Kinetic and mechanistic study of the reaction between methane sulfonamide (CH3S(O)2NH2) and OH

Abstract. Methane sulfonamide (MSAM), CH3S(O)2NH2, was recently detected for the first time in ambient air over the Red Sea and the Gulf of
Aden where peak mixing ratios of ≈60 pptv were recorded. Prior to this study the rate constant for its reaction with the OH
radical and the products thereby formed were unknown, precluding assessment of its role in the atmosphere. We have studied the OH-initiated
photo-oxidation of MSAM in air (298 K, 700 Torr total pressure) in a photochemical reactor using in situ detection of MSAM and its
products by Fourier transform infrared (FTIR) absorption spectroscopy. The relative rate technique, using three different reference compounds, was used to derive a rate
coefficient of (1.4±0.3)×10-13cm3molec.-1s-1. The main end products of the photo-oxidation observed by FTIR
were CO2, CO, SO2, and HNO3 with molar yields of (0.73±0.11), (0.28±0.04), (0.96±0.15), and
(0.62±0.09), respectively. N2O and HC(O)OH were also observed in smaller yields of (0.09±0.02) and (0.03±0.01). Both the low
rate coefficient and the products formed are consistent with hydrogen abstraction from the −CH3 group as the dominant initial
step. Based on our results MSAM has an atmospheric lifetime with respect to loss by reaction with OH of about 80 d.



Introduction
Natural emissions of organosulphur compounds from phytoplankton comprise up to 60% of the total sulphur flux into the marine boundary layer (Andreae, 1990;Bates et al., 1992;Spiro et al., 1992) and in remote oceanic areas are the main source 20 of climatically active sulphate aerosols, which can influence the radiation balance at the earth's surface (Charlson et al., 1987;Andreae and Crutzen, 1997). The main organo-sulphur trace gases in the marine boundary layer are dimethyl sulphide (CH3SCH3, DMS) and its oxidation products dimethyl sulphoxide (DMSO), dimethyl sulphone (DMSO2), methyl sulphonic acid (MSA) and methyl sulphinic acid (MSI) for which atmospheric lifetimes with respect to their degradation by the OH radical vary between hours (DMS) and several weeks (DMSO2). 25 to the observations of Edtbauer et al. (2019), there have been no laboratory studies to investigate either its spectroscopy or 30 the kinetics of its reactions with atmospheric radicals, such as OH, so that its atmospheric lifetime and the products formed during its degradation in air were unknown. Crystalline MSAM melts at 363 K, has a boiling point of approximately 453 K and a vapour pressure of < 0.02 Torr at room temperature. Combining carbon, nitrogen, sulphur and oxygen in a single, small molecule, MSAM is an intriguing species not only as an atmospheric trace gas but also from a spectroscopic and kinetic perspective. Unlike basic alkyl amines such as e.g. CH3NH2, MSAM contains an acidic -NH2 group (Remko, 2003). 35 This work presents the first kinetic and mechanistic study of the OH induced oxidation of MSAM in air. A reaction mechanism is proposed that, through numerical simulation, describes the time dependent formation of the end-products we observed. From these results, we calculate the lifetime and the likely role of MSAM in the atmosphere.

Experimental set-up 40
The experimental set-up used to study the reaction of OH with MSAM has been described in detail previously (Crowley et al., 1999;Bunkan et al., 2018). Briefly, the reaction vessel was a 44.39 L cylindrical quartz-wall chamber equipped with a White-type multiple reflection mirror system resulting in an 86.3 m optical path length for absorption spectroscopy in the infra-red. The quartz reactor was at room temperature (296 ± 2 K) and for most experiments at 700 Torr of total pressure (1 Torr = 1.333 hPa). Six external, radially mounted, low pressure Hg-lamps emitting mainly at 253.65 nm provided a 45 homogeneous light flux within the reactor for radical generation. A 1000 Torr capacitance manometer was used to measure the pressure inside the reactor.
MSAM and other gases used to generate OH (see below) were mixed in a glass vacuum-line which was connected directly to the reaction chamber by a PTFE piping. Two capacitance manometers (10 Torr and 100 Torr ranges) were used to accurately measure pressures in the vacuum-line. Owing to its low vapour pressure, MSAM was eluted into the reaction chamber by 50 flowing synthetic air (450 cm 3 STP min -1 , sccm) through a trap containing crystalline MSAM warmed to 333 K, and subsequently through a second cold trap at 298 K (to prevent condensation downstream). This way we could ensure that the saturation vapour pressure of MSAM at 298 K was achieved.
Gas-phase infrared spectra in the range of 4000-600 cm -1 were recorded with a resolution of 2 cm -1 from 16 co-added interferograms (128 scans for the background) using a Fourier Transform Infra-Red (FTIR) spectrometer (Bruker Vector 22) 55 equipped with an external photoconductive mercury-cadmium-telluride (MCT) detector cooled to liquid nitrogen temperature. OPUS-software was used to analyze and manipulate the IR spectra. Interferograms were phase-corrected (Mertz) and Boxcar apodized with a zero-filling-factor of 4. Most of the products obtained (CO2, CO, HC(O)OH, HNO3 andSO2) were identified and quantified from the IR reference spectra of pure samples under similar experimental conditions (700 Torr and 298.2 K, Figure S1).
The low vapour pressure of MSAM precluded accurate dosing into the chamber and thus generation of a calibration spectrum. In order to calibrate the infra-red absorption features of MSAM we oxidized it in air and then conducted a sulphur and nitrogen balance of the products. As discussed below, the only sulphur-containing product detected from MSAM degradation was SO2 (which can easily be calibrated) and the only nitrogen contained products were HNO3 and N2O, which can also be calibrated. Experiments in which MSAM was almost completely converted to known amounts of SO2, HNO3 and 65 N2O thus provided an indirect calibration (via assumption of 100% sulphur or nitrogen balance) of its concentration and thus IR-cross-sections.

Generation of OH
OH was generated by the 254 nm photolysis of O3 in the presence of H2.
Further reactions that cycle OH and HO2 (e.g. OH + H2, H + O3, HO2 + O3) are listed in Table S1 of the supplementary information.
In a typical experiment, the starting concentrations of O3 and H2 were ≈ 5 × 10 14 and ≈ 5 × 10 16 molecule cm -3 . The large 75 excess of H2 ensures that O( 1 D) does not react with MSAM. As described previously (Bunkan et al., 2018), this scheme generates not only OH radicals but (via e.g. (R3) also HO2. HO2 is not expected to react with MSAM but will influence the course of secondary reactions in this system (e.g. by reacting with organic peroxy radicals) and thus the end-product distribution, as described in detail in section 3.5. Simulations of the radical concentrations when generating OH in this manner indicate that the HO2 / OH ratio is approximately 30, with individual concentrations of ≈ 1 × 10 11 molecule cm -3 HO2 80 and 3 × 10 9 molecule cm -3 OH.
As OH source, the photolysis of O3 in the presence of H2 has the advantage over other photochemical sources (e.g. photolysis of H2O2, HONO or CH3ONO) that neither H2 nor O3 have strong absorption features in the infra-red, resulting in a less cluttered spectrum which simplifies retrieval of concentration-time profiles of reactants and products.

Chemicals 85
A commercially available sample of methane sulphonamide (Alfa Aesar, > 98%) was used. O3 was generated by flowing synthetic air (Westfalen) through a stainless steel tube that housed a low-pressure Hg-lamp (PenRay) emitting at 184.95 nm.

Relative rate constant determination
The rate constant (k4) of the reaction between OH and CH3SO2NH2 (Reaction R4) was measured using the relative rate method using (in different experiments) formic acid (HC(O)OH), acetone (CH3C(O)CH3) and methanol (CH3OH) as reference compounds. 95 Relative rate constants were derived by monitoring the depletion of one or more IR-features of MSAM relative to those of 100 the reference compounds. The following expression links the depletion factors (e.g. ln([MSAM]0/[MSAM]t) to the relative rate coefficient: This analysis inherently assumes that the only loss process for MSAM and the reference molecules is reaction with OH.
Experiments in which the starting gas-mixture was allowed to stand for several hours with no discernible loss of MSAM, formic acid, methanol or acetone confirmed that none of these gases are lost to the wall or react with O3 to a significant extent. From observation of MSAM and O3 mixtures we were able to derive an upper limit for the reaction of MSAM with O3 of 1 × 10 -19 cm 3 molecule -1 s -1 . 115

Vibrational characterisation of CH3SO2NH2
The experimental, infra-red absorption spectrum of MSAM ( Fig. 1) shows characteristic bands corresponding to SO2 stretching vibrations at 1385 and 1172 cm -1 , the NH2 wagging vibration at 857 cm -1 , stretching vibrations at 3476 and 3380 cm -1 , bending at 1551 cm -1 , and the CH3 wagging band at 976 cm -1 . Assignment of the infra-red features (Table 1)  should be sufficient to describe the relative energies for the isomers. Harmonic vibrational frequencies and zero-point energies (ZPE) were calculated at these levels of theory to check whether the stationary points obtained were either isomers or first-order transition states (all calculated conformers had only real frequencies). The high accuracy energy method 125 Gaussian-4 with Møller-Plesset expansion truncated at second order (G4MP2) was also used for the calculation of the barrier energies. The determination of the Hessian matrix also enabled the calculation of the thermochemical quantities for the conformers at 298.15 K. All symmetry restrictions were turned off in the calculations. All calculations were run with the Gaussian 09 program package (Frisch, 2010). Assuming that the point group for the molecule is Cs, all 24 fundamental modes should be both IR and Raman active, fourteen of them belonging to A′ representation and ten to A′′. All the 130 vibrational frequencies are real and positive. The assignments in Table 1 were made from an evaluation of the normal modes displacement vectors; as many of the modes are strongly coupled, this information is rather subjective. The frequencies of the absorption bands of the theoretical spectrum displayed in Fig.1 were adjusted by a scaling factor of 0.968 ± 0.019 recommended for the B3LYP/aug-cc-pVTZ level of theory (column "ratio" in Table 1) (Halls et al., 2001).
If the "cold trap" is removed, extra absorption bands originating from the MSAM-dimer are observed. These slowly 135 disappear with time as the condensation of the low-volatility dimer to the reactor surfaces takes place. Fig. S1 of the supplementary information shows the IR spectra of the dimer after the subtraction of the monomer. A complete characterization of the vibrational modes is presented in Table S1 of the supplementary information. According to our calculations, two hydrogen-bond interactions between the -HNH•••OSO-are formed in the dimer which produce a bathochromic shift of the absorption bands. For each kinetic experiment we ensure that no dimer band is present in the initial 140 spectrum.

Relative rate measurements: Determination of k4
Once the concentrations of MSAM and the reference compound were stable (i.e. mixing in the chamber was complete) the Hg-lamps were switched on for a period of typically one hour during which FTIR spectra (duration of ~20 seconds) were obtained every few minutes. The concentrations of the reactants in each individual experiment can be found in Table 2. 145 Figure 2 shows the loss of absorption features due to MSAM and the reference compound (in this case acetone) at different times during the experiment.
The depletion of MSAM was quantified by integrating the Q-branch of the 857, 1172, 1383 cm -1 absorption bands, and the complete absorption band at 3380 cm -1 . The relative depletion of the MSAM absorption-features agreed to within ~ 5%.
Depletion of the reference gases were quantified by integrating their absorption bands at 1221-1249 cm -1 (acetone), 2788-150 3070 cm -1 (methanol) and 1073-1133 cm -1 (formic acid). An alternative analysis procedure, in which the relative depletion of MSAM was derived by scaling a reference spectrum of MSAM (e.g. that obtained prior to photolysis) to match those at various times after photolysis was also used. The depletion factors thus obtained were indistinguishable from those using individual absorption features.  to OH radicals in 700 Torr of synthetic air at 296 K. A linear least-squares analysis of the data gives rate constant ratios k4 / k5 = (0.778 ± 0.008), k4 / k6 = (0.307 ± 0.004) and k4 / k7 = (0.158 ± 0.002) where the quoted errors are two standard deviations. Table 2 summarizes the experimental conditions and the rate coefficient ratios obtained when using each MSAM absorption band. The difference between the rate coefficient ratios obtained for the three absorption bands experiments is always less than 5%. The rate constant ratios were placed on an absolute basis using evaluated rate coefficients (Atkinson et 160 al., 2006;IUPAC, 2019) whereby k5 = (1.8 ± 0.36) × 10 -13 , k6 = (4.5 ± 1.8) × 10 -13 , and k7 = (9.0 ± 1.8) × 10 -13 cm 3 molecule -1 s -1 . We derive values of k4 (relative to acetone) = (1.40 ± 0.28) × 10 -13 , k4 (relative to formic acid) = (1.38 ± 0.55) × 10 -13 , and k4 (relative to methanol) = (1.42 ± 0.28) × 10 -13 cm 3 molecule -1 s -1 (where the uncertainties include uncertainty associated with the evaluated rate coefficients for k5, k6 and k7). The values of k4 obtained using the three deferent reference compounds are, within experimental uncertainties, identical, indicating the absence of significant systematic errors associated with the 165 use of the reference reactants. We prefer the value of k4 from the experiment using acetone as reference. For acetone, the relative rate constant is close to unity and the rate coefficient for OH has been extensively studied and is associated with low uncertainty. The preferred value of the rate coefficient, k4, is (1.4 ± 0.3) x 10 -13 cm 3 molecule -1 s -1 where the uncertainty is 2σ.

Product yields 170
In order to identify and quantify the end-products of the title reaction in air, approximately (6.25 ± 0.75) × 10 13 molecule cm -3 of MSAM, 4.04 × 10 14 molecule cm -3 of O3 and 1.00 × 10 15 molecule cm -3 of H2 were loaded into the chamber at a total pressure of 700 Torr of synthetic air and 298 K. Subsequent to initiation of the reaction between OH and MSAM by switching the Hg-lamps on, IR-spectra (700 -4000 cm -1 ) were taken at 300 seconds intervals. Figure 4 also displays reference spectra (measured at the same temperature and pressure) of the compounds we identified as 180 reaction products. Other than CO2 and CO, nitric acid (HNO3) and sulphur dioxide (SO2) are easily identified, with weak features from N2O, NO2 and formic acid (HC(O)OH) also apparent. The absorption of each product was converted to a concentration using calibration curves that were obtained at the same pressure and temperature (see  nitrogen and carbon are conserved, we can derive initial concentrations of MSAM (from the slope) of 6.12 ± 0.08 × 10 12 190 molecule cm -3 (based on the sulphur balance), 5.10 ± 0.05 × 10 12 molecule cm -3 (based on the nitrogen balance) and 7.4 ± 0.2 × 10 12 molecule cm -3 (based on the carbon balance at the maximum fractional depletion of MSAM). As already mentioned, total carbon is very likely to be overestimated due to its formation and desorption at/from the walls of the chamber. As the main nitrogen product is HNO3, which has a large affinity for surfaces and which is likely to be lost to the walls, we also expect that use of reactive nitrogen will result in an underestimation of the initial MSAM concentration. For these reasons 195 we consider that the best method to estimate the initial concentration of MSAM is via the formation of SO2. Figure S3 of the supplementary information illustrates the strict proportionality between the relative change of the SO2 and MSAM absorption features.
From this experiment we derive an initial MSAM concentration of (6.1 ± 1.0) × 10 13 molecules cm -3 and use this value to derive the absorption cross-sections for MSAM (these are given in Fig. 1 time, whereas the initial MSAM concentration was derived using all the SO2 data in this experiment as described above. As N2O contains two N-atoms, the nitrogen balance is thus 0.80 ± 0.13. It is likely that some HNO3 is lost to reactor surfaces, explaining the deviation from unity. Note that if we had used the nitrogen balance to derive the MSAM IR-cross-sections, the SO2 yield would have exceeded unity.

Reaction mechanism 215
The time dependent formation of HNO3, SO2, N2O and CO provide important clues to the reaction mechanism. Addition to the S-atom is not possible so that the initial step will be abstraction of hydrogen by the OH radical, either from the -CH3 group (Reaction R8a) or from the -NH2 group (Reaction R8b): Based on results of previous studies of the reactions of OH with trace-gases containing both CH3 and -NH2 entities (e.g. CH3NH2 or CH3C(O)NH2) we expect abstraction at the -CH3 group (Reaction R8a) to dominate (Onel et al., 2014;Borduas et al., 2015;Butkovskaya and Setser, 2016). H-abstraction at the methyl-group is also consistent with a rate coefficient for R4 that is very similar to that for OH + acetone.

Abstraction from the -CH3 group 225
In section 3.4.1 we focus on the fate of the peroxy radical, OOCH2SO2NH2, formed by reaction of initially formed CH2SO2NH2 with O2 (R9). The most important reactions of organic peroxy radicals are self-reactions (R10) or reactions with NO (R11), NO2 (R12), or HO2 (R13).
Peroxy nitrates such as the one formed in Reaction (R12) are thermally unstable with respect to dissociation back to reactants at room temperature and given the very low concentrations of NO2 in our system, Reaction (R12) will not play a 235 significant role in this study.
The oxy-radical, OCH2SO2NH2 formed in Reactions (R10) and (R11) will react with O2 to produce an aldehyde (Reaction R14). Alternatively, it could undergo C-S bond cleavage (Reaction R15) to form formaldehyde (CH2O) and the SO2NH2 radical: The fate of HC(O)SO2NH2 will be reaction with OH to form C(O)SO2NH2 (R16) which will dissociate to form CO + SO2NH2 (R17). The rate coefficient for reaction (R16) is expected to be ≈10 -11 cm 3 molecule -1 s -1 as for many similar reactions of OH with aldehydes (e.g. CH3CHO).
The predominant fate of formaldehyde will be reaction with OH to form CO and subsequently CO2: The above reactions explain, at least qualitatively, the observed formation of CO, CO2 and HC(O)OH. Note that the room 255 temperature rate coefficient for reaction of OH with HCHO is large (8.5 × 10 -12 cm 3 molecule -1 s -1 , Atkinson et al. (2006)) compared to that for reaction with CO (2.2 × 10 -13 cm 3 molecule -1 s -1 Atkinson et al. (2006)), which explains why CO was observed as an intermediate product at high concentrations whereas HCHO was not.
The likely fate of the SO2NH2 radical formed in Reaction (R15) is either reaction with O2 to generate SO2NH or dissociation by S-N bond-scission to produce SO2 and the NH2 radical. 260 We did not observe features in the IR-spectrum that that could be assigned to SO2NH based on the spectrum reported by Deng et al. (2016) and propose that reaction (R25) is the source of SO2 as a major reaction product. By analogy with the thermal decomposition of the similar CH3SO2 radical, which dissociates to CH3 and SO2 on a millisecond time scale (Ray et 265 al., 1996) The final products are thus the same as those resulting from the self-reaction of the peroxy radical. The path from MSAM to the observed end-products including the reactive intermediates that were not observed is illustrated in Fig. 7.

Abstraction from the -NH2 group
In analogy to the reaction between CH3C(O)NH2 and OH (Barnes et al., 2010), H-abstraction from the -NH2 group is expected to result in decomposition of the initially formed CH3SO2NH radical via C-S bond fission. 300 The methyl radical would react with O2 to form the methyl-peroxy radical and in subsequent reactions (via CH3O) would result in CH2O formation. As discussed above CH2O will be efficiently oxidized to CO and CO2 in this system. However, the characteristic IR-absorption bands (Deng et al., 2016) of the SO2NH product were not observed in our experiments and calculations at the G4MP2 level of theory indicate that Reaction (R40) is endothermic (by 137 kJ mol -1 ). We conclude that 305 H-abstraction from the -NH2 group is a minor channel.

Kinetic Simulation
The proposed reaction mechanism (considering initial reaction by H-abstraction from the -CH3 group only) was tested by kinetic simulation using the KINTECUS program package (Ianni, 2015). The reactions used in the chemical scheme and the associated rate coefficients are presented in Table S2. Where experimental rate coefficients were not available, we used rate 310 parameters from similar reactions, and rationalize these choices in the text associated with Table S2. The experimental and simulated concentration-time profiles are in good agreement except for CO2. As described in section 3.5, CO2 is generated from the cell walls and cannot be used quantitatively. The good agreement with the N2O (formed from NH2 in Reaction R28) and HNO3 experimental data suggests that the fate of NH2 (the only source of reactive nitrogen in this system) is accurately described in the model. Note that the wall loss rate of HNO3 (1 × 10 -5 s -1 ) in the simulation was 320 adjusted to match the HNO3 profile. The simulated amount of HNO3 lost to the wall at the end of the experiment was ≈ 14% of that formed, which helps to explain the non-unity yield of gas-phase nitrogen compounds.
The grey line in Fig. 8 represents the sum of SO2 + SO3 + H2SO4, i.e. all model trace gases containing sulphur, which, in the absence of IR absorption features of SO3 or H2SO4, we equate to SO2. We now draw attention to the fact that SO2 (the yield of which is constant with time, see Fig. 6) is only well simulated if we neglect its removal by OH (Reaction R41). 325 Otherwise, using the preferred rate constant (IUPAC, 2019) at 700 Torr and 298 K of 9.0 × 10 -13 cm 3 molecule -1 s -1 we find that the simulated SO2 concentration is significantly reduced and its yield is time dependent. At one bar of air, collisionally stabilized HOSO2 is converted within 1 µs to HO2 and SO3. In the atmosphere, SO3 reacts with H2O to form H2SO4 (R43).
The conversion of SO3 to H2SO4 may be suppressed under our "dry" conditions. 330 SO3 + (H2O)n → H2SO4 + (H2O)n-1 (R43) SO2 should therefore not behave like a stable end-product in our experiments, but be converted to more oxidized forms. In order to confirm that SO2 is a stable end product in our experiments, we measured the relative rate of loss of SO2 and acetone under the same experimental conditions (Fig. S4 of the supplementary information). The apparent, relative rate constant k41 / 335 k5 was 0.46, which converts to an effective rate constant for SO2 loss of 8.2 x 10 -14 cm 3 molecule -1 s -1 . This is more than a factor of ten lower than the preferred value, indicating that the net rate of the OH-induced SO2 loss in our system is much lower than expected and not simply governed by the rate constant for the forward reaction to form HOSO2. The reformation of SO2 under our experimental conditions is subject of ongoing experiments in this laboratory, which are beyond the scope of the present study. We note that the unexpected behaviour of SO2 does not significantly impact on the conclusions drawn 340 from the present study.

Atmospheric Implications
The rate coefficient for a number of tropospheric, organo-sulphur trace gases are listed in Table 3. The rate coefficient for the title reaction is significantly lower than those for CH3SCH3 (CH3SCH3) and CH3S(O)CH3, (DMSO) for which reaction with OH is the major atmospheric loss process (lifetimes of hours), but comparable to CH3S(O)2CH3 which also has two S=O 345 double bonds. However, as for most tropospheric trace gases, the lifetime of MSAM will be controlled by a number of processes including photolysis, reactions with the three major oxidants, OH, NO3 and O3 as well as dry deposition (kdd) and heterogeneous uptake to particles (khet), followed by wet deposition. The lack of C=C double-bonds in MSAM suggest that the reaction with O3 will be a negligible sink, which is confirmed by the low upper limit to the rate constant of 1 × 10 -19 cm 3 molecule -1 s -1 described in section 2.4. Whereas the reaction with 350 NO3 represents an important loss mechanism for DMS, we do not expect this to be important for MSAM. CH3SCH3 reacts with NO3 (despite lack of a C=C double bond) as the high-electron density around the sulphur atom enables a pre-reaction complex to form prior to H-abstraction. This mechanism is not available for MSAM because the electron density around the sulphur atom is reduced by the two oxygen atoms attached to it, which also provide steric-hindrance.
Owing to its low vapour pressure, we were unable to measure the UV-absorption spectrum of MSAM, but note that it was 355 not photolysed at a measureable rate by the 254 nm radiation in our study. We conclude that photolysis in the troposphere, where actinic flux only at wavelengths above ≥ 320 nm is available is a negligible sink of MSAM.
Therefore, the lifetime of MSAM can be approximated by: Using our overall rate coefficient, k4 = 1.4 × 10 -13 cm 3 molecule -1 s -1 for the title reaction and taking a diel-averaged OH 360 concentration of 1 x 10 6 molecules cm -3 , we can use equation (2) to calculate a first-order loss rate constant of k4[OH] = 1.4 × 10 -7 s -1 . Which is equivalent to a lifetime of ≈ 80 days.
MSAM is highly soluble and a dry deposition velocity of ≈ 1 cm s -1 to the ocean has been estimated (Edtbauer et al., 2019).
Combined with a marine boundary height of ≈ 750 ± 250 m, this results in a loss rate coefficient of 1.3 × 10 -5 s -1 or a lifetime with respect to uptake to the ocean of less than one day. Wet deposition is also likely to play a role, which may limit the 365 MSAM lifetime to days under rainy conditions to weeks in dry regions.
To a first approximation the heterogeneous loss rate of a trace gas to a particle is given by: where γ is the uptake coefficient which represents the net efficiency (on a per collision basis) of transfer of MSAM from the gas-phase to the particle phase), ̅ is the mean molecular velocity of MSAM (~26000 cm s -1 ) and A is the surface area 370 density of particles (in cm 2 cm -3 ) for which a typical value in low to moderately polluted regions would be 1 × 10 -6 cm 2 cm -3 .
A rather low uptake coefficient of ~2 × 10 -5 would then be sufficient to compete with MSAM loss due to reaction with OH, but a value of 2 × 10 -3 would be necessary to compete with dry-deposition.