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. A condensed multiphase halogen and dimethyl sulfide (DMS)chemistry mechanism for application in chemistry transport models isdeveloped by reducing the CAPRAM DMS module 1.0 (CAPRAM-DM1.0) and theCAPRAM halogen module 3.0 (CAPRAM-HM3.0). The reduction is achieved bydetermining the main oxidation pathways from analysing the mass fluxes ofcomplex multiphase chemistry simulations with the air parcel model SPACCIM (SPectral Aerosol Cloud Chemistry Interaction Model).These simulations are designed to cover both pristine and polluted marineboundary layer conditions. Overall, the reduced CAPRAM-DM1.0 contains 32gas-phase reactions, 5 phase transfers, and 12 aqueous-phase reactions, ofwhich two processes are described as equilibrium reactions. The reducedCAPRAM-HM3.0 contains 199 gas-phase reactions, 23 phase transfers, and 87aqueous-phase reactions. For the aqueous-phase chemistry, 39 processes aredescribed as chemical equilibrium reactions. A comparison of simulationsusing the complete CAPRAM-DM1.0 and CAPRAM-HM3.0 mechanisms against thereduced ones indicates that the relative deviations are below 5 % forimportant inorganic and organic air pollutants and key reactive speciesunder pristine ocean and polluted conditions. The reduced mechanism has beenimplemented into the chemical transport model COSMO-MUSCAT and tested byperforming 2D simulations under prescribed meteorological conditions thatinvestigate the effect of stable (stratiform cloud) and more unstablemeteorological conditions (convective clouds) on marine multiphasechemistry. The simulated maximum concentration of HCl is of the order of109 molecules cm−3 and that of BrO is around1×107 molecules cm−3, reproducing the range ofambient measurements. Afterwards, the oxidation pathways of DMS in a cloudymarine atmosphere have been investigated in detail. The simulationsdemonstrate that clouds have both a direct and an indirect photochemicaleffect on the multiphase processing of DMS and its oxidation products. Thedirect photochemical effect is related to in-cloud chemistry that leads tohigh dimethylsulfoxide (DMSO) oxidation rates and a subsequently enhanced formation of methanesulfonic acid compared to aerosol chemistry. The indirect photochemicaleffect is characterized by cloud shading, which occurs particularly in thecase of stratiform clouds. The lower photolysis rate affects the activationof Br atoms and consequently lowers the formation of BrO radicals. Thecorresponding DMS oxidation flux is lowered by up to 30 % under thickoptical clouds. Moreover, high updraught velocities lead to a strong verticalmixing of DMS into the free troposphere predominately under cloudyconditions. The photolysis of hypohalous acids (HOX, X = Cl, Br, or I) isreduced as well, resulting in higher HOX-driven sulfite-to-sulfate oxidationin aerosol particles below stratiform clouds. Altogether, the present modelsimulations have demonstrated the ability of the reduced mechanism to beapplied in studying marine aerosol–cloud processing effects in regionalmodels such as COSMO-MUSCAT. The reduced mechanism can be used also by otherregional models for more adequate interpretations of complex marine fieldmeasurement data.

geoscientific model development

CAPRAM reduction towards an operational multiphase halogen and dimethyl sulfide chemistry treatment in the chemistry transport model COSMO-MUSCAT(5.04e)