PM_2.5 during severe winter haze in Beijing, China, has reached levels as high as 880 μg/m³, with sulfur compounds contributing significantly to PM_2.5 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.

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 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.

Formaldehyde (HCHO), the simplest and most abundant carbonyl in the atmosphere, contributes to particulate matter (PM) formation via two in-cloud processing pathways. First, in a catalytic pathway, HCHO reacts with hydrogen peroxide (H₂O₂) to form hydroxymethyl hydroperoxide (HMHP), which rapidly oxidizes dissolved sulfur dioxide (SO_2,aq) to sulfate, regenerating HCHO. Second, HCHO reacts with dissolved SO_2,aq to form hydroxymethanesulfonate (HMS), which upon oxidation with the hydroxyl radical (OH) forms sulfate and also reforms HCHO. Chemical transport model simulations using rate coefficients from laboratory studies of the reaction rate of HMHP with SO_2,aq show that the HMHP pathways reduce the SO₂ lifetime by up to a factor of 2 and contribute up to ∼18% of global sulfate. This contribution rises to >50% in isoprene-dominated regions such as the Amazon. Combined with recent results on HMS, this work demonstrates that the one-carbon molecules HMHP and HCHO contribute significantly to global PM, with HCHO playing a crucial catalytic role. Significance Particulate matter, often formed via cloud processing, strongly influences the Earth’s climate and air quality. Particle composition depends on anthropogenic and biogenic emissions. Thus, in order to understand climate change, knowledge of the difference between preindustrial and current conditions is critical. Under preindustrial conditions, multifunctional organic hydroperoxides, which are strong oxidants and have the ability to contribute to particulate matter formation, are in higher concentrations in the atmosphere. In this work, we focus on the previously unknown importance of hydroxymethyl hydroperoxide, which can be formed by gas-phase reactions and in-cloud reaction of hydrogen peroxide with the simplest aldehyde, formaldehyde, revealing the catalytic role of formaldehyde, and demonstrate that this chemistry is of great importance for particle formation.

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 ${\mathrm{H}}_x{\mathrm{SO}}_y^{+}$ 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.

In-cloud chemistry has important ramifications for atmospheric particulate matter formation and gas-phase chemistry. Recent work has shown that, like hydrogen peroxide (H₂O₂), the two main isomers of isoprene hydroxyl hydroperoxide (ISOPOOH) oxidize sulfur dioxide dissolved in cloud droplets (SO_2,aq) to sulfate. The work revealed that the pathway of SO_2,aq oxidation with ISOPOOH differs from that of H₂O₂. We investigate the chemical mechanisms of oxidation of SO_2,aq with ISOPOOH in the cloud-relevant pH range of 3–6 and compare them with the previously reported mechanisms of oxidation of SO_2,aq with H₂O₂, methyl hydroperoxide and peroxyacetic acid. The organic products of the reaction are identified, and two pathways are proposed. For 1,2-ISOPOOH, a higher yield pathway via proposed radical intermediates yields methyl vinyl ketone (MVK) and formaldehyde, which can react to hydroxymethanesulfonate (HMS) when SO_2,aq is present. A lower yield non-fragmentation oxygen addition pathway is proposed that results in the formation of isoprene-derived diols (ISOPOH). Based on global simulations, this mechanism is not a significant pathway for formation of MVK and formaldehyde relative to their gas-phase formation but, as previously reported, it can be regionally important for sulfate production. The study adds to previous work that highlights similarities and differences between gas-phase and cloud-droplet processing of reactive organic carbon.