Chemical Speciation of Sulfur in Marine Cloud Droplets and Particles: 1. Analysis of Individual Particles Using Complementary Microprobe Methods.

Chemical speciation of dry residues of individual cloud droplets and interstitial aerosol collected from sea-fog during the Marine Stratus Experiment (MASE) in July 2005 was facilitated using a complementary combination of computer controlled scanning electron microscopy with energy dispersed analysis of x-rays (CCSEM/EDX), time-of-flight secondary ionization mass spectrometry (TOF-SIMS), and scanning transmission x-ray microscopy with near edge x-ray absorption fine structure spectroscopy (STXM/NEXAFS). Particle samples were collected at the ground site located in Pt. Reyes National Seashore, about 0.5 miles from the ocean coast over the period of time when the air plume, that originated over the open ocean, passed the area of the cold stream along the northern California coast. Based on composition, morphology, and microstructure, two externally mixed, distinct types of sea-fog particles were identified in the samples: chemically modified (aged) sea salt particles and secondary formed sulfate particles. The results indicate excessive formation of methanesulfonate (CH3SO3-) rather then non-sea-salt sulfate (nss-SO42-) in the sea salt particles. This observation is consistent with the recent modeling studies of dimethylsulfide (DMS) oxidation chemistry in the marine boundary layer (MDL). Modeling studies predict enhanced formation of CH3SO3- in activated sea salt particles under cloudy MBL conditions over the areas with low ocean surface temperatures. We discuss the climate related effects of this chemistry which likely results in: a) increasing size and hygroscopicity of the pre-existing CCN (sea salt particles), and b) reducing the production of gaseous H2SO4 and subsequent new sulfate particle formation.