J . Phys. Chem. 1990, 94, 2386-2393
2386
of the C S 2 0 H adduct with 02.The results yield a temperature cm3 independent (249-299 K ) value of (2.6 f 1.0) X molecule-' s-I for the rate coefficient of the CS20H + O2reaction.
Utter for assistance with the AMPAC calculations. This work was supported by NOAA as part of the National Acid Precipitation Assessment Program.
Acknowledgment. We thank Lester Lambert for carrying out the CS2 absorption cross section measurements and Dr. R. G .
Registry No. O H , 3352-57-6; CS,, 75-15-0; CS,OH, 123132-54-7; 02,7782-44-7.
Oxidation of CS, by Reaction with OH. 2. Yields of HO, and SO, in Oxygen Edward R. Lovejoy, Timothy P. Murrells, A. R. Ravishankara,* and Carleton J. Howard Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303, the Department of Chemistry and Biochemistry, and the Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309 (Received: May 8, 1989; In Final Form: October 6, 1989)
The products of the OH-initiated oxidation of CS2 have been investigated. Analysis of the OH loss and the HS production using a laser magnetic resonance (LMR) discharge-flow apparatus yielded k l C 3 X cm3 molecule-' s-I for the reaction OH + CS2 HS + OCS (1) (340 K and 5 Torr (He)). The oxidation of CS2is enhanced by O2due to the following mechanism: OH + CS2 2 CS20H (2), and CS20H + O2 -,products (3). H 0 2 was identified by LMR detection as a major product from this chemistry. The H 0 2 yield was measured in a pulsed photolysis experiment by modeling OH temporal profiles with NO added to convert H 0 2 to OH. The H 0 2 yield was 95 i 15% of the OH consumed by reactions 2 and 3 (50 Torr (He), 249 and 299 K). Discharge-flow experiments employing chemical ionization mass spectrometric (CIMS) detection of OH and SO2 showed that 90 & 20% of the OH consumed by reactions 2 and 3 leads to SO2 production.
-
Introduction This is the second of two papers on the oxidation of CS2 by OH. The first paper presents a study of the kinetics of the initial CS2oxidation steps.' This paper is concerned with the identification and quantification of the products of the CS2 oxidation. Recent work indicates that the metathesis reaction OH + CS2 HS + COS is slow, k I2 X cm3 molecule-' s-I at 298 K,'s2 and that O H reacts with CS2predominantly by forming an adduct which decomposes back to reactants.'q3
-
OH
+ CS2
kX
CS2OH
kb
The radical products and the elementary steps following reaction 3 have not been identified. There are many exothermic pathways for reaction 3 (see Scheme I, where AH:298 values are given after the reaction). SCHEME I CS2OH 02 H02 CS20 ? (3a) H 0 2 + OCS S -58 kcal mol-' (3b)
+
------
(2)
The enthalpy change for reaction 2 is approximately -1 1 kcal mol-' and the lifetime of C S 2 0 H is short (lifetime = 4.5 ps at 298 K and 661 Torr of N2).3 The CS20H adduct reacts with O2 C S 2 0 H + O2
k3
products
(3)
with a rate coefficient of k3 = 3 X cm3 molecule-' s-'.'s3 The reaction sequence, (2) followed by (3), is a major loss process for CS2 in the Earth's atmosphere. The rate coefficient for the loss of O H by reaction with CS2 in O2 has been measured under tropospheric condition^^-^ ( k ( 0 H CS,) = 2 X cm3 molecule-' s-l, 295 K and 700 Torr) and predicts an atmospheric lifetime which is consistent with estimates based on atmospheric measurements of CS, (7 = 10 days).6 Many studies on the products of the O H + CS2 reaction in the presence and the absence of O2 have been performed. Iyer and Rowland' observed OI4CS formation in the OH + I4CS2system but they could not rule out reactions other than OH + CS2 as the source of OCS. Leu and Smith* also detected OCS in the OH CS2 system using an electron impact ionization mass spectrometer coupled to a discharge-flow apparatus. Their experiments were complicated by wall reactions so they could not unambiguously identify the OCS source. HS production has not been observed. Chamber studies have shown that the end products of the oxidation of one CS2 in air are one SO2 and one OCS.435
+
+
*Address correspondence to this author at NOAA, R/E/AL2, 325 Broadway, Boulder, CO 80303.
0022-3654/90/2094-2386$02.50/0
--
-
+
+ + C O + S2 -19 kcal mol-' O H + SO2 + C S -23 kcal mol-' O H + SO + OCS -56 kcal mol-] HOCS + SO2 ? H + OCS + SO, -79 kcal mol-' H S 0 2 + OCS -1 13 kcal mol-' HOSO + OCS -154 kcal mol-' HCO + SO2 + S -22 kcal mol-' HSO + CO + SO -52 kcal mol-' HSO + C 0 2 + S -55 kcal mol-' HS + SO2 + CO -89 kcal mol-' HS + SO + C 0 2 -85 kcal mol-' H02
adduct
?
(3c) (3d) (3e) (30 (3g) (3h) (3i) (3j) (3k) (31) (3m) (3n) (30)
(1) Murrells, T. P.; Lovejoy, E. R.; Ravishankara, A. R. J . Phys. Chem., previous paper in this issue. (2) Wine, P. H.; Shah, R. C . ;Ravishankara, A. R. J . Phys. Chem. 1980, 84, 2499. (3) Hynes, A. J.; Wine, P. H.; Nicovich, J. M. J . Phys. Chem. 1988, 92, 3846. (4) Barnes, I.; Becker, K. H.; Fink, E. H.; Reimer, A.; Zabel, F.; Niki, H. Int. J. Chem. Kinet. 1983, 15, 631. (5) Jones, B. M. R.; Cox, R. A,; Penkett, S.A. J . Atmos. Chem. 1983.1, 65 _.
(6) Bandy, A. R.; Maroulis, P. J.; Shalaby, L.; Wilner, L. A. Geophys. Res. Lett. 1981, 8, 1180. (7) Iyer, R. S.; Rowland, F. S. Geophys. Res. Lett. 1980, 7 , 191. (8) Leu, M. T.; Smith, R. H . J . Phys. Chem. 1982, 86, 958.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2387
Oxidation of CS2 by Reaction with O H
TABLE I 1 CIMS Detection Information
TABLE I: LMR Radical Detection Information radical
far-IR laser gas/wavelength, bm
magnetic field" strength, kG
CH30D/216 CH,OD/216 CH,OH/ 163 CH?OH/163
10.4 0.2 3.7 2.2
HS HSO OH HO2
detection limit,b molecules cm-?
I 6 6 4
x 109 X X
IO9 lo7
Reaction enthalpies were calculated by using tabulated data9 and the CS20H heat of formation measured in the preceding paper (AHq298(CS20H)== 26 kcal mol-I).l The enthalpies of reactions 3h and 3i were calculated by using AHf0z98(HS02)= -53 kcal mol-' and AHfo298(HOSO)= -94 kcal mol-' which are derived from an ab initio calculation.I0 Chamber s t ~ d i e sshow ~ , ~ that reaction 3 leads to one OCS and one SO2 in excess oxygen. Therefore, reactions 3c, 3d, and 3j-3n are probably not major channels. This work was undertaken to identify and quantify the CS2 oxidation products. Three different experimental apparatuses were employed. An optically pumped far-infrared laser magnetic resonance (LMR) spectrometer coupled to a discharge-flow system was utilized to search for the potential radical products H02, HS, and HSO. Pulsed laser photolytic production of O H and pulsed-laser-induced fluorescence detection of OH (PLPLIF) were employed to quantitate the yield of HO,. Another discharge flow system coupled to a chemical ionization mass spectrometer (CIMS) was used to measure the SO, and HO, production simultaneously in the O H + CS, + O2system.
Experimental Section The LMR discharge-flow system used in these studies is described in a recent paper" and is not discussed here. The far-IR LMR spectrometer is a sensitive detector for a number of radicals important in this study. The detection information for these species is presented in Table I. The detection sensitivity of the LMR spectrometer for O H relative to HO, was determined by monitoring the changes in the H 0 2 and O H signals upon addition of various amounts of NO to a small amount of H 0 2 ([HO,], < 2 X 10" molecules ~ m - ~ The ) . H 0 2 NO reaction has been shown to stoichiometrically convert H 0 2 to OH.'z HO, was produced by the reaction sequence: CI C H 3 0 H HCI C H 2 0 H and C H 2 0 H Oz H 0 2 C H 2 0 . The calibration shows that the H 0 2 LMR signal is as strong as the OH signal for equal radical concentrations, which is consistent with an earlier mea~urement.'~ The relative O H to H S and H S to HSO responses had also been determined p r e v i ~ u s l y and ' ~ were not remeasured. The CIMS apparatus has been described p r e v i ~ u s l y and ' ~ is not discussed here. In the CIMS studies neutral species are converted to ions in a flowing afterglow (FAG) reactor by using
+
-
+
+
-
+
(9) DeMore, W. B.; Molina, M. J.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. "Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling"; Evaluation Number 8, JPL Publication 87-41; Jet Propulsion Laboratory: Pasadena, CA, 1987. ( I O ) Boyd, R. J.; Gupta, A.; Langler, R. F.; Lownie, S. P.; Pincock, J. A. Can. J . Chem. 1980, 58, 331. ( I 1) Wang, N. S.; Lovejoy, E. R.; Howard, C. J. J . Phys. Chem. 1987,91, 5743. (12) (13) 5749. (14) (15)
Howard, C. J. J . Chem. Phys. 1979, 71, 2352. Lovejoy, E. R.; Wang, N. S.; Howard, C. J. J. Phys. Chem. 1987,91,
Gleason, J. F.; Sinha, A,; Howard, C. J. J . Phys. Chem. 1987,91,719. Su, T.; Bowers, M. T. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. I, Chapter 3. (16) Bierbaum, V. M.; Grabowski, J. J.; DePuy, C. H. J . Phys. Chem. 1984, 88, 1389. (17) Ferguson, E. E. Int. J . Mass Spectrom. Ion Phys. 1976, 19, 53. (18) Streit, G . E. J . Chem. Phys. 1982, 77, 826.
--
k , cm3 molecule-!
reaction
+ +
+ +
SO2
(4) SFL O H OH- SF6 (5) OH- cs2 HS- ocs (6a) SF6- + SO2 F2S02- +
NO2
products (6b) SFC + SO2 products ( 6 ~ SFC ) + SO2 products ( 7 ) SFC NO2
OH
x 108
"All absorptions are with the electric vector of the laser radiation perpendicular to the magnetic field. bDetection limit refers to the approximate radical concentration at signal/noise = 1 with a 0.4-s time constant.
+
species
+
s-I
detection limit"
2X
5
1.5 x 10-9c 1.0 x 1 0 - 9 d
9 x 109
1.3 X
3
X lo9
FSOL +
SFC +
NO,
+ SF6
X
IO'O
"The detection limit is the approximate concentration in the discharge-flow reactor which gives signal/noise = 1 for a 5-s integration time. bEstimated using average dipole orientation (ADO) theory.I5 Reference 16. Reference 17. Reference 18.
ion-molecule reactions. The ion signal is proportional to the neutral species concentration. The CIMS detection schemes used to detect OH, SO,, and NO, in this work are presented in Table 11. The OH- formed in reaction 4 was converted completely (> 99.9%) to HS- by reaction 5 due to [CS,] = 5 X IOI3 molecules in the FAG reactor. Therefore, the HS- signal was used to monitor [OH]. SO2 was detected with the F2S02-ion since channel 6a has a good yield and there were no interferences at mass 102. Significant background signals existed at the FS02- and SF< masses. The hydroperoxyl radical was detected by adding NO to the neutral reactor to convert HO, to NO2 H02
+ NO
+
OH
+ NO2
(8)
and the NO, was detected by using reaction 7. The CIMS signals for OH, NO,, and SO2 were calibrated by using mixtures of NOz and SO2 in He. For the O H calibration, NO, was reacted with excess H atoms, H NOz O H NO ( k = 1.4 X cm3 molecule-' s-'),I9 to produce an O H concentration equal to the initial NO, concentration. Plots of the CIMS signal vs concentration were linear over an order of magnitude change in the concentration for all three species. The concentration range ( e 5 x IO1O to 5 x IO" molecules encompassed the values used in the yield experiments. The SOz and NO2 mixture compositions were measured by absorption at 366 nm (Hg atomic line, u = 5.75 X cm2 for and 214 nm (Zn atomic line, u = 3.87 X cm2 for The source reactor, movable inlet, and flow reactor were all coated with halocarbon wax to reduce radical loss on the walls in the CIMS and LMR studies. The wax coating was exposed to O H for 1-4 h at 340 K prior to use. For some LMR experiments the flow reactor was fitted with a Teflon sleeve which extended from above the source reactor inlet to the end of the reaction zone. Two OH source reactions were utilized in the LMR and CIMS discharge flow experiments. The first reaction, H + NO2 O H + NO, is very fast ( k = 1.4 X 1O-Io cm3 molecule-' s-I).l9 Hydrogen atoms were generated by passing a 0.1% H, in He mixture through a microwave discharge. A 10-20-fold excess of NO2 was added to the source reactor downstream of the discharge. The NO, was stored as a 5% mixture in He in a light-tight glass bulb. The H + NO, reaction time in the source reactor was typically 10 ms which ensured that the O H production was >99.9% complete before entering the flow tube. The second O H source reaction used in the discharge flow experiments was F H 2 0 OH HF. This reaction is reasonably fast ( k = 1.1 X lo-" cm3 molecule-' s - ' ) 9 Fluorine atoms
+
-
+
-
+
-
+
(19) Michael, J. V.; Nava, D. J.; Payne, W. A,; Lee, J. H.; Stief, L. J. J . Phys. Chem. 1979, 83, 2818. (20) Wine, P. H.; Kreutter, N. M.; Ravishankara, A. R. J . Phys. Chem. 1979, 83, 3191. (21) Wine, P. H.; Thompson, R. J.; Ravishankara, A. R.; Semmes, D. H.; Gump, C. A.; Torabi, A,; Nicovich, J. M. J . Phys. Chem. 1984, 88, 2095.
2388 The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 were produced in a microwave discharge of a 0.1% CF4 in He mixture. The water was introduced into the reactor downstream of the discharge via a bubbler system. The H 2 0 concentration in the source reactor was 4 X lOI4 molecules c m 3 which ensured that the F H 2 0 reaction was >99.9% complete prior to entering the flow tube. The pulsed laser photolysis pulsed-laser-induced fluorescence apparatus (PLPLIF) used to measure the HOz yield is described in the previous paper.' The present PLPLIF experimental procedures are the same except that in this work H202 was photolyzed exclusively at 266 nm (fourth harmonic of a Nd:YAG laser). For all the studies CS2 was evaporated from a glass reservoir immersed in a water bath at room temperature which stabilized the flow. In the discharge-flow experiments CS, and NO were added to the flow reactor through the movable injector. The main gases in the flow tube, N2 and 0,. were added upstream of the OH source reactor. In the CIMS experiments constant flows of O,, N2,and CS, were maintained in the FAG reactor. The concentration of these species in the neutral flow tube was varied by diverting a fraction of the total constant flows into the neutral flow tube. The residual flow was added to the FAG reactor across from the neutral flow tube inlet. As a result the concentrations and residence times of Oz and CS2 remained constant in the FAG reactor even though they were varied over a wide range in the neutral flow tube. This procedure ensured that the ion chemistry was not disturbed when the neutral flow tube conditions were varied. The helium (>99.999% analyzed) from the U.S. Bureau of Mines was used without further purification to prepare CF4, NO,, SOz,and H, mixtures and as a carrier gas in the neutral flow tube. The helium (>99.9%) (National Bureau of Standards) used in the FAG reactor was passed through a liquid nitrogen cooled trap containing molecular sieves. The nitrogen (>99.999%, Linde), oxygen (>99.99%, Linde), CF, (>99.7%, Matheson), SOz (> 99.98%, Scientific Gas Products), and H2 (>99.999%, Scientific Gas Products) were all used without further purification. The CS2 (>99.9%, Fisher Scientific) was degassed prior to use by pumping off vapor at 201 K. NO2 was prepared by reacting purified NO ( ~ 2 0 Torr) 0 with 0, (=700 Torr) and was purified by trap to trap distillation. The NO (>99.99'%, Scientific Gas Products) was purified by passing it through a trap filled with silica gel and cooled in a dry ice/ethanol bath. Gas flow rates were measured with linear mass flow meters or by monitoring the rate of change of pressure in a known volume. The flow meters were calibrated either by using a wet test meter or by the rate of change of pressure method. All pressures were measured with capacitance manometers.
Lovejoy et al.
O
+
Results and Discussion HS and HSO Production. The H S production in reaction 1 (OH CS2 HS + OCS) was examined by using the laser magnetic resonance discharge-flow apparatus. In these studies, the F + H 2 0 source of O H was employed. The H + NOz source was not used because NO, reacts rapidly with H S (k(HS + NO2) = 7 X lo-" cm3 molecule-' s-I.'I H S production was observed in the O H + CS, system. The O H and H S concentrations were ~) measured as a function of [CS,] (0-2 X 1015 molecule ~ m - at two reaction times, 36 and 91 ms. At both reaction times the amount of OH lost was more than twice the amount of HS produced. The total OH loss rate due to CS, was about 6 X cm3 molecule-' s-l. The H + CS, chemistry was investigated as a possible source of H S in the O H + CS, experiments since the F + H 2 0 source chemistry is known to produce H atom^.'^^^^ H S production was observed when H atoms were added to CS2 in the flow reactor. The H + CS, HS + C S reaction is endothermic by about 19 kcal mol-' and cannot be the source of the H S observed in these experiments. Due to the uncertainty in the source of the HS, it was impossible to quantify the H S yield from O H + CS,. On the basis of the observed H S yield of less than 50% of the OH
+
-
-
(22) Keyser, L. F. J . Phys. Chem. 1988, 92, 1193.
I
O
0 0 1
e
!
0.2
0
e
1
e
e ,
I I
0. 0
I
2
I
CCS,
I
1
I
(
i
3
d 5molecule
Figure 1. OH loss and H 0 2 production measured with the LMR apparatus as a function of [CS,]:(0)OH with [O,] = 0 ( 0 )OH with [O,] = 6 X loi6 molecules (m) H0, with [O,] = 6 X 10l6molecules p = 5 Torr, T = 333 K,[OH],, = 1 X IO" molecules [NO,] 3 X lo'* molecules cm-), and reaction time = 0.027 s.
consumed, and a total O H loss rate of 6 X cm3 molecule-I SKI,we report an H S production rate coefficient of 0.5 ms). The additional O H loss can be due to a number of radical loss processes: direct O H reaction, loss of the C S 2 0 H adduct, or HO, loss. The HO, + CS2 and C S 2 0 H + N O reactions are possible radical losses not accounted for in the model. All other radical losses existed in one of the other subsystems and would have shown up previously. The H 0 2 CS2 reaction was examined at 330 K in 5 Torr of O2 using the LMR apparatus. The HOz signal decreased by less than 10% with [CS,] = 3 X IOl5 molecules cm-3 and a reaction time of 110 ms, suggesting k ( H 0 2 + CS2) < 4 X cm3 molecule-I s-l. Even if the H02 + CS, reaction exhibited a linear pressure dependence it would not be fast enough at 50 Torr (He) to account for the additional radical loss observed in the photolysis experiment. The other possible O H loss mechanism is the reaction between C S 2 0 H and NO. Reaction 11 was investigated by
+
CS20H
+ NO
-
products
(1 1)
measuring the first-order CS20H loss rate coefficient as a function of [NO]. The adduct loss rate coefficient was determined by varying this parameter until the model accurately predicted the experimental O H temporal profile. A plot of the C S 2 0 H firstorder loss rate coefficient vs [NO] at 249 K is presented in Figure cm3 molecule-1 s-l 2. This data yields k , , = (1.3 f 0.3) X at 50 Torr (He). At 299 K, k l l = (9.1 f 2.6) X cm3 molecule-] s-I in 50 Torr of He. The errors are the deviations at the 95% confidence level based solely on the k1 vs [NO] plots. We estimate that the systematic errors are about &IO% and report k l l = (1.3 f 0.4) X cm3 molecule-' s-] at 249 K and (9.1 f 3.5) X cm3 molecule-1 s-I at 299 K. Finally, the complete O H CS2 O2 + N O H202reaction mixture was examined. The optimum conditions for measuring the H 0 2 yield are high [CS,] and [ 0 2 ] to maximize the H 0 2 production. However, both 0, and CS, effectively quench the O H fluorescence and decrease the O H detection sensitivity. As was only varied over a small a result, at room temperature [O,] range, (1.2-2.3) X I O l 7 molecules ~ m - and ~ , [CS,] was held constant at about 2.9 X 10l6 molecules ~ m - The ~ . N O concentration was varied over a wide range, (2.2-14.1) X lOI4 molecules ~ m - The ~ . OH profiles for the different experimental conditions were calculated by using the set of reactions and rate coefficients listed in Table 111. For each data set the branching ratio for H 0 2 production via C S 2 0 H 0, was varied until a good fit to the experimental O H profile was obtained. Typical results are shown in Figure 3. These data were best fit with approximately 95%
+
+
+
+
2390 The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
Lovejoy et al.
TABLE 111: Model Reactions and Rate Coefficientsa
PLPLIF rate coeff (50 Torr of He)
-+ - + + + + + + + + + + reaction
( I ) OH
products ( 2 0 OH CS2 CS20H ( 2 r ) CS,OH OH CS, CS,bH 0, (HO2-+ SO2) (3) CS20H O2 products (8) H 0 2 NO OH NO1 (9) OH H202 H02 H20 (IO) OH NO HONO ( 1 1 ) CS20H NO products ( I 2) OH NO2 HNO3 (13)OH+HO2+H,O+02 (14) OH (15) CS2OH (16) H02 ~I
+
+
ref
299 K
249 K
+ CS,
CIMS rate coeff (5 Torr of N, and 0,)
ref
340 K ( I .6-1.8) X 5.7 x 10-14 25900
this work b b
6.1 x 10-13 1140
3.5 x 1 0 4 3 20400
2.7 x
1044
2.3 x 10-14
1
2.3 x 10-14
b
9.7 x 2.0 x 1.0 x 1.3 X 4.0 X 1.1 x 130 150 50
lo",
8.3 X 1.9 X 5.8 X 9.0 x 2.0 x 10-12 1.0 x 10-'0 130 335 50
9 this this this 23 9 this this
7.5 x 10-12
9
7.2 x 1 0 4 4 1.0 x 10-12 2.5 X 7.0 X lo-'' 2-3 5 1
9 this work 9 9 this work
10-12 10-12
10-10
1
1
work work work work work
c
C C
"Units are s-I and cm3 molecule-' s-'. *Extrapolated from data in ref 1. 'Estimated. TABLE IV: H 0 2 Yield Data" Torr of He
laser fluence, mJ cm-2
50.1 50.2 49.9 50.2 50.0 50.0 50.2 50.1 50.0 50.2 50.3 50.3 50.3 49.9
29 33 26 22 21 21 40 38 36 13 12 12 12 12
press.,
temp, K 300 299 299 298 298 298 299 299 299 249 249 249 249 249
101'[OH]o
1014[H202]
16 18 15 12 12 12 23 23 22 5 5
5 5 5
5.1 5.1 5.2 5.1 5.2 5.2 5.5 5.6 5.5 3.8 3.8 3.8 3.8 3.8
10I6[CS2]
1017[02]
2.67 3.00 2.68 2.85 2.84 2.84 2.92 2.94 2.95 0.68 0.68 0.68 0.69 0.68
1.21 2.28 1.27 1.18 1.17 1.17 1.18 1.18 1.17 0.75 0.76 0.76 1.42 1.41
1014[NO]
H0, yield! %
3.39 3.43 7.51 6.63 2.18 11.2 4.22 14.1 8.93 3.38 10.2 7.18 2.50 5.51
80f IO >80 100 i 5 95 f 5 90 f 5 9 0 i 10 95 f 5 9 0 f 10 100f 5 95 i 5 95 f 5 9 0 f IO 95 f 5 100 f 5
OConcentration units are molecules c d . bThe errors are the range of yields which encompass >95% of the points defining the OH temporal profile
+
of the CSzOH 0, reaction giving HO,. The model profile for a 75% branching ratio is also plotted to show the sensitivity of the fits to this ratio. At 249 K a lower [CS,] (=7 X 10l5molecules ~ m - was ~ ) used because the CSzOH equilibrium constant is about 30 times larger than at room temperature. The experimental conditions and derived branching ratios for both temperatures are listed in Table IV. The quoted deviations represent the range of yields which encompassed 195% of the data points defining the OH temporal profile. There was no measurable dependence of the branching ratio on temperature. For all the data sets the best fits were obtained for a branching ratio between 80 and loo%, and the average of the best fits was 95 f 8%. The f8% deviation is a measure of the random errors at the 95% confidence level. The magnitude of the systematic errors is difficult to evaluate. The largest source of systematic error in this measurement is probably an inaccurate model mechanism. We have reduced the systematic errors by measuring the important model parameters independently and verifying the chemistry of subsets of the model for a range of reaction conditions. For example, the unknown products of reaction 1 1 could affect the yield. However, the yield was independent of [NO] over an order of magnitude variation which indicates that reaction 11 did not interfere with this measurement. We estimate that the systematic errors are comparable to the random errors and report that the HO, yield is 95 f 15%. Our measurements are insensitive to other CS20H + 0, reaction channels which produce OH. Therefore, the HO, yield measured in these studies is the yield for the channels not giving OH. However, the OH production channels are expected to be minor (
