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 wh ich includes a detailed description of the marine biogenic sulphur com pounds is used to investigate the branching of the end products of the decomposition of DMS (methane sulphonic acid (MSA) and sulphate aeros ols). Model runs for Brittany, France are compared with measurements f orm the OCEANO-NO(x) campaign in June 1989. The parameterization of th e biogenic sulphur emissions is uncertain, but the modelled concentrat ions are in the right range and fit the observed concentrations fairly well. DMS is, however, often out of phase in time with the observed c oncentrations. 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 calculatio n. The computed end products from DMS in mean for June 1989 at Brittan y, France, are distributed with 30% MSA, 25% MSEA, 5% SA and 41% SO2. The distribution between MSA and MSEA, is, however, shown to depend hi ghly on the NO(x) concentrations.