MODEL SIMULATIONS ON THE VARIABILITY OF PARTICULATE MSA TO NON-SEA-SALT SULFATE RATIO IN THE MARINE-ENVIRONMENT

A box model was constructed to investigate connections between the par ticulate MSA to non-sea-salt sulfate ratio, R, and DMS chemistry in a clean marine boundary layer. The simulations demonstrated that R varie s widely with particle size, which must be taken into account when int erpreting field measurements or comparing them with each other. In add ition to DMS gasphase chemistry, R in the submicron size range was sho wn to be sensitive to the factors dictating sulfate production via clo ud processing, to the removal of SO2 from the boundary layer by dry de position and sea-salt oxidation, to the entrainment of SO2 from the fr ee troposphere, to the relative concentration of sub- and supermicron particles, and to meteorology. Three potential explanations for the in crease of R toward high-latitudes during the summer were found: larger MSA yields from DMS oxidation at high latitudes, larger DMSO yields f rom DMS oxidation followed by the conversion of DMSO to MSA at high la titudes, or lower ambient H2O2 concentrations at high latitudes leadin g to less efficient sulfate production in clouds. Possible reasons for the large seasonal amplitude of R at mid and high latitudes include s easonal changes in the partitioning of DMS oxidation to the OH and NO3 initiated pathways, seasonal changes in the concentration of species participating the DMS-OH reaction pathway, or the existence of a SO2 s ource other than DMS oxidation in the marine boundary layer. Even smal l anthropogenic perturbations were shown to have a potential to alter the MSA to non-sea-salt sulfate ratio.