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

• Published in: Geoscientific Model Development 2020-06, Vol.13 (6), p.2587-2609
• Main Authors: Hoffmann, Erik H, Schrödner, Roland, Tilgner, Andreas, Wolke, Ralf, Herrmann, Hartmut
• Format: Article
• Language: English
• Subjects: Aerosol chemistry; Aerosol clouds; Aerosol particles; Aerosols; Air parcels; Air pollution; Air-sea interaction; Analysis; Atmospheric chemistry; Atmospheric models; Boundary conditions; Boundary layers; Chemical equilibrium; Chemical oceanography; Chemical reactions; Chemical transport; Chemistry; Cloud chemistry; Clouds; Clouds (Meteorology); Coasts; Computer simulation; Convective clouds; Dimethyl sulfide; Dimethyl sulfoxide; Emissions; Fluxes; Halogens; Humidity; Interaction models; Investigations; Iodine; Marine aerosols; Marine chemistry; Marine pollution; Meteorological conditions; Methane; Model testing; Modules; Multiphase; Oceans; Oxidation; Oxidation-reduction reactions; Photochemicals; Photochemistry; Photolysis; Pollutants; Pollution; Reduction; Shading; Simulation; Stratiform clouds; Sulfates; Sulfides; Sulfite; Sulfites; Sulfonic acid; Sulfur oxides; Sulphides; Transport; Troposphere; Two dimensional models; Vertical mixing; VOCs; Volatile organic compounds; Weather
• ISSN: 1991-9603; 1991-959X; 1991-962X; 1991-9603; 1991-962X
• DOI: 10.5194/gmd-13-2587-2020

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 D