Chemical Transformation of Methanesulfonic Acid and Sodium

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Chemical Transformation of Methanesulfonic Acid and Sodium Methanesulfonate through Heterogeneous OH Oxidation Kai Chung Kwong, Man Mei Chim, Erik Hans Hoffmann, Andreas Tilgner, Hartmut Herrmann, James Davies, Kevin R. Wilson, and ManNin Chan ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00072 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Earth and Space Chemistry

Chemical Transformation of Methanesulfonic Acid and Sodium Methanesulfonate through Heterogeneous OH Oxidation Kai Chung Kwong1, Man Mei Chim1, Erik Hans Hoffmann2, Andreas Tilgner2, Hartmut Herrmann2, James F. Davies3, Kevin R. Wilson4, Man Nin Chan1,5* 1

Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong, Hong Kong, CHINA 2 Atmospheric Chemistry Department (ACD), Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, GERMANY 3 Department of Chemistry, University of California Riverside, Riverside, USA 4

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA The Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong, Hong Kong, CHINA 5

*

Correspondence to: Man Nin Chan ([email protected])

Abstract Methanesulfonic acid (CH3SO3H, MSA) is one of the major organosulfur acids formed from the photochemical oxidation of dimethyl sulfide (DMS) produced by oceanic phytoplankton. MSA can react with metal halides (e.g. sodium chloride) in ambient aerosols to form methanesulfonate salts (e.g. sodium methanesulfonate, CH3SO3Na). While the formation processes of MSA and its salts are reasonably well understood, their subsequent chemical transformations in the atmosphere are not fully resolved. MSA and its salts accumulate near the aerosol surface due to their surface activities, which make them available to heterogeneous oxidation at the gas-aerosol interface by oxidants such as hydroxyl (OH) radicals. In this work, the compositional changes of aerosol comprised of MSA and its sodium salt (CH3SO3Na) are measured following heterogeneous OH oxidation. An aerosol flow tube reactor is coupled with a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) and a high-resolution mass spectrometer at a relative humidity (RH) of 90 %. DART-aerosol mass spectra reveal that MSA and CH3SO3Na can be detected as methanesulfonate ion (CH3SO3−) with minimal fragmentation in the negative ionization mode. Kinetic measurements 1

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show that OH oxidation with MSA and CH3SO3Na has an effective OH uptake coefficient of 0.45 ± 0.14 and 0.20 ± 0.06, respectively, revealing that MSA reacts with OH radical faster than its sodium salt. One possibility for the difference in reactivity of these two compounds is that CH3SO3Na is more hygroscopic than MSA. The increase in the coverage of water molecules at the surface of CH3SO3Na might reduce the reactive collision probability between CH3SO3− and OH radicals, resulting in a smaller reaction rate. MSA and CH3SO3Na dissociate to form CH3SO3−, which tends to fragment into formaldehyde (HCHO) and a sulfite radical (SO3•−) upon oxidation. Formaldehyde partitions back to the gas phase owing to its high volatility, and SO3•− can initiate a series of chain reactions involving various inorganic sulfur radicals and ions in the aerosol phase. Overall, the fragmentation and SO3•−-initiated chemistry are the major processes controlling the chemical evolution of MSA and its sodium salt aerosols during heterogeneous OH oxidation.

Keywords: Methanesulfonic acid, sodium methanesulfonate, heterogeneous OH oxidation, sulfite radical, inorganic sulfate

1. Introduction Dimethyl sulfide (DMS) is an important precursor of sulfur dioxide (SO2), non-sea-salt sulfate (nssSO42−), and organosulfur compounds in marine environments.1 Methanesulfonic acid (CH3SO3H, MSA) is one of the major oxidation products formed from gas-phase and aqueous-phase oxidation of DMS.2−4 MSA has been detected in ambient aerosols over oceans and coastal regions. 5−7 Based on the MSA to nss-SO42− ratio, field studies show that a maximum of 10 % of nss-SO42− can be formed from the DMS oxidation.8 Laboratory, field, and modeling studies have revealed that factors such as ambient temperature and relative humidity (RH), photochemical conditions, and cloudiness can significantly affect the production of MSA from DMS oxidation.9−12 In addition, methanesulfonate salts can be formed via acid dissociation process, generating the conjugate base in the presence of metal halides (e.g. sodium chloride (NaCl), magnesium chloride, and potassium chloride). For 2


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instance, Liu et al.13 and Tang and Zhu14 have suggested that sodium methanesulfonate (CH3SO3Na) can be formed in the presence of MSA and NaCl: NaCl(aq) + MSA(aq or g) → CH3SO3Na(aq) + HCl(aq). While the formation mechanisms are reasonably understood,15 the chemical transformation of MSA and its salts remains largely unclear. MSA and CH3SO3Na likely dissociate to form methanesulfonate ion (CH3SO3−) in an aqueous solution.16,17 CH3SO3− has a strong surface propensity and has an orientation at the surface such that the sulfite group is turned toward the bulk, and the methyl group pointed into the gas.13 The surface accumulation of CH3SO3− motivates an investigation of how MSA and its salts react with gas-phase oxidants, such as hydroxyl (OH) radicals, ozone, and nitrate radicals at or near the aerosol surface.18,19 Recently, the oxidation kinetics with gas-phase oxidants (such as ozone, OH, and nitrate radicals) with atmospheric relevant organic species at the air-water interface have been investigated.20–23 These heterogeneous oxidation processes can alter the physicochemical properties of organic aerosols, such as light scattering and absorption, water uptake, and cloud condensation nuclei activity.24,25

To gain a more fundamental understanding of the heterogeneous reactivity and chemistry, the compositional evolution of MSA and CH3SO3Na aerosols upon heterogeneous OH oxidation is investigated in an aerosol flow tube reactor at 90 % RH (Table 1). The molecular composition of the aerosols before and after OH oxidation is characterized in real-time using a soft ambient pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer. The heterogeneous reactivity of MSA and CH3SO3Na towards OH radicals is first determined by measuring the decay of the parent compound at different extent of oxidation, which enables the evaluation of their chemical stabilities against heterogeneous OH oxidation in the atmosphere. Secondly, reaction pathways are discussed based on the product ions observed in the DART-aerosol mass spectra and mechanisms proposed in the literature.

3



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Table 1. Chemical structure, properties, effective heterogeneous OH rate constant, effective OH uptake coefficient and chemical lifetime of methanesulfonic acid and sodium methanesulfonate against OH oxidation at 90 % RH. Compounds Methanesulfonic Acid Sodium Methanesulfonate (MSA) (CH3SO3Na) Structural Formula

Molecular Formula CH3SO3H −1 Molecular Weight (g mol ) 96.1057 Effective heterogeneous OH 7.42 ± 0.29 oxidation rate constant, k (×10−13 cm3 molecule−1 s−1) Effective OH 0.45 ± 0.14 uptake coefficient, γeff Chemical lifetime against 10.4 ± 0.4 heterogeneous OH oxidation (day)a a 24-hour averaged OH concentration of 1.5 × 106 molecules cm−3

CH3SO3Na 118.0875 5.90 ± 0.11 0.20 ± 0.06 13.1 ± 0.2

2.Experimental Method 2.1 Heterogeneous Oxidation An atmospheric pressure aerosol flow tube reactor was used to investigate the heterogeneous OH oxidation of MSA and its sodium salt (CH3SO3Na) aerosols at 90 % RH and room temperature. Experimental procedures have been described previously.26−28 Briefly, aqueous droplets generated by an atomizer were mixed with humidified nitrogen (N2), oxygen (O2), ozone (O3), and hexane (a gasphase OH tracer) before introducing into the reactor. Inside the reactor, the aerosols were oxidized by gas-phase OH radicals, which were generated by the photolysis of O3 under ultraviolet light with a wavelength of 254 nm in the presence of water vapor. The RH inside the reactor was controlled by the ratio of dry and humidified N2. The gas-phase OH concentration was varied by changing the O3 concentration and determined by measuring the decay of the hexane using a gas chromatograph with flame ionization detection. The OH exposure, defined as the product of OH concentration and the aerosol residence time, t, was determined as follows 29 𝑂𝐻 𝐸𝑥𝑝𝑜𝑠𝑢𝑟𝑒 = −

𝑙𝑛([𝐻𝑒𝑥]/[𝐻𝑒𝑥]0 ) 𝑘𝐻𝑒𝑥

𝑡

= ∫𝑜 [𝑂𝐻]𝑑𝑡

[1] 4


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where [Hex] is the hexane concentration leaving the reactor, [Hex]0 is the hexane concentration before oxidation, and kHex is the second order rate constant of the gas-phase OH-hexane reaction (5.2 × 10−12 cm3 molecule−1 s−1).29 The OH exposure for MSA and CH3SO3Na ranged from 0 to 1.36 × 1012 molecule cm−3 s and 1.33 × 1012 molecule cm−3 s, respectively. The aerosol stream leaving the reactor passed through an annular Carulite catalyst denuder and an activated charcoal denuder to remove O3 and other gas-phase species, respectively. A portion of the aerosol stream was then sampled by a scanning mobility particle sizer (SMPS) for aerosol size distribution measurement and the remaining flow was delivered into a stainless-steel tube heater to vaporize the aerosols. The heater was maintained at 200 − 250 oC for MSA and at 350 − 400 oC for CH3SO3Na. The resulting gas-phase species were directed into the ionization region, an open space between the DART ionization source (IonSense: DART SVP) and the atmospheric inlet of the high-resolution mass spectrometer (ThermoFisher, Q Exactive Orbitrap) for real-time chemical characterization.28 Mass spectra were collected over a scan range of m/z 70 − 700. Each spectrum was averaged for 5 minutes at a mass resolution of 140,000, and analyzed using the Xcalibur software (Xcalibur Software, Inc., Herndon, VA, USA).

In the ionization region, gas-phase species were ionized by reactive species produced by the DART ionization source. The working principle of DART ionization has been given by Cody et al.30 The DART ionization source was operated in the negative ion mode using helium as the reagent. Ions, electrons, and metastable helium atoms were generated under glow discharge from a 4 kV potential in the device. Two electrostatic lenses were used to remove unwanted ions and electrons, and only the metastable helium atoms remained entrained in the gas. The gas stream was then heated to 500 o

C before exiting the ionization source. For the ionization mechanism, MSA (pKa = −1.86)31 and its

sodium salt likely dissociate to form CH3SO3−, which can be ionized via direct ionization.32,33

The physical state of the aerosol is known to be a key factor in controlling the bulk and surface 5



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composition and properties (e.g. viscosity) of the aerosol and thus the heterogeneous reactivity.34−36 For MSA, before oxidation, the composition of the aerosol (e.g. mass fraction of solute, mfs) can be determined from the hygroscopicity data reported by Johnson et al. 37, which used a hygroscopicity tandam differential mobility analyzer (H-TDMA) at 90 % RH. The growth factor, Gf, defined as the ratio of the aerosol diameter measured at high RH (Dp(RH2)) to that measured at low RH (Dp(RH1)), is converted into mfs using the following equation:38 1

𝐷𝑝 (𝑅𝐻2 )

𝐺𝑓 = 𝐷

𝑝 (𝑅𝐻1 )

𝑚𝑓𝑠𝑅𝐻,1 𝜌𝑅𝐻,1 3

= (𝑚𝑓𝑠

𝑅𝐻,2

𝜌𝑅𝐻,2

)

[2]

where mfsRH,i and ρRH,i are the mass fraction of MSA and particle density at a given RH, respectively. The aerosol density is estimated using the volume additivity rule with the density of water and MSA (1.475 g cm−3) with an uncertainty of 20 − 30 %. As a first approximation, it is assumed that MSA aerosols are anhydrous at the low RH (RH < 10 %) (i.e. mfsRH,1 = 1). Using Eqn. 2, with the reported Gf of 1.57 at 90 % RH,37 the mfs is estimated to be 0.33. For CH3SO3Na, Liu and Laskin39 and Peng and Chan40 have reported that CH3SO3Na aerosols exhibit distinct solid to liquid phase transitions (i.e., deliquescence) at 71 % RH and liquid to solid phase transitions (i.e., efflorescence) at about 49 % RH. The mfs of CH3SO3Na was reported to be 0.24 at 90 % RH.40 Given the aerosols were always exposed to high humidity in the system, they likely remained as aqueous droplets before oxidation.

Control experiments were conducted to investigate to what extent MSA and its sodium salt reacts with ozone or undergoes photolysis (Figure S1, supporting information). For both MSA and CH3SO3Na, no change in DART-aerosol mass spectra was observed in the presence of ozone and the absence of UV light, suggesting that the ozone reactions with MSA and CH3SO3Na are not significant. Additionally, the composition of the aerosols did not show significant changes when the UV lights were on in the absence of ozone, suggesting that MSA and CH3SO3Na are not likely photolyzed. MSA may re-evaporate into the gas phase at low relative humidity;41 however, reevaporation did not occur under our experimental conditions as MSA aerosols were always exposed 6


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to high RH. Furthermore, the intensity of the parent ion in the DART-aerosol mass spectra was very small after filtering out the aerosols, suggesting that volatilization and gas-phase oxidation of MSA and CH3SO3Na are not significant.

3. Results and Discussions 3.1 DART-Aerosol Mass Spectra

Figure 1. DART-aerosol mass spectra of MSA (left panel): (a) before oxidation; (b) after oxidation; and CH3SO3Na (right panel): (c) before oxidation; (d) after oxidation.

Figure 1 shows the DART-aerosol mass spectra of MSA and CH3SO3Na before and after OH oxidation at 90 % RH. For MSA, the dominant peak before oxidation (Figure 1a) is at m/z 95 with a chemical formula of CH3SO3−, which corresponds to the methanesulfonate ion. Figure 1b shows a decrease in intensity of the parent peak after oxidation at the maximum OH exposure. The intensity of a product peak at m/z 97 with a chemical formula of bisulfate (HSO4−) increases significantly. At the maximum OH exposure, the intensity of HSO4− increases by a factor of about 4.8 (Figure 2a), suggesting that HSO4− is likely formed due to oxidation. Small product peaks at m/z 96, 112, and 113 are detected, which have a chemical formula of SO4−, SO5−, and HSO5−, respectively. The intensities 7



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of these product ions increase with increasing OH exposure (Figures 2b − 2d). Like CH3SO3−, the detection of these ions likely originates from direct ionization.32,33 For instance, HSO4− and SO4− have been detected as product ions in the heterogeneous OH oxidation of sodium methyl sulfate and sodium ethyl sulfate.42 The DART-aerosol mass spectra of CH3SO3Na look very similar to that of MSA before and after OH oxidation (Figure 1). Before oxidation (Figure 1c), CH3SO3− is the dominant peak. After oxidation, CH3SO3− remains the largest peak together with the product peaks of HSO4−, SO4−, HSO5−, and SO5−. Overall, the same product ions are detected for both MSA and CH3SO3Na after oxidation. In the following sections, the heterogeneous OH kinetics and chemistry of MSA and CH3SO3Na aerosols will be discussed.

Figure 2. The kinetic evolution of (a) HSO4−, (b) SO4−, (c) HSO5−, and (d) SO5− as a function of OH exposure during heterogeneous OH oxidation of MSA and CH3SO3Na at 90 % RH. 3.2 Oxidation Kinetics To quantify the kinetics, the effective OH uptake coefficient, γeff, defined as the fraction of gas-phase OH collisions with aerosol surface that result in a reaction, is calculated:43 𝛾eff =

2 𝑘 𝐷0 𝜌 mfs 𝑁𝐴 ̅̅̅̅̅̅ 3 𝑀𝑤 𝑐 𝑂𝐻

[3] 8


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where D0 is the mean surface-weighted aerosol diameter, ρ is the aerosol density, NA is the Avogadro’s number, Mw is the molecular weight of the parent compound (i.e. MSA and CH3SO3Na), and cOH is the average speed of gas-phase OH radicals. The k is the effective heterogeneous OH oxidation rate constant and is obtained by applying an exponential fit to the normalized parent decay as a function of OH exposure (Figure 3). The fitted k value for the MSA and CH3SO3Na are 7.42 ± 0.29 × 10−13 cm3 molecule−1 s−1 and 5.90 ± 0.11 ×10−13 cm3 molecule−1 s−1, respectively. Before oxidation, the diameter of MSA and CH3SO3Na aerosols was measured to be 229.9 nm and 228.1 nm, respectively. These aerosol sizes are near the maximum measured aerosol number distribution in the accumulation mode.44 The mfs of MSA and CH3SO3Na aerosols were obtained from the hygroscopic data reported in the literature.37,40 Based on the aerosol composition (i.e., mfs), the aerosol density is estimated using the volume additivity rule with the density of water and MSA or CH3SO3Na.

Figure 3. The normalized parent decay of MSA and CH3SO3Na as a function of OH exposure during heterogeneous OH oxidation at 90 % RH.

Using Eqn. 3, the γeff are calculated to be 0.45 ± 0.14 and 0.20 ± 0.06 for MSA and CH3SO3Na, respectively. The difference in reactivity of these two compounds could be explained by that CH3SO3Na (mfs = 0.24) is more hygroscopic than MSA (mfs = 0.33). During OH oxidation of 9



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CH3SO3Na, the increase in surface coverage of water molecules might lower the collision probability between CH3SO3− and gas-phase OH radicals at the interface.28,43 However, the effects of Na+ on the surface partitioning and configuration of CH3SO3− are not well understood and further investigations would be needed to better understand the difference in the heterogeneous reactivity of MSA and CH3SO3Na toward gas-phase OH radicals. The rate of OH reaction with MSA and its salts can affect the interpretation of MSA to nss-SO42− ratio in ambient aerosols, which is often used to evaluate the relative contribution of natural versus anthropogenic sulfur in ambient marine aerosols since MSA has no known anthropogenic source in a marine environment.8 Based on our kinetic data, the chemical lifetime of MSA and its sodium salt during heterogeneous OH reaction, 𝜏 is estimated: [MSA]

1

𝜏 = 𝑑[𝑀𝑆𝐴]⁄𝑑𝑡 = 𝑘[𝑂𝐻]

[4]

Using a 24-hour averaged gas-phase OH concentration of 1.5 × 106 molecules cm−3, the lifetime of MSA and CH3SO3Na against heterogeneous OH oxidation is calculated to be 10.4 ± 0.4 days and 13.1 ± 0.2 days, respectively. These characteristic timescales are slightly longer than other major aerosol removal processes such as dry and wet deposition (7 − 10 days).44 These suggest that the heterogeneous OH reaction might not be an important atmospheric sink. It is known that OH oxidation with MSA in aqueous solution can produce inorganic sulfate.45 Although the OH oxidation lowers the concentration of MSA, it has a minor effect on the total sulfate concentration but can alter the MSA to nss-SO42− ratio. For instance, field measurements have shown that the MSA to nss-SO42− ratio ranges from 0.0024 – 0.06 in the tropical regions, 0.06 – 0.12 in the unpolluted midlatitudes and 0.15 – 0.93 in polar regions.5 For instance, using the determined lifetime of 10.4 days for MSA oxidation by OH, the amount of MSA will decrease to 1/e (~ 37%) of its initial value after 10 days of OH oxidation. If the oxidative loss of MSA due to OH oxidation is not considered, the MSA to nssSO42− ratio could be underestimated. Since MSA and CH3SO3Na exhibit different reactivity toward OH radicals, considering the heterogeneous OH oxidation of MSA and its salts (e.g. sodium salt) in 10


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the model could bring a better prediction of MSA concentration and determine the contribution of natural oceanic sulfur to total sulfate concentration more accurately.

Mungall et al.46 have measured the heterogeneous OH oxidation of ammonium sulfate aerosols with an MSA mass fraction of 0.16 using an aerosol flow tube reactor coupled with an Aerodyne aerosol mass spectrometer at 75 % RH. The γeff was measured to be 0.05 ± 0.03, which is smaller than that measured for pure MSA and its sodium salt aerosols in this work. While the difference in γeff could attribute to that since ammonium sulfate is more hygroscopic than MSA and CH3SO3Na, more water molecules likely present at the interface of ammonium sulfate aerosols with an MSA compare to pure MSA. This may lower the overall heterogeneous reactivity. We would like to note that secondary chemistry initiated by sulfite radicals (SO3•−) is likely occurred in the aerosol phase, which can explain the detected products and can further contribute to MSA oxidation in the bulk. Therefore, in the following section, the chemistry of SO3•− will be discussed.

3.3 Reaction Mechanisms As shown in Figure 1, given the same product ions were observed in the DART-aerosol mass spectra, generalized reaction mechanisms are proposed for the heterogeneous OH reaction with MSA and CH3SO3Na aerosols. As shown in Scheme 1, MSA and its sodium salts likely dissociate to form CH3SO3−. The reaction of OH with CH3SO3− is suggested to be first initiated by hydrogen abstraction, generating an alky radical (•CH2SO3). Subsequently, a peroxy radical (•O2CH2SO3) is formed via O2 addition to the alkyl radical. The self-reactions of two peroxy radicals yields an alkoxy radical (•OCH2SO3), which subsequently converts into formaldehyde (HCHO) and a sulfite radical (SO3•−) as a result of decomposition.15 Formaldehyde is volatile and partitions back to the gas phase, and SO3•− can initiate a series of chain reactions in the aerosol phase, involving sulfate (SO 4•−) and peroxymonosulfate (SO5•−) radical chemistry. 11



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Scheme 1. Proposed reaction mechanism of heterogeneous OH oxidation of MSA and CH3SO3Na leading to the formation of sulfite radical and formaldehyde.

Aqueous-phase SO3•−-initiated oxidation chemistry has been reviewed in the literature.47−52 We acknowledge that many reaction intermediates and radicals present in the chain reactions initiated by SO3•− cannot be detected by the DART ionization source. Without authentic standards and surrogates, the ionization efficiencies of the product ions are not known, and the concentrations of the ions cannot be quantified. A full detailed discussion of reaction mechanisms is beyond the scope of this work. In the following, based on reaction mechanisms proposed in the literature, the production of ions detected in the DART-aerosol mass spectra upon heterogeneous OH oxidation via SO3•−initialized chemistry are outlined below.

3.3.1 Formation of HSO4− (m/z = 97) and HSO5− (m/z = 113) As shown in Figures 1 and 2, HSO4− and HSO5− are detected at m/z 97 and 113, respectively, after OH oxidation. The production of these two ions can be originated from the formation of SO 3•− which subsequently initiates a series of chain reactions, involving sulfate (SO4•−) and peroxymonosulfate (SO5•−) radical chemistry. During oxidation, SO3•−, once formed, can react quickly with O2 to form SO5•− (R1). Alternatively, SO3•− can react with another SO3•− to yield a sulfur trioxide (SO3) and a sulfite ion (SO32−) (R2), which can combine with a hydrogen ion (H+) to form a bisulfite ion (HSO3−) (R3). In the presence of aerosol phase water, SO3 instantaneously hydrolyze into sulfuric acid (H2SO4), which dissociates to form HSO4− and H+ (R4). It also acknowledges that the reaction rate of SO3•− with O2 is reported to be 1.1 × 109 M−1 s−1, which is about of a factor of 3 faster than that of self-reaction of SO3•− (= 3.2 × 108 M−1 s−1).47 In the presence of O2, SO3•− mainly react with O2 to form SO5•− (R1)53. Although the self-reactions of SO3•− (R2) is expected to be minor, the formation 12


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of SO32− and HSO3− (R2 and R3) is necessary for initializing the chain reactions in the presence of SO5•− as discussed below. SO3 •− + O2 → SO5 •−

R1

SO3 •− + SO3 •− → SO3 2− + SO3

R2

SO3 2− + H + ⇌ HSO3

−

SO3 + H2 O → H2 SO4 ⇌ HSO4 − + H +

R3 R4

SO5•− Chemistry. The formation of SO5•− and SO32−/HSO3− reaction pair can trigger a series of chain reactions, which eventually generate HSO4− and HSO5−. For instance, the SO5•− reactions with SO32−/HSO3− can either regenerate SO3•− while forming SO52−/HSO5− (R5a and R5b) or produce a sulfate radical (SO4•−) while forming SO42−/HSO4− (R6a and R6b).54 Furthermore, SO5•− can undergo self-reactions to form either a S2O102− (R7) or a peroxydisulfate ion (S2O82−) (R8),51 which can further decompose into two SO4•− (R9).49 SO5 •− + SO3 2− → SO3 •− + SO5 2−

R5a

SO5 •− + HSO3 − → SO3 •− + HSO5 −

R5b

SO5 •− + SO3 2− → SO4 •− + SO4 2−

R6a

SO5 •− + HSO3 − → SO4 •− + HSO4 −

R6b

SO5 •− + SO5 •− → S2 O10 2−

R7

S2 O10 2− → S2 O8 2− + O2

R8

S2 O8 2− → SO4 •− + SO4 •−

R9

SO4•− Chemistry. Similar to SO5•−, the formation and subsequent reactions of SO4•− can propagate the chain reactions and produce HSO4−. For example, SO4•− can react with SO32−/HSO3− and SO52−/HSO5− reaction pairs to regenerate SO3•− (R10a and R10b) and SO5•− (R11a and R11b), respectively.47,49,54,55 The self−reaction of two SO4•− can produce S2O82− (R12), 56 which can possibly react with SO4•− to produce SO42− (R13).47 SO4•− is a strong oxidant and can possibly react with CH3SO3− via hydrogen abstraction to form a HSO4− (R14).57 Alternatively, SO4•− can react with CH3SO3− via electron transfer reaction to yield CH3SO3• and SO42− (R15).58 This reaction can lead to another radical chemistry as CH3• and SO3 can be formed from the decomposition of CH3SO3•. The 13



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CH3• can quickly react with O2 to CH3O2•, which can recombine with another CH3O2• to yield formaldehyde (HCHO) and HO2. SO4 •− + SO3 2− → SO3 •− + SO4

2−

R10a

−

R10b

2−

R11a

SO4 •− + HSO5 − → SO5 •− + HSO4 −

R11b

SO4 •− + SO4 •− → S2 O8 2−

R12

SO4 •− + HSO3 − → SO3 •− + HSO4 SO4 •− + SO5 2− → SO5 •− + SO4

SO4 •− + S2 O8

2−

→ S2 O8 •− + SO4

2−

R13

SO4 •− + CH3 SO3 − → CH2 SO3 •− + HSO4 −

R14

SO4 •− + CH3 SO3 − → CH3 SO3 • + SO4 2−

R15

3.3.2 Formation of SO4− (m/z = 96) and SO5− (m/z = 112) From the proposed reaction pathways, although the SO4•− and SO5•− has the same chemical formula as the peaks observed at m/z 96 (SO4−) and 112 (SO5−), these two peaks are not likely originated from SO4•− and SO5•− owing to their high reactivity. Instead, SO4− and SO5−, based on the mass-tocharge ratio, are likely originated from S2O82− and S2O102−, respectively, in which they are more stable species (R7 and R12). It is noted that the chain reactions could be slowed down by the selfreactions of SO5•− and SO4•−, which can be considered as the termination steps when S2O102− and S2O82− are formed. In addition, SO4•− is reactive and reacts with CH3SO3− (R14) and other organic compounds to form organic peroxy radicals. For instance, the rate constant for SO4•− with other organics such as alcohols, ethers, alkanes, and aromatic compounds ranges from 106 − 109 M−1 s−1.59,60 This could terminate its role as a charge carrier in S(IV) oxidation, and SO4•− induced chain reactions might not be very effective in aerosol phase S(IV) oxidation under conditions with large amounts of organic compounds.

3.3.3 Absence of Functionalization Products and Reaction Intermediates proposed in the Reaction Mechanisms As shown in Figures 1 and 2, no formation of alcohol (HOCH2SO3−) and carbonyl (O=CHSO3−) 14


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functionalization products are observed in the DART-aerosol mass spectra for both MSA and CH3SO3Na. This observation agrees with the proposed reaction mechanisms that the peroxy-peroxy radical reactions likely result in the decomposition of CH3SO3− into formaldehyde and SO3•− (Scheme 1). Some reaction intermediates (SO32−, SO42−, SO52−, and HSO3−) have not been detected. If SO32−, SO42−, and SO52− were formed in sufficient amount, they are likely detected by the DART ionization source via direct ionization. However, these ions have a mass which is below the mass range of the mass spectrometer (m/z 70 – 700) and do not show in the mass spectra. The absence of HSO3− could be partly explained by its consumption by subsequent reactions.

3.4 Heterogeneous OH Oxidation of Two C1 Organosulfur Compounds: MSA versus Methyl Sulfate We have recently investigated the heterogeneous OH oxidation of sodium methyl sulfate, (CH3SO4Na, Table S1, supporting information)42 and would like to discuss the difference in the heterogeneous OH kinetics and chemistry for these two structurally similar C1 organosulfur compounds. Kinetic measurements show that at a high humidity (85 – 90 %), MSA reacts with OH at a faster rate than sodium methyl sulfate. For MSA, the effective OH uptake coefficient, γeff is 0.45 ± 0.14, which is about a factor of 2.6 larger than that of sodium methyl sulfate (γeff = 0.17 ± 0.03). This might suggest that when the methyl group is bonded directly with sulfur (C-S), the reactivity is faster than it is bonded indirectly by oxygen atom (C-O-S). For the chemistry, we found that the OH reaction with sodium methyl sulfate leads to the formation of a formaldehyde and a SO4•−, which can then initiate secondary chain reactions (Scheme S1, supporting information). In contrast to that of MSA and CH3SO3Na, HSO4− and SO4− are the only two major product ions observed in the DARTaerosol mass spectra of sodium methyl sulfate (Figure S2, supporting information). HSO4− is likely originated from the hydrogen abstraction of sodium methyl sulfate by SO4•− (similar to SO4•− reaction with CH3SO3− via R14), while SO4− is likely originated from S2O82− formed via the self-reactions of two SO4•− (R12). Together these results suggest that SO3•− and SO4•− initiated oxidation reactions involve different reaction intermediates and charge carriers during the OH oxidation of these two C1 15



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organosulfur compounds. Moreover, we have shown that OH reaction with these two organosulfur compounds can produce inorganic sulfate (i.e. bisulfate ions, HSO4−). This could have an atmospheric implication that organosulfur compounds could be a minor source of inorganic sulfate through heterogeneous OH oxidation or other oxidative processes such as aqueous-phase OH oxidation. Further work on investigating the formation of inorganic sulfate from OH oxidation of the organosulfur compounds is desirable.

3.5 Aerosol Mass Lost via Fragmentation and Volatilization Processes Fragmentation and the volatilization of formaldehyde can be considered as the major reaction pathways for OH oxidations with MSA and CH3SO3Na (Scheme 1), however, the size of the aerosols does not significantly decrease after oxidation (Figure 4). For instance, the diameter of MSA aerosols decreases slightly from 229.9 to 219.1 nm (about 4.7 %) after oxidation. One likely explanation is that when the fragmentation processes occur, one methyl group is lost via volatilization in the form of formaldehyde (Scheme 1). The methyl group (CH3, molar mass = 15 g mol−1) contributes about 15.6 % of the molecular mass of MSA. At the maximum OH exposure, about 63 % of MSA is oxidized. If only fragmentation processes occurred, and all formaldehyde partitioned back to the gas phase upon oxidation, this leads to a 9.9 % loss in aerosol mass. This mass loss due to fragmentation and volatilization processes corresponds to a decrease in aerosol diameter of about 3.4 % with an assumption that the aerosol density does not change upon oxidation. It is noted that sulfate has a higher density than MSA. When one mole of MSA is oxidized, one mole of sulfate is eventually formed. The change in aerosol density may lead to a decrease in the aerosol diameter upon oxidation. Generally, the results of this simple analysis agree well with the SMPS data that only a small decrease in aerosol size and mass was observed after OH oxidation for these two species. The decrease in observed aerosol diameter is mainly due to fragmentation and volatilization processes, while the change in aerosol density would play a minor role. 16


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Figure 4. The surface-weighted particle diameter of MSA and CH3SO3Na as a function of OH exposure during the heterogeneous OH oxidation at 90 % RH.

4. Conclusions and Atmospheric Implications Oxidation of MSA and its salts can take place at or near the aerosol surface through reactions with gas-phase OH radicals throughout their atmospheric lifetime. Kinetic measurements reveal that MSA and its sodium salt can be oxidized by gas-phase OH radicals. Although the changes in MSA concentration might be small over atmospheric timescales, this could have strong effect on the MSA to nss-SO42− ratio as one mole of MSA and its sodium salt is lost, and one mole of sulfate is eventually formed. OH radical-initiated heterogeneous oxidation may need to be considered in order to better predict the atmospheric abundance of MSA and interpret field observations of MSA to nssSO42− ratios. MSA and its sodium salt likely fragment into formaldehyde and SO3•−, which subsequently initiates a series of chain reactions in aerosol phase. The product ions detected in the DART-aerosol mass spectra agree with the SO3•− initiated chain reactions which also include SO4•− and SO5•− chemistry. SO3•−-initiated chemistry, fragmentation, and volatilization processes are likely the major reaction pathways during the heterogeneous OH oxidation of MSA and its sodium salt aerosols. Our work has demonstrated that the heterogeneous reactivity of MSA towards gas-phase OH radicals is different from its sodium salt. 17



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At present, the observed latitudinal trend of MSA concentration is suggested to be correlated with OH concentrations.46 For instance, lower MSA concentration is observed in the tropics with higher OH levels. The OH reaction with MSA has been relatively well understood, but information about the heterogeneous oxidation of MSA with nitrate and halogen radicals is still lacking. The nitrate radical is the most important nighttime radical and is well known to be of high importance for DMS oxidation at polluted coastlines.61 Information about the oxidation by halogen radicals is needed to better understand the chemical processing of MSA during polar nights and polar regions with high abundance of reactive halogen species.62 Overall, understanding the heterogeneous kinetics and chemistry of MSA and methanesulfonate salts by gas-phase oxidants such as OH radicals, nitrate radicals, and halogen radicals (e.g. Cl) is crucial to better understand the oxidation pathways, transformation, and global distribution of MSA.

Acknowledgement K. C. Kwong, M. M. Chim, and M. N. Chan are supported by the Hong Kong Research Grants Council (HKRGC) Project ID: 2191111 (Ref 24300516). E. H. Hoffmann, A. Tilgner, and H. Herrmann are supported by the EC HORIZON 2020 project MARSU as project 690958 in RISE is also acknowledged. E. H. Hoffmann is also supported by Deutsche Bundesstiftung Umwelt (DBU). K. R. Wilson is supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, in Condensed Phase and Interfacial Molecular Science Program of the U.S. Department of Energy under Contract No. DE−AC02−05CH11231.

Supporting Information Supporting information contains the DART-aerosol mass spectra of MSA and its salt for control experiments (Figure S1), DART-aerosol mass spectra of sodium methyl sulfate before and after heterogeneous OH oxidation (Figure S2), proposed reaction mechanism scheme for the 18


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heterogeneous OH oxidation of sodium methyl sulfate (Scheme S1), and the chemical structure of sodium methyl sulfate (Table S1).

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Reactions

of

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Radicals

with

Dimethylsulfoxide,

Dimethylsulfone,

and

Methanesulfonate. J. Photochem. Photobiol., A. 2003, 157, 311−319. 59. Clifton, C. L.; Huie, R. E. Rate Constants for Hydrogen Abstraction Reaction of the Sulfate Radical, SO4−. Alcohols. Int. J. Chem. Kinetics. 1989, 21, 677−687. 60. Padmaja, S.; Alfassi, Z. B.; Neta, P.; Huie, R. E. Rate Constants for Reactions of SO4•− Radicals in Acetonitrile. Int. J. Chem. Kinetics. 1993, 25, 193−198. 61. Breider, T. J.; Chipperfield, M. P.; Richards, N. A. D.; Carslaw, K. S.; Mann, G. W.; Spracklen, D. V.: Impact of BrO on Dimethylsulfide in the Remote Marine Boundary Layer, Geophys. Res. Letters, 37, L02807 10.1029/2009gl040868, 2010. 26


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62. Saiz-Lopez, A.; von Glasow, R. Reactive Halogen Chemistry in the Troposphere, Chem. Soc. Rev., 2012, 41, 6448−6472.

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Table of Contents (TOC) graphic:

For TOC only

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Figure 1. Aerosol mass spectra of MSA (left panel): (a) before oxidation; (b) after oxidation; and CH3SO3Na (right panel): (c) before oxidation; (d) after oxidation

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Figure 2. The kinetic evolution of (a) HSO4−, (b) SO4−, (c) HSO5−, and (d) SO5− as a function of OH exposure during heterogeneous OH oxidation of MSA and CH3SO3Na at 90 % RH.

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Figure 3. The normalized parent decay of MSA and CH3SO3Na as a function of OH exposure during heterogeneous OH oxidation at 90 % RH.

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Figure 4. The surface-weighted particle diameter of MSA and CH3SO3Na as a function of OH exposure during the heterogeneous OH oxidation at 90 % RH.

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Scheme 1. Proposed reaction mechanism of heterogeneous OH oxidation of MSA and CH3SO3Na leading to the formation of sulfite radial and formaldehyde.

5


Chemical Transformation of Methanesulfonic Acid and Sodium

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