PNNL

Abstract

Detailed chemical speciation of the dry residue particles from individual cloud droplets and interstitial aerosol collected during the Marine Stratus Experiment (MASE) was performed using a complementary combination of microanalysis techniques. Techniques include computer controlled scanning electron microscopy with energy dispersed analysis of X-rays (CCSEM/EDX), time-of-flight secondary ionization mass spectrometry (TOFSIMS), and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). Samples were collected at the ground site located in Point Reyes National Seashore, approximately 1 km from the coast. This manuscript focuses on the analysis of individual particles sampled from an air mass that originated over the open ocean and then passed through the area of the California current located along the northern California coast. Based on composition, morphology, and chemical bonding information, two externally mixed, distinct classes of sulfur containing particles were identified: chemically modified (aged) sea salt particles and secondary formed sulfate particles. The results indicate substantial heterogeneous replacement of chloride by methanesulfonate (CH3SO3 -) and non-sea salt sulfate (nss-SO4 2-) in sea-salt particles with the characteristic ratios of CH3SO3 -/nss-SO4 2-> 0.6. Although this value seems too high for a mid-latitude site, our model calculations suggest that high CH3SO3 -/nss-SO4 2- ratios are expected during the early stages of dimethyl sulfide (DMS) oxidation when CH3SO3H forms more rapidly than H2SO4.