Contribution of hydroxymethanesulfonate (HMS) to severe winter haze in the North China Plain

Severe winter hazes accompanied by high concentrations of fine particulate matter (PM2.5) occur frequently in the 20 North China Plain and threaten public health. Organic matter (OM) and sulfate are recognized as major components of PM2.5, while atmospheric models often fail to predict their high concentrations during severe winter hazes due to incomplete understanding of secondary aerosol formation mechanisms. By using a novel combination of single particle mass spectrometer and optimized ion chromatography measurement, here we show that hydroxymethanesulfonate (HMS), formed by the reaction between formaldehyde (HCHO) and dissolved SO2 in aerosol water, is ubiquitous in Beijing winter. The HMS concentration 25 and the molar ratio of HMS to sulfate increased with the deterioration of winter haze. High concentrations of precursors (SO2 and HCHO) coupled with low oxidant levels, low temperature, high relative humidity, and moderately acid pH facilitate the heterogeneous formation of HMS, which could account for up to 15% of OM in winter haze and lead to 36% overestimates of sulfate when using traditional ion chromatography measurements. Despite the clean air actions have substantially reduced SO2 emissions, HMS concentration and molar ratio of HMS to sulfate during severe winter hazes increased from 2015 to 2016 with 30 the growth of HCHO concentration. Our findings illustrate the significant contribution of heterogeneous HMS chemistry to severe winter hazes in Beijing, which help to improve the prediction of OM and sulfate, and suggest that the reduction in HCHO can help to mitigate haze pollution. https://doi.org/10.5194/acp-2020-113 Preprint. Discussion started: 13 February 2020 c © Author(s) 2020. CC BY 4.0 License.


Introduction
Severe winter haze pollution with high PM2.5 (particles with aerodynamic diameter ≤ 2.5 μm) concentration occurs frequently 35 in the North China Plain (NCP), exerting adverse impacts on the environment and human health (Huang et al., 2014;Lelieveld et al., 2015). Secondary components, constituting a large fraction of PM2.5, are key drivers of haze formation (Huang et al., 2014), however, atmospheric models with known formation mechanisms often fail to predict high levels of secondary organic matter (OM) and sulfate during severe winter hazes.
Traditional models with gas-phase photochemical mechanisms and aqueous chemistry involving glyoxal and methylglyoxal 40 significantly underestimate the high OM levels observed in NCP winter (Wang et al., 2014;Zheng et al., 2015a). Adding heterogeneous reactions involving isoprene epoxide, glyoxal, and methylglyoxal, and accounting for organic aerosol aging and oxidation of intermediate-volatility organic compounds can improve the model predictions of OM (Hu et al., 2017;Zhao et al., 2016;Yang et al., 2018), however, high OM concentrations observed in Beijing winter are still underpredicted (Hu et al., 2017), especially during the periods with low oxidant concentrations and weak photochemical activity. 45 In addition to OM, high levels of particulate sulfate are often observed in NCP winter and it increases sharply with increasing PM2.5 pollution levels (Zheng et al., 2015b). Traditional atmospheric models containing both gas-phase oxidation of SO2 by OH radicals and aqueous-phase reaction pathways involving H2O2, O3, and O2 catalyzed by Fe 3+ and Mn 2+ fail to reproduce the observed high sulfate levels, and revised models with heterogeneous chemistry greatly improve the sulfate simulation (Wang et al., 2014;Zheng et al., 2015a). Cheng et al. and Wang et al. reported that the oxidation of SO2 by NO2 in aerosol 50 water under high pH could explain the difference between modeled and observed sulfate (Cheng et al., 2016;Wang et al., 2016). On the other hand, the misidentification of organosulfur compounds as inorganic sulfate in conventional measurements can lead to overestimation in the observed particulate sulfate (Chen et al., 2019). Recently, Moch et al. and Song et al. reported the potential contribution of hydroxymethanesulfonate (HMS, HOCH2SO3 − ) to particulate sulfur during winter haze in Beijing (Moch et al., 2018;Song et al., 2019). However, there is no direct evidence for the content of HMS in haze particles, and the 55 formation mechanism of HMS in NCP winter hazes is still unclear.
HMS was previously found at appreciable concentrations in cloud and fog (Munger et al., 1986;Rao and Collett, 1995), whereas the HMS concentrations observed in atmospheric aerosols in the United States, Germany, and Japan were low (Dixon and Aasen, 1999;Scheinhardt et al., 2014;Suzuki et al., 2001). If HMS does play a role in NCP winter haze, accurate identification and quantification of aerosol HMS are essential. Low oxidant concentrations, high water content, moderate pH, 60 and low temperatures for typical cloud and fog environments together with the presence of SO2 and formaldehyde (HCHO) favor HMS formation (Deister et al., 1986;Boyce and Hoffmann, 1984;Kok et al., 1986;Lagrange et al., 1999). Such conditions are common during severe winter hazes in the NCP, such as Beijing (Zheng et al., 2015b;Cheng et al., 2016;Rao et al., 2016).
In this study, combining aerosol time-of-flight mass spectrometer (ATOFMS) measurement and optimized ion chromatography 65 method, we identified the ubiquity of HMS in Beijing winter and quantified its contribution to severe winter hazes. We https://doi.org/10.5194/acp-2020-113 Preprint. Discussion started: 13 February 2020 c Author(s) 2020. CC BY 4.0 License. demonstrated that the reaction between HCHO and dissolved SO2 to form HMS in aerosol water was an important pathway that contributed to winter haze pollution in Beijing, not only contributing a substantial mass of OM but also leading to overestimation of sulfate in conventional measurements. High concentrations of precursors (i.e., SO2 and HCHO) coupled with appropriate conditions (i.e., low oxidants, low temperature, high relative humidity, and moderately acid pH) in severe 70 winter hazes favored the heterogeneous HMS formation. Furthermore, two-year continuous winter measurements from 2015 to 2016 indicated that HMS concentrations increased with the increase of HCHO. Finally, we discussed the implications of heterogeneous HMS chemistry for haze chemistry and control strategies. Details of the measurements and analysis are described below. Ambient aerosols between 100 nm and 3000 nm enter the ATOFMS through an aerodynamic focusing lens and are accelerated 85 to their size-dependent terminal velocities. The particle velocity is determined by measuring the time-of-flight between two solid state green lasers (λ = 532 nm, 50 mW, CL532-050-L, CrystaLaser, NV, USA). The particle size is calculated from the measured velocity based on the calibration curve between particle size and velocity. In addition, the velocity is used to trigger the 266 nm Nd:YAG laser (~1 mJ/pulse), which desorbs and ionizes the particle. The generated positive and negative ions are detected using a bipolar reflectron ToF-MS. The ATOFMS aerodynamic sizing was calibrated by standard polystyrene 90 latex spheres (PSL) with different sizes (d = 151,199,269,350,499,and 799 nm,Duke Scientific Corp.,USA). Mass calibration of ATOFMS was conducted with the standard solution (Ba, K, Pb, Na, Li, V in HNO3).

ATOFMS measurement and data analysis
During the winter campaign, ATOFMS detected 4,495,233 particles containing both size and chemical information, accounting for 49% of all sized particles. Single particle mass spectrometers identify the peak at m/z −111 as HMS (Whiteaker and Prather, negative ion mass spectrum (Whiteaker and Prather, 2003).

Offline sample collection and ion chromatography analysis
PM2.5 samples were collected on 47 mm quartz filters at a flow rate of 15.4 L min −1 for 23.5 hours every day in 2015 and 2016.
Here we analyzed some samples during haze episodes in four seasons in 2015 and winter of 2016. In the winter of 2016, we 100 also collected PM2.5 samples on 90 mm quartz filters at a flow rate of 100 L min −1 in the daytime and nighttime, respectively.
The filters were baked in Muffle furnace at 550 °C for 4 h and put in the cassettes and packed using aluminum foil prior to sampling, and all samples were stored at −20 °C before analysis. A quarter of each 47 mm filter or 3.14 cm 2 punch from each 90 mm filter was extracted twice with 5 mL 0.1% HCHO solution, ultrasonic oscillation for 20 min in an ice water bath and then filtered through the 0.45 μm membrane syringe filter. Two extracts were combined for ion chromatography analysis. We 105 found that HMS slowly converted to sulfate during conventional sample preparation (i.e. water extraction), leading to HMS underestimation and sulfate overestimation, and extraction with 0.1% HCHO solution can counteract the HMS decomposition ( Fig. S2a, b).
A Dionex Integrion HPIC Ion Chromatography system with AS11-HC analytical column and AG11-HC guard column (Dionex Corp., CA, US), typical columns used in previous studies during winter hazes in Beijing (Cao et al., 2014), was used for the 110 anion analysis. The separation of HMS and sulfate depends on ion chromatography conditions (i.e., column and eluent). We found the separation of HMS and sulfate peak was not good under conventional condition (eluent: 30 mM KOH, flow rate: 1.5 mL min −1 ) (Fig. S2c). Here we used an eluent of 11 mM KOH (pH≈12) with a flow rate 1.5 mL min −1 , and successfully distinguished HMS from sulfate ( Fig. S2d). In addition, Moch et al. and Dovrou et al. found that the AS22 column could not fully separate HMS and sulfate, while AS12A was able to successfully separate HMS and sulfate (Moch et al., 2018;Dovrou 115 et al., 2019). Actually, HMS dissociates into SO3 2− and HCHO rapidly in the eluent due to the short characteristic time for HMS dissociation at pH 12, and HMS concentration is measured in the form of sulfite in ion chromatography ( Fig. S2e) (Dasgupta, 1982). Thus, ion chromatography method cannot distinguish HMS from sulfite directly. Previous studies indicated that the concentration of sulfite in atmospheric aerosols was much lower than that of HMS (Dixon and Aasen, 1999;Dabek-Zlotorzynska et al., 2002). In order to distinguish between sulfite and HMS, a second analysis was performed using dilute 120 nitric acid (pH≈3) to extract samples. In the second analysis, sulfite is oxidized to sulfate, while HMS is stable. We tested some samples collected during severe winter hazes in Beijing, and found that the influence of sulfite on HMS measurement was negligible (Fig. S2f). The method detection limit was 0.02 mg L −1 for SO3 2− , equal to 0.03 mg L −1 for HMS. The blank quartz filter was analyzed as control. We also tested the accuracy (through recovery analysis) and precision (through repetitive analysis) of the method on HMS analysis. The recovery of blank and sample was 95.6% and 112.5%, respectively. The relative 125 standard deviation (RSD) of triple repetitive analysis of the sample (average: 3.17 mg L −1 ) was 4.7%.

Supplementary data and analysis
Online measurements of gaseous pollutants, particulate matter, and meteorological parameters were conducted on the roof of https://doi.org/10.5194/acp-2020-113 Preprint. Discussion started: 13 February 2020 c Author(s) 2020. CC BY 4.0 License.
School of Economics and Management, approximately 20 m above ground level and 100 m away from the ATOFMS observation site, on the campus of Tsinghua University as described in previous works (Xu et al., 2017;Zheng et al., 2015b). 130 In brief, hourly mass concentrations of PM2.5 and PM1 were monitored based on the β-ray absorption method by using two dichotomous monitors (PM-712 and PM-714; Kimoto Electric Co., Ltd., Japan). The hourly concentrations of carbonaceous species including organic carbon (OC) and elemental carbon (EC) in PM2.5 in 2015 were monitored by APC-710 (Kimoto Electric Co., Ltd., Japan). The hourly OC and EC concentrations in PM2.5 in 2016 winter were measured by a Sunset Model 4 Semi-Continuous Carbon Analyzer (Beaverton, OR, USA). We adopted a factor of 1.6 to convert the OC mass into OM mass 135 (Xing et al., 2013;Zhang et al., 2017). The hourly concentrations of gaseous pollutants including SO2, CO, and O3 were monitored with MCSAM-13 system (Kimoto Electric, Ltd., Japan). The hourly meteorological parameters including temperature and relative humidity (RH) were simultaneously monitored with an automatic meteorological observation instrument (Milos 520, VAISALA Inc., Finland).
Online concentrations of water-soluble ions in PM2.5 and inorganic gases were measured by the Monitor for AeRosols and 140 Gases (MARGA, Metrohm Ltd., Switzerland) in 2016 winter at the Chinese Research Academy of Environmental Sciences, about 9 kilometers from Tsinghua University. We calculated the aerosol water content and pH with the ISORROPIA-II thermodynamic equilibrium model (Fountoukis and Nenes, 2007). Briefly, we adopted the forward mode constrained by gas (HNO3, HCl, and NH3) + aerosol (SO4 2− , NO3 − , Cl − , K + , Ca 2+ , Na + , Mg 2+ , NH4 + ) measurements, and assumed the aerosol phase state to be metastable (Hennigan et al., 2015). The model inputs were taken from the MARGA measurements. 145 Online HCHO measurement was conducted at the Chinese Academy of Meteorological Sciences, about 6 kilometers from Tsinghua University. The HCHO was measured by an Aero-Laser GmbH HCHO analyzer (model AL4021) based on the Hantzsch reaction, as described in the previous work (Song et al., 2019). The Hantzsch reagents were prepared every 3 days and stored in a refrigerator. This analyzer was calibrated with a 1 µM HCHO standard solution every 2 to 3 days. The detection limit is 150 ppt in the field, and the accuracy and precision are ±15% or 150 ppt and ±10% or 150 ppt, respectively (Hak et al., 150 2005).
The anthropogenic emission inventory data of HCHO was derived from the Multi-resolution Emission Inventory of China (MEIC) model framework (available at http://www.meicmodel.org/), as described in detail by earlier papers (Li et al., 2019b).
Briefly, the emissions were calculated based on a technology-based methodology using updated activity data from the MEIC model framework and a collection of state-of-the-art emission factors and source profiles. The uncertainty of volatile organic 155 compounds (VOCs) emission inventory in MEIC was estimated to be ±68% (Cheng et al., 2019).

Identification of HMS in atmospheric particles
Our field measurements with ATOFMS showed that HMS was ubiquitous in aerosols during Beijing winter. During the winter observation, we found that 76% of particles contained the peak at m/z −111, and screened HMS-containing particles (the relative peak area at m/z −111 greater than 2%, Fig. 1) accounted for 9% of the total particles. It should be noted that KCl2 − and methyl sulfate (CH3SO4 − ) could also contribute to the peak at m/z −111. According to the natural isotopic compositions, we considered that the contribution of KCl2 − was insignificant. The peak area ratio of m/z −111 to m/z −113 in all screened particles was 18.7, which was consistent with the natural isotopic distribution of HMS (18.7) and much larger than that of KCl2 − (4.8). Also, the peak area ratio of m/z −109 to m/z −111 in all screened particles was 0.03, which was much smaller than 165 that of KCl2 − (1.4). Considering the moderate aerosol pH (4-5; see Sect. 3.3) in Beijing winter haze and the tendency of m/z −111 to exist in supermicrometer particles (see Sect. 3.2) in this study, the observed peak at m/z −111 was unlikely to be methyl sulfate since methyl sulfate formation requires relatively high acid conditions (Lee, 2003) and organosulfates tend to exist in submicrometer particles (Hatch et al., 2011). Therefore, the peak at m/z −111 in ambient particles was almost entirely assigned to HMS. Ambient particles in Beijing winter contained a large amount of ammonium relative to sodium, and the ammonium 170 was more common than sodium in HMS-containing particles (Fig. S3), indicating that the matrix effects on HMS detection due to counterions was not significant, because ammonium promotes the presence of the HMS marker peak in the negative ion spectrum (Neubauer et al., 1997;Whiteaker and Prather, 2003).

Quantification of HMS in atmospheric particles
Based on the optimized ion chromatography method, we quantified the HMS concentration and its contribution to haze 175 pollution. We found that HMS concentration was appreciable in humid winter haze conditions, but low in clean winter periods, dry winter haze conditions, and other seasons (Fig. S4). HMS concentration exhibited similar periodic variation to that of PM2.5 and sulfate concentration in winter, and was consistent with RH variation. With the deterioration of winter haze in 2015, i.e., from clean (PM2.5 ≤ 75 μg m −3 ), polluted (75 < PM2.5 ≤ 150 μg m −3 ) to heavily polluted (PM2.5 > 150 μg m −3 ), HMS concentration increased rapidly (Fig. 2a). Also, the molar ratio of HMS to sulfate increased from 0 (clean), 0.02 (polluted), to 180 0.06 (heavily polluted). During the HMS increase process, RH, SO2, and HCHO concentration increases, while O3 concentration declines (Fig. 2b, c). HMS tends to exist in supermicrometer particles. During the HMS events, the ratio of PM1-2.5 to PM2.5 was generally greater than 0.4, indicating the large contribution of supermicrometer aerosols. The size distribution of HMS-containing particles displayed a mode at larger sizes compared with the total particle size distribution, and the percentage of HMS-containing particles increased with particle size and remained relatively constant when the diameter is 185 greater than 1 μm (Fig. 2d), indicating the predominance of HMS in large particles.

Factors influencing HMS formation
HMS is formed in the aqueous phase, such as cloud (Moch et al., 2018), fog (Munger et al., 1986), and aerosol water (Song et al., 2019). We find that HMS is present in aerosols regardless of the presence of cloud/fog (Fig. S8a, b), and HMS 200 concentrations show a good correlation (r = 0.92, P < 0.01) with aerosol water content (Fig. S8b), indicating that aerosol water serves as a medium for HMS formation. In some HMS events, cloud/fog processes exist (Fig. S8a, b) and may contribute together with heterogeneous processes in aerosol water to the formation of HMS. Our results indicate that high concentrations of precursors (i.e., SO2 and HCHO) coupled with appropriate conditions (i.e., low oxidants, low temperature, high RH, and moderately acid pH) in severe winter hazes in the NCP facilitate the heterogeneous HMS formation. 205 Low oxidants and low temperature during winter haze facilitate the HMS formation. The S(IV) oxidation reactions are major competitions with HMS formation (Pandis and Seinfeld, 1989). The low O3 concentration and low solar radiation during HMS events ( Fig. S4 and S5) indicate the weak photochemical activity. Previous measurements also showed that OH radical and H2O2 concentrations were low in winter, especially during severe winter hazes (Tan et al., 2018;Zhang et al., 2012;Ye et al., 2018). Low temperature increases the solubility of gas (Sander, 2015), whereas decreases the reaction rate constant for HMS 210 production (Boyce and Hoffmann, 1984). According to the kinetics calculation (Song et al., 2019), the increase of gas solubility in water at low temperature is greater than the decrease of reaction rate constant, leading to higher HMS formation rate in winter hazes. Once formed, HMS is relatively stable due to the self-acidification and resistance to oxidation by O3, H2O2, and O2 (Hoigne et al., 1985;Kok et al., 1986;Munger et al., 1986;Dasgupta et al., 1980). High RH is a key factor driving fast HMS formation in winter hazes. The HMS concentration increases slowly under low RH, 215 whereas it increases rapidly under high RH (Fig. S9). Similarly, the aerosol water content exhibits an exponential increase with RH (Fig. S10), providing abundant reaction interfaces for HMS formation. HMS concentration started to increase significantly with the enhancement in RH when the RH > 60%, coinciding with the reported deliquesce RH of particles in Beijing winter (Liu et al., 2017b). With the increase of RH, atmospheric sulfur distribution shifts toward particle phase and more particulate sulfur exist in the form of HMS (Fig. 3). The molar ratio of HMS to sulfate also started to increase rapidly at the RH ~60%, 220 and high values usually occurred under severe winter hazes with high PM2.5 and HMS concentrations.
Moderately acid pH in Beijing winter hazes favors the HMS formation. Previous studies indicated that both HMS formation and decomposition rate increased rapidly with pH, thereby high HMS concentrations were usually observed in moderate pH conditions, since low pH retards the formation of HMS, while high pH is not suitable for its preservation (Munger et al., 1986).
The calculation based on the ISORROPIA-II thermodynamic equilibrium model constrained by in situ gas and aerosol 225 measurements showed an average pH value of 4.5 (from 4 to 5) for aerosol water under severe winter hazes in 2016, which https://doi.org/10.5194/acp-2020-113 Preprint. Discussion started: 13 February 2020 c Author(s) 2020. CC BY 4.0 License.
agreed reasonably with previous studies in the NCP (Liu et al., 2017a;Song et al., 2018;Ding et al., 2019;Li et al., 2019a) and was higher than those in the United States and Europe (Bougiatioti et al., 2016;Weber et al., 2016). Under such conditions, the HMS formation is favored, whereas the decomposition of HMS is negligible, thereby resulting in the observed high HMS concentrations during severe winter hazes. 230

Implications for haze chemistry and control strategies
Our findings reveal the significant contribution of HMS to severe winter hazes. We propose a more comprehensive conceptual model of heterogeneous sulfur chemistry in NCP haze events, including traditional sulfate formation, and HMS formation under high HCHO concentration, low oxidants, low temperature, high RH, and moderate pH (Fig. 4a). With the deterioration of winter haze, atmospheric oxidation capacity decreases associated with the weak photochemistry activity, while 235 heterogeneous HMS chemistry is enhanced, resulting in the increase of HMS concentration and the ratio of HMS to sulfate (Fig. 4b). Adding heterogeneous HMS chemistry into the model could improve the simulation of OM. In addition, the presence of HMS could lead to sulfate overestimation in conventional measurements, such as ion chromatography and aerosol mass spectrometry (Song et al., 2019). This can partly explain the discrepancy between sulfate simulation and observation during severe NCP winter hazes. Furthermore, HMS can be used as a tracer for heterogeneous chemistry and moderate pH during 240 typical winter haze pollution.
Our results suggest that the reduction in HCHO concentration can help to mitigate severe winter haze pollution in the NCP.
Previous studies show that HCHO is an important source of ROx (OH+HO2+RO2) radicals and ozone (Tan et al., 2018;Li et al., 2019b;Niu et al., 2016), and has high toxicity as a Group 1 human carcinogen (Niu et al., 2016), here we further demonstrate its significant contribution to particulate matter in winter. Since the implementation of 'Air Pollution Prevention 245 and Control Action Plan' in 2013, SO2 concentrations during winter hazes in Beijing have decreased significantly due to the implementation of desulfurization measures and controls on emission activities (Zheng et al., 2015b;Zheng et al., 2018), resulting in the decrease of sulfate, while HCHO concentrations show an increasing trend (Fig. S7), leading to the increased importance of HMS in winter haze. HCHO comes from primary emission and secondary formation (Chen et al., 2014;Sheng et al., 2018). Therefore, the cooperative emission reduction in primary HCHO, which mainly comes from residential solid fuel 250 (biofuel and coal) combustion and transportation (Fig. S11) (Li et al., 2019b), and VOCs (e.g., alkenes, aromatics, and alkanes) related to secondary HCHO formation should be considered in the future pollutant control strategies in the NCP. Furthermore, the HMS chemistry and related control strategies can be applicable to other regions with high SO2 and HCHO concentrations, such as India (De Smedt et al., 2015;Li et al., 2017).

Conclusion 255
Combining field measurements and laboratory experiments, we show the ubiquity of HMS in aerosols and the quantification of the large amounts of HMS in PM2.5 in Beijing winter, and elucidate the heterogeneous HMS chemistry in winter haze. High https://doi.org/10.5194/acp-2020-113 Preprint. Discussion started: 13 February 2020 c Author(s) 2020. CC BY 4.0 License.
concentrations of precursors (SO2 and HCHO), low oxidant levels, low temperature, high RH, and moderately acid pH during severe winter hazes facilitate the heterogeneous formation of HMS, which could account for up to 15% of OM in winter haze and lead to 36% overestimates of sulfate. The HMS concentration and the molar ratio of HMS to sulfate increased with the 260 deterioration of winter haze, as well as from winter 2015 to winter 2016 with the growth of HCHO concentration. Our results reveal the significant contribution of HMS to severe winter hazes, which help to improve the prediction of OM and sulfate, and suggest that the reduction in HCHO can help to mitigate severe winter haze pollution.
Data availability. All data that support the findings of this study are available in this article and its Supplement or from the 265 corresponding author on request.   atmospheric sulfur distribution shifts toward particle phase and more particulate sulfur exists in the form of HMS. The gray, red, and green colors represent SO2, SO4 2− and HMS, respectively. https://doi.org/10.5194/acp-2020-113 Preprint. Discussion started: 13 February 2020 c Author(s) 2020. CC BY 4.0 License.