Fast oxidation of sulfur dioxide by hydrogen peroxide in deliquesced aerosol particles Tengyu Liua,1, Simon L. Cleggb, and Jonathan P. D. Abbatta,1 aDepartment of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada; and bSchool of Environmental Sciences, University of East Anglia, NR4 7TJ Norwich, United Kingdom Edited by John H. Seinfeld, California Institute of Technology, Pasadena, CA, and approved December 2, 2019 (received for review September 20, 2019) Atmospheric sulfate aerosols have important impacts on air quality, Oxidation experiments with high solute strength aerosol face climate, and human and ecosystem health. However, current air- significant challenges due to the need for online measurement of quality models generally underestimate the rate of conversion of the reaction kinetics using aerosol particles, and the necessity for sulfur dioxide (SO2) to sulfate during severe haze pollution events, good control of AWC and aerosol pH (27). Sulfate formation indicating that our understanding of sulfate formation chemistry is rates for many aqueous SO2 oxidation pathways involving O3, incomplete. This may arise because the air-quality models rely upon O2+TMI, and NO2 are strongly pH-dependent (6) and are kinetics studies of SO2 oxidation conducted in dilute aqueous solu- subject to 1 to 2 orders of magnitude change if the pH changes by tions, and not at the high solute strengths of atmospheric aerosol 1 unit because the solubility and effective Henry’s law constant of particles. Here, we utilize an aerosol flow reactor to perform direct SO2 positively depend on pH (28). This sensitivity of sulfate investigation on the kinetics of aqueous oxidation of dissolved SO2 formation rates to pH poses experimental challenges in controlling + by hydrogen peroxide (H2O2) using pH-buffered, submicrometer, aerosol pH because product hydrogen ions (H ) will perturb the deliquesced aerosol particles at relative humidity of 73 to 90%. aerosol pH. As an exception, the rate of aqueous oxidation of We find that the high solute strength of the aerosol particles signif- dissolved SO2 by H2O2 is largely pH-independent for pH above 2 icantly enhances the sulfate formation rate for the H O oxidation 2 2 because the effects arising from the pH dependence of the SO2 pathway compared to the dilute solution. By taking these effects solubility and the reaction rate constant offset each other (10). into account, our results indicate that the oxidation of SO2 by H2O2 This characteristic makes the SO2–H2O2 reaction a useful system in the liquid water present in atmospheric aerosol particles can con- to isolate the effects of solute strength from aerosol pH on the tribute to the missing sulfate source during severe haze episodes. sulfate formation rate. EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Here, we study pH-buffered submicrometer, deliquesced aerosol air pollution | Chinese haze | sulfate aerosol | multiphase chemistry | particles in an aerosol flow tube to create high solute strengths, aerosol kinetics enabling direct investigation of the kinetics of aqueous oxidation of dissolved SO2 by H2O2 in aerosol particles. Six types of seed ulfate aerosol is an important component of fine particulate aerosols were investigated, with aerosol pH buffered at 2.3 to 4.8 as Smatter that impacts air quality, climate, and human and calculated using the extended aerosol inorganics model (E-AIM) ecosystem health (1–3). Atmospheric models currently generate (29, 30) and a Pitzer activity coefficient model (31): A) a mixture of aerosol sulfate either via condensation of H2SO4, which is formed sodium chloride (NaCl)/malonic acid/sodium bimalonate (molar via gas-phase oxidation of SO2, or via a suite of oxidation processes ratios 20:1:1, 6:1:1, 2:1:1 and aerosol pH 2.3 to 2.5); B) NaCl/ involving SO2 dissolved in cloudwater. Although the aerosol liquid sodium bimalonate/sodium malonate (molar ratios 20:1:1, 6:1:1, 2:1:1 water content (AWC) is generally much lower than cloud liquid water, it is possible that such aerosol multiphase oxidation pro- Significance cesses may be important in polluted and high relative humidity conditions. However, it is uncertain whether the kinetics of Atmospheric sulfate aerosol particles contribute significantly to aqueous oxidation of dissolved SO by different oxidants in- 2 poor air quality and direct forcing of the Earth’s climate. How- vestigated in bulk solution with low ionic strength (<5molal) ever, air pollution and climate models simulate the formation of are applicable to the high solute concentration situations that sulfate using acid rain chemistry known to be appropriate only prevail for aerosol particles. Measuring the kinetics of aqueous for cloudwater conditions. By measuring the oxidation of oxidation of dissolved SO2 in aerosol particles is thus critical to sulfur dioxide (SO2) by hydrogen peroxide (H2O2)directlyin the accurate modeling of aerosol sulfate in the atmosphere. hygroscopic, pH-buffered aerosol particles with high solute Assessing the rate of aerosol sulfate formation in polluted strength characteristic of many tropospheric conditions, we conditions can evaluate the atmospheric importance of multiphase show that sulfate formation occurs significantly faster than oxidation processes. In particular, rapid sulfate production has under the cloudwater conditions previously explored. In part, been observed during cloud-free, severe haze events in China, with ionic strength and general acid catalysis effects drive the fast the peak sulfate mass concentration reaching as high as ∼300 − chemistry. These results indicate that the H2O2-driven oxidation μgm3 (4–8). However, current air-quality models that include of SO2 in aqueous aerosol particles can contribute to the missing gas-phase oxidation of SO2 by the hydroxyl radical (9) and aqueous sulfate source during severe haze pollution events. oxidation of dissolved SO2 by hydrogen peroxide (H2O2) (10), O3 (10), O2 catalyzed by transition-metal ions [TMI, i.e., Fe (III) and Author contributions: T.L. and J.P.D.A. designed research; T.L. performed research; T.L., Mn (II)] (11–14), methyl hydrogen peroxide (15), and peroxyacetic S.L.C., and J.P.D.A. analyzed data; and T.L., S.L.C., and J.P.D.A. wrote the paper. acid (15) cannot capture these high levels of aerosol sulfate (5, The authors declare no competing interest. 16, 17), indicating that our understanding of sulfate formation This article is a PNAS Direct Submission. chemistry is fundamentally incomplete. Oxidation of dissolved Published under the PNAS license. SO2 by NO2 may be important if the aerosol pH is high (4, 6) and 1To whom correspondence may be addressed. Email: [email protected] or jonathan. inclusion of a hypothetical heterogeneous oxidation process in [email protected]. aerosol particles can greatly improve the model performance (17). This article contains supporting information online at https://www.pnas.org/lookup/suppl/ Overall, the formation mechanism of the missing sulfate source doi:10.1073/pnas.1916401117/-/DCSupplemental. remains unclear and controversial (4, 6, 18–26). www.pnas.org/cgi/doi/10.1073/pnas.1916401117 PNAS Latest Articles | 1of6 Downloaded by guest on September 24, 2021 2- and aerosol pH 4.8); C) sodium nitrate (NaNO3)/malonic acid/ dissolved sulfate concentrations, [SO4 ] (molality units), show sodium bimalonate (molar ratio 20:1:1 and aerosol pH 2.8); D) strong linear correlations (r2 > 0.96) with the reaction time (Fig. 1 A C NaNO3/sodium bimalonate/sodium malonate (molar ratio 20:1:1 and ). As well, the slopes of the sulfate formation rate versus and aerosol pH 4.0); E) malonic acid/sodium bimalonate (molar initial SO2 and H2O2 concentrations using log–log plots (Fig. 1 ratio of 1:1 and aerosol pH 2.8); and F) sodium bimalonate/sodium B and D) are close to unity (1.03 ± 0.14 and 1.19 ± 0.03, re- malonate (molar ratio of 1:1 and aerosol pH 3.9). spectively), suggesting first-order reactions in dissolved SO2 In part, the individual aerosol particles components were cho- and H2O2. Except for some data points associated with the sen to be representative of species found in the atmosphere. More particles containing NaCl or NaNO3, the estimated buffer ca- + importantly, they satisfy the demands of the experiment (see de- pacity is higher than the amount of H formed (SI Appendix, tailed explanation in SI Appendix,sectionS1) by providing different section S1). It is possible that HCl or HNO3 evaporate from the aerosol pH and different AWC (32), and by enabling explicit ex- particles under those conditions, removing acidity. Nevertheless, amination of the effects of aerosol pH and solute strength on the the strong correlations in Fig. 1 A and C show no signs of a slower sulfate formation rate. Aerosol mass spectrometry (33) (AMS) reaction when a large amount of sulfate forms. quantitatively characterized the composition of seed aerosols and Overall, these observations are consistent with the assumed the sulfate that forms. A scanning mobility particle sizer (SMPS) mechanism for the reaction (34): was used to measure particle-size distributions and to determine HSO− + H O ⇌ HOOSO− + H O [R1] the AWC. All experiments were performed at 21 to 25 °C and high 3 2 2 2 2 , relative humidity (RH) (73 to 90%) to ensure that the seed aerosol Methods SI Appendix HOOSO− + H+ ⇌ HOOSO [R2] particles are deliquesced; see and , section S1 2 2H, for details on the experimental conditions, choice of aerosol sys- + − tems to study, instrument operation, and data analysis. HOOSO H → 2H + SO2 . [R3] The overall goal of this work is to measure the sulfate formation 2 4 rates on aerosol particles with high solute concentrations, to en- As well, it is known that weak acids, HX, can act as proton able comparison with the literature parameters that have pre- donors to promote the reaction through general acid catalysis: viously been obtained in bulk solutions.