions are around Paris, Southern England, Moscow, the Po Valley, and Western Germany. Carbon monoxide contributions range from 30 nmol mol−1 to more than 75 nmol mol−1 near emission hot spots such as Paris or Moscow. The contribution of land transport emissions to ozone show a strong seasonal cycle which absolute contributions of 3 nmol mol−1 during winter and 5 to 10 nmol mol−1 during summer. This corresponds to relative contributions of 8 to 10 % during winter and up to 16 % during summer. Those largest values during summer are confined to the Po Valley, while the contribution in Western Europa ranges from 12 to 14 %. The ozone contributions are robust. Only during summer the ozone contributions are slightly influenced by the emission inventory, but these differences are smaller than the range of the seasonal cycle of the contribution. This cycle is caused by a complex interplay of seasonal cycles of other emissions (e.g. biogenic) and seasonal difference of the ozone regimes. This small difference of the ozone contributions due to the emission inventory is remarkable as the precursor concentrations (NOx and CO) are much more affected by the change. In addition, our results suggest that during events with large ozone values the contribution of land transport emissions and biogenic emissions increase strongly. Here, the contribution of land transport emission peak up to 28 %. Hence, land transport is an important contributor to events of large ozone values.