J. Phys. Chem. 1996, 100, 17207-17217
17207
FTIR Kinetic and Mechanistic Study of the Atmospheric Chemistry of Methyl Thiolformate Iulia V. Patroescu, Ian Barnes,* and Karl H. Becker Physikalische Chemie/Fachbereich 9, Bergische UniVersita¨ tsGH Wuppertal, D-42097 Wuppertal, F.R.G. ReceiVed: May 21, 1996; In Final Form: August 6, 1996X
Some aspects of the atmospheric chemistry of methyl thiolformate (CH3SCHO), a recently detected intermediate in the oxidation of dimethyl sulfide, have been investigated at 298 K and 1000 mbar total pressure in large reaction chambers using long path in situ FTIR absorption spectroscopy for the analysis. Rate coefficients of (1.11 ( 0.22) × 10-11 and (5.80 ( 0.80) × 10-11 cm3 molecule-1 s-1 have been determined for its reaction with OH radicals and Cl atoms, respectively. The UV spectrum of CH3SCHO has been measured in the range 220-355 nm and a lower limit of 5.4 days determined for its atmospheric photolytic lifetime. Detailed product analyses have made for the OH and Cl initiated photooxidation of CH3SCHO. Strong SO absorption bands observed in both systems are tentatively assigned to CH3SOCHO in the OH system and to CH3SOCl in the Cl system. The first gas-phase spectra of CH3SCl and CH3SOCl are also presented. The results are discussed with respect to the atmospheric chemistry of CH3SCHO and possible consequences for the photooxidation mechanism of dimethyl sulfide.
Introduction Emissions of dimethyl sulfide (DMS: CH3SCH3), principally from the world’s oceans, account for nearly half of the biogenic sulfur emitted to the atmosphere and approximately 25% of the total global sulfur emissions.1,2 Reactions with OH and NO3 radicals represent the major atmospheric sinks for DMS during the daytime and nighttime, respectively.3-8 Although the exact nature of the mechanisms of the reactions of these radicals with DMS is still uncertain, production of (methylthio)methyl radicals (CH3SCH2) via an overall abstraction mechanism is thought to represent the major reaction channel for both radicals at room temperature. In the atmosphere the further reactions of these radicals are thought to result in the formation of methylthiyl radicals and HCHO:5-8
CH3SCH3 + OH/NO3 f CH3SCH2 + H2O/HNO3 (1) CH3SCH2 + O2 f CH3SCH2O2
(2)
CH3SCH2O2 + NO f CH3SCH2O + NO2
(3)
CH3SCH2O f CH3S + HCHO
(4)
The results from recent laboratory investigations support that in the presence of NOx this will be the major fate of CH3SCH2.6-10 However, a recent study from this laboratory11 has shown that reaction of these radicals results in the formation of methyl thiolformate (MTF, CH3SCHO) either via mutual reaction of methylthiylperoxy radicals (CH3SCH2O2) and cross reaction with HO2 or other RO2:
CH3SCH2O2 + CH3SCH2O2 f CH3SCHO + CH3SCH2OH + O2 (5) CH3SCH2O2 + HO2 f CH3SCHO + OH + O2
(6)
In the experiments the photolysis of bis(methylthio)methane (CH3SCH2SCH3) was used to produce the CH3SCH2 radicals and a formation yield of ∼15% was observed for methyl thiolformate in this system. In the study it was found that when NOx was present in the system formation of MTF was not observed. Since in the presence of NO any CH3SCH2O2 radicals X
Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01452-9 CCC: $12.00
will be quantitatively converted to CH3SCH2O, this is presently being interpreted as indicating that MTF can only be formed in mutual or cross reactions of CH3SCH2O2 and that the other possible formation channel
CH3SCH2O + O2 f CH3SCHO + HO2
(7)
is negligible. However, simulations of the above system and also current work on the oxidation of DMS in this laboratory using a value of k ) 5 × 10-15 cm3 molecules-1 s-1 for reaction 7, which is typical for reactions of alkoxy radicals with O2,12 and a value of 1 × 105 s-1 for the thermal decomposition channel, reaction 4, as suggested by recent work in the literature on the oxidation of DMS, show that the concentration of MTF would not be formed at levels very much above the MTF experimental detection limit (50 ppb) because of its fast removal via UV photolysis. Higher rate coefficients for reaction 7 and lower thermal decomposition rates for reaction 4 raise the MTF level significantly above the detection limit. Therefore, given the complexity of the reaction systems and until rate coefficients are available for reactions 4 and 7, it cannot presently be completely excluded that reaction 7 is not operative to some extent and that the failure to detect MTF is simply its rapid further oxidization to levels below the detection limit of the experimental setup. Experiments are ongoing to clarify this point. Other recent studies in the literature, however, support that thermal decomposition of CH3SCH2O is the major pathway.7,9 If we assume that reaction 7 is negligible the laboratory results imply that MTF will only have a possibility of being formed in the atmospheric oxidation of DMS in areas of very low NO such as is the case in the tropical and subtropical remote marine boundary layer where NO is typically 2-8 ppt.13,14 Under such conditions the major fate of CH3SCH2OO radicals will be reaction with other peroxy radicals, in particular HO2 radicals and to a lesser extent CH3OO radicals. It is well established that a major channel in the reactions of HO2 with other peroxy radicals is formation of hydroperoxides:
CH3SCH2O2 + HO2 f CH3SCH2OOH + O2
(8a)
The atmospheric fate of hydroperoxides is wet and dry deposition, photolysis, and reaction with OH radicals. Photolysis will probably result in the formation of CH3SCH2O radicals. There © 1996 American Chemical Society
17208 J. Phys. Chem., Vol. 100, No. 43, 1996
Patroescu et al.
are several possible channels for reaction with OH such as reformation of CH2SCH2OO radicals and formation of species such as CH2SCH2OOH and CH3SOH radicals and CH3S(O)CH2OOH. None of the reaction products is likely to result in the formation of MTF. However, there is direct experimental evidence that the reaction of HO2 radicals with substituted peroxy radicals can also proceed via a second channel producing carbonyl products. For example, the reaction of HO2 with methoxymethylperoxy radical (CH3OCH2O2), the oxygenated analogue of CH3SCH2O2, has been shown to produce methyl formate (CH3OCHO) with a yield in the range 0.3-0.5.12 Thus, it is not unreasonable to expect that the yield of MTF from the reaction of HO2 with CH3SCH2O2 will be of the same order of magnitude.
CH3SCH2O2 + HO2 f CH3SCHO + O2 + H2O (8b) The cross reactions of organic peroxy radicals generally have two main pathways, reaction 9a which produces an alkoxy radical and reaction 9b which produces an alcohol and carbonyl product:15
RO + RO f 2RO + O2
(9a)
f ROH + R′CHO + O2 (9b) Therefore, reaction of CH3SCH2O2 with other alkyl peroxy radicals present in the atmosphere will certainly result, to some extent, in the formation of MTF. There have not been very many determinations of the carbonyl yields in the cross reactions of organic peroxy radicals; however, from the reported values,15 yields of 0.5 are not uncommon. Nothing is presently known about the atmospheric chemistry of MTF. Attempts have been made in this laboratory to detect MTF in the OH-initiated oxidation of DMS using the UV photolysis of DMS/H2O2/air reaction mixtures.16,17 Formation of MTF was observed but only in the initial few minutes of the irradiation, indicating that MTF is being rapidly further oxidized in the reaction system. Addition of NOx to the system was observed to suppress the formation of MTF. Tentative identification of MTF has been reported in a low-pressure discharge flow-mass spectrometry study of the reaction of NO3 radicals with DMS.4 Apart from this study, no other reports have been made in the literature of the observation of MTF either in laboratory or field investigations of DMS chemistry. This is probably due to lack of a concerted effort by workers to look for this compound both in laboratory and field measurements and also because its reactivity may be such that its concentration in both laboratory reaction systems and in the atmosphere may be very low which would render its detection extremely difficult. Since MTF is a potentially important intermediate in the oxidation of DMS, some aspects of the atmospheric chemistry of this compound have been investigated. The UV absorption spectrum and photolysis products of methyl thiolformate have been measured together with the kinetics and products of its reaction with OH radicals. A recent study has indicated the possible importance of the reaction of Cl atoms with DMS in marine environments.18 The kinetics and products of the reaction of Cl atoms with MTF have, therefore, also been investigated. Experimental Section The kinetic and photolysis studies on the reaction of methyl thiolformate with OH and Cl radicals were performed in a quartz glass reactor of 1080-L volume equipped with a built-in White mirror system. Long-path in situ FTIR absorption spectroscopy (492 m, Bruker IFS 88 spectrometer) was used to monitor the
concentration-time behavior of both reactants and products. The reactor can be irradiated with either 32 low-pressure mercury vapor lamps (Philips TUV 40, λmax ) 254 nm) or 32 fluorescent lamps (Philips Tl 40W/05, 320 < λ < 450 nm, λmax ) 365 nm). Details of the experimental setup can be found elsewhere.19,20 The UV spectrum of methyl thiolformate was measured in a glass reactor of 480-L volume which contains two White mirror systems and allows simultaneous long-path in situ measurements in the infrared and UV. The infrared spectra were recorded at a total path length of 50 m with a Nicolet 520 FTIR spectrometer and the UV spectra at a total path length of 51.6 m in combination with a 22 cm spectrometer (SPEX) equipped with a diode array detector (PAR 1412). Details of this experimental setup can be found in references 21 and 22. Irradiations were performed on MTF and MTF/CH3ONO, MTF/reference hydrocarbon/CH3ONO, MTF/Cl2, and MTF/ hydrocarbon/Cl2 reaction mixtures at 1000 mbar total pressure (N2 + O2). The initial concentrations of MTF and the reference hydrocarbon were in the range 1-3 ppm (1 ppm ) 2.46 × 1013 molecules cm-3 at 1000 mbar and 298 K) and those of methyl nitrite (CH3ONO) and Cl2 were typically 2-4 and 2040 ppm, respectively. All the substances were injected into the reactor using gas syringes. In all of the experiments, infrared spectra were recorded with a resolution of 1 cm-1 in the range 600-4000 cm-1 using a KBr beamsplitter and a liquid nitrogen cooled MCT detector. Typically, 32-128 interferograms were co-added per spectrum and up to 10-20 spectra were recorded per experiment. The rate coefficients for the reactions of OH and Cl radicals with methyl thiolformate were determined at 1000 mbar of synthetic air at 298 ( 2 K using the relative kinetic method.23 The photolysis of methyl nitrite with the vis lamps was used as the OH radical source and the photolysis of molecular Cl2, also with the vis lamps, as the Cl atom source. Provided that gas phase reaction with either OH or Cl represents the only loss for methyl thiolformate and the reference hydrocarbon in the reaction system
CH3SCHO + OH/Cl f products, k1
(10)
reference + OH/Cl f products, k2
(11)
then the kinetic data can be treated using eq I:
ln
(
[CH3SCHO]t0
[CH3SCHOl]t
) ( )
)
k1 [Reference]t0 ln k2 [Reference]t
(I)
where [CH3SCHO]t0 and [reference]t0 are the concentrations of the reactant and reference organics, respectively, at time t0, [CH3SCHO]t and [reference]t are the corresponding concentrations at time t, and k1 and k2 are the rate coefficients for the reaction of either OH or Cl with the carbonyl and reference compounds, respectively. Plots of ln([CH3SCHO]t0/[CH3SCHO]t) against ln([reference]t0/[reference]t) should yield a straight line of slope k1/k2 and zero intercept. The experiments were performed relative to the reaction of OH with ethene (k298 K ) 8.5 × 10-12 cm3 molecule-1 s-1)24,25 and Cl with ethane (k298 K ) 5.9 × 10-11 cm3 molecule-1 s-1).26 Methyl thiolformate was synthesized according to a method described in the literature.27,28 Briefly, methanethiol was added slowly to a cooled equimolar mixture of formic acid and acetic anhydride and the resulting solution was refluxed and distilled into an acetone/CO2 cold trap. The distillate was washed with sodium bicarbonate, extracted with ether, and washed with water. The ether extract was then distilled at approximately 70 °C and purified by chromatography. The purity of the methyl
Atmospheric Chemistry of Methyl Thioformate
J. Phys. Chem., Vol. 100, No. 43, 1996 17209 TABLE 1: UV Absorption Cross Sections for Methyl Thiolformate in the Region 220-355 nm at 5 nm Intervals
Figure 1. Infrared spectrum of 0.84 ppm methyl thiolformate in the spectral range 4000-700 cm-1 recorded in 1000 mbar of synthetic air at 298 K with a resolution of 1 cm-1.
Figure 2. Gas-phase UV absorption spectrum of methyl thiolformate.
thiolformate, based on its infrared spectrum, was generally better than 98%. Typical impurities included minor amounts of formic and acetic acid. Results and Discussion (a) Infrared and UV Spectrum of Methyl Thiolformate. Figure 1 shows the gas phase infrared spectrum of 0.84 ppm methyl thiolformate recorded in 1000 mbar of synthetic air. The spectrum is in good agreement with a vapor phase spectrum published in the literature.28 The three absorptions with band centers at 2838, 1698, and 767 cm-1 can be assigned to the CH stretch (ν3), CO stretch (ν4), and C-S-C symmetric stretch (ν10), respectively. From a series of calibration experiments, absorption coefficients of ) (2.0 ( 0.1) × 10-18 cm2 molecule-1 and ) (9.0 ( 0.3) × 10-19 have been determined for the bands with maxima at 1698 and 767 cm-1, respectively. Calibrated infrared spectra of MTF were obtained by injecting weighed amounts of MTF into heated inlet manifold and flushing it into the reactor with N2. This was performed for several different quantities of MFT and the reproducibility was generally better than (5%. Figure 2 shows the UV spectrum of MTF in the region 220355 nm. The measured cross sections in units of cm2 molecule-1 are given for 5 nm intervals in Table 1 and are represented in Figure 2 by open circles. The measured absorption values were converted to cross sections by determining the absolute concentrations from the simultaneously recorded infrared spectra. The 2σ uncertainties of the cross sections are
wavelength (nm)
absorpn cross section (cm2 molecule-1)
wavelength (nm)
absorpn cross section (cm2 molecule-1)
220 225 230 235 240 245 250 255 260 265 270 275 280 285
2.137 × 10-18 2.836 × 10-18 3.075 × 10-18 2.637 × 10-18 1.752 × 10-18 9.133 × 10-19 3.978 × 10-19 1.631 × 10-19 7.323 × 10-20 4.381 × 10-20 3.322 × 10-20 2.876 × 10-20 2.575 × 10-20 2.299 × 10-20
290 295 300 305 310 315 320 325 330 335 340 345 350 355
2.031 × 10-20 1.642 × 10-20 1.063 × 10-20 6.217 × 10-21 3.074 × 10-21 1.642 × 10-21 1.024 × 10-21 6.958 × 10-22 4.909 × 10-22 4.278 × 10-22 2.982 × 10-22 2.863 × 10-22 2.384 × 10-22 2.057 × 10-22
15-20% up to 325 nm, 30% over 330 nm, and 50% for 355 nm. The UV spectrum shows an apparent center at approximately 270 nm with a rise to a second center at 230 nm. The low- and high-intensity absorptions are probably due to carbonyl and sulfur chromophores, respectively. The extension of the absorption spectrum into the near-UV can be attributed to the presence of the carbonyl chromophore. The UV absorption cross sections from the present study can be used to estimate the photolysis frequency of MTF in the atmosphere. Assuming a quantum yield for photodissociation of Φ ) 1, for noontime solar fluxes at 40 °N, July 1,29,30 a photolysis frequency of J ) (2.14 ( 0.20) × 10-6 s-1 is obtained. Over the Equator the corresponding values for the same day and time would be J ) (3.08 ( 0.29) × 10-6 s-1 and τ ) 3.7 days. Since the quantum yield for photodissociation is probably much less than unity, a lower limit of 5.4 days can be put on the photolytic lifetime of methyl thiolformate at 40° N. Using ultraactinic fluorescent lamps, which approximately simulate sunlight, the photolysis loss of MTF in the reactor was negligibly slow. This lack of activity allows a limit of e10-5 s-1 to be put on the MTF photolysis frequency which is in agreement with the calculated frequency from the UV spectrum. It is presently difficult to speculate on possible limits for the photolysis quantum yield of MTF in the atmosphere. (b) Kinetics of the Reactions of OH and Cl Radicals with MTF. Loss of MTF in the reactor due to absorption on the walls was found to be negligible over the time period of the experiments. Similarly, photolysis with the vis lamps used for the generation of the OH radicals and Cl atoms was also negligibly small. Figure 3, panels a and b, shows the data from a minimum of four experiments obtained from irradiation of MTF/ethene/CH3ONO/air and MTF/ethane/Cl2/air reaction mixtures plotted according to eq I. From the slopes of these plots the following rate coefficients have been derived for the reaction of OH radicals and Cl atoms with MTF:
k(OH + MTF, 298 K) ) (1.11 ( 0.22) × 10-11 cm3 molecule-1 s-1 k(Cl + MTF, 298 K) ) (5.80 ( 0.80) × 10-11 cm3 molecule-1 s-1 The errors are estimates for the uncertainty at the 95% confidence level. The rate coefficient for the reaction of OH with MTF is of the same order of magnitude as those reported in the literature for the reactions of OH with simple aldehydes and R-dicarbonyls and intermediate between those reported for dimethyl sulfide and methanethiol.24-26 The dominant reaction pathway for
17210 J. Phys. Chem., Vol. 100, No. 43, 1996
Patroescu et al. for the reactions of Cl with DMS and methanethiol.31-33 As for OH, the reaction of Cl with MTF can proceed by three channels:
Figure 3. Kinetic data plotted according to eq I for the reactions of (a) OH radicals and (b) Cl atoms with methyl thiolformate.
simple aldehydes5,25-27 is a direct H abstraction; however, for sulfur compounds the situation is more complex. In the case of CH3SH the reaction proceeds via a CH3S(OH)H adduct which decomposes to give CH3S and H2O.24-26 For DMS it is now known that the reaction proceeds via two channels, (i) an overall H atom abstraction to form CH3SCH2 radicals and H2O and (ii) formation of an CH3S(OH)CH3 adduct which rapidly decomposes back to reactants but in the presence of O2 will react to give products.5 Because of the chemical structure of MTF, three reaction pathways are possible: (i) direct abstraction of the aldehydic H atom, (ii) abstraction of an H atom from the methyl group, and (iii) OH addition to the sulfur atom, reactions 12, 13, and 14, respectively.
CH3SCHO + OH f CH3SCO + H2O
(12)
CH3SCHO + OH f CH2SCHO + H2O
(13)
CH3SCHO + OH T CH3S(OH)CHO
(14)
There are several possibilities for the fate of the OH adduct which are analogous to those which have been discussed for the OH + DMS reaction:5-8 (i) it could react with O2 to form products (reaction 15), (ii) undergo intramolecular H-atom abstraction (reaction 16) which would be indistinguishable from reaction 12, or (iii) thermally decompose (reaction 17).
CH3S(OH)CHO + O2 f products (possibly CH3SOCHO) (15) CH3S(OH)CHO + ∆ f CH3SCO + H2O
(16)
CH3S(OH)CHO + ∆ f CH3S + HCOOH and CH3SOH + HCO (17) The rate coefficient for the reaction of Cl atoms with MTF is also of the same order of magnitude as those reported for the reaction of Cl with simple aldehydes and R-dicarbonyls;26 however, it is an order of magnitude slower than that reported
CH3SCHO + Cl f CH3SCO + HCl
(18)
CH3SCHO + Cl f CH2SHCO + HCl
(19)
CH3SCHO + Cl T CH3S(Cl)CHO
(20)
Both the reactions of Cl and Br atoms with DMS are known to involve two channels, a pressure-independent H abstraction and a pressure-dependent channel involving a collisionally stabilized adduct. The reactions of Cl and Br with CH3SH are thought to proceed via an addition-elimination reaction mechanism rather than a direct H abstraction.33,35 A comparison of the magnitude of the rate coefficients for the reactions of OH and Cl with MTF with those of the literature rate coefficients for the reactions of OH and Cl with aldehydes, CH3SH and DMS, leads to the conclusion that abstraction of the aldehyde H atom will be the major channel for these reactions; this is also supported by the product analyses described below. The same comparison suggests that abstraction from the methyl group will probably not be significant, particularly for the Cl reaction. The presence of the carbonyl group next to the S atom in MTF will have a deactivating effect toward the addition of electrophilic reactants such as OH and Cl and, therefore, it is to be expected that, although the addition channel is probably occurring, it will not have the same significance as in the reactions of OH and Cl with DMS. The relative importance of the various possible reaction channels is discussed further below in conjunction with the product analyses of the UV photolysis and the OH and Cl reactions. (c) Reaction Product Studies. The products identified in the various reaction systems, UV photolysis, OH radical reaction, and Cl atom reaction, are collected in Table 2. For the UV photolysis and Cl atom systems, the product analyses were performed in both synthetic air and N2. For the purpose of the following discussion it should, however, be borne in mind that when using N2 as a bath gas in large-volume chambers, O2 concentrations of up to 100 ppm can be present due to minor leaks in the system. The product analysis of the OH reaction system was only carried out in synthetic air since the OH radical source, photolysis of methyl nitrite, requires the presence of an adequate concentration of O2. Although yields are given for CO and HCHO in the OH-initiated oxidation of MTF, the values contain contributions from of the photooxidation of CH3ONO, the OH radical precursor, which also produces these species. As a consequence, quantification of the yields of CO and HCHO due solely to the photooxidation of MTF has not been possible. UV Photolysis Products. Figure 4, panels a and b, shows the concentration-time profiles of the products measured in the 254 nm photolysis of ppm levels of methyl thiolformate in 1000 mbar of synthetic air and N2, respectively. As noted above, traces of up to 100 ppm O2 can be present due to minor leaks in the system. The same products, CO, HCHO, HCOOH, CH3OOH, SO2, and OCS, were observed in both systems. The individual yields of the products and the overall sulfur and carbon balances are given in Table 2. Apart from CH3OOH and OCS, whose yields were higher in air than in N2, the yields of the other measurable products were generally somewhat higher in N2 than in air. Very significant differences in formation yields were observed for CH3OH, CH3OOH, and OCS between synthetic air and N2. In the N2 system the sulfur and carbon balance were both approximately 100%, indicating that virtually all of the carbon and sulfur is accounted for. In synthetic air, however, the sulfur and carbon balances were
Atmospheric Chemistry of Methyl Thioformate
J. Phys. Chem., Vol. 100, No. 43, 1996 17211
TABLE 2: Product Formation Yields (% C or S) Measured in the UV Photolysis, OH Radical, and Cl Atom Initiated Oxidation of MTF in 1000 mbar of N2 and Synthetic Air at 298 K by ∼70% Conversion of MTF UV photolysis
Cl reaction
OH reaction
product
N2
air
N2
air
air
CO COCl2 HCHO HCOCl CH3OH HCOOH CH3OOH SO2 OCS CH3SO2(O2)NO2 CH3SCl CH3S(O)Cl CH3S(O2)Cl SOCl2
51.6% C
47.2% C 21.8% C
12.2% C 5.4% C 2.0% C 96.7% S 0.24% S
1.2% C 4.4% C 12.3% C 86.7% S 0.84% S
39.3% C 0.3% C 15.1% C 0.8% C 0.3% C 1.3% C
74% C
28.1% C
42.2% C 0.4% C 9.8% C 0.2% C 0.5% C 0.2% C 15.8% S n.d.
56.6% S 0.6% S
total % S total % C
(10% S) (18.5% S) 3.7% S (4% S) 97 99
86.5 87.0
44 65
300% C 7% C 13% C 40% S n.d. (20% S)
(4% S) (23% S) 4.3% S (4% S) 83 72
60
certainly in the form of CO2; however, saturation of its infrared absorption band due to background CO2 prevented quantification of its yield. Figure 5, panel a, shows the residual infrared spectrum obtained for the photolysis of methyl thiolformate in air after subtraction of all the identified products. The spectrum is dominated by strong absorption bands at ∼1190 and ∼1746 cm-1; other weaker but significant absorption features are centered around 1400, 1245, and 1050 cm-1. Strong absorptions in the regions around ∼1190 cm-1 and ∼1746 cm-1 are typical for the sulfoxide -SO-36,37 and -CO- groups, respectively.36 There are two possible channels for the 254 nm photolysis of methyl thiolformate involving either cleavage of the C-H bond or the S-C bond, pathways 21 and 22, respectively:
CH3SCHO + hν f CH3SCO + H
(21)
f CH3S + HCO
(22)
Further reactions of either CH3SCO or HCO could be responsible for the large measured yields of CO, 47 and 51% C in synthetic air and N2, respectively. Although the end formation yield of CO does not show a large dependence on the nature of the bath gas, there are differences in its behavior as a function of time between N2 and synthetic air. In N2 the CO yield is independent of time whereas in synthetic air the yield increases with time (Figure 6, panel a). If reaction 22 was the major channel, then the yield of CO in synthetic air would be expected to be independent of time due to the fast reaction:
HCO + O2 f HO2 + CO
(23)
The difference in behavior of the CO yield between air and N2 supports that reaction 21 is the dominant UV photolysis pathway for MTF. The thermal decomposition of CH3SCO, reaction 24, is probably the main source of CO in the N2 photolysis system:
CH3SCO + ∆ f CH3S + CO
(24)
In synthetic air, addition of O2 to CH3SCO will be the dominant fate of CH3SCO radicals: Figure 4. Concentration-time profiles of the products formed in the UV photolysis of methyl thiolformate in 1000 mbar of synthetic air and N2 containing trace amounts of O2 (up to 100 ppm), panels a and b, respectively.
somewhat poorer with yields of ∼87% S and not all the reaction products were identified. Some of the missing carbon is
CH3SCO + O2 f CH3SCO(O2)
(25)
The further reaction of CH3SCO(O2) will eventually yield CH3S radicals and CO2. Further secondary reactions of CH3S will produce CO, i.e., through formation of HCHO and is further oxidation, and result in the observed time dependence of the CO yield.
17212 J. Phys. Chem., Vol. 100, No. 43, 1996
Patroescu et al.
Figure 5. Residual infrared spectra obtained after subtraction of all identified products from (a) a MTF/air UV photolysis system, (b) a MTF/ CH3ONO/air visible photolysis system, (c) a MTF/Cl2/air visible photolysis system, and (d) a MTF/Cl2/N2 visible photolysis system.
The CH3SCO radical could also thermally decompose to produce OCS:
CH3SCO + ∆ f CH3 + OCS
(26)
This reaction could explain the formation of OCS observed in the UV photolysis of MTF; however, the time behavior and formation yields of OCS in synthetic air and N2 are very different (Figure 6, panel b). The OCS yield is much higher in air than in N2 and the OCS yield is independent of reaction time whereas as in N2 there is a time dependence. If reaction 26 was the main source of OCS the yields in air and N2 should both be independent of time. The yield in air would also be expected to be lower than in N2 due to scavenging of CH3SCO by O2. The results imply that, although reaction 26 may possibly contribute to the OCS formation, it is not the main source. The main pathway for OCS formation is currently thought to be reaction of CH3S radicals with O2 to form thioformaldehyde (H2CdS) which further oxidizes to form, in part, OCS.16,17 The further reactions of CH3S radicals are responsible for the formation of SO2, HCHO, and CH3OH observed in the photolysis of MTF. The various pathways possible for the formation of these compounds are described in the literature and will not be discussed further here.7,8 The SO2 formation yield of ∼90% S obtained in air is very similar in magnitude to that observed recently in the 254 nm photolysis of DMDS in air 19 which produces exclusively CH3S radicals. The results support the conclusions of the DMDS photolysis study19 that in the absence of NOx the major fate of the CH3S radical will be oxidation to SO2..
It is known that OH radicals can be formed in the photooxidation of organic sulfur compounds, particularly in the presence of oxygen,19 and it is probable that the generation of OH radicals in the UV photolysis of MTF in air is mainly responsible for many of the differences in the product distributions between air and N2 and also for the unidentified products. In synthetic air, self-reactions and cross reactions of peroxy radicals will also be much more important than in the “pure” N2 system. The residual spectrum obtained from the UV photolysis of MTF as shown in Figure 5, panel a, has many similarities with the residual spectrum obtained from the OH-initiated oxidation of MTF (Figure 5, panel b), particularly in the -CO- and -SOabsorption regions 1700-1800 and 1100-1200 cm-1, respectively. As will be discussed below, the dominant spectral features of Figure 5, panels a and b, are ascribed to methylformyl sulfoxide (CH3S(O)-C(O)-H) formed via the addition of OH to MTF and subsequent reaction of the adduct with O2, reactions 14 and 15. OH Radical Reaction System. Table 2 lists the products from the OH-initiated oxidation of MTF using the photolysis of methyl nitrite as the OH source. The yield of SO2 (∼40% S) is similar to that observed in the OH-initiated oxidation of DMS in the presence of NOx20,38 and is the only positively identified S-containing compound in the reaction system. In the presence of excess NO2 the formation of methanesulfonyl peroxynitrate (CH3SO2O2NO2) is also observed; under such conditions estimations indicate that the yield of this compound is of the order of 20% S. Identified carbon-containing products include CO, HCHO, CH3OH, and HCOOH. As mentioned above, since the photolysis of the OH source, methyl nitrite,
Atmospheric Chemistry of Methyl Thioformate
J. Phys. Chem., Vol. 100, No. 43, 1996 17213 this pathway would explain the “prompt” formation of CO observed in the reaction system (i.e., the shape of the measured concentration-time profile supports that CO is a primary reaction product and is not formed in secondary reactions). In principle, abstraction of an H atom from the methyl group and further oxidation of the resulting CH2SCHO radical could also produce high yields of CO. However, the CO would be formed in secondary reactions which again would not be consistent with the observed concentration-time profile. Further, work on the Cl initiated oxidation of methyl chlorothiolformate (CH3SCOCl) performed in this laboratory, where the only H-abstraction route possible is from the methyl group to form CH2SCOCl radicals, shows that although high yields of CO are formed in this oxidation they are accompanied by relatively high formation yields of OCS (∼50% S) probably via the series of reactions:
2CH2SCOCl + O2 f 2O2CH2SCOCl f 2OCH2SCOCl + O2 (27) OCH2SCOCl f HCHO + OCS + Cl
(28)
Since similar oxidation pathways can be expected for CH2SCOCl and CH2SCHO, formation of CH2SCHO in the OHinitiated oxidation of MTF would be expected to give high yields of OCS. Formation of OCS is not observed in the OH-initiated oxidation of MTF and is taken as indicating that H abstraction from the methyl group is probably negligible. The identified products do not give any information on the possibility of an addition channel such as has been observed for the reaction of OH with DMS.5-8 The addition reaction of OH with DMS in the presence of O2 is now known to lead to the formation of dimethyl sulfoxide (DMSO).5,16,17 The mechanism of DMSO formation is not presently known but probably involves the formation of an intermediate OH adduct which further reacts with O2: Figure 6. Yields of CO and OCS observed as a function of time in the UV photolysis of methyl thiolformate in synthetic air and N2, panels a and b, respectively.
also produces CO and HCHO, it is difficult to give quantitative estimates of the yields of these compounds for the MTF oxidation. The residual spectrum obtained after subtraction of all the identified products is shown in Figure 5, panel b. The major features are absorptions in the carbonyl region with sharp features at 1758.7 and 1745.9 cm-1, a broad absorption in the region typical for the SO-group with a maximum about 1192 cm-1, and a broad absorption in the 800-750 cm-1 region with a maximum at 775.5 cm-1. As discussed in the kinetic section there are three possible pathways for the attack of the OH radical aldehyde H-atom abstraction, methyl H-atom abstraction or addition to the sulfur atom. On the basis of a comparison of the rate coefficients of OH with various aldehydes and DMS,26 one would expect that the aldehyde H-atom abstraction pathway probably dominates with perhaps some contribution from the addition channel and a minor contribution from the methyl group H-atom abstraction channel. A very high formation yield of CO is observed in the product studies (Table 2). The photooxidation of CH3ONO certainly contributes to the CO yield; however, from a consideration of the magnitude of the measured OH radical rate coefficient and the concentration-time behavior of CO and HCHO, it is concluded that a major proportion of the CO yield must stem from the reaction of OH with MTF. Although this evidence is by no means conclusive, it implies that abstraction of the aldehyde H atom is a major reaction pathway since only
(CH3)2S + OH f (CH3)2S-OH
(29)
(CH3)2S-OH + O2 f {(CH3)2S(O2)-OH} f (CH3)2SO + HO2 (30) If the addition channel is operative in the case of MTF, the OH-initiated oxidation process would lead to the formation of methylformyl sulfoxide (CH3S(O)-C(O)-H) which would give rise to infrared absorptions in the 1700-1800 and 1100-1200 cm-1 regions:
CH3SCHO + OH (+ M) f CH3S(OH)CHO (+ M) (31) CH3S(OH)CHO + O2 f CH3S(O)CHO + HO2 (32) In the residual spectrum from the OH/MTF reaction system (Figure 5, panel b) the strong absorption feature with maximum at ∼1190 cm-1 is typical for a compound containing the sulfoxide group, -SO-,36,37 and a sulfoxide compound is probably responsible for at least part of this absorption. Because of the presence of NOx in the system, absorptions from nitrate compounds may also contribute in this region. The concentration-time behavior of the absorption at ∼1190 cm-1 correlates with the carbonyl absorption in the 1750 cm-1 region, suggesting that they probably belong to the same compound. As mentioned above, the absorptions at ∼1750 and ∼1190 cm-1 observed in the both the OH-initiated and UV photolysis of MTF are very similar. Based on the recent observed formation of DMSO in the OH-initiated oxidation of DMS,16,17 the absorptions are being
17214 J. Phys. Chem., Vol. 100, No. 43, 1996
Patroescu et al.
tentatively assigned to the formation of methylformyl sulfoxide in the reaction system. Another compound which could explain the absorptions is diformyl sulfoxide (OHC-SO-CHO) which could be formed either by the oxidation of MTF (reaction 13 followed by reactions 33-36) or the further oxidation of methylformyl sulfoxide (reaction 36): several steps
CH2SCHO + O2 98 OHC-S-CHO
(33)
OHC-S-CHO + OH (+ M) f OHC-S(OH)-CHO (+M) (34) OHC-S(OH)-CHO + O2 f OHC-SO-CHO (35) several steps
CH3S(O)CHO + OH + O2 98 OHC-SO-CHO (36) However, since H-atom abstraction from the methyl group is considered to be very minor, formation of diformyl sulfoxide, if occurring, is also expected to be very minor. Cl-Atom Reaction System. Figure 7, panels a and b, shows the concentration-time profiles of the calibrated products identified in the reaction of Cl atoms with MTF in synthetic air and N2, respectively. The yields of all the identified products determined for ∼70% conversion of MTF are listed in Table 2. The list of products in Table 2 also contains chlorinated compounds such as CH3SCl, CH3S(O)Cl, CH3S(O2)Cl, and SOCl2 which have been identified or assigned in the reaction system. The yields of these compounds should be treated as preliminary due to difficulties in accurate calibration; because of the uncertainty in the yields the concentration-time profiles of these compounds are not shown in Figure 7. The analysis of the results leading to the identification of these compound is outlined below. In synthetic air the carbon and sulfur balances are both approximately 70-80%. SO2 accounts for a large fraction of the chlorine-free detected sulfur in synthetic air along with low yields of OCS and the major carbon-containing product is CO which accounts for approximately 55% of the detected carbon. In nitrogen, SO2 accounts for only 15% S and a large fraction the sulfur has been shown to be in the form of chlorinated organosulfur compounds. The total carbon balance (65% C) is a little higher than that obtained in air and again CO is the major carbon containing product. Figure 5, panels c and d, shows the residual spectra obtained from the reaction of Cl atoms with MTF in air and N2, respectively, after subtraction of all the non-chlorine-containing compounds listed in Table 2. In both residual spectra the dominant feature is a strong absorption feature with maxima at 1190, 1180, and 1175 cm-1. The relative intensities of the maxima are different between air and N2 suggesting that more than one compound is probably contributing to the absorption. There is another band on the right-hand flank of this absorption with a center at approximately 1145 cm-1. Other features presence in both spectra are absorptions due to C-H stretching vibrations at approximately 2998 and 2945 cm-1 (not shown), a broad absorption in the carbonyl region from approximately 1700 to 1780 cm-1, and various absorptions in the fingerprint region centered at 1402, 1255, 1025, 944, and 745 cm-1. The absorption features at 2998, 2945 1255, 1145, 1025, 944, and 745 cm-1 are more intense in N2 compared to air. The absorption centered at 1255 cm-1 has been identified as thionyl chloride (SOCl2) from an authentic spectrum of the substance.38 This compound is difficult to calibrate and as stated above the preliminary yield should be treated with caution. The mechanism of SOCl2 formation is presently unclear; its absorp-
Figure 7. Concentration-time profiles of the products formed in the reaction of Cl atoms with methyl thiolformate in 1000 mbar of synthetic air and N2, panels a and b, respectively.
tion appears fairly late in the irradiation and increases with time. It could be formed by reactions of Cl with CH3SO or SO radicals formed in the system by oxidation of CH3S. However, the intensity of the 1255 cm-1 absorption increases as that of the absorption centered around 1180 cm-1 decreases, suggesting that it is probably an oxidation product of the compound giving rise to this absorption. As already mentioned, strong absorption features in the region 1100-1200 cm-1 are typical for compounds containing the sulfoxide group, -SO-,36,37 and a sulfoxide compound is probably responsible for the major fraction the residual absorptions in Figure 5. Inspection of the broad carbonyl region 1780-1700 cm-1 suggests that at least two compounds contribute these CO absorptions. In the product analysis all of the potential carbonyl-containing products formed in bond cleavage reactions such as HCHO, HCOCl, COCl2, and HCOOH have all been accounted for. Therefore, the compounds giving rise to the residual carbonyl absorptions must come from carbonylcontaining compounds produced in the MTF oxidation which leave the MTF entity intact; i.e., C-S bond fission does not occur.
Atmospheric Chemistry of Methyl Thioformate
J. Phys. Chem., Vol. 100, No. 43, 1996 17215
The high yields of CO observed in N2 support that abstraction of the aldehydic H atom must be a major reaction pathway:
CH3SCHO + Cl f CH3SCO + HCl
(18)
CH3SCO + ∆ f CH3S + CO
(24)
However, as discussed in the kinetic section there are several other reaction pathways for the Cl atom attack, an H atom abstraction from the methyl group (reaction 19) and formation of a weakly bound Cl-MTF adduct (reaction 20). Adduct formation is known to occur between the reactions of Cl and Br atoms with DMS.31,32,34 The fate of the Cl-MTF adduct could be either decomposition for which a number of pathways are possible
(CH3)(CHO)S-Cl + ∆ f CH3S + HClCO
(37)
(CH3)(CHO)S-Cl + ∆ f CH3S-Cl + HCO
(38)
or further reaction with O2 to form products (reaction 39),
(CH3)(CHO)S-Cl + O2 f products
(39)
Formyl chloride is observed in the reaction system; however, the formation yields are extremely low and thus reaction 37, if occurring, is of negligible importance. No evidence was found for a channel producing CH3SCl 32 in a discharge flow-mass spectroscopy investigation of the reaction of Cl atoms with DMS at low pressure, and a laser flash photolysis-resonance fluorescence study of the reaction of Br atoms with DMS was inconclusive concerning the fate of the (CH3)2SBr adduct.39 However, the formation of CH3SF has been observed in the reaction of F atoms with DMS 32,40 and there is evidence that the formation of CH3SOH in the reactions of OH radicals with DMS is possible.7,8 Further, ongoing work in this laboratory on the reaction of Br atoms with DMS at room temperature has shown that the fate of the (CH3)2SBr adduct in synthetic air apart from reforming DMS and Br atoms is decomposition to form methanesulfenyl bromide CH3SBr and CH3 radicals. The results support that reaction of the adduct with molecular oxygen is negligible. The infrared spectrum of methanesulfenyl chloride (CH3SCl) is known,41 and it can also be easily generated by reacting DMDS with molecular chlorine.37
CH3SSCH3 + Cl2 f CH3SCl + CH3SCl
(40)
Figure 8, panel a, shows a gas-phase reference spectrum of approximately 18 ppm of CH3SCl produced in situ in the 1080 L reactor by mixing stoichiometric quantities of DMDS (9 ppm) and Cl2 (9 ppm) in 1000 mbar of synthetic air. This is to our knowledge the first calibrated gas-phase spectrum of CH3SCl. Using this spectrum as a reference, formation of CH3SCl was confirmed in the Cl-initiated oxidation of MTF. Methanesulfenyl chloride is known to be very reactive37,42 and, if formed in the present reaction system, its major fate is expected to be oxidation initially to methanesulfinyl chloride (CH3SOCl) which is itself very reactive and will rapidly further oxidize to methanesulfonyl chloride (CH3SO2Cl). According to Bellamy,36 CH3SOCl would be expected to have an SO absorption at ∼1180 cm-1 (i.e., estimated by determining the middle value of the sum of DMSO and SOCl2 SO absorption frequencies: (1256 + 1103)/2 ) 1180 cm-1). This is in agreement with the strong absorption observed at 1180 cm-1 in the Cl initiated oxidation of MTF (Figure 5, panels c and d); however, it would not explain the carbonyl adsorption. Infrared spectra of CH3SOCl recorded in an aqueous medium43,44 report
Figure 8. (a) Infrared spectrum of 18 ppm of methanesulfenyl chlorine (CH3SCl) produced by the reaction of Cl2 with dimethyl disulfide (DMDS) in 1000 mbar of N2. (b) Residual spectrum obtained after 2 min irradiation of a DMDS/Cl2/N2 mixture in which the strongest residual absorption with centered at 1180 cm-1 is tentatively assigned to methanesulfinyl chloride (CH3SOCl). (c) Residual spectrum obtained from the mixture in (b) after complete consumption of DMDS in which absorption features attributable to methanesulfonyl chloride (CH3SO2Cl) and thionyl chloride (SOCl2) are clearly visible.
a strong SO absorption at approximately 1150 cm-1. Figure 8, panel b, shows the spectrum obtained on photolysis of a DMDS/ Cl2 mixture in synthetic air for 5 min. The dominant feature here is an SO absorption with maxima at 1188, 1180, and 1175 cm-1. Since the major fate of DMDS in this system is initially formation of CH3SCl, the absorptions entered around 1180 cm-1 are assigned to CH3SOCl. Figure 8, panel c, shows the product spectrum obtained upon further irradiation. The SO absorption diminishes with formation of SO2, CH3SO2Cl, and SOCl2. Comparison of the absorptions around 1180 cm-1 in Figure 5, panels c and d, with those in Figure 8, panel b, shows that they are very similar, particularly for the Cl-initiated oxidation of MTF in synthetic air. From this comparison it is suggested that a major portion of the SO absorption observed in the residual spectrum from the reaction of Cl with MTF (Figure 5, panels c and d) is due to CH3SOCl formed by the oxidation of CH3SCl produced via reactions 36 and 38.
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Low yields of methanesulfonyl chloride (CH3SO2Cl) are observed in the MTF/Cl2 oxidation systems and also in the DMDS/Cl2 system. Experiments carried out with high starting concentrations of Cl2 led to an increase of the CH3SO2Cl yields in the MTF/Cl2 systems. The source of CH3SO2Cl is thought to be primarily further oxidation of CH3SOCl as has been observed in liquid phase studies of CH3SCl chemistry37,45 and is supported by the present gas-phase study of the DMDS/Cl2 reaction system. Under the present gas-phase conditions reaction of CH3SOCl with ClO radicals is probably the most likely process responsible for the oxidation:
CH3SOCl + ClO f CH3SO2Cl + Cl
(41)
As discussed above, the reaction of OH with MTF is believed to produce methylformyl sulfoxide. A similar reaction sequence is possible for the reaction between Cl atoms and MTF leading to the formation of CH3SOCHO:
CH3SCHO + Cl + O2 f {(CH3)(CHO)S(O2)-Cl} f CH3-SO-CHO + ClO (42) Formation of this compound would account partly for the carbonyl absorption in the 1700 cm-1 region in Figure 5, panels c and d, and also the differences observed for the SO absorptions assigned to CH3SOCl in the MTF/Cl2 reaction system compared to the DMDS/Cl2 system. There is another, and more probable, pathway to formation of CH3SOCHO in the MTF/Cl2 system, reaction of Cl atoms with alkyl peroxy radicals produced in the oxidation could result in the formation of ClO radicals which could react with MTF to produce methylformyl sulfoxide:
Cl + organic peroxy f ClO + organic alkoxy
(43)
ClO + CH3-S-CHO f CH3-SO-CHO + Cl (44) The reaction of Cl atoms with methylperoxy radicals (CH3O2) is known to produce ClO 26 radicals and it is now well established that reactions of ClO with DMS produce DMSO with unit yield.46 Recent experience on the formation of DMSO in both the Br- and Cl-initiated oxidation of DMS from ongoing studies in this laboratory lead us to believe that formation of CH3SOCHO via reaction sequence (43) and (44) is more probable. Conclusions The atmospheric chemistry of methyl thiolformate, an intermediate in the oxidation of dimethyl sulfide, has been investigated. Its UV absorption spectrum has been measured in the range 220-355 nm at 298 K and a lower limit of 5.4 days determined for its atmospheric photolytic lifetime in the summertime at 40° N. It is presently not known what effect temperature will have on the UV spectrum; however, since the absorption in the atmospheric actinic region is relatively weak it is unlikely that significant changes will occur over the tropospherically relevant temperatures. Rate coefficients have been determined for its reaction with OH radicals and Cl atoms. Since the concentration of Cl atoms in the atmosphere is, with the possible exception of a few specific marine regions, generally very low, loss by this reaction will be negligible. However, OH radicals concentrations are much higher, using a globally-averaged 24 h mean value for [OH] of the order of 7.7 × 105 molecules cm-3 47 results in an atmospheric lifetime due to reaction with OH of approximately 1.4 days. Since the reaction of OH with MTF is believed to be an overall abstraction process similar to those of the simple aldehydes the temperature dependence of the rate coefficient is also expected to be similar.
Thus, no large temperature dependence is expected for the reaction which could alter the importance of the OH sink reaction for MTF. This is, however, speculation and needs to be confirmed by experiment. MTF is only very sparingly soluble in water and has a boiling point of 70-75 °C at 1 atm; therefore, it is unlikely that heterogeneous processes such as wet deposition or aerosol formation will be able to compete with oxidation via OH radicals. Therefore, reaction of OH radicals will most probably be the dominant atmospheric sink for MTF. Although not described here, investigations were also carried out to determine the rate coefficient for the reaction of MFT with NO3 radicals using the thermal decomposition of N2O5 as the radical source. The decay of MTF under the experimental conditions was very slow and did not allow an accurate determination of the rate coefficient for the reaction with NO3 radicals. However, from the measurements it can be estimated that the rate coefficient must be of the order of 2 × 10-16 molecules cm-3 s-1; i.e., of the same order of magnitude as for the NO3 radical reaction with HCHO 26 and thus of negligible importance as a nighttime sink for MTF in the atmosphere. Acknowledgment. Financial support of this work by the EC (European Commission) is gratefully acknowledged. The authors thank Dr. H. G. Libuda for the invaluable assistance in recording the UV spectra References and Notes (1) Bates, T. S.; Lamb, B. K.; Guenther, A.; Dignon, J.; Stoiber, R. E. J. Atmos. Chem. 1992, 14, 315. (2) Spiro, P. A.; Jacob, D. J.; Logan, J. A. J. Geophys. Res. 1992, 97, 6023. (3) Jensen, N. R.; Hjorth, J.; Lohse, C.; Skov, H.; Restelli, G. J. Atmos. Chem. 1992, 14, 95. (4) Butkovskaya, N. I.; LeBras, G. J. Phys. Chem. 1994, 98, 2582. (5) a) Hynes, A. J.; Stoker, R. B.; Pounds, A. J.; McKay, T.; Bradshaw, J. D.; Nicovich, J. M.; Wine, P. H. J. Phys. Chem. 1995, 99, 16967. (b) Hynes, A. J.; Wine, P. H.; Semmes, D. H. J. Phys. Chem. 1986, 90, 4148. (6) Stickel, R. E.; Zhao, Z.; Wine, P. H. Chem. Phys. Lett. 1993, 212, 312. (7) Turnipseed, A. A.; Ravishankara, A. R. In Proceedings of the International Symposium on Dimethylsulphide: Oceans, Atmosphere, and Climate, Belgirate, Italy, 13-15, Oct. 1992; Restelli, G., Angeletti, G., Eds.; Kluwer Academic Publishers: Dordrecht, 1993; pp 185-195. (8) Barone, S. B.; Turnipseed, A. A.; Ravishankara, A. R. Faraday Discuss. 1995, 100, 39. (9) Wallington, T. J.; Ellermann, T.; Nielsen, O. J. J. Phys. Chem. 1993, 97, 8442. (10) Nielsen, O. J.; Sehested, J.; Wallington, T. J. Chem. Phys. Lett. 1995, 236, 385. (11) Barnes, I.; Becker, K. H.; Patroescu, I.; Ruppert, L. In Second Progress Report, Contract No. EV5V-CT91-0038; Moortgat, G. K., coordinator; Commission European Communities, Brussels, 1994. (12) (a) Wallington, T. J.; Hurley, M. D.; Ball, J. C.; Jenkin, M. E. Chem. Phys. Lett. 211, 1993, 41. (b) Jenkin, M. E.; Hayman, G. D.; Wallington, T. J.; Hurley, M. D.; Ball, J. C.; Nielsen, O. J.; Ellermann, T. J. Phys. Chem. 1993, 97, 11712. (13) Berresheim, H.; Wine, P. H.; Davis, D. D. In Composition, Chemistry, and Climate of the Atmosphere; Singh, H. B., Ed.; Van Nostrand Reinhold: New York, 1995; pp 251-307. (14) Davis, D. D.; Bradshaw, J. D.; Rodgers, M. O.; Sandholm, S. T.; KeSheng, S. J. Geophys. Res. 1987, 92, 2049. (15) (a) Lightfoot, P. D.; Cox, R. A.; Crowley, J. N.; Destriau, M.; Hayman, G. D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. Atmos. EnViron. 1992, 26A, 1805. (b) Wallinton, T. J.; Dagaut, P.; Kurylo, M. J. Chem. ReV. 1992, 92, 667. (16) Barnes, I.; Becker, K. H.; Patroescu, I. Geophys. Res. Lett. 1994, 21, 2389. (17) Barnes, I.; Becker, K. H.; Patroescu, I. Atmos. EnViron. 1996, 30, 1805. (18) Pszenny, A. A.; Keene, W. C.; Jacob, D. J.; Fan, S.; Maben, J. R.; Zetwo, M. P.; Springer-Young, M.; Galloway, J. N. Geophys. Res. Lett. 1993, 20, 699. (19) Barnes, I.; Becker, K. H.; Mihalopoulos, N. J. Atmos. Chem. 1994, 18, 267. (20) Barnes, I.; Bastian, V.; Becker, K. H Int. J. Chem. Kinet. 1988, 20, 415.
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