MODELING OF THE END-PRODUCTS OF THE CHEMICAL DECOMPOSITION OF DMS IN THE MARINE BOUNDARY-LAYER

A one-dimensional Lagrangian trajectory model (1DIM) with chemistry which includes a detailed description of the marine biogenic sulphur compounds is used to investigate the branching of the end products of the decomposition of DMS (methane sulphonic acid (MSA) and sulphate aerosols). Model runs for Brittany, France are compared with measurements form the OCEANO-NO(x) campaign in June 1989. The parameterization of the biogenic sulphur emissions is uncertain, but the modelled concentrations are in the right range and fit the observed concentrations fairly well. DMS is, however, often out of phase in time with the observed concentrations. This is probably due to a coastal effect linked to the shifting tide, which influences the DMS emissions. The observed ratio of MSA to SO2-/4 is reproduced well. However, methane sulphinic acid (MSEA) concentrations comparable to and some times even higher than MSA are computed. No measured MSEA concentrations support this calculation. The computed end products from DMS in mean for June 1989 at Brittany, France, are distributed with 30% MSA, 25% MSEA, 5% SA and 41% SO2. The distribution between MSA and MSEA, is, however, shown to depend highly on the NO(x) concentrations.