CAPRAM reduction towards an operational multiphase halogen and dimethyl sulfide chemistry treatment in the chemistry transport model COSMO-MUSCAT(5.04e)
Authors: Hoffmann Erik H. , Schrödner Roland , Tilgner Andreas , Wolke Ralf , Herrmann Hartmut 
Published: June 2020

Abstract. A condensed multiphase halogen and dimethyl sulfide (DMS)
chemistry mechanism for application in chemistry transport models is
developed by reducing the CAPRAM DMS module 1.0 (CAPRAM-DM1.0) and the
CAPRAM halogen module 3.0 (CAPRAM-HM3.0). The reduction is achieved by
determining the main oxidation pathways from analysing the mass fluxes of
complex 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 marine
boundary layer conditions. Overall, the reduced CAPRAM-DM1.0 contains 32
gas-phase reactions, 5 phase transfers, and 12 aqueous-phase reactions, of
which two processes are described as equilibrium reactions. The reduced
CAPRAM-HM3.0 contains 199 gas-phase reactions, 23 phase transfers, and 87
aqueous-phase reactions. For the aqueous-phase chemistry, 39 processes are
described as chemical equilibrium reactions. A comparison of simulations
using the complete CAPRAM-DM1.0 and CAPRAM-HM3.0 mechanisms against the
reduced ones indicates that the relative deviations are below 5 % for
important inorganic and organic air pollutants and key reactive species
under pristine ocean and polluted conditions. The reduced mechanism has been
implemented into the chemical transport model COSMO-MUSCAT and tested by
performing 2D simulations under prescribed meteorological conditions that
investigate the effect of stable (stratiform cloud) and more unstable
meteorological conditions (convective clouds) on marine multiphase
chemistry. The simulated maximum concentration of HCl is of the order of
109 molecules cm−3 and that of BrO is around
1×107 molecules cm−3, reproducing the range of
ambient measurements. Afterwards, the oxidation pathways of DMS in a cloudy
marine atmosphere have been investigated in detail. The simulations
demonstrate that clouds have both a direct and an indirect photochemical
effect on the multiphase processing of DMS and its oxidation products. The
direct photochemical effect is related to in-cloud chemistry that leads to
high dimethyl
sulfoxide (DMSO) oxidation rates and a subsequently enhanced formation of methane
sulfonic acid compared to aerosol chemistry. The indirect photochemical
effect is characterized by cloud shading, which occurs particularly in the
case of stratiform clouds. The lower photolysis rate affects the activation
of Br atoms and consequently lowers the formation of BrO radicals. The
corresponding DMS oxidation flux is lowered by up to 30 % under thick
optical clouds. Moreover, high updraught velocities lead to a strong vertical
mixing of DMS into the free troposphere predominately under cloudy
conditions. The photolysis of hypohalous acids (HOX, X = Cl, Br, or I) is
reduced as well, resulting in higher HOX-driven sulfite-to-sulfate oxidation
in aerosol particles below stratiform clouds. Altogether, the present model
simulations have demonstrated the ability of the reduced mechanism to be
applied in studying marine aerosol–cloud processing effects in regional
models such as COSMO-MUSCAT. The reduced mechanism can be used also by other
regional models for more adequate interpretations of complex marine field
measurement data.