Chemical Speciation of Sulfur in Marine Cloud Droplets and Particles: 2. Partitioning of Methanesulfonate and non-Sea Salt Sulfate in Individual Sea Salt Particles.

The gas phase precursor for sulfate aerosol over the oceans is dimethyl sulfide (DMS). DMS is emitted to the atmosphere by phytoplankton and oxidized, yielding both intermediate methane sulfonate (CH3SO3-) and final non-sea salt sulfate (nss-SO42-) products. Knowledge about the CH3SO3- to nss-SO42- partitioning in the sea salt particles over specific geographic locations is important for understanding DMS oxidation chemistry and its possible effects on clouds, i.e. the potential for new particle formation versus growth of existing droplets and particles. Unambiguous, quantitative assessment of CH3SO3- /nss-SO42- ratios in individual sea salt particles has been made possible using combined data sets from two analytical techniques: computer controlled scanning electron microscopy with energy dispersed analysis of X-rays (CCSEM/EDX) and scanning transmission x-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). In this presentation, we report the particle size specific data on the nss-S/Na and the CH3SO3- /nss-SO4^{2- } ratios measured in dry residues of marine cloud droplets and particles collected during the Marine Stratus Experiment (MASE) in July 2005. Characteristic ratios of nss-S/Na > 0.10 are reported for sea salt particles, with higher values for small particles indicating extensive formation of sulfur containing salts in small particles. Characteristic ratios of CH3SO3- /nss-SO42- > 0.70 are reported with higher values for large particles, indicating the higher capacity for CH3SO3- (lower conversion to SO42- ) for large particles. To the best of our knowledge, this is the first time that CH3SO3- /nss-SO42- have been quantitatively reported based on the individual particle measurements.