Heterogeneous Oxidation of Particulate Methanesulfonic Acid by the

Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Heterogeneous Oxidation of Particulate Methanesulfonic Acid by the Hydroxyl Radical: Kinetics and Atmospheric Implications Emma L. Mungall,† Jenny P. S. Wong,†,‡ and Jonathan P. D. Abbatt*,† †

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, United States

‡

ABSTRACT: Dimethyl sulfide (DMS) is a major source of sulfur to the marine boundary layer (MBL), and methanesulfonic acid (MSA) is one of its two main final oxidation products. MSA can participate in the nucleation and growth of aerosol particles, thereby affecting clouds and climate, and is used as a tracer of biological sulfur inputs. Unlike MSA, the other major oxidation product of DMS, sulfate, has several other sources, including volcanic and anthropogenic inputs. As a result, MSA to non-sea-salt sulfate (nss-sulfate) ratios are often used as proxies for biological activity; i.e., the MSA to nss-sulfate ratio in aerosol particles is used to estimate the marine biological contribution to nss-sulfate in the MBL. We present here a determination of the reactive uptake coefficient, γ, for the heterogeneous oxidation of MSA by hydroxyl radicals within deliquesced ammonium sulfate aerosol particles with an MSA mass fraction of 0.16 (a typical marine value) at room temperature. We find γ = 0.05 ± 0.03. For high ambient gas-phase concentrations of the hydroxyl radical, this uptake coefficient corresponds to an estimated lifetime against heterogeneous oxidation of only a few days for MSA in MBL aerosol particles. Significantly, for typical gas-phase and condensed concentrations of the hydroxyl radical, the lifetime estimated here for heterogeneous oxidation is shorter than that for condensed-phase oxidation of MSA. This finding should be taken into consideration when using MSA to nss-sulfate ratios as tracers for DMS and biological activity, especially for air masses that have been exposed to considerable photochemical oxidation. KEYWORDS: MSA, heterogeneous kinetics, MSA to nss-sulfate ratio, marine aerosol, photo-oxidation, aerosol mass spectrometer

1. INTRODUCTION Dimethyl sulfide (DMS), which is produced biogenically in the oceans, is an important source of sulfur to the marine boundary layer (MBL) globally.1 The two major oxidation products of DMS are sulfate and methanesulfonic acid (MSA).2,3 These compounds have such low volatilities that they are found largely in the condensed phase in the atmosphere3 and, thus, can form new aerosol particles or condense upon existing aerosol particles.4 Recent work has shown that DMS oxidation products may play a direct role in new particle formation and growth in the Arctic troposphere in the summer.5−9 While the overall effects of MSA and sulfate on MBL cloud condensation nuclei (CCN) globally remain uncertain,10,11 DMS oxidation products certainly contribute significantly to marine aerosol particle populations and likely played an important role in MBL CCN concentrations in the pre-industrial past.12 Furthermore, as anthropogenic sulfur emissions decrease, natural sulfur will perforce play a more important role in determining MBL CCN concentrations. While the oxidation of DMS is the only known source of MSA to the atmosphere, aerosol sulfate can also be formed by the oxidation of anthropogenic or volcanic SO2. Sulfate is also emitted directly from the ocean as an important component of sea spray aerosol. DMS is widely used as a proxy for marine biological activity,13−15 and MSA concentrations as well as the ratio of MSA to non-sea-salt sulfate (nss-sulfate) have often © XXXX American Chemical Society

been used as tracers for DMS inputs to aerosol. MSA in ice cores has also been used to draw conclusions regarding how sea ice extent and marine productivity have changed in the past.16−19 While the presence of MSA is an indisputable marker for marine biogenic influence, the validity of using it as a conservative or even semi-conservative tracer was first called into question over 30 years ago20 and has often been questioned since.21−24 The DMS oxidation mechanism is very complex and not yet fully characterized.2,3 Furthermore, the branching ratios of this complex oxidation mechanism are strongly dependent upon environmental conditions, such as the temperature, prevalence of different oxidants (i.e., OH•, NO•, O3, and BrO), and presence of clouds.2,3,23,24 As a result of the interest in the climate effects of DMS via aerosol particles, many attempts have been made to parametrize this reaction scheme for inclusion in chemical transport or climate models.23−30 The level of detail in these model mechanisms varies, but all are missing chemical processes which strongly affect the relative yields of MSA and sulfate.3 Received: Revised: Accepted: Published: A

October 10, 2017 December 4, 2017 December 5, 2017 December 5, 2017 DOI: 10.1021/acsearthspacechem.7b00114 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Article

ACS Earth and Space Chemistry

Figure 1. Schematic of the aerosol flow tube system used for the kinetic experiments. Abbreviations are defined in the main text.

bound MSA and not in the gas phase on the time scale of the experiment.2,3 2.1. Aerosol Generation. A dilute solution of ammonium sulfate and MSA is prepared and placed in an atomizer. An atomizer flow of 3500 standard cubic centimeter per minute (sccm) N2 generates polydisperse particles, which, after passing through a dryer, are size-selected by a differential mobility analyzer (DMA, TSI model 3080) to 100 nm. These particles pass into a mixing volume, where they are subjected to 80% relative humidity (RH) conditions by means of dilution with a humidified flow (Figure 1). The deliquesced particles then pass into the flow tube, where the RH is 75%. 2.2. Flow Tube. The flow tube used was the Toronto Photo-Oxidation Tube (TPOT), which has been described in detail previously.38−40 This setup allows for the introduction of deliquesced particles into a flow tube, where they are subjected to OH• oxidation over a time scale of 90 s. Hydroxyl radicals are generated in the gas phase by the photolysis of ozone by ultraviolet (UV) lamps (λ = 254 nm) in the presence of water vapor. Ozone is generated by passing 100 sccm of O2 gas through a Jelight ozone generator (model 600). This flow is combined with the deliquesced-aerosol-containing flow in a mixing volume, ensuring that the flow entering the TPOT is well-mixed. Ozone mixing ratios in the TPOT were determined by an ozone monitor (2B Technologies model 202). Ozone mixing ratios were set between 300 and 2000 ppbv in the experiments presented here. The OH• exposure resulting from a given ozone mixing ratio and residence time were determined by monitoring the decay of SO2 (supplied to the flow tube from a pressurized glass bulb containing 3% SO2 in N2) using a SO2 monitor (Thermo 40C), as described previously.39 Assuming a second-order rate constant of 9.6 × 10−13 cm3 molecules−1 s−1 for the reaction of SO2 with OH•,41 the OH• exposure can then be determined from eq 1. The OH• exposures used for the kinetic experiments were equivalent to 2−13 days of exposure to ambient OH• concentrations of 1 × 106 molecules cm−3. The highest OH• exposure used in this study, 1.2 × 1012 molecules cm−3 s, was generated by doubling the residence time in the flow tube (to 175 s) at the highest O3 mixing ratio.

A further complication in using MSA and, in particular, its ratio to sulfate as a tracer for DMS emissions is the oxidation of MSA to sulfate by the hydroxyl radical (OH•) and the Cl2− anion radical. The latter reaction is expected to be particularly important in sea salt aerosol particles.3 The gas-phase reaction with the hydroxyl radical is so slow as to be negligible,2 but the aqueous-phase reaction has been shown to proceed somewhat more quickly. The OH• concentration in atmospheric condensed phases (cloudwater and aerosol) is highly uncertain, with estimates ranging over 4 orders of magnitude.31,32 The lifetime of MSA against oxidation by OH• generated in situ in aerosol particles or cloud droplets would be just a few days if average condensed-phase OH• concentrations lie at the higher end of these estimates (∼1 × 10−13 M). While this chemistry has been known for over 30 years20,33 and has periodically been flagged as a potential obstacle to using MSA as a tracer,3,20,22,23,34,35 the majority of studies in the literature ignore this process. Perhaps this is reasonable, given that most estimates of condensed-phase OH• concentrations are much lower than 1 × 10−13 M.31,32,36 Hoffmann et al.3 estimate that, under most conditions, only a few percentages of MSA will be converted to sulfate by OH• or the Cl2− anion radical as a result of aqueous-phase processing (in cloudwater or aerosol particles) during the tropospheric lifetime of an aerosol particle of about a week. Heterogeneous oxidation, in which gas-phase OH• impacts an aerosol particle and reacts with MSA, provides another route to MSA oxidation by rendering MSA in aerosol particles susceptible to oxidation by gas-phase OH• radicals. It is now well-recognized that heterogeneous oxidation processes are widely prevalent for aerosol organic compounds,37 but heterogeneous oxidation of MSA has been overlooked to this point by the laboratory community and, hence, is not currently included in any modeled DMS oxidation schemes. In this work, we present the first exploration of the heterogeneous kinetics of MSA. In some settings, MSA may be lost quickly via heterogeneous oxidation, with a lifetime of a few days for high ambient gas-phase OH• concentrations (1 × 107 molecules cm−3) or weeks for more typical values (1 × 106 molecules cm−3). This has potentially important implications for the use of MSA as a tracer, reinforcing previous calls for caution in interpreting MSA to nss-sulfate ratios as an indication of biological input to marine aerosol.

[OH•]t = −ln

2. EXPERIMENTAL SECTION The experimental setup is shown in Figure 1. It is adapted slightly from a setup that has previously been described in detail.38,39 In a typical experiment, size-selected deliquesced ammonium sulfate particles containing MSA are introduced to a flow tube, where they are exposed to gas-phase OH•. The decay of MSA is monitored using an aerosol mass spectrometer (AMS), allowing for the quantification of the loss of MSA as a function of OH• exposure. MSA Henry’s law constant and gasphase rate constant for reaction with OH• are so high and low, respectively, that reaction with OH• occurs only with aerosol-

[SO2 ]i [SO2 ]f

(1)

Control experiments with lights only or ozone only were used to confirm that MSA does not react with O3 or photolyze at 254 nm. As expected from the literature,2 no loss of MSA was detected by the AMS in the absence of OH•. 2.3. Detection. The particle number and volume were monitored continuously using a scanning mobility particle sizer (SMPS, TSI DMA model 3080 and CPC model 3772). Deliquescence of the aerosol particles was confirmed by a ∼45 nm increase in the peak of the particle size distribution under wet (80% RH) as opposed to dry (20% RH) conditions, consistent with an expected growth factor of 1.5 for B

DOI: 10.1021/acsearthspacechem.7b00114 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Article

ACS Earth and Space Chemistry deliquescence of ammonium sulfate.42 Particle sizes did not change over the course of the kinetic experiments, indicating that particles remained deliquesced throughout. The decay of MSA in the aerosol particles was monitored using the Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne), which measures the composition and mass of non-refractory components of aerosol particles.43 The method used for quantitative detection of MSA has been detailed previously.8 The fragmentation table and calibration determined in that work were used here. Data analysis was carried out in IGOR Pro version 6.37 with the ToF-AMS Analysis Toolkit 1.56D (HR Analysis 1.15D). Normalizing the observed MSA signals to sulfate ensured that any changes in collection efficiency or transmission through the aerodynamic lens of the AMS would not affect the kinetic results. The average mass ratio of MSA to sulfate before OH• exposure was 0.16. In addition to being an environmentally relevant ratio, this excess of ammonium sulfate was used such that the conversion of MSA to sulfate would not significantly affect the measured mass of sulfate. The largest change to the sulfate mass possible from MSA oxidation in these experiments would be ∼7%. As discussed in section 3.2, this error is negligible compared to the error associated with determining the mass of sulfate (estimated at 35%).44 Ammonium sulfate particles were chosen over equally atmospherically relevant but non-sulfur-containing sodium chloride particles because of the difficulties associated with measuring the latter using the AMS45 as well as the potential for a competing reaction between MSA and Cl2−.3

Figure 2. Decay of particulate MSA upon gas-phase OH• exposure is plotted as the natural logarithm of the ratio of the initial signal of MSA as measured by the AMS to the signal after exposure to OH•. The bottom axis shows the OH• exposure in molecules cm−3 s, while the top axis shows the equivalent atmospheric exposure for heterogeneous oxidation in days, assuming an ambient average OH• concentration of 1 × 106 molecules cm−3. The colored lines show the calculated decay of MSA at each OH• exposure for three uptake coefficients bracketing the value determined in this study.

measured RH.46 Having determined the mass fraction of sulfate 2− 4 (mSO , the fraction of sulfate mass to the total particulate f mass), the measured mass ratio of MSA to sulfate (mMSA/ mSO42−) is then used to determine mMSA according to eq 4. f mMSA SO4 2− MSA mf mf = mSO4 2− (4)

3. RESULTS 3.1. Calculation of the Uptake Coefficient. Two quantities are extracted from the AMS data for each experiment. The first is the ratio of the MSA signal following OH exposure (MSAt) to the initial MSA signal (MSA0), which is used to calculate the second-order rate constant for the reaction, and the second is the mass of particulate MSA and sulfate in each experiment, which is used in the calculation of the uptake coefficient. During a given kinetic experiment, the O3 mixing ratio entering the TPOT is held constant and the UV lights are turned on and off approximately every 10 min, repeatedly generating a reproducible OH• exposure. In this manner, MSA0 and MSAt are measured repeatedly and the ratio of each pair of MSA0 and MSAt values (averaged over a 5 min interval) is calculated. That ratio is then considered a single data point. Each of the 32 points plotted in Figure 2 represents one of these ratios. The second-order rate constant, k, for the loss of MSA was determined to be 6.2 × 10−13 cm3 molecule−1 s−1 from the data presented in Figure 2 according to eq 3. • MSA t = e(−k[OH ]t ) MSA 0

ln

MSA t = −k[OH•]t MSA 0

Finally, the uptake coefficient for OH to the MSA-containing deliquesced ammonium sulfate particles in these experiments is calculated according to eq 547 γ=

2kDρaq mfMSA NA 3 cOH ̅ MMSA

(5)

where D is the wet diameter of the aerosol particles (as determined by the SMPS, 146 nm on average in our experiments), ρaq is the density of the deliquesced particles, NA is Avogadro’s number, cO̅ H is the mean free speed of OH•, and MMSA is the molecular mass of MSA. We find that γ = 0.05 ± 0.03. 3.2. Uncertainties in the Uptake Coefficient. Because the uncertainty on the MSA signal is low (i.e., there is a high signal-to-noise ratio), the uncertainty in k is taken as the error of the slope from the linear regression. The error on the slope is only a few percentages, in keeping with an uncertainty on that order arising from the OH• exposure calibration. The uncertainty in the masses of MSA and sulfate determined by the AMS is much larger, estimated at 35%.44 Because the ratio is used, this error is propagated, yielding a 50% error on the mass fraction of MSA. The major source of uncertainty in the uptake coefficient thus arises from the uncertainty in the mass fraction of MSA. The uncertainties associated with the other quantities necessary for the calculation of γ are minor. Uncertainties also arise from variations in RH between experiments, leading to variability in the diameters and calculated densities of the particles. However, these uncertain-

(2)

(3)

The second quantity, the ratio of the mass of MSA to the mass of sulfate, is used to calculate the MSA mass fraction (mMSA ). f The E-AIM model II with solid formation suppressed was used to estimate the density of the aerosol particles and the mass fractions of the major components (sulfate, ammonium, and water) given the initial ratio of ammonium to sulfate and the C


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Figure 3. (a) Lifetime of MSA against OH oxidation within a deliquesced ammonium sulfate aerosol particle is plotted against the gas-phase OH• concentration for a 146 nm particle with an MSA to sulfate ratio of 0.16, such as those in this experiment. The black line displays the relationship for an uptake coefficient of 0.05, while the dotted and dashed lines display the relationship for the upper and lower uncertainty bounds of the uptake coefficient, respectively. (b) Lifetime of MSA against OH oxidation within an aerosol particle with the same characteristics as in panel a is shown for varying aerosol particle diameters according to the color scale at the far right as a function of the gas-phase OH• concentration. (c) Lifetime of MSA with respect to oxidation by condensed-phase OH• as calculated from a literature rate constant2 is shown according to the color scale at the far left as a function of the hours per day available for oxidation (y axis; 24 h for aerosol particles and ∼3 h for cloudwater32,36) and condensed-phase OH• concentration (x axis).

heterogeneous oxidation process, we need to compute lifetimes, which is done in section 4.2. A large number of studies of OH• heterogeneous kinetics for reactions with a wide range of organic compounds have found uptake coefficients of at least 0.1, with values sometimes exceeding unity due to radical recycling within the particle.37 A reported value of at least 0.1 for the uptake coefficient of OH to halide-containing aerosols is also relevant to the marine aerosol situation.51 The smaller value measured here for reactive uptake of OH• to MSA-containing particles is likely a result of the low inherent reactivity between OH• and MSA; i.e., the gas-phase rate constant is only 2.24 × 10−14 s−1,3 consistent with a considerable activation energy that will be prevalent in the heterogeneous reaction as well. Because these other common components of marine aerosol (halides and organic species) are likely to be present in higher concentrations than MSA, they may preferentially consume OH• both within the condensed phase and at the surface, effectively shielding MSA from loss by OH• oxidation. As a further caveat, it must be stated that the uptake coefficient determined in this work applies to roomtemperature oxidation of deliquesced ammonium sulfate particles. It is possible that this reaction does not proceed as rapidly under cold conditions, and we do not have any information on how this reaction proceeds in the case of an effloresced particle, although we note that efflorescence is unlikely in MBL conditions and that ammonium sulfate particles remain deliquesced over a wide range of conditions, including at temperatures well below 0 °C.52 4.2. Rates of MSA Loss in the Atmosphere. The lifetime of MSA in an aerosol particle exposed to a given OH• concentration can be estimated from eq 7

ties are less than 2%. Finally, consideration of mass transfer limitations for the gas-phase diffusion of OH• to the surface of the particles according to Fuchs and Sutugin requires a correction of ∼1%,48 which is also negligible compared to the uncertainty arising from the mass fraction determination. Propagation of errors using the uncertainties in each quantity in eq 5 yields the reported absolute uncertainty of 0.03.

4. DISCUSSION 4.1. Interpretation of the Uptake Coefficient. As mentioned above, we are not assessing the loss rate resulting from OH• that could be generated photochemically in the particles in an atmospheric setting. The only source of OH• in this experiment is in the gas phase. In that context, the conventional manner to express the kinetics is by way of an OH• uptake coefficient that reflects the probability that gasphase OH• will react with MSA upon collision with a particle. We cannot, however, be certain as to the location of the oxidation reaction. The reaction may occur in the bulk of the particle away from the interface. However, if MSA concentrations are enhanced at the surface of the particles, as suggested for liquid solutions,49 then it becomes more likely that the oxidation reaction is occurring at the interface. If we consider a homogeneously mixed particle in which the only reactive species is MSA, we can estimate the reacto-diffusive length of OH• (l, the mean distance into the particle OH• that will travel before reacting) using a diffusion constant for OH•, DOH = 2.2 × 10−5 cm2 s−1,50 the second-order rate constant for the aqueous reaction of OH• and MSA, kaq = 1.2 × 107 M−1 s−1,2 and the concentration of MSA within the particles (calculated using the quantities in eq 5) according to eq 6. l=

DOH kaq[MSA]

τMSA,g = (6)

For our experimental conditions, l = 16 nm or ∼10% of the particle diameter, suggesting that the oxidation reaction is not occurring at equal rates throughout the bulk of the particle but rather close to the interface. The question which then naturally arises is how the kinetics in this heterogeneous chemistry compare to the kinetics of MSA loss by OH• generated in the condensed phase. To assess the atmospheric significance of this

NMSA 1 • γAcOH ̅ [OH ]g 4

(7)

where A is the area of that particle and NMSA is the number of MSA molecules in that particle. NMSA is calculated from eq 8 NMSA = mfMSA

mp MMSA

(8)

where mp is the mass of that particle. D


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ACS Earth and Space Chemistry A literature value for the reaction rate constant2 (kaq = 1.2 × 107 M−1 s−1) can be used to estimate its aqueous-phase lifetime in cloud or aerosol water from eq 9. τMSA,aq =

1 kaq[OH•]aq

such as in the tropics or above the Greenland or Antarctic ice sheets.53−56 Ascertaining whether the oxidation of MSA by OH• is an important loss process or not will have to await more definite determinations of OH• concentrations in atmospheric liquid water and a reduction of the uncertainty in the reactive uptake coefficient determined here. Overall, we would argue that there is potential for significant decay of MSA in atmospheric aerosol particles during transport. At longer transport times, the effects of both heterogeneous and aqueous-phase losses of MSA will become more pronounced. Indeed, work in Antarctica has shown that MSA measured in snowpack and ice core samples decreases with the distance inland, possibly due to the loss of MSA during transport.27 This additional source of variability in the MSA to nss-sulfate ratio is not currently accounted for in the use of that quantity as a tracer for biogenic influence. 4.3. Atmospheric Implications. The MSA to nss-sulfate ratio is lower in the tropics than it is at higher latitudes.14,57 Isotopic studies have confirmed that this is not due to larger anthropogenic nss-sulfate contributions in the tropics.58 It has been suggested that this trend is due to the temperature dependence of the relative yields of MSA to sulfate in the oxidation of DMS,14 but other studies have questioned this result.27 In light of the decay process studied in this work, we can speculate that the higher OH• levels in the tropics (annual mean from ∼3 × 106 to 4 × 106 molecules cm−3)54,59 may be contributing to the lower MSA to nss-sulfate ratio. Some studies of Antarctic ice cores have found higher MSA concentrations during the last glacial maximum.16 Intriguingly, there have been suggestions that OH• levels were lower during that time and that reactive halogen chemistry was more active.60 Because DMS can be oxidized not only by OH• but also by BrO, O3, and NO3•, while MSA is only known to be oxidized by OH• and Cl2−, changes in the oxidant balance may well lead to changes in the ratios of products of DMS oxidation, That is, higher MSA may be present when OH• mixing ratios are lower.

(9)

The lifetime of MSA calculated from eqs 7 and 9 is shown as a function of the OH• concentration in Figure 3. Panels a, b, and c of Figure 3 show the dependence of the lifetime of MSA on the uncertainty of the γ value determined in this work, the diameter of the aerosol particle, and the hours per day available for oxidation, respectively. In the case of an aerosol particle, the latter quantity is 24 h, but when cloudwater is considered, it is likely to be closer to 3 h.32,36 The lifetime of MSA against OH• oxidation depends strongly upon each of these variables. The range in Figure 3a from the lower bound of the uptake coefficient to the upper bound is 35 days for an OH• concentration of 1 × 106 molecules cm−3. Figure 3b shows that the lifetime of MSA against heterogeneous OH• oxidation ranges from days to years depending upon the diameter of the aerosol particle in question and the ambient gas-phase OH• concentration. Estimates of the OH• concentration in atmospheric liquid water range over several orders of magnitude,31,32 as shown on the x axis of Figure 3c. Some of the spread in these estimates is due to the consideration of different sources of OH•, and some of it results from both the variety and the uncertainty regarding condensed-phase OH• sinks. In summary, the lifetime of MSA depends very strongly upon the particular environment that it finds itself in and, in particular, on the ambient OH• concentration in that environment. Furthermore, while we have not examined the reaction of MSA with Cl2− here, we note that it could represent a significant sink of MSA in marine aerosols, where concentrations may be elevated.3 Clearly, the effect of these competing processes on the lifetime of MSA in the MBL is not easily understood. The uncertainty and variability in these environmental parameters and the large uncertainty in the uptake coefficient determined in this work preclude a quantitative comparison of these loss pathways, either among themselves or to other losses of particulate phase MSA such as wet or dry deposition. However, if we use a best guess of important values, we can place the decay of MSA as a result of heterogeneous OH• oxidation in the context of these other loss processes. The average lifetime against deposition of aerosol particles in the troposphere is on the order of a week.42 An oft-cited average value for tropospheric OH• is 1 × 106 molecules cm−3 (shown at the center of Figure 3b). An average value for condensedphase OH• is harder to come by, but for the sake of argument, we will choose 1 × 10−14 M and consider the aerosol case with its potential for 24 h of oxidation in a day. According to Figure 3c, for those values, the lifetime of MSA against oxidation by OH• generated in the condensed phase is on the order of a year. In contrast, for a deliquesced ammonium sulfate aerosol with a diameter of 100 nm, Figure 3b suggests that the lifetime of MSA against heterogeneous OH• oxidation and deposition will be roughly equivalent at about a week. At higher gas-phase OH• concentrations, heterogeneous MSA oxidation might be the predominant loss process. It is worth noting that higher gasphase OH• levels (closer to 1 × 107 molecules cm−3; shown to the right of Figure 3b) can be found in some environments,

5. CONCLUSION Assumptions regarding the stability of MSA have led to it and, in particular, its ratio to sulfate being used as a tracer for DMS emissions (and by proxy, marine biological activity) in both present day aerosol and ice cores. The fact that MSA within aerosol particles is subject to heterogeneous oxidation by OH•, as demonstrated in this study, contributes to the complexity inherent in using MSA to nss-sulfate ratios for the determination of the relative biological contribution to MBL sulfur. We determined the uptake coefficient of the hydroxyl radical to MSA-containing deliquesced ammonium sulfate aerosol particles (γ = 0.05). For an accumulation-mode aerosol particle with a MSA to sulfate ratio of 0.16 (a value typical of MBL aerosol),8 the lifetime of MSA will be on the order of days to weeks, depending upon the ambient gas-phase OH• mixing ratios to which it is exposed. This decay of MSA in aerosol particles is potentially significant, because atmospheric lifetimes for aerosol particles are on the order of a week. It is very difficult to estimate the aqueous-phase lifetime given the high degree of uncertainty in condensed-phase OH• concentrations. If measured values31 from 1 × 10−15 to 1 × 10−14 mol L−1 are representative, the lifetime of MSA against oxidation by OH• generated within the condensed phase will be on the order of years. It is thus possible that the majority of MSA oxidation in E


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the atmosphere is occurring as a result of heterogeneous oxidation. This result adds a new perspective to our understanding of the stability of MSA in the MBL. We suggest that including the heterogeneous oxidation of MSA in modeling efforts might help to resolve outstanding questions in the literature concerning MSA to nss-sulfate ratios. In light of the complexity of the DMS oxidation scheme, the MSA to nss-sulfate ratio appears to have limitations that may preclude its use in quantitatively unravelling the chemical and biological processes at play in the MBL, although it remains useful as a qualitative indicator of marine biological influence.8,61 An improved parametrization of DMS oxidation and the marine sulfur cycle, one which includes the oxidation of MSA in aerosols, might allow for a more robust interpretation of MSA concentrations and MSA to nss-sulfate ratios in both the present day MBL and ice cores. Understanding the effect of this loss process on measured MSA concentrations may also help to constrain models of DMS oxidation in the atmosphere. Finally, we suggest that an excellent next step would be the exploration of MSA oxidation in snow and ice. Such an investigation is long overdue. Oxidation of MSA may be even more important when considering ice cores, because once aerosol particles are deposited on a snow surface, they will not immediately be protected from the oxidative capacity of the atmosphere, particularly in areas with low accumulation rates. Regardless of the mechanism at play, post-depositional oxidation has the potential to drastically alter the MSA to nss-sulfate ratios that are ultimately preserved in ice cores.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Emma L. Mungall: 0000-0003-2567-5090 Jenny P. S. Wong: 0000-0002-8729-8166 Jonathan P. D. Abbatt: 0000-0002-3372-334X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Megan Willis for help with the MSA fragmentation table. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).

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REFERENCES

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Heterogeneous Oxidation of Particulate Methanesulfonic Acid by the

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