Measurement techniques for identifying and quantifying hydroxymethanesulfonate (HMS) in an aqueous matrix and particulate matter using aerosol mass spectrometry and ion chromatography

Abstract

Abstract. Oxidation of sulfur dioxide\n( SO2 ) in the gas phase and in cloud and fog\nwater leads to the formation of sulfate that contributes to ambient\nparticulate matter (PM). For severe haze events with low-light conditions,\ncurrent models underestimate the levels of sulfate formation that occur\nexclusively via the oxidation of sulfur dioxide. We show here that\nmeasurement techniques commonly used in the field to analyze PM composition\ncan fail to efficiently separate sulfur-containing species, resulting in\nthe possible misidentification of compounds. Hydroxymethanesulfonate (HMS), a\nsulfur(IV) species that can be present in fog and cloud water, has been\nlargely neglected in both chemical models and field measurements of PM\ncomposition. As HMS is formed without oxidation, it represents a pathway for\n SO2 to contribute to PM under low-light\nconditions. In this work, we evaluate two techniques for the specific\nquantification of HMS and sulfate in PM, ion chromatography (IC) and aerosol\nmass spectrometry (AMS). In cases in which the dominant sulfur-containing\nspecies are ammonium sulfate or HMS, differences in AMS fragmentation\npatterns can be used to identify HMS. However, the AMS quantification of HMS\nin complex ambient mixtures containing multiple inorganic and organic sulfur\nspecies is challenging due to the lack of unique organic fragments and\nthe variability of fractional contributions of\n H x SO y + ions\nas a function of the matrix. We describe an improved IC method that provides\nefficient separation of sulfate and HMS and thus allows for the identification and\nquantification of both. The results of this work provide a technical\ndescription of the efficiency and limitations of these techniques as well as\na method that enables further studies of the contribution of S(IV)\nversus S(VI) species to PM under low-light atmospheric conditions.