Relative importance of organic coatings for the heterogeneous hydrolysis of N₂O₅ during summer in Europe

The heterogeneous reaction probability γ_N2O5 of N ₂O₅ depends largely on the aerosol chemical composition. Several recent laboratory studies have shown that the presence of organic coatings on aqueous aerosols can suppress heterogeneous N₂O ₅ hydrolysis. In this study we investigated the relative importance of organic coatings, formed via gas-toparticle conversion on aqueous aerosols, to N₂O₅ hydrolysis on local and regional scales during summer in Europe. We present results of one-dimensional process studies and of regional-scale model simulations for Europe with the comprehensive model system COSMO-ART. To treat N₂O₅ hydrolysis, we used recent results from laboratory studies that quantified γ_N2O5 on the basis of the organic coating thickness. The simulations showed that during any episode the conditions for formation of N₂O₅ and secondary organic compounds were very variable and depended strongly on the meteorological conditions. In regions of the model domain where both components were built-up, the formation of organic coatings could decrease particulate nitrate concentrations by up to 90%. Where these conditions were not met, the impact of the organic coating was negligible.