An advanced scheme for wet scavenging and liquid-phase chemistry in a regional online-coupled chemistry transport model

Clouds are reaction chambers for atmospheric trace gases and aerosols, and the associated precipitation is a major sink for atmospheric constituents. The regional chemistry-climate model COSMO-ART has been lacking a description of wet scavenging of gases and aqueous-phase chemistry. In this work we present a coupling of COSMO-ART with a wet scavenging and aqueous-phase chemistry scheme. The coupling is made consistent with the cloud microphysics scheme of the underlying meteorological model COSMO. While the choice of the aqueous-chemistry mechanism is flexible, the effects of a simple sulfur oxidation scheme are shown in the application of the coupled system in this work. We give details explaining the coupling and extensions made, then present results from idealized flow-over-hill experiments in a 2-D model setup and finally results from a full 3-D simulation. Comparison against measurement data shows that the scheme efficiently reduces SO<sub>2</sub> trace gas concentrations by 0.3 ppbv (−30%) on average, while leaving O<sub>3</sub> and NO<sub>x</sub> unchanged. PM<sub>10</sub> aerosol mass was increased by 10% on average. While total PM<sub>2.5</sub> changes only little, chemical composition is improved notably. Overestimations of nitrate aerosols are reduced by typically 0.5–1 μg m<sup>−3</sup> (up to −2 μg m<sup>−3</sup> in the Po Valley) while sulfate mass is increased by 1–1.5 μg m<sup>−3</sup> on average (up to 2.5 μg m<sup>−3</sup> in Eastern Europe). The effect of cloud processing of aerosols on its size distribution, i.e. a shift towards larger diameters, is observed. Compared against wet deposition measurements the system tends to underestimate the total wet deposited mass for the simulated case study