Determination of the Photolysis Rate Coefficient of

Article pubs.acs.org/est

Determination of the Photolysis Rate Coefficient of Monochlorodimethyl Sulfide (MClDMS) in the Atmosphere and Its Implications for the Enhancement of SO2 Production from the DMS + Cl2 Reaction G. Copeland,† E. P. F. Lee,† R. G. Williams,‡ A. T. Archibald,§ D. E. Shallcross,§ and J. M. Dyke*,† †

School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, U.K. Molecular Spectroscopy Facility, RAL SPACE, STFC, Rutherford Appleton Laboratory, Harwell, Oxford OX11 0QX, U.K. § Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. ‡

S Supporting Information *

ABSTRACT: In this work, the photolysis rate coefficient of CH3SCH2Cl (MClDMS) in the lower atmosphere has been determined and has been used in a marine boundary layer (MBL) box model to determine the enhancement of SO2 production arising from the reaction DMS + Cl2. Absorption cross sections measured in the 28000−34000 cm−1 region have been used to determine photolysis rate coefficients of MClDMS in the troposphere at 10 solar zenith angles (SZAs). These have been used to determine the lifetimes of MClDMS in the troposphere. At 0° SZA, a photolysis lifetime of 3−4 h has been obtained. The results show that the photolysis lifetime of MClDMS is significantly smaller than the lifetimes with respect to reaction with OH (≈4.6 days) and with Cl atoms (≈1.2 days). It has also been shown, using experimentally derived dissociation energies with supporting quantumchemical calculations, that the dominant photodissocation route of MClDMS is dissociation of the C−S bond to give CH3S and CH2Cl. MBL box modeling calculations show that buildup of MClDMS at night from the Cl2 + DMS reaction leads to enhanced SO2 production during the day. The extra SO2 arises from photolysis of MClDMS to give CH3S and CH2Cl, followed by subsequent oxidation of CH3S.

1. INTRODUCTION Dimethyl sulfide (DMS, CH3SCH3) is a naturally emitted compound produced as a byproduct of the biodegradation of organosulfur compounds from phytoplankton throughout the marine environment.1,2 Current estimates show that DMS contributes between 20 and 30% of the total global sulfur budget, and in the southern ocean, it is the single largest source of atmospheric sulfur.3 Oxidation of DMS leads to the production of oxidized sulfur compounds, particularly sulfur dioxide (SO2), methanesulfonic acid (MSA, CH3SO3H), dimethyl sulfoxide [DMSO, (CH3)2SO], methylsulfonylmethane [(CH3)2SO2], and sulfuric acid (H2SO4),4−8 which serve as cloud condensation nuclei, contributing to the natural acidity of precipitation. DMS oxidation is also thought to provide a negative feedback mechanism to the Earth’s radiative balance, which will have an effect on the climate. This was first proposed by Charlson et al. and is known as the CLAW hypothesis,7 although recent work suggests that this feedback mechanism is not as significant as first thought.8 There have been many field and laboratory studies involving DMS oxidation in the past decade (e.g., refs 9−11), although © 2013 American Chemical Society

there are still considerable uncertainties concerning the oxidation of DMS in the atmosphere.12 The dominant daytime oxidant of DMS is the OH radical, with the NO3 radical being an important oxidant at night in coastal areas;13 the significance of NO3 in open ocean areas is thought to be small. Modeling studies using the oxidation of DMS with OH give rise to levels of SO2 that are lower than actually observed, and the oxidation of DMS with halogens has been suggested as a way of increasing the computed SO2 yields.14,15 In field measurements in the southern ocean,9,14 the DMS concentration shows a smooth variation with time with a maximum near sunrise (∼07.00 h) and a minimum near ∼15.00 h. The SO2 mixing ratios are typically half of the DMS mixing ratios, and the SO2 concentration shows a similar smooth variation with time but is out-of-phase with the DMS curve, showing a minimum at ∼07.00 h and a maximum at ∼15.00 h. Received: Revised: Accepted: Published: 1557

July 19, 2013 November 10, 2013 November 26, 2013 November 26, 2013 dx.doi.org/10.1021/es402956r | Environ. Sci. Technol. 2014, 48, 1557−1565

Environmental Science & Technology

Article

Table 1. (i) DMS Oxidation Mechanism Used in BAMBO44 (j⟨no.⟩ Refers to the Photolysis Rates Derived by the MCM Protocol45) and (ii) Chlorine Chemistry (j data* Means That Absorption Cross Sections and Quantum Yields Needed To Calculate j Values Were Taken from These References)a k/10−14 cm3 molecule−1 s−1

ref

(i) DMS Oxidation Mechanism

DMS + OH + O2 → CH3SCH 2OO + H 2O

500

44

DMS + OH + O2 → DMSO + HO2

282

44

DMS + NO3 + O2 → CH3SCH 2OO + HNO3

109

44

DMSO + OH + O2 → DMSO2 + HO2

14.9

44

CH3SCH 2OO + NO → CH3S + HCHO + NO2

1200

44

CH3SCH 2OO + NO → CH3SCH 2NO3

750

44

CH3SCH 2OO + HO2 → CH3SCH 2OOH + O2

500

44

CH3SCH 2NO3 + hν → CH3S + HCHO + NO2

j⟨ 53⟩

44

CH3SCH 2OOH + hν → CH3S + HCHO + OH

j⟨ 41⟩

44

CH3S (+ O2 ) → CH3SOO

2.54

CH3SOO → CH3SO2

7.00 × 10

44

44 13

CH3S + O3 → CH3SO + O2

490

44

CH3S + NO2 → CH3SO + NO

6070

44

CH3SO + O3 → CH3SO2 + O2

60

44

CH3SO + NO2 → CH3SO2 + NO

1200

44

DMSO + OH + O2 → CH3S(O)OH + CH3OO

9000

44

DMSO + OH + O2 → CH3SOCH 2OO + H 2O

100

44

CH3S(O)OH + OH → CH3SO2 + H 2O

9000

44

CH3SOCH 2OO + NO → CH3SO + HCHO + NO2

750

44

CH3SOCH 2OO + HO2 → CH3SOCH 2OOH + O2

995

44

CH3SOCH 2OOH + hν → CH3SO + HCHO + OH

j41

44

CH3SO (+ O2 ) → CH3SOO2

6.61

44

CH3SOO2 + NO → CH3SO2 + NO2

800

44

CH3SO2 (+ O2 ) → CH3O2 + SO2

7.00 × 1013

44

CH3SO2 + OH → MSA

5000

44

CH3SO2 + O3 → CH3SO3 + O2

30

44

CH3SO2 + NO2 → CH3SO3 + NO

400

44

CH3SO3 + HO2 → MSA + O2

5000

44

(ii) Chlorine Chemistry

DMS + Cl 2 → CH3SCH 2Cl + HCl

3.4

16, 17

CH3SCH 2Cl + OH + O2 → CH3SOH + CH 2ClOO

300

19

CH3SCH 2Cl + O2 → CH3S + CH 2ClOO

j2

this work

CH3SOH + OH → CH3SO + H 2O

5000

44

CH 2ClOO + NO + O2 → HCClO + HO2 + NO2

1900

46

CH 2ClOO + HO2 → CH 2ClOOH + O2

517

46

CH 2ClOO + CH3O2 → CH 2ClOH + HCHO + O2

40

47

CH 2ClOO + CH3O2 → HCClO + CH3OH + O2

40

47

CH 2ClOO + CH3O2 → CH 2ClO + CH3O + O2

120

47

CH 2ClOO + NO3 → HCClO + HO2 + NO2

250

45

CH 2ClOH + OH + O2 → HCClO + HO2 + H 2O

108

44

HCClO + OH → CO + Cl + H 2O

612

48

HCClO + hν + O2 → CO + Cl + HO2

j data*

46

HCClO + NO3 → CO + Cl + HNO3

0.3

45

CH 2ClO + O2 → HCClO + HO2

6.0

46

1558

dx.doi.org/10.1021/es402956r | Environ. Sci. Technol. 2014, 48, 1557−1565


Article

Table 1. continued k/10−14 cm3 molecule−1 s−1

ref

(ii) Chlorine Chemistry 0.2

CH3O + O2 → HCHO + HO2 a

46 12,49

The standard DMS mechanism used in BAMBO was derived based on the work of Lucas and Prinn with updates based on the recent reviews by Barnes et al.50 and gas kinetic database information provided by JPL46 and IUPAC (Atkinson et al.30). All rate coefficients are reported at 298 K.

CH3SCH 2Cl → CH3SCH 2 + Cl

With this in mind, we have previously studied the reaction between DMS and Cl2 using a flow tube interfaced with a photoelectron spectrometer,16,17 reaction (1). The products from this reaction were established as CH3SCH2Cl (MClDMS) and HCl, and the rate coefficient was determined to be k1 = (3.4 ± 0.7) × 10−14 cm3 molecules−1 s−1 at 294 ± 2 K. CH3SCH3 + Cl 2 → CH3SCH 2Cl + HCl

Each photolysis route is expected to lead to some SO2 production. For example, if photolysis cleaves the S−CH2Cl bond, then CH3S formed will be oxidized in air to form CH3SO2, which decomposes to SO2.20 In this work, electronic absorption spectroscopy will be used to record the absorption spectrum of MClDMS in the solar actinic region at known sample pressures and photoabsorption cross sections will be derived. This information will be used with known actinic solar fluxes at various solar zenith angles (SZAs) to calculate the photolysis rate coefficient, on the assumption that photodissociation occurs in the actinic region. This will then be incorporated into a marine boundary layer (MBL) model to determine the impact on SO2 production in the troposphere.

(1)

This work has shown that, although the reaction between DMS and Cl2 is moderately slow, it could lead to the production of extra SO2 in the atmosphere, depending on the fate of MClDMS. In a series of nighttime measurements of molecular chlorine, using chemical ionization mass spectrometry at a North American coastal site during on-shore wind conditions, Spicer et al.18 measured Cl2 mixing ratios in the range 1 to >7 days. The lifetime of MClDMS with respect to reaction with OH can be estimated because the rate coefficient of OH + MClDMS had been measured previously, with a discharge-flow technique.19 The rate coefficient obtained at 298 K is (2.5 ± 1.3) × 10−12 cm3 molecule−1 s−1. In this work,19 an estimate was also made of the room temperature rate coefficient of the reaction Cl + MClDMS as 1 × 10−10 cm3 molecules−1 s−1, estimated from known rate coefficients of Cl and OH reactions with related molecules as well as the known OH + MClDMS rate coefficient. Assuming average values of [OH] = 1 × 106 molecules cm−3 and [Cl] = 1 × 105 molecules cm−3 in the MBL,32 lifetimes of MClDMS with respect to reaction with [OH] and [Cl] can be calculated as 4.6 and 1.2 days, respectively. (These were determined as 1/kX[X], where X = [OH] or [Cl]). Upon comparison of these values with the lifetimes shown in Table 2, it is clear that, apart from when the sun is low in the sky, at SZA values greater than 70°, the lifetime of MClDMS with respect to photolysis is much lower than the lifetime with respect to reaction with OH and Cl. In this comparison, it must be borne in mind that the photolysis rate coefficients listed in Table 2 will be upper limits (and the derived lifetimes lower limits) because of the assumption that the quantum yield of photolysis of MClDMS is 1.00 in the actinic region. It is felt, however, that this is a reasonable approximation given the evidence from the electronic structure calculations that follows. Electronic Structure Calculations. i. Dissociation Energies of Reactions (2a)−(2c). Electronic structure calculations were carried out in order to obtain the dissociation energies of reactions (2a)−(2c) and to estimate the dissociation energy of MClDMS in its lowest triplet and singlet excited states. In order 1561



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energy of 5.55 eV (44766 cm−1). A comparison of Figure 1 with Table A6 in the Supporting Information indicates that the weak band centered at 34500 ± 100 cm−1 can be assigned to the formally forbidden (1)3A″ ← X̃ 1A′ transition with a computed excitation energy of 4.65 eV (37507 cm−1). It presumably gains some intensity via spin−orbit interaction of the (1)3A″ state with neighboring 1A″ states. The strong band centered at 42800 ± 50 cm−1 is assigned to the spin-allowed transition (3)1A″ ← X̃ 1A′ with a computed vertical excitation energy of 5.63 eV (45411 cm−1) (computed oscillator strength, f, of 0.0013). The experimental separation of the vertical positions of these two bands is 8300 ± 150 cm−1 compared to the computed separation of 7904 cm−1. The main contributors to the (1)3A″ ← X̃ 1A′ and (3)1A″ ← X̃ 1A′ transitions are one-electron excitations from molecular orbitals 25 to 27 and 25 to 28, respectively. These are both excitations from an S 3p nonbonding lone pair with some small C 2pπ antibonding contributions to a delocalized σ antibonding orbital. It is also noted that, with these assignments, both excited states, the (1)3A″ and (3)1A″ states, are dissociative in character, with dissociation occurring by breaking of the CH3S−CH2Cl bond (see Table A6 in the Supporting Information). Other bands will contribute to the spectrum with the (1)1A″ ← X̃ 1A′ transition, with a computed vertical excitation energy of 5.03 eV (40572 cm−1) and a computed oscillator strength, f, of 0.0001 being the most significant contributors (the main contributor to this excitation is the 25 → 26 transition, which is from an S 3p nonbonding lone pair, with small antibonding contributions from C 2pπ orbitals, to a delocalized σ antibonding orbital). The two bands observed in the absorption spectrum of CH3SCH2Cl have therefore been assigned on the basis of TD-DFT calculations to valence− valence transitions. On the basis of the above evidence, it is concluded that, in the overlap region shown in Figure 1 of the actinic flux and the weak absorption of MClDMS (the 28000−34000 cm−1 region), the excited state of MClDMS is dissociative and the quantum yield for photodissociation is close to unity. MBL Modeling Results. The results of the MBL modeling calculations are shown in Figures 2−5. The DMS oxidation mechanism used incorporates the MClDMS photolysis rate coefficient determined in this work at SZA = 0° (j2; see Table 2) as well as the MClDMS + Cl2 [reaction (1)] and MClDMS + OH rate coefficients measured in earlier work.16,19 A strong anticorrelation between the DMS and SO2 concentrations has been observed in several field campaigns.9,14 Figure 2 shows the output data from BAMBO for DMS and SO2 using a constant DMS source of 3.40 × 109 molecules cm−2 s−1 (as used by Chen et al.14) and a SO2 combined physical loss rate of 1.6 × 10−5 s−1, but with no Cl2 present. These model base-case results well reproduce the diurnal profiles derived by Chen et al.14 Under these conditions, DMS oxidation mainly occurs via reaction with OH (during the day) and NO3 (at night). As has been observed in several field studies,9,14 the SO2 concentration is anticorrelated with the DMS concentration, with the highest SO2 concentration predicted at late afternoon when the DMS concentration shows a minimum. Figure 3 shows the effect of adding Cl2 on the DMS mixing ratio. In this figure, the black line shows the DMS mixing ratio with no added Cl2 and the red line shows the effect of adding a flux of Cl2 of 6.0 × 109 molecules cm−2 s−1. As can be seen, the DMS concentration is reduced significantly (from approximately 290 to 75 pptv, at 12.00 h). The blue and

Figure 2. Plot of DMS and SO2 against time. DMS (black line) is shown to peak in concentration toward the early morning, after which the concentration is shown to be reduced to a minimum at late afternoon. The horizontal axis is time (day/h:min).

Figure 3. Plot of the DMS concentration against time. The black line is the DMS concentration when oxidation is considered for OH and NO3 initiation only, and the red line includes initiation by Cl2, with the Cl2 flux set to 6.0 × 109 molecules cm−2 s−1. The blue and pink lines show the data obtained by reducing the flux of Cl2 by a half (blue) and to a tenth (pink) of the base-case flux. The horizontal axis is time (day/h:min).

pink lines show the DMS mixing ratio obtained by reducing the Cl2 flux by a half (blue line) and by a tenth (pink line) of 6.0 × 109 molecules cm−2 s−1. By far the greatest impact on inclusion of reactions involving Cl2, notably reaction (1) and the photolysis process (2a), is on the predicted mixing ratio of DMS, which is shown to be reduced by up to two-thirds (see Figure 3). By including a flux of Cl2 suitable to maintain average nighttime Cl2 levels observed by Spicer et al.18 (i.e., ∼70 ppt; the red line in Figure 4; the base-case Cl2 plot), the DMS time profile (the red curve in Figure 3) looks dissimilar to any of the reconstructed timeseries measurements, which to date have clearly shown that there is a definite diurnal pattern with midday minima.9,14 1562



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(Figure 3) and a nighttime Cl2 mixing ratio (∼30 ppt) that is within the lower end of the range observed by Spicer et al.18 (see Figure 4). Another feature that is as equally important as the decrease in DMS upon inclusion of reactions (1) and (2a) is the increase in SO2. SO2 time profiles are shown in Figure 5. The base-case model, without reactions (1) and (2a), is shown by the red curve in this figure. This shows a clear diurnal profile that is out of phase with the DMS profile, where a minimum is produced during the early morning and a maximum occurs at late afternoon. Upon inclusion of reactions (1) and (2a), the SO2 maximum is found to shift earlier, toward the early morning (see the black curve in Figure 5), reflecting the photochemical source of SO2 coming from photolysis of MClDMS. This leads to a peak in SO2 at ca. 08.00 h around 80 ppt greater than the base case run at 08.00 h. By reduction of the Cl2 flux to half of the initially selected flux, this peak moves to later in the day, with the SO2 mixing ratio being 40 ppt greater than the base case at 08.00 h (blue curve in Figure 5), while reducing the initially selected Cl2 flux by a factor of 10 leads to a peak that is approximately equal to the base case at 17.00 h and 10 ppt greater than the base case at 08.00 h (see the pink curve in Figure 5). Therefore, the combined effects of reactions (1) and (2a) are consistent with the available measurements provided that an appropriate flux of Cl2 is used (e.g., 3.0 × 109 molecules cm−2 s−1). The rapid rise in SO2 in the highest Cl2 flux integration arises from direct formation of the CH3S radical from photolysis of MClDMS and its oxidation to SO2. The impact of the direct reaction between Cl and DMS is small in all simulations in keeping with the work of Chen et al.14 The central target in this paper has been to estimate the photolysis lifetime of MClDMS in the atmosphere and compare it with the lifetime with respect to reaction with OH and Cl. As can be seen from Table 2, the lifetime of MClDMS is 3.6 h at a SZA of 0°. This increases to 8.3 h at SZA = 60° and 15.0 h at SZA = 70°. At SZA = 78° and 86°, the photolysis lifetime increases to 35.8 h (1.5 days) and 185.1 h (7.7 days). The lifetimes with respect to OH and Cl reactions have been estimated as 4.6 and 1.2 days, respectively. It is clear therefore that, for SZA values in the region 0−70°, photolysis is the main daytime loss mechanism of MClDMS. It should be borne in mind, however, that the photolysis lifetimes calculated in this work have been derived on the assumption that the quantum yield for photolysis of MClDMS to give CH3S and CH2Cl at wavelengths in the actinic range is 1.0. This means that the photolysis rate coefficients and photolysis lifetimes determined in this work must be viewed as upper and lower limits, respectively. However, the results of the electronic structure calculations on the lowest excited state of MClDMS obtained in this work, combined with the experimental electronic spectra, indicate that the lowest excited state is dissociative and therefore the quantum yield for dissociation via the lowest electronic excitation must be close to unity. This leads to the conclusion that photolysis of MClDMS is the dominant loss mechanism in the troposphere for SZA values in the region 0−70°. The results obtained support the hypothesis that when DMS and Cl2 react at night to form MClDMS and HCl, the MClDMS formed is photolyzed during the following morning to produce CH3S and CH2Cl. CH3S leads to extra SO2 production in the atmosphere during the day because CH3S is oxidized by O3 and/or NO2 to form CH3SO2, which gives rise to SO2 from photolysis and/or thermal decomposition.

Figure 4. Cl2 concentrations, derived from the base case run where the title reaction is omitted, plotted in black and shown to match well with the reported values of Spicer et al.18 Inclusion of the reaction between DMS and Cl2 (red) is shown to reduce the Cl2 concentration by approximately two-thirds. The blue and pink plots show data for inclusion of the reaction but by varying the flux of Cl2 by a half (blue) and reducing to a tenth (pink). The horizontal axis is time (day/ h:min).

Figure 5. Base-case model run (without the DMS and Cl2 reaction) shown by the red line. Inclusion of the reaction (black line) is shown to dramatically alter the SO2 time profile with a sharp peak in the SO2 concentration predicted in the early morning. The blue and pink lines reflect SO2 time profiles based on including the reaction but by reducing the flux of Cl2 by a half (blue) and by a tenth (pink). The horizontal axis is time (day/h:min).

Given the error bars suggested by Chen et al.,14 such high Cl2 would significantly perturb both the DMS diurnal cycle and that of SO2 (black curve in Figure 5). Reducing the Cl2 flux to half of the initially selected flux is sufficient to increase the DMS concentration by 50 ppt, and a slight diurnal pattern can be seen where a shallow dip in [DMS] is present at ca. 16.00 h. Further reducing the Cl2 flux to a tenth of the initial flux produces a DMS pattern that matches very well with the basecase run with no Cl2, but the DMS mixing ratios are around 50 ppt lower (see Figure 3). It is notable that the base-case 0.5Cl2 integration produces a DMS plot that shows a diurnal variation 1563



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Inclusion of reactions (1) (DMS + Cl2 → MClDMS + HCl) and (2a) (MClDMS + hν → CH3S + CH2Cl) in the MBL box model has an impact on both SO2 production and DMS loss. Enhanced production of SO2 early in the day is the result of including these reactions in the box model. However, enhanced [Cl2] values affect the diurnal profiles of both SO2 and DMS and suggest that mixing ratios of Cl2 > 100 ppt are rare events and that more modest levels of 30−40 ppt are more appropriate in marine air because at these levels the diurnal behavior of both SO2 and DMS are consistent with field observations of these species, whereas higher Cl2 mixing ratios give diurnal plots that are not consistent with SO2 and DMS field measurements. Clearly, this work would be aided considerably by the detection of MClDMS and measurement of its concentration as a function of time in the troposphere.

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(8) Quinn, P. K.; Bates, T. S. The case against regulation via oceanic phytoplankton sulphur emissions. Nature 2011, 480, 51−56. (9) de Bruyn, W. J.; Harvey, M.; Cainey, J. M.; Saltzman, E. S. DMS and SO2 at Baring Head, New Zealand: Implications for the Yield of SO2 from DMS. J. Atmos. Chem. 2002, 41, 189−209. (10) Sciare, J.; Baboukas, E.; Mihalopoulos, N. Short-term Variability of Atmospheric DMS and its Oxidation Products at Amsterdam Island during Summer time. J. Atmos. Chem. 2001, 39, 281−302. (11) Knight, G. P.; Crowley, J. N. The reactions of IO with HO2, NO and CH3SCH3: Flow tube studies of kinetics and product formation. Phys. Chem. Chem. Phys. 2001, 3, 393−401. (12) Lucas, D. D.; Prinn, R. G. Mechanistic studies of dimethylsulfide oxidation products using an observationally constrained model. J. Geophys. Res. 2002, 107 (D14), ACH-12−1−ACH-12−26. (13) Bardouki, H.; Berresheim, H.; Vrekoussis, M.; Sciare, J.; Kouvarakis, G.; Oikonomou, K.; Schneider, J.; Mihalopoulos, N. Gaseous (DMSA, MSA, SO2, H2SO4, and DMSO) and particulate (sulfate and methanesulphonate) sulfur species over the northeastern coast of Crete. Atmos. Chem. Phys. 2003, 3, 1871−1886. (14) Chen, G.; Davis, D. D.; Kasibhatla, P.; Bandy, A. R.; Thornton, D. C.; Huebert, B. J.; Clarke, A. D.; Blomquist, B. W. A Study of DMS Oxidation in the Tropics: Comparison of Christmas Island Field Observations of DMS, SO2 and DMSO with Model Simulations. J. Atmos. Chem. 2000, 37, 137−160. (15) 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 boundary layer. Geophys. Res. Lett. 2010, 37, L02807. (16) Dyke, J. M.; Ghosh, M. V.; Kinnison, D. J.; Levita, G.; Morris, A.; Shallcross, D. E. A kinetics and mechanistic study of the atmospherically relevant reaction between molecular chlorine and dimethyl sulfide (DMS). Phys. Chem. Chem. Phys. 2005, 7, 866−873. (17) Dyke, J. M.; Ghosh, M. V.; Goubet, M.; Lee, E. P. F.; Levita, G. A study of the atmospherically relevant reaction between molecular chlorine and dimethyl sulphide (DMS): Establishing the reaction intermediate and measurement of absolute photoionization crosssections. Chem. Phys. 2006, 324, 85−95. (18) Spicer, C. W.; Chapman, E. G.; Finlayson-Pitts, B. J.; Plastridge, R. A.; Hubbe, J. M.; Fast, J. D.; Berkowitz, C. M. Unexpectedly high concentrations of molecular chlorine in coastal air. Nature 1998, 394, 353−356. (19) Shallcross, D. E.; Vaughan, S.; Trease, D. R.; Canosa-Mas, C. E.; Ghosh, M. V.; Dyke, J. M.; Wayne, R. P. Kinetics of the reaction between OH radicals and monochlorodimethylsulphide (CH3SCH2Cl). Atmos. Environ. 2006, 40, 6899−6904. (20) Chu, L. K.; Lee, Y. P. Transient infrared spectra of CH3SOO and CH 3 SO observed with a step-scan Fourier Transform spectrometer. J. Chem. Phys. 2010, 133, 184303. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. (22) Werner, H. J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; Celani, P.; Korona, T.; Lindh, R.; Mitrushenkov, A.; Rauhut, G.; Shamasundar, K. R.; Adler, T. B.; Amos, R. D.; Bernhardsson, A.; Berning, A.; Cooper, D. L.; Deegan, M. J. O.; Dobbyn, A. J.; Eckert, F.; Goll, E.; Hampel, C.; Hesselmann, A.; Hetzer, G.; Hrenar, T.; Jansen, G.; Köppl, C.; Liu, Y.; Lloyd, A. W.; Mata, R. A.; May, A. J.;

ASSOCIATED CONTENT

S Supporting Information *

UV−visible and IR experiments, IR absorption spectrum, electronic structure calculations, heats of formation, photolysis rate coefficient and photolysis lifetime, further details of the BAMBO calculations used for MBL modeling. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the NERC for supporting this work. G.C. thanks the NERC, and A.T.A. thanks the UK Met Office and Great Western Research for postgraduate studentships. The authors thank the EPSRC (UK) National Service for Computational Chemistry Software for a provision of computational resources. J.M.D. acknowledges support from the Leverhulme Trust for an Emeritus Fellowship.

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Determination of the Photolysis Rate Coefficient of

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