PM2.5 during severe winter haze in Beijing, China, has reached levels as high as 880 μg/m3, with sulfur compounds contributing significantly to PM2.5 composition. This sulfur has been traditionally assumed to be sulfate, although atmospheric chemistry models are unable to account for such large sulfate enhancements under dim winter conditions. Using a 1‐D model, we show that well‐characterized but previously overlooked chemistry of aqueous‐phase HCHO and S(IV) in cloud droplets to form a S(IV)‐HCHO adduct, hydroxymethane sulfonate, may explain high particulate sulfur in wintertime Beijing. We also demonstrate in the laboratory that methods of ion chromatography typically used to measure ambient particulates easily misinterpret hydroxymethane sulfonate as sulfate. Our findings suggest that HCHO and not SO2 has been the limiting factor in many haze events in Beijing and that to reduce severe winter pollution in this region, policymakers may need to address HCHO sources such as transportation.

Air pollution is a major risk factor for human health. Chemical reactions in the epithelial lining fluid (ELF) of the human respiratory tract result in the formation of reactive oxygen species (ROS), which can lead to oxidative stress and adverse health effects. We use kinetic modeling to quantify the effects of fine particulate matter (PM2.5), ozone (O 3 ), and nitrogen dioxide (NO 2 ) on ROS formation, interconversion, and reactivity, and discuss different chemical metrics for oxidative stress, such as cumulative production of ROS and hydrogen peroxide (H 2 O 2 ) to hydroxyl radical (OH) conversion. All three air pollutants produce ROS that accumulate in the ELF as H 2 O 2 , which serves as reservoir for radical species. At low PM2.5 concentrations (<10 μg m −3 ), we find that less than 4% of all produced H 2 O 2 is converted into highly reactive OH, while the rest is intercepted by antioxidants and enzymes that serve as ROS buffering agents. At elevated PM2.5 concentrations (>10 μg m −3 ), however, Fenton chemistry overwhelms the ROS buffering effect and leads to a tipping point in H 2 O 2 fate, causing a strong nonlinear increase in OH production. This shift in ROS chemistry and the enhanced OH production provide a tentative mechanistic explanation for how the inhalation of PM2.5 induces oxidative stress and adverse health effects.

The oxidation of sulfur dioxide (SO2) by peroxides leads to the formation of sulfate in cloudwater, contributing to particulate matter (PM) formation. The reaction with hydrogen peroxide (H2O2) is considered to be the main cloud oxidation pathway. Previous studies have examined the oxidation of SO2 in cloudwater by small organic peroxides with one functional group; however, oxidation by multifunctional organic hydroperoxides, which are expected to have higher water solubility and reactivity, has not been examined. We investigate the aqueous oxidation of SO2 by the two main isomers of isoprene hydroxyl hydroperoxide (ISOPOOH), the primary low-NO x isoprene oxidation products in the atmosphere. Having large Henry’s law constants and being among the most abundant multifunctional hydroperoxides, they are among the most important organic hydroperoxides present in clouds. The pH dependence of the reactions was investigated at cloud relevant pH of 3–6, and the results reveal their importance compared to the oxidation of SO2 via H2O2. Model simulations in GEOS-Chem, updated with the chemistry described herein, highlight the importance of these pathways for sulfate formation in regions with high isoprene emissions and low-NO x atmospheric conditions, especially if they maintain significant SO2 emissions.

Abstract. Oxidation of sulfur dioxide (SO2) in the gas phase and in cloud and fog water leads to the formation of sulfate that contributes to ambient particulate matter (PM). For severe haze events with low-light conditions, current models underestimate the levels of sulfate formation that occur exclusively via the oxidation of sulfur dioxide. We show here that measurement techniques commonly used in the field to analyze PM composition can fail to efficiently separate sulfur-containing species, resulting in the possible misidentification of compounds. Hydroxymethanesulfonate (HMS), a sulfur(IV) species that can be present in fog and cloud water, has been largely neglected in both chemical models and field measurements of PM composition. As HMS is formed without oxidation, it represents a pathway for SO2 to contribute to PM under low-light conditions. In this work, we evaluate two techniques for the specific quantification of HMS and sulfate in PM, ion chromatography (IC) and aerosol mass spectrometry (AMS). In cases in which the dominant sulfur-containing species are ammonium sulfate or HMS, differences in AMS fragmentation patterns can be used to identify HMS. However, the AMS quantification of HMS in complex ambient mixtures containing multiple inorganic and organic sulfur species is challenging due to the lack of unique organic fragments and the variability of fractional contributions of HxSOy+ ions as a function of the matrix. We describe an improved IC method that provides efficient separation of sulfate and HMS and thus allows for the identification and quantification of both. The results of this work provide a technical description of the efficiency and limitations of these techniques as well as a method that enables further studies of the contribution of S(IV) versus S(VI) species to PM under low-light atmospheric conditions.

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 SO 2 , 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 μg m −3 of HMS, while samples from Singapore examined 9-18 months after collection reveal~0.6 μg 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.