Biogenic Sulfur in the Environment - American Chemical Society

Chapter 25

OH-Initiated Oxidation of Biogenic Sulfur Compounds

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Kinetics and Mechanisms Under Atmospheric Conditions A. J. Hynes and P. H. Wine Molecular Sciences Branch, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, GA 30332 Our current understanding of the rates and mechanisms of the OH-initiated oxidation of several reduced sulfur compounds is reviewed with particular emphasis on recent studies in our laboratory. Important uncertainties are highlighted and directions for future research are suggested. Much progress has been made in our understanding of the mechanism of the oxidation of naturally and anthropogenically produced sulfur compounds (1). We focus here on our recent studies of the OH-initiated oxidation of reduced sulfur in the gas phase (2-4) and try to address some questions posed by current work. Recent reviews (5.6) have summarized progress in laboratoiy studies, which can be conveniently divided between direct studies, aimed primarily at isolating and determining absolute rates of elementary steps in the oxidation process, and competitive studies aimed at determining relative reaction rates and also the identities and yields of stable end products. Clearly, the success of these efforts can be measured by our ability to model the observed concentration profiles obtained in controlled laboratory studies (2) and the concentrations and lifetimes observed in the atmosphere (8). In this paper, we review the current level of understanding of the rates and mechanisms of O H reactions with several reduced sulfur compounds, and suggest directions for future research.

QH + CS -> Products 2

Direct studies of reaction (1) which have O H + CS — > Products

(1)

2

been performed in the absence of oxygen indicate that the reaction proceeds slowly, if at all (£). In apparent contradiction to this, both competitive rate studies performed under atmospheric conditions and atmospheric lifetime measurements of C S are consistent with a fast rate for (1) (10-12). Adduct formation followed by adduct reaction with 0 has been suggested (10-12) as a mechanism for reaction (1) which would remove the discrepancy between these results. We have studied reaction (1) (4) using 'Pulsed Laser 2

2

0097-6156/89/0393-0424S06.00/0 • 1989 American Chemical Society

Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

25. HYNES AND WINE

OH-Initiated Oxidation ofBiogenic Sulfur Compounds 425

Photolysis - Pulsed Laser Induced Fluorescence' (PLP-PLIF), a technique which allows us to directly measure O H reaction rates under atmospheric conditions and also permits the sub-microsecond time resolution necessary to observe fast equilibration processes. In the absence of oxygen, we observed the rapid formation of a CSoOH adduct and were able to extract elementary rates for its formation and decay (k ,k. ), as a function of temperature. la

la

O H + C S + M CS2OH + M

(la,-la)

2

The temperature dependence of the forward rate in 700 Torr N is described by the Arrhenius expression 2

4

k

l a

1

= 6.9 x IO-* exp (1150/T) cm^molecule-V .

The ratio of the forward and reverse rates gives the equilibrium constant and a Van't Hoff plot of its variation with temperature, shown in Figure 1, gives a heat of reaction A H = -9.9 ± 0.8 kcal/mole. Using known values for the heats of formation of O H and C S (12) leads to a value of 27.5 kcal/mole for the adduct heat of formation. In the presence of oxygen, the observed reaction rate increases rapidly as a function 01 oxygen pressure as shown in Figure 2, and a rate of (1.5 ± 0.1) x lO-^cnAnolecule'V in 700 Torr of air at 298K is obtained. This is in good agreement with the competitive studies (10-12) which report rates of ~2 x 10 cm molecule" s* at this pressure and temperature. The temperature dependence of the observed bimolecular rate constant, k t> , defined as the slope of a plot of the pseudo-first order O H decay rate versus the C S concentration, is shown in Figure 3, plotted in Arrhenius form. The rate increases rapidly as the temperature is decreased which is consistent with a mechanism involving reversible adduct formation followed by adduct reaction with oxygen: 2

1

_12

3

1

1

0

s

2

C S ^ O H + 0 — > products.

(2)

2

At 25 IK, the observed bimolecular rate constant reaches its limiting value, k * 6 x lO-^cmSmolecule'V , at an oxygen partial pressure of *100 Torr. At this point, adduct formation becomes the rate limiting step and further increases in oxygen partial pressure produce no increase in the observed rate constant. Applying a steady state analysis to the reaction sequence (la,-la,2) gives the expression 1

l a

kobs(T,[02]) = k (T)X(T)[02] la

1+X(T)[02]

(I)

where x(T) = kjCiyk^cr), and allows us to obtain a value of k , the adduct + 0 rate constant (units are cnAnolecules'V ): 2

2

1

k = 1.4 ; J J x 10-M exp [217 ± 301/T]. 4

2


426

BIOGENIC SULFUR IN THE ENVIRONMENT

T

I

i 3.4

1

1

1

1

1

r

i

i 3.6

i

i 3.8

i

I 4.0

1

1000/T(K)

Figure 1. Van't Hoff plot for the equilibrium O H + C S < — > C S O H . (Reproduced from Reference 4. Copyright 1988 American Chemical Society.) 2

30

2

(c) (d) ( b >

20

/o

-

10 (a) _ 0 0.0

0.5

1.0 [CS 1 (10 2

1.5 16

2.0

3

cm" )

Figure 2. Plots of k' versus [CS ] for data obtained at 295K and 700 Torr N + Oo. Solid lines are obtained from linear least squares analyses and give the following bimolecular rate constants (units are lO'^cnAnolecule'V , errors are 2a and represent precision only): (a) 0.20 ± 0.01, (b) 1.51 ± 0.09, (c) 2.06 ± 0.10, and (d) 2.63 ± 0.19. For the sake of clarity, data points obtained for [CSJ = 0 are not shown. (Reproduced from Reference 4. Copyright 1988 American Chemical Society.) 2

2

1


25. HYNES AND WINE

OH-Initiated Oxidation ofBiogenic Sulfur Compounds All

Figure 3. Arrhenius plot for the O H + C S reaction in 680 ± 20 Torr air. The solid line is obtained from Equation II. (Reproduced from Reference 4. Copyright 1988 American Chemical Society.) 2


428


A temperature independent rate constant of (2.9 ± 1 . 1 ) x lO-^cmôiecule-V is also consistent with our data within the estimated error limits. Fitting (I) to the k ^ data shown in Figure 3 gives an expression for the temperature dependence of k t, in one atmosphere of air (units are cmSmolecule-V ): 1

0

0

s

1

k

o b s

= 1.25 x 10-16 exp (4550/T) T + 1.81 x 10-3 exp (3400K).

(II)

This work directly confirms the role of O H in the atmospheric oxidation of CS . A l l of our results are consistent with a simple three step reaction mechanism involving adduct formation followed by decay or reaction with oxygen. COS, S 0 and CO are all potential end products of reaction (2), and its mechanism and relative yields are of great importance in assessing the role of reaction (1) in the global sulfur cycle. We have identified thirteen exothermic channels tor (2), three of which, (2c), (2g) and (2j), regenerate the O H molecule and would not be measured in our experiments. 2

2

CS2OH + 0 — > S H + S 0 + C O 2

2

(2b)

81

(2c)

+ COS

79

(2d)

+ C0

79

(2e)

C0

OH + S +

C0

2

S0

2

H + S 0 2

(2a)

86

SH + SO +

H +

- A H = 91 kcal/mole

HOCS +

S0

2

2

2

(2f)

2

50

(2g)

45

(2h)

SO + H S O + C O

42

(2i)

S0

+ CS

34

(2j)

+ COS

27

(2k)

+ HCO

23

(21)

+ CO

20

(2m)

O H + SO + COS

C0

S + HSO +

OH +

SH + O S + S

2

+

S0

Z

2

H0

2

2

2

The consistency between our elementary rate constants, k ( T ) and our values for k^fr), combined with the agreement between our k ^ (298K) values and those or Jones, et al. (10.11). leads us to believe that tne O H regeneration channels are relatively minor. The competitive rate studies of Jones, et al. (10.11) and Barnes, et al. (12) monitored both C S decay and the appearance of COS and SO^ and found the reaction stoichiometry to be la

2

C S + W2 0 2

2

- > COS + S 0 . 2


(lb)

25. HYNES AND WINE


Rowland, et al. (14) have used radiochemical tracer techniques to study reaction (1) and obtain equal yields of CO and COS. C S oxidation could account for a significant traction of the atmospheric COS budget (8) and definitive measurements of the mechanism of reaction (2), its primary products and the routes to, and yields of the ultimate stable products are needed. 2

O H + Dimethyl Sulfide Marine dimethyl sulfide emissions are thought to account for about half of the global flux of biogenic sulfur into the atmosphere (15). While there appears to be reasonable agreement on concentration levels, which are in the 100 ppt range in the marine boundary layer, (1£) estimates of the atmospheric lifetime vary by several orders of magnitude (17.18). O H oxidation has been considered to be the major sink for DMS, although N O j and possibly IO could play significant roles. There have been numerous studies of reaction (3), (5) O H + CH3SCH3 —> Products.

(3)

However, agreement between studies is poor and the overall mechanism of oxidation is unclear, although SO2, methane sulfonic acid (MSA), and dimethyl sulfone (DMSO2) have been identified as stable end products in steady state photolysis experiments (19-22). Studies m our laboratory, utilizing both conventional flash photolysisresonance fluorescence (FP-RF) (2.23) and PLP-PLIF (2) indicate that reaction (3) proceeds via a complex mechanism which involves abstraction, reversible addition, and adduct reaction with 0 . In the absence of 0% the FP-RF studies obtain a 298K rate constant of 4.4 x 10 cm molecule" s- . The observed kinetic isotope effect and positive activation energy are consistent with the reaction proceeding via H atom abstraction, 2

_12

3

Y

1

O H + CH3SCH3 - - > CH3SCH2 + H 0 .

(3a)

2

In the presence of 0 , however, a significant rate enhancement occurs which shows no isotope dependence and increases dramatically as the temperature is reduced, as shown in the Arrhenius plot, Figure 4, for O H + DMS-d$. As in the case of CS , this behavior is consistent with adduct formation followed by adduct reaction with 0 . 2

2

2

OH O H + CH3SCH3 + M C H - S - C H + M 3

3

(3b,-3b)

OH C H - S - C H + 0 —- > products. 3

3

2

(4)

As the temperature is decreased the adduct lifetime increases, resulting in more collisions with O3 and an increase in the observed reaction rate. Applying a steady state analysis to the reaction sequence (3a, 3b, -3b, 4) gives an expression similar to Equation (I), and assumption of a small negative activation energy for reaction (3b) allows us to fit the data in Figure 4 to the steady state expression and extract values for k3b(T) and k4(T)/k_3b(T). Although direct observation of equilibration kinetics was not possible because of the fast abstraction channel and short adduct lifetime, observation of a non linear dependence of k t, on [DMS-d ] at high DMS-ds and low temperature 0

s

6


430


20

....... o

JL

2

—•— Air • 3

10

V

O

E Eo

: -

o

2h

=

2.6

i

i

3.0

i

__J

3.4

1

>

—

3.8

1000/T0O

Figure 4. Arrhenius plots for the O H + CD3SCD3 reaction in 700 Torr % air, and 0 . (Reproduced from Reference 35. Copyright 1987 American Chemical Society.) 2


25. HYNES AND WINE

OH-initiated Oxidation ofBiogenic Sulfur Compounds 431

allowed us to obtain an adduct lifetime toward unimolecular decomposition, l/k.35, of 400 ns and, hence, a rate for the adduct + 0 reaction of (4.2 ± 2.2) x lO-ânîolecule-V at 261K and 700 Torr N + 0 . Our results indicate that, at 300K, reaction proceeds 25% via addition followed by adduct reaction with 0 , whereas, at 250K, the addition route accounts for 70% of the overall reaction. In their modeling study of DMS oxidation, Yin, et al. consider both addition and abstraction channels to produce S 0 and MSA (2). They do not consider other energetically accessible channels such as 2

1

2

2

2

2

OH

O

C H 3 - S - C H 3 + 0 —- > C H 3 - S - C H 3 + H 0 2

OH

(4a)

2

O

C H - S - C H + 0 — > CH3 -1 - C H + O H I O 3

3

2

(4b)

3

Barnes, et al. (22) have studied reaction (3) in the presence of 0 using a competitive rate technique which utilizes H 0 as the O H photolytic precursor. This avoids the use of N O containing precursors which nave been shown to produce unreliable results in studies of O H reactions with organo-sulfur compounds (24.25). They find up to 30% yields of dimethylsulfone, ( C J ^ S O ^ and suggest that reaction (4b) is important. If reaction (4b) is an elementary reaction and O H is regenerated, then our study of reaction (3) would have obtained values of k ^ which were too low and, hence, underestimated the importance of the addition channel. One apparent inconsistency between the recent work of Barnes, et al. (21) and our study (2) is that Barnes, et al. observed only D M S 0 as a product of reaction (4); neither DMSO nor MSA was observed. Since we would not have observed an O? rate enhancement if the adduct + 0 reaction proceeded entirely via channel (4b), our results imply that an addition channel other than (4b) must be important. The only addition pathways which seem reasonable are (4a), (4b), and 2

2

2

x

0

s

2

2

OH CH - S -CH + 0 3

3

2

-> CH3SOH + C H 0 . 3

(4c)

2

The C H 3 S O H product of reaction (4c) is expected to react with 0 to produce MSA. Further studies are needed to establish whether DMSO? is formed directly via reaction (4b) or via a multistep pathway involving production and subsequent oxidation of DMSO. If D M S 0 is, indeed, formed via reaction (4b), then the source of the oxygen enhancement observed in our experiments needs to be identified. On the basis of our results, one might expect the end product yields from the OH-initiated oxidation of DMS to be strongly temperature dependent. No temperature dependent laboratory studies have been performed which would allow this hypothesis to be tested. However, atmospheric measurements suggest higher MSA-to-S0 yield ratios at higher latitudes (i.e. lower temperatures) 2

2

2


432


At present, modeling simulations of both laboratory experiments (2) and measured atmospheric trace species distributions (8) are unable to reproduce observed concentrations of DMS and its oxidation products. It is clear that direct studies aimed at identifying reactive intermediates, and a better understanding of the chemistry via which these intermediates are converted to end products, is required. O H + Methvl Mercaptan While methyl mercaptan (MM) has been observed in the marine atmosphere under favorable conditions at the ppt level (12), its mean concentration appears to be sub ppt and it is not considered to be a significant component of marine sulfur emissions. FP-RF studies (23.29) from our laboratory show that O H reacts rapidly with M M (k = 3.3 x lfrUcnAnolecule-V at 298K), 1

5

O H + C H S H —> products,

(5)

3

and that k5 shows very small kinetic isotope effects and a slight negative activation energy; these results are consistent with an addition mechanism. While direct laboratory studies are in reasonable agreement, two competitive studies report considerably faster rate constants in the presence of O2, suggesting the possibility of reversible adduct formation, followed by adduct reaction with 0 (26.27). In our PLP-PLIF studies (2) of reaction (5) in one atmosphere of nitrogen, air, and oxygen, shown in Figure 5, we observe rate constants in good agreement with the F P - R F studies with no 0 rate enhancement. The competitive studies used N O containing compounds as photolytic precursors. It has recently been demonstrated that secondary chemistry in these systems produces unreliable rate constants (24.25). The photolysis studies of Hatakeyama and Akimoto (12) appear to indicate that the C H 3 S O H adduct has a reasonable lifetime and reacts with N O containing compounds or decomposes to CH S and H 0 ; its reaction with oxygen appears to be slow. Again, direct measurement of C H 3 S yields together with a better understanding of CH3S chemistry are important in understanding M M oxidation. 2

2

x

x

3

2

Reactivity Trends Comparison of reactions (1), (3), (5) and others which might be expected to behave similarly, i.e. O H + COS —> products

(6)

O H + H S —> products,

(7)

2

show no consistent trends. Reaction (6) would appear to be a good candidate for "O^rate enhancement." However, a recent study saw no evidence for adduct formation or 0 rate enhancement (2&). Reaction (7) appears to proceed solely via an abstraction mechanism and no 0 enhancement" is observed in competitive rate studies (26). Additionally, the reactivity towards Cb of the adducts formed in reactions (1), (3) and (5) appears to be dramatically different with ( C H 3 ) S O H reacting 100 times faster than C S O H (2>4), while CH SH(OH) appears to react very slowly (12). It is difficult to rationalize the very different behavior of rather similar chemical systems, and future experimental studies might be expected to produce more surprises. A 2

M

2

2

?

3


25. HYNES AND WINE


I

l

o o

•

l

l

I

2

Air

- •

/

A

12

8

v

4

(3.29*0.15) x 1 0 " c m molecule" s~ 3

i

1

i

0

i

i

3 [CH SH] 3

11

1

6

( 1 0 molecules per cm ) 14

3

Figure 5. Plot of the pseudo-first order O H decay rate versus [CH^SH] for data obtained in a series of back-to-back experiments at 300K and 700 Torr total pressure but with different buffer gases. The 0 data points labeled with an Y and a"-" were obtained at particularly low and high laser powers, respectively. (Reproduced from Reference 3. Copyright 1987 American Chemical Society.) 2


434


theoretical framework for understanding the observed reactivity trends remains to be developed. Fyffljrg Wprk We believe that our studies of reactions (1), (3) and (5) have largely resolved the question of the mechanism of initial O H attack and the focus of our attention has shifted to the yields and chemistry of reactive intermediates and stable products. Direct observation of C H S and quantitative yield measurements as a function of temperature, pressure, and [OT] will clarify the detailed mechanisms of reactions (3) and (5). Additionally, the oxidation pathway for CH3S under atmospheric conditions is not well understood. Reactions of CH S with O ^ 0 3 , N 6 and H 0 are all potentially important, but data from these reactions is either limited (30-34) or non-existent and the kinetics and mechanisms of these reactions all require further study. Based on chemical kinetic evidence, it is now clear that O H reactions with C&2, CH3SCH3, and CH3SH proceed primarily via formation of relatively short lived addition complexes. However, it should be emphasized that none of these short lived complexes have been observed, and no structural information, either experimental or theoretical, is available for any of them. In the case of the OH...CS adduct, for example, it is not obvious whether O H binds to the carbon atom or to a sulfur atom. A detailed understanding of the reactivity trends discussed above will not be forthcoming until adduct structures are elucidated. 3

3

2

2

2

Acknowledgments This work was supported by the National Science Foundation through grant no. ATM-86-00892.

Literature Cited 1. Plane, J. M . C. In Biogenic Sulfar in the Environment; Saltzman, E.; Cooper, W. J., Eds.; American Chemical Society: Washington, DC, 1988, this volume. 2. Hynes, A. J.; Wine, P. H.; Semmes, D. H. J. Phys. Chem. 1986,90,4148. 3. Hynes, A. J.; Wine, P. H. J. Phys. Chem. 1987,91,3672. 4. Hynes,A.J.; Wine, P. H.; Nicovich, J. M . J. Phys. Chem. 1988,92,3846. 5. Atkinson, R. Chem. Rev. 1985, 85, 69. 6. Heicklen, J. Rev. Chem. Intermed. 1985,6,175. 7. Yin, F.; Grosjean, D.; Seinfeld, J. H . J. Geophys. Res. 1986, 91, 14417. 8. Toon, O. B.; Kasting, J. F.; Turco, R. P.; Lui, M.S. J. Geophys. Res. 1987, 91, 943. 9. Bierman, H. W.; Harris, G . W.; Pitts, J. N., Jr. J. Phys. Chem. 1982, 86, 2958, and references cited therein. 10. Jones, B. M . R.; Burrows, J. P.; Cox, R. A.; Penkett, S. A. Chem. Phys. Lett. 1982, 88, 372. 11. Jones, B. M . R.; Cox, R. A.; Penkett, S. A. J. Atmos. Chem. 1983, 1, 65.


25. HYNES AND WINE

OH-initiated Oxidation of Biogenic Sulfur Compounds 435

12. Barnes, I.; Becker, K. H.; Fink, E. H.; Reiner, A.; Zabel, F.; Niki, H . Int. J. Chem. Kinet. 1983, 15, 631. 13. Benson, S. W. Thermochemical Kinetics; Wiley-Interscience: New York, 1976. 14. Yang, W. X.; Iyer, R. S.; Rowland, F. S. CACGB Meeting; Peterborough: Ontario, 1987. 15. Bates, T. S.; Cline, J. D.; Gammon, R. H.; Kelly-Hansen, S. R. J. Geophys. Res. 1987,92,2930. 16. Andreae, M . O.; Ferek, R. J.; Bernard, F.; Byrd, K. P.; Engstrom, R. T.; Hardin, S.; Houmere, P. D.; LeMarres, F.; Raemdanck, H.; Chatfield, R. B. J. Geohys. Res. 1985,90,12891. 17. Berresheim, H . J. Geophys. Res. 1987, 92, 13245, and references therein. 18. Nguyen, B. C.; Bergeret, C.; Lambert, G . In Gas Transfer at Water Surfaces; Brutseert, W.; Jurke G. H., Eds.; Reidel: Dordrecht, 1984, p 549. 19. Hatakeyama, S.; Akimoto, H . J. Phys. Chem. 1983, 87, 2387. 20. Hatakeyama, S.; Okuda, S. M . ; Akimoto, H . Geophys. Res. Lett. 1982, 9, 583. 21. Grosjean, D. Environ. Sci. Technol. 1984,18,460. 22. Barnes, I.; Bastian, V.; Becker, K. H . Int. J. Chem. Kinet. 1988, 20, 415. 23. Wine, P. H.; Kreutter, N . M . ; Gump, C. A.; Ravishankara, A . R. J. Phys. Chem. 1981, 85, 2660. 24. Wallington, T. J.; Atkinson, R.; Tuazon, E . C.; Aschmann, S. M . Int. J. Chem. Kinet. 1986, 18, 837. 25. Barnes, I.; Bastian, V.; Becker, K. H.; Fink, E. H.; Nelsen, W. J. Atmos.

Chem.1986,4, 445. 26. Cox, R. A.; Sheppard, D. Nature 1980, 284, 330. 27. Barnes, I; Bastian, V.; Becker, K. H.; Fink, E. H . Physi-Chemical Behavior of Atmospheric Pollutants; Proceedings of the 3rd European Symposium: Varese, Italy; Reidel: New York, 1984, pp. 149-57. 28. Wahner, A.; Ravishankara, A R. J. Geophys. Res. 1987,92,2189. 29. Wine, P. H.; Thompson, R. J.; Semmes, D. H . Int. J. Chem. Kinet. 1984, 16, 1623. 30. Balla, R. J.; Nelson, H . H.; McDonald, J. R. Chem. Phys. 1986,109,101. 31. Black, G.; Jusinski, L. E. J. Chem. Soc. Faraday Trans. II 1986, 82, 2143. 32. Barnes, I.; Bastian, V.; Becker, K. H . Chem. Phys. Lett. 1987,140,451.


436

BIOGENIC SULFUR I N T H E ENVIRONMENT

33. Tyndall, G. S.; Ravishankara, A. R. In Biogenic Sulfur in the Environment; Saltzman, E . ; Cooper, W. J., Eds.; American Chemical Society: Washington, DC, 1988, this volume. 34. Hatakeyama, S. In Biogenic Sulfur in the Environment; Saltzman, E.; Cooper, W. J., Eds.; American Chemical Society: Washington, DC, 1988, this volume. 35. Hynes, A . J.; Wine, P. H . In The Chemistry of Acid Rain. Sources and Atmospheric Processes; Johnson, R. W.; Gordon, G . E., Eds.; American Chemical Society Symposium Series 349; Washington, DC, 1987, pp 133-41. RECEIVED August 30, 1988


Biogenic Sulfur in the Environment - American Chemical Society

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