Environ. Sci. Technol. 1984, 18, 116-118
(5) Lonneman, W. A.; Bufalini, J. J.; Namie, G. R. Enuiron. Sei. Technol. 1982, 16, 655. (6) Stephens, E. R. "Advances in Environmental Sciences"; Wiley-Interscience: New York, 1969; Vol. I, p 119. (7) Kravetz, T. M.; Martin, S. W.; Mendenhall, G. D. Environ. Sei. Technol. 1980, 14, 1262. (8) Nielsen, T.; Hansen, A. M.; Thomsen,E. L. Atmos. Environ. 1982,16, 2447.
(9) Stephens, E. R. Anal. Chem. 1964, 36, 928.
Received for review March 28,1983. Revised manuscript received August 17, 1983. Accepted August 25, 1983. Portions of this work were supported by the Environmental Science Research Laboratory of the U.S. Environmental Protection Agency (Cooperative Agreement R-805812) and Battelle's Pacific Northwest Laboratories.
NOTES Redetermination of the Rate Constant for the Reaction of HO Radicals with SO, Katsuyukl Iruml, * Motoyukl Mlzuochl, M8SaO Yoshloka,t Kentaro Murano, and Tsutomu Fukuyama The National Institute for Environmental Studies, P. 0. Tsukuba-gakuen, Ibaraki, 305 Japan
The rate constant (k,)for the reaction of HO radicals with SO2 has been determined by a competitive rate technique with n-C4Hloused as a reference reactant. The HO radicals were generated from CH30N0, and the rate of decrease in the SO2 concentration has been measured in dry air ([H20]< 1ppm) at 780-820 torr in the presence of NO. A value of kl = (1.22 f 0.13) X cm3molecule-l s-l at 303 f 1 K has been obtained supporting a recent estimate of Calvert and Stockwell (1). Introduction The addition reaction of a hydroxyl radical to sulfur dioxide HO
+ SO2 + M
L HOS02 + M
(1) is considered to be the major gas-phase depletion pathway for SO2 in the atmosphere. Because of its importance in the SO2oxidation processes, reaction 1 has been the subject of many experimental studies, and an extensive review article on those studies has recently been written by Calvert and Stockwell (1). Reaction 1, which is a third-order reaction at low pressures, is in the transition region between third- and second-order kinetics in the pressure region characteristic of the troposphere. The effective second-order rate constant, kl,depends on the pressure of a third body M, and the nature of M complicates the determination of kl. Absolute measurements of kl at low pressures have been carried out by using the flash photolysis-resonance fluorescence technique (2-6). However, because of the quenching of excited HO radicals by oxygen and nitrogen, this technique is not applicable to the tropospheric pressure of air, the very conditions which are most important for the environmental studies. In order to measure the kl value at high pressures, one must use a competitive rate technique such as employed by Castleman and Tang Cox (81, and Cox and Sheppard (9). Among these, however, only in the last study was the rate of SO2 loss followed directly. Moreover, the rate constant for the reaction HO CO H C02,which was used in the former studies
(a,
+
-
+
'Permanent address: Nihon Kagaku Kogyo Co., 2-1 Shimizu, Suita, Osaka, 565 Japan. 116
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18, No. 2, 1984
as a reference, has been found to have an unexpected pressure dependence, and hence, it has turned out that the kl values reported in those studies must be corrected. On the other hand, in the last study, Cox and Sheppard used C2H4 as a reference organic; however, their k1 value was also questioned by Calvert and Stockwell, who pointed out that the use of an alkene as a reference might lead to an erroneously low kl value because of the reaction between the alkene and the radical product of the HO SO2 reaction. By correcting for the pressure dependence of the rate constant for the HO CO reaction, Calvert and Stockwell derived an estimate of kl = 1.14 X cm3molecule-l s-l from the data of Castleman and Tang and showed that this klvalue meshed well with the result of the low-pressure experiment of Leu (4). The kl value was also consistent with extrapolations from the low-pressure measurements of Davis et al. (3), who reported kl = ((0.9 (+0.25, -0.15)) X 10-l2,and of Harris et al., who reported a rather rough estimate of kl N 1.0 X 10-l2. In these circumstances, it is desirable to examine the accuracy of the k l value at typical tropospheric pressures by using a more appropriate reference reaction and by directly tracing the SO2 concentration. Recently Atkinson et al. (10) proposed an improved competitive rate technique, which employs CH30N0 as an HO radical source, for the determination of HO radical rate constants in air at the atmospheric pressure. This technique allows an accurate determination of the rate constants on the order of 3 X cm3 molecule-'s-l or larger. Being motivated by this proposal, we have chosen n-C4Hloas a reference to avoid the problem accompanying the use of alkene and attempted to get a kl value under conditions which simulate the real atmosphere more closely than before by using an aerosol chamber recently constructed in our institute.
+
+
Experimental Section An evacuable aerosol chamber which is 4 m3 in volume and has inner walls lined with borosilicate glass and Teflon was used as a reaction vessel. Prefiltered and predried air was purified by allowing it to pass through a platinum catalyst at 400 OC, through molecular sieves cooled down to -160 OC, and finally through an absolute filter. The air
0013-936X/84/0918-0116$01.50/0
0 1984 American Chemical Society
Table I. Initial Conditions and the Slope of the Plot Based o n Eq 8a initial Concentration, ppm run no. [C,H,,] [SO,] [CH,ONOl [NO1 1 1.32 0.201 1.95 2.40 2 1.28 0.199 3.0 3.08 2.48 0.402 1.95 3 1.32 4.0 0.387 4.0 4 1.31 0.376 6.0 5.81 5 1.29 2.24 0.784 1.95 6 1.26 7 1.29 2.04 1.95 2.43
k w , b ppm rnin-'
-1.9 x 8.4 X 1.4 x -5.3 x 6.9 x 2.6 x -9.1 x
10-5 10-5 10-5 10-5 10-5
a All runs were conducted at 303 t 1K and with an NO, photolysis rate constant k = 0.20 stant of SO, calculated as first order to [SO,].
thus purified contained less than 1ppmV of H20, 2 ppbV of nitrogen oxides (NO,), 0.04 ppmC of total hydrocarbons, and 5 cm-3 of particulates (condensation nuclei), and it was fed into the chamber with a pressure of about 820 torr. Reactant gases were measured volumetrically with injection tubes and put into the chamber. After two electric fans installed in the chamber were operated for a few minutes to mix the reactants, the irradiation started. The stirring of the mixture was continued during the irradiation time to keep the temperature and the reactant concentrations homogeneous. Initial concentrations of the reactants are given in Table I. The purest grade S02, n-C4Hlo,and NO, obtained from Matheson Gas Products, were used after further purification by distillation under vacuum. NO was added to suppress the ozone formation, since, in the preliminary experiments, the presence of ozone (0.1-0.5 ppm) was found to increase the wall decay rate of SO2. Following the prescription of Atkinson et al. (IO),CH30N0 was prepared from NaN02 and CH30H (11),distilled twice under vacuum at -78 OC, and used as an HO radical source after being stored in the dark at -198 "C. The reaction mixture was irradiated at 303 f 1K by a light source composed of 1 2 1.6-kW Xe lamps. Pyrex filters were installed into the light paths to match the spectral distribution of the light to the actinic irradiance of the real sun, cutting off the wavelengths less than 290 nm. Intensity of the light was determined in advance by measuring the photolysis rate constant (k)of NO2. Chemiluminescent analyzers and a UV fluorescent analyzer were used to monitor NO, and ozone, and SO2, respectively. CH30N0 and n-C4Hlowere monitored every 10 min by gas chromatography with a flame ionization detector. After 70 min of irradiation the light source was turned off, but the monitoring of SO2 was allowed to continue for an additional 2 h to estimate decay rate (k,) of SO2to the chamber wall. The SOz analyzer and the gas chromatograph were then recalibrated. The pressure of the chamber was lowered to about 780 torr by successive sampling out by the monitors. Results and Discussion A typical reaction profile (run 1) for the mixture of SO2, n-C4Hlo,CH30N0, and NO is shown in Figure 1. No ozone formation was observed during the experiment. The concentration of CH30N0 decreased exponentially with time and disappeared in about 1 h, indicating a rapid progress of reactions 2-4. The concentrations of n-C4Hlo CH30N0 + hv CH30 + NO (2) CH,O 02 CH2O + HOz (3) HOP NO -,HO + NOz (4)
+ +
-
--+
and SO2 decreased in accordance with the diminution of CH30N0. The HO radical concentration was calculated
slope, (-1 0.423 ?r 0.011 0.435 * 0,018 0.445 r. 0.011 0.475 ?r 0.028 0.424 t 0.030 0.388 2 0.008 0.378 r. 0.011 Wall decay rate conmin-I.
0 25
' N
005-
4 0-
0
05
10
Irradiation Time (h)
Figure I.Typical reaction profile for the mixture of CH,ONO, and NO. Run 1.
so,,
~-C~HIO,
Irradiation Time (h)
Figure 2. Time variations of decrements in SOz (0)and n-C4HIo(0) concentratlons. Run 1.
from the diminution rate of n-C4Hloconcentration through eq 5 by using the k6 value reported in ref 12. The con-
centration of HO radicals thus estimated exceeded 1.5 X lo+ ppm in the initial 10 min and then gradually lowered. On the other hand, the concentration of NO decreased by 50% in about 20 min and then increased slightly. This might be attributed to an accumulation of HONO by reaction 6 in the initial stage due to very high concentrations HO + NO (+M) HONO(+M) (6) HONO hu HO + NO (7) of HO and NO. This reaction is followed by a slow decomposition reaction 7, whose photodissociation coefficient J7 (=1.27 X s-l) (13) is one-fifth of that for reaction 2(J= ~ 6.3 x 10-3 s-1) (14). Decrease in the SO2and n-C4Hloconcentrations (A[S02] and A[n-C4Hlo])with irradiation time in run 1is shown in Figure 2. Net depletion of SO2by the reaction is obtained by correcting for the decay of SO2to the chamber wall. After subtracting the wall decay contribution k,t, the depletion of SO2 is correlated with that of n-C4Hlo through eq 8 where subscripts 0 and t denote the con[~Ozlo ki [n-C&ioIo In -- k,t = - In (8) [S021, k.5 [n-C4HioIt
+
--
Environ. Sci. Technol., Vol. 18, No. 2, 1984
117
[CH30NOIo
(pprn)
Flgure 4. Relation between [CH3ONOloand the slope of the plot based on eq 8. Symbols (0),(e),(D), (A),and (A)correspond to runs 1, 2, 3, 4, and 5, respectively. Flgure 3. Plot of In [S02]o/[S02],- k,tvs. In [n-C4H,o]oI[n-C4H,o],. Run 1.
centration at the beginning of the reaction and that at time t , respectively. In Figure 3, the quantity corresponding to the left side of eq 8 is plotted vs. In [n-C4Hlo]o/[n-C4Hlo], for run 1. As expected the plot is found to yield a line passing through the origin. By a least-squares fitting of a linear relation with zero intercept, the slope is determined to be 0.423 f 0.011. Six more runs were performed, and their results were analyzed in the same way. The slope obtained from each run is given in Table I together with 3 times the standard deviation. Strictly speaking, eq 8 is valid only when reaction 1 is the sole depletion pathway of SOz. In fact, there is possibility of a few side reactions which may affect the concentration of either SOz or n-C4Hio. One of them is a reaction of HOz radical with SOz, the former being produced through reaction 3. However, in recent studies by Graham et al. (15) and Burrows et al. (16),the rate constant for HOz + SOz was found to be smaller than that for reaction 1by about 5 orders of magnitude. Therefore, the presence of HOz will not affect the analysis based on eq 8. Another possible side reaction is the radical attack of CH30 on SOz to form an adduct: CH30 + SOz(+M) CH30S0,(+M) (9) If this reaction interferes with reaction 1, the slope of the plot in Figure 3 will be apparently dependent on the initial concentration of CH30N0, since the CH30 radical is produced from CH30N0 (reaction 2) in the course of formation of HO. In order to check the effect of reaction 9, several different initial concentrations of CH30N0 were employed in runs 1-5 to see whether or not the slope was dependent on [CH30NO],-,. The results are shown in Figure 4; it was thus found that the slope is independent of [CH30NOIo. This finding supports the conclusion drawn by Kan et al. (17)that reaction 1is not affected by reaction 9. On the other hand, in the last two runs, where higher initial concentrations of SOz were employed, the slope values were found to be slightly but significantly lower. This suggests the possibility of the secondary reaction between n-C4Hlo and the radical products (for example HOSOJ of reaction 1. Therefore, the results of these runs were eliminated from the data to derive a final value of the slope. When the weighted average was calculated (the weight for each run was put inversely proportional to square of the standard deviation), the final value of the
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slope was determined to be 0.436 f 0.021, the error limits being 3 times the standard deviation, and this was equated to the ratio k l / k 5 . The rate constant k5 of the reference reaction was calculated from the Arrhenius expression of Perry et al. (12) to be k , = (2.79 0.28) X cm3 molecule-’ s-’ for the reaction temperature of 303 K. This k , value combined with the k l / k 5 ratio gives kl = (1.22 f 0.13) X cm3molecule-l s-’. The k1 value obtained in this study agrees with the estimate given by Calvert and Stockwell. Thus, the accuracy of their k l value has been verified by the present direct measurement.
*
Registry No. Hydroxyl, 3352-57-6; sulfur dioxide, 7446-09-5.
Literature Cited Calvert, J. G.; Stockwell, W. R. In “Acid Precipitation: SO2, NO, and NO2 Oxidation Mechanism: Atmospheric Considerations”; Ann Arbor Science Publishers Inc.: Ann Arbor, MI, in press. Harris, G. W.; Wayne, R. P. J. Chem. Soc., Faraday Trans. 1 1975, 71, 610-617. Davis, D. D.; Ravishankara, A. R.; Fischer, S. Geophys. Res. Lett. 1979, 6, 113-116. Leu, M. T, J. Phys. Chem. 1982,86,4558-4562. Atkinson, R.; Perry, R. A.; Pitts, J. N., Jr. J. Chem. Phys. 1976, 65, 306-310. Harris, G. W.; Atkinson, R.; Pitts, J. N., Jr. Chem. Phys. Lett. 1980, 69, 378-382. Castleman, A. W., Jr.; Tang, I. N. J. Photochem. 1977,6, 349-354. Cox, R. A. J. Photochem, 1974,3, 291-304. Cox, R. A.; Sheppard, D. Nature (London) 1980, 284, 330-331. Atkinson, R.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J. Air Pollut; Control Assoc. 1981, 31, 1090-1092. Blatt, A. H., Ed. “Organic Synthesis”; Wiley: New York, 1966; Collect. Vol. 11, pp 363-364. Perry, R. A.; Atkinson, R.; Pitts, J. N., Jr. J. Chem. Phys. 1976, 64, 5314-5316. Demerjian, K. L.; Schere, K. L.; Peterson, J. T. Adu. Environ. Sei. Technol. 1980, 10, 369-459. Taylor, W. D.; Allston, T. D.; Moscato, M. J.; Fazekas, G . B.; Kozlowski, R.; Takacs, G. A. Int. J. Chern. Kinet. 1980, 12, 231-240. Graham, R. A.; Winer, A. M.; Atkinson, R.; Pitts, J. N., Jr. J. Phvs. Chem. 1979. 83. 1563-1567. Burrd;vs, J. P.; Cliff, D. I.; Harris, G. W.; Thrush, B. A,; Wilkinson, J. P. T. R o c . R. Soc. London, Ser. A 1979,A368, 463-481. Kan, C. S.; Calvert, J. G.; Shaw, J. H. J. Phys. Chem. 1981, 85, 1126-1132. Received for review February 9,1983. Accepted August 10,1983.
