14
J. Phys. Chem. 1980, 84, 14-16
Acknowledgment. The research described in this paper was carried out a t the Jet Propulsion Laboratory, California Institute of Technology, under NASA Contract NAS7-100. References and Notes (a) P. J. Crutzen, I. S. A. Isaksen, and J. R. McAfee, J. Geophys. Res., 83, 345 (1978); (b) S. C. Wofsy, ibid., 83, 364 (1978). L. J. Stief, J. V. Michael, W. A. Payne, D. F. Nava, D. M. Butler, and R. S. Stolarski, Geophys. Res. Left., 5, 829 (1978). M. T. Leu and W. B. DeMore, Chem. Phys. Left., 41, 121 (1976). G. Pouiet, G. LeBras, and J. Combourieu, J . Chem. Phys., 69, 767 (1978). B. A. Thrush, results presented at the WMO Symposium, Toronto, 1978. (a) R. Watson, G. Machado, S. Fischer, and D. D. Davis, J. Chem. Phys., 65, 2126 (1976); (b) R. T. Watson, J. Phys. Chem. Ref. Data, 6, 871 (1977). J. V. Michael, D. A. Whytock, J. H. Lee, W. A. Payne, and L. J. Stief, J. Chem. Phys., 67, 3533 (1977).
(8) (a) R. T. Watson, E. S. Machado, R. L. Schiff, S.Fischer, and D. D. Davis, Proceedings of the 4th CIAP Conference 1975; (b) R. S. Lewis, S. P. Sander, S. G. Wagner, and R. T. Watson, J . Phys. Chem., submitted for publicatlon. (9) D. A. Whytock, J. H. Lee, J. V. Michael, W. A. Payne, and L. J. Stief, J. Chem. Soc., Faraday Trans. I , 73, 1530 (1977). (10) L. F. Keyser, J . Chem. Phys., 69, 214 (1978). (11) C. L. Lin, M. T. Leu, and W. B. DeMore. J. Phvs. Chem.. 82, 1772 (1978). (12) R. Mannlng and M. J. Kurylo, J. Phys. Chem., 81, 291 (1977). (13) J. V. Michael, D. F. Nava, W. A. Payne, and L. J. Stief, J. Chem. Phys., 70, 1147 (1979). (14) R. Bailey and D. F. Boltz, Anal. Chem., 31, 117 (1959). (15) L. T. Molina, S. D. Schinke, and M. J. Molina, Geophys. Res. Left., 4, 580 (1977). (16) C. L. Lin, N. K. Rohatgi and W. 8. DeMore, Geophys. Res. Left., 5, 113 (1978). (17) F. Kaufman, Prog. React. Kinet., 1, 1 (1961). (18) T. R. Marrero and E. A. Mason, J . Phys. Chem. Ref. Data, 1, 3 (1972). (19) R. H. Perry and C. H. Chitton, Ed., "Chemical Engineering Handbook", 5th ed, McGraw-Hill, 1973, pp 3-230-3-234.
Fourier Transform Infrared Study of the HO Radical Initiated Oxidation of SOz H. Niki," P.
D. Maker, C. M. Savage, and L.
P. Breitenbach
Ford Motor Company, Research Staff, Dearborn, Michigan 48 121 (Received April 30, 1979) Publication costs assisted by Ford Motor Company
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To elucidate the atmospheric fate of HOS02radicals formed via HO + SO2 + M HOSOz + M, the photolysis of mixtures containing C12 (20 ppm), H2 (1000 ppm), SOz (20 ppm), and NO (5 ppm) in 700 torr of purified air was studied by using the long path FT IR method. IR absorption bands corresponding to those of slightly hydrated liquid H2S04were detected in situ during the photolysis due to the formation of air-borne H2SO4 aerosols. A 15N0-labelingexperiment was also used to examine the possible formation of sulfur- and nitrogen-containing products, e.g., HO(S02)0N0and HO(S02)0N02. The results were negative. Thus, these nitrogenous sulfuric acids do not appear to serve as a significant sink for SO2 and NO, in the atmosphere.
Introduction Gas phase chemical reactions of atmospheric interest often result in the formation of condensable products. The most notable example is aerosol formation in the photooxidation of mixtures containing SO2and various atmospheric trace constituents.' In order to establish reaction mechanisms for such chemical systems, it is necessary to characterize the chemical composition of the ensuing aerosol as well as of the gaseous products. Recently, in situ measurements of photochemically generated NH4C1and organic aerosols have been made in this laboratory by using the long-path Fourier transform infrared (FT IR) method.2 The present paper describes the use of this method for the characterization of aerosols formed by the HO radical initiated oxidation of SOzin the presence of nitrogen oxides. The HO radical reaction of SOz, reaction 1, is well-recHO + SO2 + M HO(S02) + M (1) AH = -37 kcal/mol
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ognized as one of the most important homogeneous reacHowever, there exists tions of SOz in the atm~sphere.~,"~ considerable uncertainty as to the subsequent reactions of the HO(S02)radical. On the basis of thermochemical estimates by Benson,6 the first step should be reaction 2a rather than 2b. In the presence of NO and NOz, the HO(S02) + 0 2 HO(S02)02 (24 AH = -16 kcal/mol -+
0022-3654/80/2084-0014$01.OO/O
HO(SO2) + 02
-
HOz + SO3
(2b)
AH = -8 kcal/mol following are some of the likely reactions of the HO(S02)02 radica1:l HO(S02)02 + NO --* HO(S02)O + NO2 (3) AH = -25 kcal/mol
-
HO(S02)O + NO HO(SO2)ONO AH = -26 kcal/mol HO(SOJ0
+ NO2
+
HO(SO2)ONOz
(4) (5)
AH = -22 kcal/mol The HO(SOZ)0 radical can also undergo H-atom abstraction to form H2S04,e.g., reaction 6. Thus, nitrosylHO(S0z)O + HO2 HO(S0z)OH + 0 2 (6) AH = -57 kcal/mol
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and nitrylsulfuric acid, HO(S0,)ONO and HO(S02)0NOZ, as well as H2S04may be present in the product aerosols. The formation of solid HO(S0,)ONO has been reported by Schroeder and Urone in the photolysis of mixtures containing high concentrations (-50 torr) of SO2 and NO2 in air.' Notably, both nitrosyl- and nitrylsulfuric acid have relatively weak 0-N bonds, and reactions 4 and 5 may be reversible. Benson has estimated their half-lives to be 0 1980 American Chemical Society
HO Radical Initiated Oxidation of SO2
about 1-10 s in the gas phase at STPe6On the other hand, nitrosylsulfuric acid i s known to be stable in H2S04solution but dissociate readily to NO, NO2,"OB, and H2S04 in the presence of water.lJ Therefore, it is of interest to determine whether these products can be incorporated in the aerosols at ppm reactant concentrations. The present study was made under relatively dry conditions to gain insight into these mechanistic questions concerning the HO radical initiated oxidation of SO2 in the presence of NO and NOz.
Experimental Section The long-path FT XR system was used for the photolysis and prloduct analysis. Details of the facility and computer-aided spectral analysis methods have been described elsewhere.* Typically, interferograms recorded in 1.5 min (16 scans) yielded 1/16 cm-l resolution spectra in the frequency range from 500 to 4000 cm-l with an optimum signal-to-noise ratio of about 200:l at 1000 cm--l. Various photochemical systems were examined for the generation of HO radical in a long-path IR cell (1m long, 40 Pam, 70-L, Pyrex cylinder) equipped with fluorescent lamps (G.E. F40 BLB and Westinghouse FS 40). The main scheme chosen involved the C1-atom sensitized oxidation of H2 in the presence of NO, Le., reactions 7-10.9 C12 + hu (300-400 nm) -+ 2C1 (7) C1+ H2 H + €IC1 (8) H + 02 + M-+HOz + M (9) HO2 + NO -+ HO +. NO2 (10) Typically, low pressure mixtures containing C12,NO, and SO2 at
