J. Phys. Chem. 1996, 100, 8895-8900
8895
Kinetics of the Thermal Decomposition of the CH3SO2 Radical and Its Reaction with NO2 at 1 Torr and 298 K Alok Ray,† Isabelle Vassalli, Ge´ rard Laverdet, and Georges Le Bras* Laboratoire de Combustion et Syste` mes Re´ actifs, C.N.R.S., and UniVersite´ d’Orle´ ans, 45071 Orle´ ans Cedex, France ReceiVed: January 2, 1996; In Final Form: March 12, 1996X
The kinetics and mechanism of the thermal decomposition of CH3SO2 and its reaction with NO2 have been investigated in a discharge flow reactor at 298 K and 1 Torr pressure using Cl/Cl2/CH3SH/NO2/He mixtures: CH3SO2 + NO2 f CH3SO3 + NO (3); CH3SO2 + M f CH3 + SO2 + M (4). The rate constants k3 and k4 were derived from the fitting of calculated and experimental kinetic profiles of CH3O and SO2 monitored by laser-induced fluorescence (LIF) analysis. CH3 produced in reaction 4 was rapidly converted into CH3O by reaction with NO2. The values obtained were k3 ) (2.2 ( 1.1) × 10-12 cm3 molecule-1 s-1 and k4 ) 510 ( 150 s-1. Errors are 1 standard deviation. The role of these reactions in the atmospheric oxidation mechanism of dimethyl sulfide is discussed.
Introduction Dimethyl sulfide (DMS) is the main component of the natural sulfur emissions which represents approximately 60 and 15% of the total sulfur flux in the Southern and Northern hemispheres, respectively. It has been hypothesized in recent years that DMS has a regulatory influence on the Earth’s climate through the formation of aerosols and cloud condensation nuclei (CCN).1 The capacity of DMS to produce new aerosols through its atmospheric oxidation is greatly dependent on the distribution of the end products, particularly SO2 and CH3SO3H (methanesulfonic acid, MSA). SO2 can lead to new particle production through oxidation into gaseous H2SO4. In contrast, MSA is believed to condense on preexisting particles. The MSA and SO2 yields in the atmospheric DMS oxidation are dependent on a very complex gas phase mechanism (see reviews of e.g. refs 2-4). The atmospheric DMS oxidation is initiated mainly by OH reaction during daytime and also NO3 during nighttime. The reactions proceed for a large part (for OH5) and exclusively (for NO36) through a net H-abstraction mechanism, producing the peroxy radical CH3SCH2O2. This peroxy radical is further oxidized into CH3SCH2O, which is considered to readily decompose into CH3S and CH2O. CH3S is sequentially oxidized to CH3SO and CH3SO2 by NO2 and O3, although reaction of CH3S with O2 to produce CH3SOO adduct has been suggested as a potentially important additional pathway at low temperature.7 The fate of CH3SO2 is a key process in the DMS oxidation mechanism, since it can influence the MSA/SO2 yield (e.g. ref 8). CH3SO2 can decompose or be oxidized by species like NO2 or O3 to produce CH3SO3 which is likely to yield MSA by reaction with hydrogenated species. In this work, the thermal decomposition of CH3SO2 and its reaction with NO2 have been investigated in a discharge-flow study of the Cl/CH3SH/NO2 system. In this system, CH3S, produced by reaction of Cl atoms with CH3SH, initiated the following sequence of reactions: † Present address: Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Bombay 400 085, India. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00012-3 CCC: $12.00
CH3S + NO2 f CH3SO + NO
(1)
CH3SO + NO2 f CH3SO2 + NO
(2)
CH3SO2 + NO2 f products
(3)
CH3SO2 + M f CH3 + SO2 + M
(4)
In this mechanism, CH3 further reacted with NO2 to form CH3O. The rate constants k3 and k4 have been determined by a fitting procedure of calculated kinetic profiles to experimental profiles of SO2 and CH3O obtained by laser induced fluorescence (LIF) analysis. No experimental determination of k3 has been reported so far. The only one previous determination of k4 had been obtained in our laboratory,9 also from a discharge flow study of the Cl/CH3SH/NO2 system, but in a more indirect way, and omitting reaction 3 in the chemical mechanism used in model calculations. Experimental Section The discharge flow apparatus equipped with laser-induced fluorescence and quadrupole mass spectrometric detection, shown in Figure 1, was similar to that used in recent work.10,11 The flow tube (2.4 cm i.d.) had a 50 cm long reaction zone coated with halocarbon wax. Helium was used as the carrier gas and the average gas flow velocity in the flow tube was ∼1700 cm s-1. A constant fraction of the flow was effusively sampled through a 100 µm pinhole into the chamber of the mass spectrometer. The pressure in this chamber was ∼8 × 10-7 Torr. The pressure in the reactor, around 1 Torr, was measured through a port located at the middle of the reaction zone by a 10 Torr full scale capacitance manometer. The flow rate of the carrier gas was measured by a linear mass flowmeter. The flow rates of the other chemicals used (CH3SH, Cl2, NO2, and C2H3Br) were obtained by measuring the decrease of their pressure in a known volume. In the case of NO2, a correction was made to account for the equilibrium with its dimer. The LIF cell was located upstream of the mass spectrometer, and the distance between the LIF and mass spectrometric detection points was ca. 7 cm. The LIF excitation source was frequency-doubled radiation of a Lambda Physik FL3002 dye laser pumped by a Lambda Physik excimer (XeCl) laser. The © 1996 American Chemical Society
8896 J. Phys. Chem., Vol. 100, No. 21, 1996
Ray et al.
Figure 1. Schematic diagram of the discharge flow reactor coupled to laser-induced fluorescence and mass spectrometric analysis.
dye Rhodamine-6G was used to obtain the excitation wavelengths for SO2 and CH3O. As discussed by Mellouki et al.9 the reaction of Cl with CH3SH is an appropriate source of CH3S radicals:
Cl + CH3SH f CH3S + HCl
(5)
Nesbitt and Leone12 reported that the alternative channel producing CH2SH and HCl accounts for less than 2%. Cl atoms were generated in the sliding injector by flowing ∼1% Cl2 in helium through a microwave discharge, while the other reactants CH3SH and NO2 were introduced directly into the reactor through a side arm tube (see Figure 1). The discharge tube was coated with diluted phosphoric acid in order to get a high dissociation yield of Cl2 (a yield of 40-70%, which is the fraction of dissociated Cl2, was observed by mass spectrometric analysis of Cl2 at m/e ) 70). Under these conditions, CH3S reacted predominantly with NO2, making negligible its reaction with undissociated Cl2:9
CH3S + Cl2 f CH3SCl + Cl
(6)
k6 ) 1.0 × 10-12 cm3 molecule-1 s-1, at 298 K13 (Rate constants are given at 298 K, unless specified.) The rate ratio k1[NO2]/k6[Cl2] was always higher than 6 × 103. In addition, as CH3SH was allowed to react with Cl atoms directly in the reactor, and subsequently CH3S with Cl2, possible complication was avoided due to CH3S wall reaction, as observed by Domine et al.14 Also, the mass spectrometric signals at m/e ) 79 corresponding to CH3SS and at m/e ) 95 corresponding to CH3SSO (upon addition of NO2) were not observed, as found by Domine et al.14 Besides the selfrecombination of the CH3S radical was also unimportant due to its fast reaction with NO2. Our mass spectrometric observations were consistent with these arguments. In the absence of NO2, signals were observed at m/e ) 82 and 94 corresponding to CH3SCl and CH3SSCH3 (DMDS) produced by reaction 6 and the self-recombination of CH3S, respectively. When NO2 was added at typical concentrations of the present study, signals at m/e ) 82 and 94 became negligible. The appearence of a new peak at m/e ) 96, possibly due to CH3SO3H (MSA), will be discussed latter. The absence of dark reaction between CH3SH and NO2 was verified by mass spectrometric analysis at their parent peak (m/e ) 48 and 46, respectively). Also, no LIF signal of SO2 and CH3O was observed. Also, CH3SH was not used in large excess
over Cl atoms. The CH3SH concentration was high enough to favor the Cl + CH3SH reaction over the slow reaction of Cl with NO2 (k ) 4.7 × 10-14 cm3 molecule-1 s-1 13). Thus, the kinetic study was performed using the following ranges of initial concentrations of reactants: [Cl]0 ) (1.4-2.4) × 1012, [Cl2]0 ) (1.9-9.4) × 1011, [CH3SH]0 ) (2.3-6.4) × 1012, [NO2]0 ) (0.4-7.5) × 1014 (units are in molecule cm-3). [Cl2]0 is the residual chlorine concentration with the discharge on. The absolute Cl atom concentration was determined by a chemical titration before and after each experiment using the fast reaction:
Cl + C2H3Br f Br + C2H3Cl
(7)
k7 ) 1.43 × 10-10 cm3 molecule-1 s-1 15 The consumption of excess C2H3Br was monitored at its molecular peak (m/e ) 106), whereas the concentration of undissociated Cl2 concentration, [Cl2]0, was measured at m/e ) 70. In the course of this work, kinetic profiles of CH3O and SO2 were monitored by pulsed laser-induced fluorescence. The methane sulfonyl radical, CH3SO2, generated by reaction 2, decomposed to the SO2 molecule and the CH3 radical. The latter radical reacted with NO2 to produce the CH3O radical
CH3SO2 + M f CH3 + SO2 + M
(4)
CH3 + NO2 f CH3O + NO
(8)
k8 ) 2.3 × 10-11 cm3 molecule-1 s-1 13 the time constant of CH3/CH3O conversion (τ ) 1/k8[NO2]) being less than 0.4 ms. The CH3O radical was detected by LIF via its electronic transition (A2A1-X2E) either at 292.8 nm (ν3′ ) 4 band) or at 298.3 nm (ν3′ ) 3 band). SO2 has three main regions of absorption in the near-UV, a very weak absorption in the region from 390 to 340 nm, a stronger absorption in the region 320-250 nm, and an even stronger absorption in the 220-190 nm region.16 We have selected to excite SO2 at a wavelength around 300 nm in the second transition (A ˜ 1B11 17 X A1) region, so that both kinetic profiles of SO2 and CH3O could be monitored simultaneously. Excitation of SO2 at 300 nm produces broad emission spectrum with a maximum around 360 nm. Interference due to fluorescence of other reaction products in the range 290-305 nm of the CH3O and SO2 detection is unlikely. Especially, CH3SO2 interference is not expected,
Thermal Decomposition of the CH3SO2 Radical
Figure 2. Fluorescence excitation spectra of SO2 and CH3O: (a) SO2 alone; (b) CH3O (produced from CH4 + Cl + NO2 system); (c) SO2 and CH3O produced in the CH3S + NO2 system. The upward (v) and downward (V) arrows indicate characteristic peaks of SO2 and CH3O, respectively.
considering the liquid phase absorption of this species which peaks at ca. 350 nm.18 The excitation spectra of SO2 and CH3O were obtained using a box-car integrator (Model EG & G) while the dye laser was scanned from 290 to 302 nm. The source of SO2 was a 1% SO2-He mixture, while CH3O was generated by reaction of Cl atom with CH4 in the presence of NO2. In both cases, the total pressure in the reactor was around 1 Torr. In kinetic experiments with the CH3S + NO2 system, the excitation spectra of the products were recorded in the same wavelength region, 290-302 nm, with a similar scan rate of the laser. The observed CH3O excitation spectrum agrees well with that reported in the same region by Inoue et al.19 and Kappert et al.20 The observed excitation spectrum of SO2 can be compared to that reported by Mettee21 in this region. The excitation spectrum of SO2 was found to be much weaker than that of CH3O. In general, the stronger bands of SO2 overlapped with that of CH3O. However, the characteristic peaks of SO2 and CH3O were clearly observed in the recorded excitation spectra of the products of the CH3S + NO2 system, as illustrated in Figure 2. The observed fluorescence lifetime of CH3O was 1.8 µs for excitation at 298.3 nm (ν′ ) 3), which agrees well with a reported lifetime of 1.93 µs by Inoue et al.19 The fluorescence lifetime of SO2 (SO2 + He mixture) at 300.06 nm was ∼1.8 µs, but it reduced to ∼1.2 µs in the presence of CH3SH and NO2. The latter value agreed with the same value measured for SO2 in the CH3S + NO2 system. These spectroscopic observations confirmed that SO2 and CH3O were indeed the products in the CH3S + NO2 system. Also, the linearity of fluorescence intensity with SO2 concentration was observed for SO2 e 2 × 1012 molecule cm-3. As shown in Figure 2, overlaps exist in the bands of CH3O and SO2. The SO2 contribution at CH3O bands at 292.8 and 298.3 nm was typically around 10% under the conditions of the kinetic experiments. On the other hand, the CH3O contribution was around 5% at the bands at 294.7 and 300.06 nm, which were selected to monitor SO2.
J. Phys. Chem., Vol. 100, No. 21, 1996 8897 The fluorescence signal intensity of CH3O was corrected for SO2 and vice versa. This was done by calibration of the fluorescence signals of both SO2 ([SO2] ∼ 1 × 1012 molecule cm-3) in the presence of NO2 and CH3SH flows, and CH3O (produced by CH4 + Cl + NO2 reaction), at the exciting wavelengths of both SO2 and CH3O. Besides, the CH3O fluorescence signal intensity was corrected for quenching by CH4, considering the rate constant, kq ) 1.1 × 10-10 cm3 molecule-1 s-1.22 The undispersed fluorescence was collected at right angle to the laser beam and flow reactor axis, and imaged onto the PMT (Hamamatsu R2560, spectral response 300-650 nm) using two plano-convex quartz lenses. A cutoff filter (Schott UG3) at 310 nm with maximum transmittance at ∼350 nm was used to discriminate fluorescence against exciting radiation. The photomultiplier was operated in the pulse counting mode, and pulses were preamplified (SR445) before discrimination and integrated in a multichannel analyser (SR 430). The fluorescence signal was integrated over 300 laser pulses for each record. The laser was operated at 10 Hz. The pulse energy of exciting radiation was estimated to be higher than 1 mJ with a bandwidth of 0.3 cm-1. The background signal due to the scattered laser radiation with discharge on, but without NO2 or CH3SH, was measured at the two exciting wavelengths (298.3 and 300.06 nm) and subtracted from the total signal to obtain the fluorescence signal. At S/N ) 1, the detection sensitivity of SO2 and CH3O was ∼5 × 1010 and 1 × 109 molecule cm-3, respectively. The purity of reactants used was as follows: He (Alphagaz, 99.9995%) was used as the diluent passed through a liquid N2 trap before entering the reactor; Cl2 (Ucar, laser quality), C2H3Br (Ucar, >99.5%), CH4 (Ucar, 99.9%), and CH3SH (Ucar, >99.5%) were used without further purification; NO2 (Alphagaz, 99%) was purified by trap to trap distillation at ∼-80 °C and then collected at room temperature. Results and Discussion The reaction of CH3S with NO2 produced CH3SO2 by consecutive reactions 1 and 2. The generated CH3SO2 decomposed to CH3 and SO2; the CH3 radical in turn reacted with NO2 to form CH3O. This was confirmed by observing characteristic excitation bands of SO2 and CH3O in the kinetic system (see Experimental Section):
CH3S + NO2 f CH3SO + NO
(1)
CH3SO + NO2 f CH3SO2 + NO
(2)
CH3SO2 + M f CH3 + SO2 + M
(4)
CH3 + NO2 f CH3O + NO
(8)
Fluorescence signal intensity of CH3O and SO2 was measured as a function of reaction time in the reactor. Typical temporal profiles of CH3O and SO2 LIF signals are shown in Figure 3. Such temporal profiles were measured for different NO2 concentrations in the range (1-5) × 1014 molecule cm-3. The temporal profiles of SO2 showed fast rise during about 10 ms followed by a plateau at long reaction times. The fast rise of SO2 concentration was found to depend on NO2 concentration. On the other hand, the CH3O profiles exhibited fast rise followed by fast decay. The observed profiles of CH3O were very sensitive to NO2 concentration. This was consistent with the occurrence of the fast reaction of CH3 with NO2 to produce CH3O followed by fast removal of CH3O by NO2.
8898 J. Phys. Chem., Vol. 100, No. 21, 1996
Ray et al. TABLE 1: Reaction Mechanism Used in Model Calculation To Derive k3 and k4 by Fitting the Experimental and Calculated Profiles of SO2 and CH3O, Obtained at 298 K and 1 Torr Pressure k (cm3 molecule-1 s-1) at 298 K
reactions
Figure 3. Typical temporal concentration profiles of SO2 and CH3O in the CH3S + NO2 system. Experimental points are shown by symbols; solid lines are obtained by simulation from the reaction mechanism of Table 1. The initial concentrations of the reactants, in molecule cm-3, are [CH3SH]0 ) 2.7 × 1012, [NO2]0 ) 2.0 × 1014, [Cl]0 ) 1.4 × 1012, [Cl2]0 ) 9.9 × 1011.
The high sensitivity of the temporal profiles of SO2 and CH3O to NO2 suggested a possible occurrence of a reaction between CH3SO2 and NO2:
CH3SO2 + NO2 f products
(3)
The observed concentration-time profiles of SO2 and CH3O were compared to those obtained from simulation by the reaction mechanism reported in Table 1. A nonlinear least-squares program, FACSIMILE,31 was used to simulate profiles of SO2 and CH3O, using k3 and k4 as variable parameters (see Figure 3). The important reactions in model calculations were sequential reactions 1, 2, 3, 4, and 8, and the reaction of CH3O with NO2 (9). In this mechanism, reaction 3
CH3O + NO2 f products
(9)
is assumed to produce CH3SO3 and NO (see below). However, the values of k3 and k4 do not change even if we consider products of reaction 3 as unknown. Besides, reaction between CH3SO and NO2 is assumed to only produce CH3SO2 and NO (∆H ) -27.3 kcal mol-1). However, the channel producing CH3, SO2, and NO is also exothermic (∆H ) -6.3 kcal mol-1) and might occur. Model calculations have been carried out considering this second channel. Acceptable fits of experimental and calculated CH3O and SO2 profiles were also obtained for branching ratios of this second channel up to 0.2, but not for larger branching ratios. The overall picture for k3 and k4 was nevertheless not significantly changed. Then, although this second channel could not be ruled out, it is certainly minor and has not been considered in the modeling calculations. The input parameters in the modeling calculations were the initial concentrations of Cl atoms, undissociated Cl2, CH3SH, and NO2. The rate constants of reactions listed in Table 1 are from the literature (see references in Table 1), except for the wall loss rate of CH3O, which was measured in this work in independent experiments and was found to be 23 s-1. Although not measured in our work, the heterogeneous loss of CH3S and CH3SO was considered to be negligible compared to reaction with NO2, at the NO2 concentrations used. This relies upon the loss rate value of 6 s-1 measured by Domine et al.14 in a flow reactor coated with halocarbon wax as in the present study.
CH3SH + Cl f CH3S + HCl CH3SH + Cl f CH2SH + HCl CH3S + CH2SH + M f CH3SCH2SH + M CH3S + CH3S + M f CH3SSCH3 + M CH3S + CH3S f CH3SH + CH2S CH2SH + CH2SH + M f CH3SCH2SH + M CH3S + Cl2 f CH3SCl + Cl CH3S + NO2 f CH3SO + NO CH3SO + NO2 f CH3SO2 + NO CH3SO2 + M f CH3 + SO2 + M CH3SO2 + NO2 f CH3SO3 + NO CH3 + CH3 + M f C2H6 + M CH3 + CH3S + M f CH3SCH3 + M CH3 + CH3SH f CH3S + CH4 CH3 + NO2 f CH3O + NO CH3 + NO2 + M f CH3NO2 + M CH3O + NO2 f products CH3O + CH3O f products CH3O + Cl f CH2O + HCl CH3O + CH3 f products CH3O + wall f products
refs
1.4 × 10-10 4.3 × 10-12 1.2 × 10-10
13 12 23
1.0 × 10-12
9b
1.0 × 10-12 1.5 × 10-10
9b 23
1.0 × 10-12 6.1 × 10-11 1.2 × 10-11 variable variable 3.7 × 10-11 2.0 × 10-11 2.0 × 10-12 2.3 × 10-11 2.0 × 10-12 9.5 × 10-13 7.0 × 10-12 1.8 × 10-11 6.0 × 10-11 23a
9 13 13 24 9b 9b 25 26 27 28 29 30 this work
a Units are s-1. b Estimated from Amano et al. Nippon Kagaku Kaishi 1983, 3, 385.
TABLE 2: Rate Constants k3 and k4 at 298 K Derived from the Fitting of SO2 and CH3O Profiles:
CH3SO2 + NO2 f CH3SO3 + NO
(3)
CH3SO2 + M f CH3 + SO2 + M
(4)
(M ) He, PHe ) 1 Torr) [NO2]a
k4b
k3c
1.07 1.52 2.04 2.33 2.86 3.49 3.52 5.14 mean values
500 ( 126 236 ( 8 574 ( 93 376 ( 54 588 ( 306 722 ( 366 545 ( 326 543 ( 164 510 ( 150
3.04 ( 0.91 2.72 ( 0.09 3.21 ( 0.58 1.53 ( 0.26 2.91 ( 2.00 2.83 ( 1.46 0.80 ( 0.49 0.63 ( 0.18 2.2 ( 1.1
a Units are 1014 molecule cm-3. b Units are s-1. c Units are 10-12 cm3 molecule-1 s-1.
The best fitted values of k3 and k4 with 95% confidence limit are reported in Table 2. The mean values of k3 and k4, with 1 standard deviation, are the following:
k3 ) (2.2 ( 1.1) × 10-12 cm3 molecule-1 s-1 and k4 ) 510 ( 150 s-1 Sensitivity analysis was performed to confirm the occurrence of reactions 3 and 4. When reaction 3 was removed from the mechanism of Table 1, the simulated profiles of both SO2 and CH3O could not be properly fitted (Figure 4). The simulated profiles of Figure 4 given by the fitting program yielded a value k4 ∼ 50 s-1, which is much lower than the values obtained using both k3 and k4 as variable parameters. Also, when reaction 4 was removed from the mechanism of Table 1 and assuming that the alternative source of CH3O was reaction 3 (producing CH3SO3 and NO), followed by reactions
Thermal Decomposition of the CH3SO2 Radical
J. Phys. Chem., Vol. 100, No. 21, 1996 8899
Figure 4. Sensitivity of calculated SO2 and CH3O profiles to reaction 3: CH3SO2 + NO2 f products. Experimental points are shown by symbols; solid lines are same as in Figure 3; dotted lines are obtained from the simulation of mechanism of Table 1 without reaction 3. k4 derived from the calculation was ca. 50 s-1. The initial concentrations of the reactants, in molecule cm-3, are [CH3SH]0 ) 2.7 × 1012, [NO2]0 ) 2.0 × 1014, [Cl]0 ) 1.4 × 1012, and [Cl2]0 ) 9.9 × 1011.
10 and 8,the fast rise of CH3O profile could not be simulated.
CH3SO3 + M f CH3 + SO3 + M
(10)
Also sensitivity of the kinetic profiles of CH3O and SO2 to reaction 10 was verified by varying k3, k4, and k10 simultaneously. The negligible value of k10 showed that decomposition of CH3SO3 did not occur. Therefore, the observed kinetic profiles of CH3O and SO2 were indeed sensitive to reactions 3 and 4. As mentioned in the Experimental Section, the mass spectrometric signal at m/e ) 96 was observed in the CH3S/NO2 system. The signal intensity was dependent on NO2 concentration and reaction time. We did not observe the signal at m/e ) 95 corresponding to the parent peak of CH3SO3. In the absence of any other species in our system, the peak at m/e ) 96 was attributed to methanesulfonic acid (MSA), CH3SO3H. The experimental profiles of CH3SO3H could be correctly fitted with the calculated ones obtained by simulation of the mechanism of Table 1, using k3 and k4 values determined in this work. This strongly indicates that CH3SO3 is a product of reaction 3 and it is rapidly converted into CH3SO3H either in the reactor or at the sampling point into the mass spectrometer. A fast conversion of CH3SO3 to CH3SO3H by the reaction
CH3SO3 + CH3SH f CH3SO3H + CH3S
(11)
is very unlikely. This would require very high values of k11 (k11 g 10-10 cm3 molecule-1 s-1) at the CH3SH concentrations of the experiments. However, even with such high values of k11, it was impossible to fit the experimental and calculated profiles of SO2 and CH3O, due to generation of CH3S in reaction 11. In the absence of any other hydrogenated species present at significant concentrations in the reactor, it is suggested that the fast CH3SO3/MSA conversion could result from reaction of CH3SO3 with adsorbed species at the wall of the reactor, or at the sampling surface from the reactor to the mass spectrometer. The present study of the CH3S/NO2 system has shown evidence for a competition between the thermal decomposition of CH3SO2 to CH3 + SO2 and its reaction with NO2, to form CH3SO3 + NO. While the kinetics and mechanism for the CH3SO2 formation from the CH3S + NO2 reaction are now well
established (ref 13 and refs 9, 14, 32, and 33), no or very few data exist for reactions 3 and 4. a. Reaction of CH3SO2 with NO2 (3). The reaction of CH3SO2 with NO2 had not been investigated so far and the present value of k3 (k3 ) (2.2 ( 1.06) × 10-12 cm3 molecule-1 s-1, at 298 K) is the first experimental determination. It can be mentioned that an estimated value, k3 ) 4 × 10-12 cm3 molecule-1 s-1, was reported recently8 and is comparable to our determination. However, nothing was mentioned about the basis of this estimation. The error on k3 is fairly high and mainly results from the complexity of the chemical mechanism used for modeling calculation and from the absolute calibration of SO2 and particularly CH3O. b. Thermal Decomposition of CH3SO2 (4). Although indirect evidence for reaction 4 was obtained in several smogchamber type studies of DMS oxidation, the only reported rate constant for reaction 4 is from Mellouki et al.9 In this dischargeflow mass spectrometric study performed in our laboratory, k4 was derived from a fitting procedure of experimental and calculated SO2 profiles also in the Cl/CH3SH/NO2 system. The SO2 profiles were obtained from SO2+ profiles (m/e ) 64) taking into account the contribution of CH3SO2 at m/e ) 64. Only an upper limit, k4 e 10 s-1, was obtained at 298 K and P ) 0.3 Torr of helium. Similar reaction mechanism was used for the simulation calculation, except that reaction 3 was not included. New calculations of these previous experiments were performed from the reaction mechanism of Table 1, including reaction 3 with k3 from the present study. The fitting procedure gave k4 ≈ 350 s-1, which is fairly consistent with the present determination. However, this former value is probably less reliable than the present one since it is only based on a fitting procedure of SO2 profiles indirectly obtained from the calculated contribution of SO2 to the SO2+ peak intensity. The present value of k4 is nevertheless much higher than those estimated (k4 ) 1015 s-1 7,8) at 298 K for use in atmospheric modeling. The difference may be even larger if k4 at 1 Torr of helium has not reached its high-pressure limit. The present value of k4 at 298 K, combined with an activation energy, E ) 18 kcal mol-1, reported by Benson34 gives the following Arrhenius expression: k4 ) 8 × 1015 exp(-18 000/RT) (in s-1). Atmospheric Importance The present data provide some insight into the mechanism which determines the product distribution of DMS oxidation, which is crucial to assess the role of DMS in cloud formation. As mentioned in the Introduction, the cloud formation from CCN is determined by the MSA/SO42- ratio, the SO42- species being produced from SO2 oxidation. The MSA/SO42- ratio is likely to be influenced by reactions of intermediate adducts like CH3SO2, as already suggested, especially in the recent paper of Barone et al.8 From the present data for k3 and k4, we can calculate the relative rates of reactions 3 and 4, which can influence the MSA/SO42- ratio since the products of reactions 3 and 4, CH3SO3 and SO2, are precursors of MSA and SO42-, respectively. In the calculations it was assumed that k3 is not temperature dependent and that k4 is valid at atmospheric pressures. The rate ratios k3[NO2]/k4 are the following: 10-6, 3 × 10-4, and 10-2 at 300, 250, and 220 K, respectively, for 10 pptv of NO2 (ground level conditions); 10-4, 3 × 10-2 and 1 at 300, 250 and 220 K respectively, for 1 ppbv of NO2. The 10 pptv and 1 ppbv levels of NO2 are typical of remote oceanic and coastal regions, respectively. These calculations indicate that the MSA/SO42- ratio would not be sensitive to NO2 levels in remote atmosphere but it would in regions with significant levels of NOx. In that case, the MSA/SO42- ratio is indeed
8900 J. Phys. Chem., Vol. 100, No. 21, 1996 very dependent on temperature. Besides, the reactivity between CH3SO2 and NO2, observed in this work, leads one to expect significant reactivity between CH3SO2 and O3, which also likely leads to MSA formation. In contrast with NO2, reaction of CH3SO2 with O3 ([O3] ∼ 30-40 ppbv) may compete with its thermal decomposition in the whole marine atmosphere, even at moderate temperatures. This would be consistent with field measurements which generally show an increase of the MSA/ SO42- ratio with decreasing temperature [e.g. refs 35 and 36]. However, the importance of the CH3SO2 + O3 route depends on the yield of CH3SO2 from the sequence +O3
+O3
CH3S 98 CH3SO (+O2) 98 CH3SO2 (+O2) which is still uncertain (a 15% yield of CH3SO from the CH3S + O3 reaction has been measured37). Besides, the atmospheric role of the CH3SO2 discussed here is dependent on the importance of alternative routes involving adducts formed from the reactions of the CH3SOx (x ) 0, 1, or 2) radicals with O2 which is still uncertain.8 Acknowledgment. The European Commission is acknowledged for support through its environmental program. References and Notes (1) Charlson, R. J.; Lovelock, J. E.; Andreae, M. O.; Warren, S. G. Nature 1987, 326, 655. (2) Tyndall, G. S.; Ravishankara, A. R. Int. J. Chem. Kinet. 1991, 23, 483. (3) Yin, F.; Grosjean, D.; Seinfeld, J. H. J. Atmos. Chem. 1990, 11, 309. (4) Turnipseed, A. A.; Ravishankara, A. R. Dimethylsulfide: Oceans, Atmosphere and Climate; Restelli, G., Angeletti, G., Eds.; Proceedings of the International Symposium held in Belgirate, Italy, 13-15 October 1992; Kluwer Academic: New York, 1992; p 185. (5) Hynes, A. J.; Wine, P. H.; Semmes, D. H. J. Phys. Chem. 1986, 90, 4148. (6) Butkovskaya, N. I.; Le Bras, G. J. Phys. Chem. 1994, 21, 2582. (7) Turnipseed, A. A.; Barone, S. B.; Ravishankara, A. R. J. Phys. Chem. 1993, 97, 5926. (8) Barone, S. B.; Turnipseed, A. A.; Ravishankara, A. R. Faraday Discuss., in press. (9) Mellouki, A.; Jourdain, J. L.; Le Bras, G. Chem. Phys. Lett. 1988, 148 (2,3), 231.
Ray et al. (10) Dae¨le, V.; Ray, A.; Vassalli, I.; Poulet, G.; Le Bras, G. Int. J. Chem. Kinet. 1995, 27, 1121. (11) Ray, A.; Dae¨le, V.; Vassalli, I.; Poulet, G.; Le Bras, G. J. Phys. Chem., in press. (12) Nesbitt, D. J.; Leone, S. R. J. Phys. Chem. 1981, 75, 4949. (13) De More et al. “Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling”; Evaluation No. 11, JPL Publication 9426, 1994. (14) Domine´, F.; Murrells, T. P.; Howard, C. J. J. Phys. Chem. 1990, 94, 5839. (15) Park, J.-Y.; Slagle, I. R.; Gutman, D. J. Phys. Chem. 1983, 87, 1812. (16) Strickler, S. J.; Howell, D. B. J. Chem. Phys. 1968, 49(4), 1947. (17) Herzberg, G. Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966. (18) Chatgilialoglu, C.; Griller, D.; Guerra, M. J. Phys. Chem. 1987, 91, 3747. (19) Inoue, G.; Akimoto, H.; Okuda, M. J. Chem. Phys. 1980, 72, 1769. (20) Kappert, J.; Temps, F. Chem. Phys. 1989, 132, 197. (21) Mettee, H. D. J. Chem. Phys. 1968, 49, 1784. (22) Wantuck, P. J.; Oldenborg, R. C.; Baugheum, S. L.; Winn, K. R. J. Phys. Chem. 1987, 91, 3253. (23) Rahman, M. M.; Becker, E.; Wille, U.; Schindler, R. N. Ber. Bunsenges. Phys. Chem. 1992, 96, 783. (24) Sagle, I. R.; Gutman, D.; Davies, J. W.; Pilling, M. J. J. Phys. Chem. 1988, 92, 2455. (25) Yamada, F.; Sagle, I. R.; Gutman, D. Chem. Phys. Lett. 1981, 83, 409. (26) McCaulley, J. A.; Moyle, A. M.; Golde, M. F.; Anderson, S. M.; Kaufman, F. J. Chem. Soc., Faraday Trans. 1990, 86, 4001. (27) McCaulley, J. A.; Anderson, S. M.; Jeffries, J. B.; Kaufman, F. Chem. Phys. Lett. 1985, 115, 180. (28) Zellner, R.; Hartmann, D.; Kartha¨user, J.; Rha¨sa, D.; Weibring, G. J. Chem. Soc., Faraday Trans. 1994, 90, 1197. (29) Dae¨le, V.; Laverdet, G.; Poulet, G. Int. J. Chem. Kinet., in press. (30) Tsang, W.; Hampson, R. F., Jr. J. Chem. Phys. Ref. Data 1986, 15(3). (31) FACSIMILE Programme. A. R. Curtis and W. P. Sweetenhan, U.K. Atomic Energy Res. Establ. [Rep.], R-12805, 1987. (32) Tyndall, G. S.; Ravishankara, A. R. J. Phys. Chem. 1989, 93, 2426. (33) Tyndall, G. S.; Ravishankara, A. R. J. Phys. Chem. 1989, 93, 4707. (34) Benson, S. W. Chem. ReV. 1978, 78, 23. (35) Staubes, R.; Georgii, H. W. Dimethylsulfide: Oceans, Atmosphere and Climate; Restelli, G., Angeletti, G., Eds.; Proceedings of the International Symposium held in Belgirate, Italy, 13-15 October 1992; Kluwer Academic: Dordrecht, 1992; p 95. (36) Legrand, M.; Saltzman, E. S.; Germain, C.; Barkov, N. I.; Petrov, V. N. Nature (London) 1991, 350, 144. (37) Domine´, F.; Ravishankara, A. R.; Howard, C. J. J. Phys. Chem. 1992, 96, 2171.
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