Global importance of hydroxymethanesulfonate in ambient particulate matter

Abstract

Sulfur compounds are an important constituent of particulate matter, with impacts on climate and public health. While most sulfur observed in particulate matter has been assumed to be sulfate, laboratory experiments reveal that hydroxymethanesulfonate (HMS), an adduct formed by aqueous phase chemical reaction of dissolved HCHO and SO2, may be easily misinterpreted in measurements as sulfate. Here we present observational and modeling evidence for a ubiquitous global presence of HMS. We find that filter samples collected in Shijiazhuang, China, and examined with ion chromatography within 9 days show as much as 7.6 mu g m(-3)of HMS, while samples from Singapore examined 9-18 months after collection reveal similar to 0.6 mu g m(-3)of HMS. The Shijiazhuang samples show only minor traces of HMS 4 months later, suggesting that HMS had decomposed over time during sample storage. In contrast, the Singapore samples do not clearly show a decline in HMS concentration over 2 months of monitoring. Measurements from over 150 sites, primarily derived from the IMPROVE network across the United States, suggest the ubiquitous presence of HMS in at least trace amounts as much as 60 days after collection. The degree of possible HMS decomposition in the IMPROVE observations is unknown. Using the GEOS-Chem chemical transport model, we estimate that HMS may account for 10% of global particulate sulfur in continental surface air and over 25% in many polluted regions. Our results suggest that reducing emissions of HCHO and other volatile organic compounds may have a co-benefit of decreasing particulate sulfur.Plain Language Summary Sulfur molecules are a major part of the tiny particles in air that are both a significant cause of death worldwide and exert a large influence on climate. Scientists have previously assumed that the sulfur in particle air pollution is in the form of sulfate. However, recent work has shown that a different sulfur molecule called hydroxymethanesulfonate may be confused for sulfate in the machines used for determining the chemical composition of particles. By reanalyzing observations of particle chemical composition and conducting new observations we find a significant global presence of hydroxymethanesulfonate. Using a computer simulation of global air pollution chemistry, we find that hydroxymethanesulfonate may comprise more than one quarter of sulfur in particles in many polluted regions. The computer simulation also indicates that in multiple regions the formation of hydroxymethanesulfonate is controlled by levels of formaldehyde. Controlling emissions of formaldehyde or emissions of the volatile organic compounds that can create formaldehyde in the atmosphere can therefore have the added co-benefit of also reducing the levels of sulfur in particle air pollution.