J. Phys. Chem. 1995, 99, 6570-6579
6570
Atmospheric Chemistry of l,l,l-Trichloroethane: UV Absorption Spectra and Self-Reaction Kinetics of CCl3CH2 and CCl3CH202 Radicals, Kinetics of the Reactions of the CCl3CH202 Radical with NO and NO2, and the Fate of the Alkoxy Radical CCl3CH20 Jesper Platz, Ole J. Nielsen,* and Jens Sehested Section for Chemical Reactivity, Environmental Science and Technology Department, Ris@National Laboratory, DK-4000 Roskilde, Denmark
Timothy J. Wallington* Ford Research Laboratory, SRL-3083, Ford Motor Company, Dearbom, P.O.Box 2053, Michigan 48121-2053 Received: October 25, 1994; In Final Form: February 10, 1995@
A pulse radiolysis technique was used to measure the UV absorption spectra of CCl3CH2 and CCl3CH202 radicals over the range 220-300 nm. At 220 nm, o(CCl3CH2) = (9.4 f 1.1) x lo-'* cm2 molecule-'; at 250 nm, a(CC13CH202) = (2.9 f 0.3) x cm2 molecule-'. The observed self-reaction rate constants, defined as -d[CCl3CH2]ldt = 2k4[CC13CH~]and -d[CC4CH202]/dt = 2k~,~[CC13CH202]~, were k4 = (9.1 f cm3 molecule-' s-'. The reaction of CCl3CH202 radicals with 1.1) x and k50bs = (4.7 f 1.6) x NO gives CC12CH20 radicals. In the atmosphere, >96% of the CCl3CH20 radicals react with 0 2 to give CC13CH0. The rate constants for the reactions of CC13CH202 radicals with NO and NO2 were determined to be k6 L 6.1 x cm3 molecule-' s-' and k7 = (6.5 f 0.4) x lo-'' cm3 molecule-' s-', respectively. As a part of this work, the rate constant for the reaction between F atoms and CCl3CH3 was determined to be k3 = (6.8 f 1.5) x lo-', cm3 molecule-' s-'. A relative rate technique was used to measure rate constants for the reactions of C1 atoms with: CCl3CH3, (9.9 f 2.0) x lo-? CCl3CH0, (6.0 f 1.3) x lo-',; and CC13CH20H (3.0 f 0.6) x (units of cm3 molecule-' s-') at 296 K. Results are discussed in the context of the atmospheric chemistry of 1, 1, l-trichloroethane.
Introduction
+ NO - products + NO, + M - CCl3CH2O,NO, + M
CC13CH,02
By international agreement, industrial production of chlorofluorocarbons (CFCs), together with hydrochlorofluorocarbons (HCFCs), will be phased out. In 1990 there was a production of 726 x lo6 kg of CC13CH3, and the estimated release to the atmosphere the same year was 705 x lo6 kg of CCbCH3.l It is therefore important to establish the environmental impact of the release of CC13CH3. It has been estimated that 10-15% of CCl3CH3 will enter the strato~phere.~,~ In the stratosphere C1 atoms will be generated by photolysis of CC13CH3. In the troposphere CCl3CH3 will react with OH radicals to produce alkyl radicals which will, in tum,react with 0 2 to give peroxy radicals:
+
CC13CH3 OH CCl,CH,
-
+ 0, iM
CCl,CH,
+ H,O
CCl,CH,O,
+M
(1)
CCl,CH,O,
of CC13CH20.
Experimental Section
Pulse Radiolysis System. The pulse radiolysis transient UV absorption setup used for the present experiments has been described in detail previously and will only be dealt with briefly here.4 CCLCH;, radicals were generated by the radiolysis of SFdCCl3CH3 gas mixtures in a 1 L stainless steel reactor with a 30 ns pulse of 2 MeV electrons from a Febetron 705B field emission accelerator. SF6 was always in great excess and was used to generate fluorine atoms:
-+ + + + SF,
CH202 radicals, and the kinetics of the reactions 3-7. CC1,CH3 CCl,CH,
+ F - CC13CH2i- HF + CC13CH2 products
-
CC13CH,02 i- CCl,CH,O,
-
products
* Authors to whom correspondence may be addressed.
* Abstract published in Advance ACS Abstracts, April 1, 1995
F
CCl,CH, (3) (4)
(5)
(7)
FTIR product studies were used to study the atmospheric fate
(2)
In the present work we have used pulse radiolysis to determine the UV absorption spectra of CCl3CH2 and cc13-
(6)
2MeV e-
F
CCl,CH,
0,
products
+ HF CCl,CH,O, + M
CCl,CH,
M
(3) (2)
Four sets of experiments were performed. First, the ultraviolet absorption spectra of CCl3CH2 and CC13CH202 radicals were determined by observing the maximum of the transient UV absorption at short times (0-20 ps) following the pulse radiolysis of SFdCC13CH3, and SFdCC13CH3102 mixtures, respectively. Second, using a longer time scale (100-400 ps), the subsequent decay of the absorption was monitored to determine the kinetics of the self-reactions 4 and 5 :
0022-365419512099-6570$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 17, 1995 6571
Atmospheric Chemistry of 1,1,1-Trichloroethane CC1,CH2
+ CC13CH, - products
(4)
+
(5)
-
CC13CH202 CC13CH202
products
Third, the rate of NO2 formation following the pulse radiolysis of SFdCC13CH3IO2INO mixtures was used to measure k6. Fourth, the rate of decay of NO2 following the pulse radiolysis of SFdCC13CH3/0dN02 mixtures was used to measure k7. To monitor the transient W absorption, the output of a pulsed 150 W xenon arc lamp was multi-passed through the reaction cell using internal White type optics (40, 80, or 120 cm path length). Reagent concentrations used were: SF6, 500- 1000 mbar; 0 2 , 0-40 mbar; CC13CH3, 0-10.0 mbar; NO, 0-1.5 mbar; and NO2,0-2.0 mbar. All experiments were performed at 295 K. Ultrahigh purity 0 2 was supplied by L'Air Liquide, SF6 (99.9%) was supplied by Gerling and Holz, CC13CH3 (>99%) was obtained from the Fluka AG and was purified on a A1203 column, NO ('99.8%) was supplied by Messer Griesheim, and NO2 ('98%) was supplied by Linde Technische Gase. The CCl3CH3 sample was degassed before use by repeated freeze-pump-thaw cycling. All other reagents were used as received. FTIR Smog Chamber System. The M?R system was interfaced to a 140 L pyrex reactor. Radicals were generated by the UV irradiation of mixtures of 110-890 mTorr of CCl3CH3,0.1-0.6 Torr of Fz, 3.4-700 Torr of 0 2 in 700 Torr total pressure with N2 diluent at 296 K using 22 blacklamps (760 Torr = 1013 mbar). The loss of reactants and the formation of products were monitored by FTIR spectroscopy, using an analyzing path length of 27 m and a resolution of 0.25 cm-'. Infrared spectra were derived from 32 co-added spectra. CCl3CH3, CC13CH0, and COCl2 were monitored using their characteristic features over the wavenumber ranges 1030- 1500, 950-1050 and 1740-1800, and 800-900 cm-', respectively. Reference spectra were acquired by expanding known volumes of reference materials into the reactor.
Results and Discussion Spectrum of CCl3CHz Radicals. Following the pulse radiolysis of CC13CH3/SF6 mixtures, a rapid increase in absorption in the UV (230 nm) was observed, followed by a slower decay. Figure 1A shows the transient absorption at 230 nm following the radiolysis of a mixture of 5 mbar of CC13CH3, and of 995 mbar of SF6. No absorption was observed when either 5 mbar of CCl3CH3 or 995 mbar of SF6 was radiolyzed separately. We ascribe the absorption shown in Figure 1 to the formation of CC13CH2 radicals and their subsequent loss by self-reaction. In this work we assume that F atoms react exclusively with CCl3CH3 by a H atom abstraction mechanism to give CC13CH2 radicals, reaction 3. The validity of this assumption is discussed in a following section dealing with the atmospheric fate of CCl3CH20 radicals. Measurement of the absolute absorption spectrum of CCl3CH2 radicals requires calibration of the initial F atom concentration. Additionally, experimental conditions are needed to ensure that there is 100% conversion of F atoms to CCl3CH2 radicals. The yield of F atoms was established by monitoring the transient absorbance at 260 nm due to methylperoxy radicals produced by radiolysis of SFdCHd02 mixtures as described previ~usly.~The yield of F atoms at 1000 mbar of SF6 was determined to be (2.8 i 0.3) x 1015 molecules cm-3 at full irradiation dose, using a value of 3.18 x cm2 molecule-' for o(CH302) at 260 nm.6 The quoted error on the F atom calibration includes both statistical (&2 standard deviations) and potential systematic errors associated with the 10% uncertainty
0.5
,
A I
I
1
i
9
0.0
-0.1
I-' '
'
1
,
,
0
250
500
750
4 I
1000
Time ( p s e c )
0.3 I
$ 1
.
B I
I
I
I
1
1
I
1
I
r
4
in o(CH302). In the following, errors are propagated using conventional error analysis methods and are equal to 5 2 standard deviations. To work under conditions where the F atoms are converted stoichiometrically into CCl3CH2 radicals, it is necessary to consider potential interfering secondary chemistry. A complication could be reaction 9:
F
+ CC13CH2- products
(9)
To check for complications caused by reaction 9, experiments were performed using [CCl3CH3] = 5 mbar and [SF61 = 995 mbar in which the maximum transient absorption at 220 nm was measured as a function of the radiolysis dose. Figure 2A shows the observed maximum transient ascribed to CCl3CH2 radicals as a function of the dose; the optical path length was 40 cm. As shown in Figure 2A, the maximum absorption observed in experiments using high dose was significantly less than expected on the basis of a linear extrapolation of the data obtained at low dose. This observation suggests that reaction 9 is important in experiments using the maximum initial F atom concentration. The solid line drawn through the CC13CH2 data in Figure 2A is a linear least-squares fit to the low dose data and has a slope of (4.5 f 0.3) x lo-'. Combining the data with the calibrated yield of F atoms of (2.8 f 0.3) x 1015 molecules cm-3 (full dose and [SF61 = 1000 mbar) gives a(CC13CH2) at 220 nm = (9.4 f 1.1) x lo-'* cm2 molecule-'. Quoted errors reflect uncertainties in both the F atom calibration and the slope of the line drawn in Figure 2A. To map out the absorption
6572 J. Phys. Chem:, Vol. 99, No. 17, 1995
Platz et al. A
A
,
,
I
--
0 CCI,CH,
1200
I
-
1000
0 -
E
800 600
'o N
400 v
b
200
Relative dose
200
220
240
260
280
Wavelength (nm)
B
500
I
I
I
I
0 CC13CH202
V CH,CIO, 0 CF,CH,02 300
200 Relative d o s e
Figure 2. (A) Maximum transient absorbance at 220 nm following the pulsed radiolysis of mixtures of 5 mbar of CCl3CH3 and 995 mbar of SF6 as a function of the radiolysis dose; path length was 40 cm. (B) Maximum transient absorbance at 250 nm following the pulsed radiolysis of mixtures of 10 mbar of CCl3CH3,20 mbar of 0 2 , and 970 mbar of SF6 as a function of the radiolysis dose; path length was 120 cm. The solid lines in A and B are linear regressions of the low dose data (filled circles). The dotted lines are second-orderfits to the entire data set to aid in visual inspection of the data trend.
TABLE 1 wavelength (nm) 220 230 240 250 260 270 280 290 300
U(CCI~CH~) (10-20 cm2 molecule-1) 938 648
296 169 121 77 54
(
0(CC13CH202) cm2molecule-I) 327 336 348 290 213 141
220
260
280
300
Wavelength (nm)
Figure 3. (A) W-absorption spectrum of the CCl3CH2 radical in the range 220-280 nm. (B) UV-absorption spectrum of the CC13CH202 radical in the range 220-300 nm.
complications include: (i) loss of F atoms by reaction with molecular oxygen:
F
+ 0, + M -FO, + M
(10)
(ii) unwanted radical-radical reactions such as reaction 4, 5, 11, and 12.
81 44
+ CC13CH,0, -products + CC1,CH20, - CCl,CH,O + CC13CH,0 F
25 CCl,CH,
spectrum of CCl3CH2 radicals, experiments were performed to measure the initial absorption between 220 and 280 nm following the pulsed irradiation of mixtures of 5 mbar of CCl3CH3 and 995 mbar of SF6. Initial absorptions were then scaled to that at 220 nm and converted into absolute absorption cross sections. Values obtained are given in Table 1 and shown in Figure 3A. The spectrum is distinctly different from that of the CH2C1 radical7 also shown in Figure 3A. Spectrum of CC13CHz02 Radicals. Following the pulsed radiolysis of mixtures of 10 mbar of CCl3CH3, 20 mbar of 0 2 , and 970 mbar of SF6, a rapid increase, complete within 5-10 p s , in W absorbance in the region 220-300 nm was observed, followed by a slower decay. An example is shown in Figure 1B. We ascribe the UV absorbance resulting from radiolysis of SFdCC13CH3I02 mixtures to CCl3CH202 radicals. To work under conditions where the F atoms are converted stoichiometrically into CCl3CH202 radicals, it is necessary to consider potential interfering secondary chemistry. Potential
240
(11)
(12)
Using rate constants for reaction 3 and 10 measured in our laboratory, k3 = (6.8 f 1.5) x lo-', cm3 molecule-' s-l and klo = (1.9 f 0.3) x cm3 molecule-' s-',* we calculate that 5.5% of the F atoms are converted into FOz radicals and 94.5% into CCl3CH202 radicals. Corrections for the presence of 5.5% of F02 radicals were calculated using the expression a(CCl3CH202) = [o(observed) - 0.055a(F02)]/0.945. Values of a(FO2) are taken from the l i t e r a t ~ r e . ~ , ~ There are no literature data concerning the kinetics of reactions 9, 11, and 12; hence we cannot calculate their importance. To check for these unwanted radical-radical reactions, the transient absorbance at 250 nm was measured in experiments using [CCl3CH3] = 10 mbar, [02] = 20 mbar, and [SF61 = 970 mbar with the radiolysis dose varied over an order of magnitude. The UV path length was 120 cm. Figure 2B shows the observed maximum of the transient absorbance of CC13CH202 radicals at 250 nm as a function of dose. As seen
J. Phys. Chem., Vol. 99, No. 17, 1995 6573
Atmospheric Chemistry of l , l ,1-Trichloroethane from Figure 2B, the maximum absorbance is linear with the radiolysis dose up to about 40% of the maximum dose. At maximum dose and half-dose, the maximum transient absorbance falls below that expected from a linear extrapolation of the low dose results. We ascribe the curvature in Figure 2B to incomplete conversion of F atoms into CCl3CH202 radicals caused by secondary radical-radical reactions 9, 11, and 12 at high initial F atoms concentrations. The solid line drawn through the data in Figure 2B is a linear least-squares fit of the low dose data. The slope is (3.98 f 0.14) x lo-'. From this value and three additional pieces of information, (i) the yield of F atoms of (2.8 f 0.3) x 1015 molecule cm-3 (full dose and [S&] = 1000 mbar), (ii) the conversion of F atoms into 94.5% CC13CH202 radicals and 5.5% F02, and (iii) the absorption cross section for FOz at 250 nm ((T= 1.3 x cm2 molecule-'),8 we derive a(CC13CH202) at 250 nm = (2.9 f 0.3) x cm2 molecule-'. The quoted error includes both statistical and potential systematic errors and so reflects the accuracy of the measurement. To map out the spectrum of the CCl3CH202 radical, experiments were performed to measure the initial absorbance between 220 and 300 nm following the pulsed irradiation of SFdCC13CH3/02 mixtures. The initial absorbances were scaled to that at 250 nm and corrected for FOz to obtain absolute absorption cross sections. Absorption cross sections are given in Table 1 and compared to the spectra of C H Z C ~ and O ~ CF3CH202l0 ~ in Figure 3B. As seen from Figure 3B, the CC13CHz02 spectrum is closely related to spectra of other halogenated peroxy radicals. The position of the maximum is consistent with the literature on peroxy radical spectra." The C1 atoms on the carbon atom adjacent to the carbon bearing the 0-0' group lower the absorption maximum. Kinetic Study of the Self-Reaction of CC13CH2 Radicals. Figure 1A shows a typical absorption trace obtained for the selfreaction of CC13CH2 radicals, together with a nonlinear leastsquares second-order decay fit. As discussed in previous publication^,'^^^^ the kinetic analysis of second-order decays can be complicated by the formation of products which absorb at the monitoring wavelength. This leads to a positive absorbance after all reactions have ceased. Accordingly, the experimental data were fit with a three parameter expression
1/B - l/Bo = (In 10)(2kt)/ue&.
(1)
where B = log(ZJfl, Bo = log(ZJZ'), where Z' is the minimum transmitted light intensity following the radiolysis pulse, I, is the final light intensity, and Z is the transmitted light at time t. k is the second-order rate constant for the self-reaction of the radicals, u,ff is the absorption cross section of the radical minus that of any absorbing product formed at the monitoring wavelength (in the case of CC13CH2 radical there was no observable residual absorption and u,ff = a(CCl3CH2)), and L is the monitoring UV path length (80 cm). The decay of the transient absorption following radiolysis of CCl3CH3/SF6 mixtures was monitored at a wavelength of 230 m. The decay was well represented by this second-order leastsquares fit. No residual absorption was observed, indicating the absence of any long-lived product which absorbs significantly at 230 nm. Figure 4A shows the reciprocal half-life for the decay of the absorption at 230 nm as a function of the initial absorbance due to CC13CH2 radicals. A linear least-squares fit of the data in Figure 4A gives a slope of (5.7 f 0.2) x IO4 s-' = (k4 x 2 In 10)/(a(CC13CH2)L). The intercept of the linear regression of the data is (1.7 f 0.6) x IO3 s-l and is significantly different from 0. This could be explained by a small amount of 0 2 present in the SF6. Using a(CC13CH2) =
A
30
25 20
15 10
5 0'
I
0.0
I
I
0.1
0.2
0.3
,
I
,
0.08
0.12
,
0.4 0.5 Absorbance (Base 1 0 ) of CC13CH, a t 230 nm
0.00
0.04
Absorbance (Base 10) o f CCI,CH,O,
B
1
0.16
0.20
a t 250 nm
Figure 4. (A) Plot of the reciprocal half-life for the self-reaction of CC13CH2 radicals as a function of A,, at 230 nm. (B) Plot of the reciprocal half-life for the self-reaction of CCl3CH202 radicals as a function of A,, at 250 nm.
(9.4 f 1.1) x IO-'* cm2 molecule-' at 230 nm and the slope (5.7 f 0.2) x 104 s-l, k4 = (9.1 f 1.1) x cm3molecule-' s-' was derived. This rate constant value can be compared to typical halogenated alkyl radical self-reaction rate constants in the case of CF3CC1214 to 3.2 x ranging from 2.4 x 10-l' cm3 molecule-' s-l for CH2C17and follow the sequence CH2C1 > CHClz > CCl3 > CF3CC12. This order of reactivity can be rationalized in simple terms on the basis of steric arguments with bulky substituents, F and C1 atoms, slowing down the association reaction. Kinetic Study of the Self-Reaction of the CCl3CH2Oz Radicals. Following the pulsed radiolysis of 10 mbar of CC13CH3, 20 mbar of 02, and 970 mbar of SF6, a rapid increase in UV absorbance in the region 220-300 nm was observed followed by a slower decay, as shown in Figure 1B. As discussed previously, we ascribe the absorbance to the CCl3CH2Oz radicals. The decay was fit by expression I and in all cases, within the experimental uncertainties, the decays followed second-order kinetics. The observed self-reaction rate constant of reaction 5 is defined as -d[CCl3CH202]/dt = 2k50t,s[CC13CH~0~]2. Figure 4B shows the reciprocal half-life for the decay of the absorption at 250 nm as a function of the initial absorbance due to CC13CH202 radicals. The absorbance is corrected for the absorption of FOz. A linear least-squares fit of the corrected data in Figure 4B gives a slope of (9.3 f 0.4) x lo4 s-'. The intercept of the linear regression of the data is (-3.9 f 4.4) x lo3 s-' and is not statistically significant. Using a(CCl3CH202) = (2.9 f 0.3) x 10-'8 cm2 molecule-' at 250 nm gives k5obs = (4.7 f 1.6) x 10-'2 cm3 molecule-' s-'. k5& may be an overestimate of the true bimolecular rate constant for reaction 5 , as the CCl3CH202 radicals might react with the reaction products, as discussed in
6574 J. Phys. Chem., Vol. 99, No. 17, 1995
Platz et al.
,
I
0.15
- I
I
0
M 0.05 n
0.001 0.0
"
"
"
"
0.2
0.4
0.6
0.8
'
I
1 .o
[CC~,C~,l/[O,I
Figure 5. Plot of the maximum absorbance as a function of the
concentration ratio [CC13CH3]/[02] observed following the radiolysis of CC13CH3/02/SF6mixtures; see text for details. ref 15. The rate constant for the self-reaction of CCl3CH202 radicals is consistent with those for self-reactions of other halogenated peroxy radicals." It should be pointed out that there is a large residual absorption as seen in Figure 1B. As will be discussed in the subsequent section dealing with the atmospheric fate of CCl3CH20 radicals, under the present experimental conditions with [02] = 20 mbar, 27% of the CCl3CH20 radicals formed in reaction 5 will decompose to give CCl3 radicals which eliminate C1 atoms to give COC12. By analogy to the behavior of other peroxy radicals, CCl3CH202 radicals probably react rapidly with C1 atoms to give C10. The C10 concentration does not change on the timescale of our experiment. If all C1 atoms gave C10, and using the recommended C10 absorption cross section at 250 nm, 3.8 x cm2 molecule-',16 then the residual absorption expected would be A,, = 3.8 x x 0.27 x 2.8 x lOI5 x 80 x 0.97/2.303 = 0.097, which compares reasonably with the observed value of 0.07. At this point it should be noted that CCl3CHO and COC12 absorb weakly at 250 nm with absorption cross sections of 2 x l7 and 8.1 x cm2 molecule-1,18respectively. The formation of CCl3CHO and COC12 does not contribute significantly to the residual absorption seen in Figure 1B. In light of the potential complications noted above, the value of k50bs derived in the present work should be treated as an observed rate constant for the decay of absorption at 250 nm. Reaction of F Atoms with CC13CH3. To measure k3, experiments were performed in which the maximum in the transient absorbance at 250 nm was observed following the radiolysis of SFdCC13CH3I02 mixtures. The radiolysis dose (41.6% of full dose) and the concentration of sF6 (980 mbar) were held fixed. The concentrations of CCl3CH3 and 0 2 were varied over the ranges 0-20 mbar and 20-40 mbar, respectively. Figure 5 shows the observed variation of the maximum absorbance as a function of the concentration ratio [CC13CH2]/ [02]. In this reaction system there is a competition between 3 and 10:
F
-
+ CC13CH3 CC13CH, + HF F + 0, + M -.. F02 + M
(3) (10)
At 250 nm, FO2 radicals absorb less than the same amount of CCl3CH202 radicals. As the CCl3CH3 concentration is increased, CC13CH202 radicals are formed at the expense of FO2, and the maximum absorbance increases; see Figure 5. A,,, increases until the ratio [CC13CH3]/[02] is about 0.2. Further increase in this ratio does not affect the maximum absorbance. The solid line in Figure 5 is a three parameter fit of the following expression to the data:
where A,, is the observed maximum absorbance, A F O ~is the maximum absorbance expected if only F02 were produced, RH is CC13CH3, and A~o2is the maximum absorbance expected if only CCbCH202 were formed. Parameters AFOZ, ARO~, and (kd klo) were varied simultaneously. The best fit was obtained with k3/klO = 36.0 f 5.3. Using klo = (1.9 k 0.3) x wi3 cm3 molecule-' s-' gives k3 = (6.8 f 1.5) x cm3molecule-1 s-l. This value is used for the calculations in this work. Kinetic Data for the Reaction CC4CH202 NO Products. The kinetics of reaction 6 were studied by monitoring the increase in the absorbance at 400 nm, attributed to the formation of NO2, following the pulsed radiolysis of SFdCC13CHdOdNO mixtures. The mixtures were combinations of [NO] = 0.28-1.5 mbar, [CC13CH31 = 10 mbar, [021 = 20 mbar, and [SF61 = 970 mbar. This method of determining rates constants of reactions of peroxy radicals with NO has been used extensively in our laboratory and is discussed in detail elsehere.'^^^^ Figure 6A shows the result from a mixture with [NO] = 1.03 mbar. After the pulse radiolysis the CC13CH202 radicals will form within a few microseconds. The smooth curve in Figure 6A is a first-order fit starting 3 ps after the radiolysis pulse. The absorption transient is fitted using the expression:
+
A($) = (A, - Ao)[l - exp(-k'"'r)]
+Ao
-
(11)
where A(t) is the absorbance as a function of time, A, is the absorbance at infinite time, kist is the pseudo-first-order appearance rate of NO*, and A0 is the extrapolated absorbance at t = 0. For data shown in Figure 6A, kist = 1.67 x lo5 s-l. In all cases, the rise in absorbance was well fitted by first-order kinetics. It seems reasonable to conclude that NO2 is the species responsible for the absorbance change following pulsed radiolysis of SFdCCLCH3I02INO mixtures. As seen from Figure 7A, the pseudo-first-order rate constant, kist, increased linearly with [NO]. The initial F atom concentration employed in the present experiments (1.2 x 1015molecule ~ m - is ~ )a significant fraction (3-17%) of that of the initial NO concentration and deviations from pseudo-first-order kinetics may be expected. However, no such deviations were discernible within the experimental data scatter. Assuming that all F atoms either react directly with NO or produce species that react with NO, then the average NO concentration in a given experiment is [NO],, = [NO30 - [Fld2. l k s expression was used to correct the data shown in Figure 7. Corrections applied were in the range 2-8%. Linear least-squares analysis of the data in Figure 7A gives k6 = (6.6 f 0.5) x cm3 molecule-' s-l. The y-axis intercept in Figure 7A is (-0.2 f 1.2) x lo4 s-l and is not statistically significant. F atom also react with NO producing FNO,k(F NO FNO)= 5.5 x cm3molecule-' s-1.21 The absorption of FNO at 400 nm is negligiblez2 and hence has no influence on the kinetics analysis deriving ks. The reaction of CC13CH202 radicals with NO produces NO2 and, by inference, CCl3CH20 radicals. The alkoxy radical CCl3CH20 can, as discussed in a following section, react with 0 2 or undergo C-C bond fission. In either case peroxy radicals (HOz or CCl3O2) will be formed which can undergo subsequent reaction with NO to give more NO2. In the present experiments the yield of NO;?,calculated using a~02(400nm) = 6 x cm2 m o l e c ~ l e - ~was , ~ ' 162 f 33%. The fact that the NO2 yield exceeds 100% shows that reaction with 0 2 , and/or decomposition of CCl3CH2O radicals, takes place within our experimental
+
-
J. Phys. Chem., Vol. 99, No. 17, 1995 6515
Atmospheric Chemistry of 1,1,1-Trichloroethane A
h
0 c
I
Q
-25.0
0.0
25.0
50.0
75.0
100.0
Time (gsec)
B
I 0.00
.0
h
I
tw
+
.................................................................................................
.......
L
-25.0
0.0
+
25.0
50.0 Time ( p s e c )
75.0
100.0
Figure 6. (A) Transient absorbance at 400 nm observed following pulsed radiolysis (dose = 42% of maximum) of a mixture of 1.03 mbar of NO, 10 mbar of CClsCH3,20 mbar of 0 2 , and 970 mbar of SF6. (B) Transient absorbance at 400 nm observed following pulsed radiolysis (dose = 42% of maximum) of a mixture of 0.61 mbar of NO2, 20 mbar of CC13CH3, 20 mbar of 0 2 , and 960 mbar of SF6. The path length was 120 cm in A and B.
time scale of 0-40 p s . The presence of additional processes forming NO2 subsequent to reaction 6 increases the time taken for the NO2 to reach a maximum, and so the value of ks = (6.6 f 0.5) x cm3 molecule-' s-l is actually a lower limit. Hence, we choose to give k6 > 6.1 x lo-'* cm3 molecule-' s-l. This value is in accordance with rate constants previously determined for RO2 NO reactions.20 Kinetic Data of the Reaction CCl3CHzOz NO2 M CCl3CH202N02 M. The kinetics of reaction 7 were studied by monitoring the absorbance at 400 nm following the pulsed radiolysis of mixtures of 20 mbar of CC13CH3, 20 mbar of 02, 0.5-2.0 mbar NO,, and SFg to 1000 mbar total pressure. Figure 6B shows the absorbance as a function of time after the pulsed radiolysis of a mixture with [NO21 = 0.61 mbar. The absorbance before the pulsed radiolysis is due solely to the N02. The decay rate of the absorbance at 400 nm increased with increasing NO;?concentration. It seems reasonable to explain the loss in absorbance by a loss of NO,. Three reactions could be responsible:
+
+
CCl,CH,O,
+
+ NO, + M -FN02/FON0 + M CC13CH2+ NO, - products + NO, + M - CC13CH202N02+ M
F
The smooth curve in Figure 6B is the first-order fit of the absorption transient from 4 ps. The curve gives a pseudo-firstorder rate constant kist. kist values for six experiments with different concentrations of NO2 are shown in Figure 7B. The initial F atom concentration employed in the present experiments (1.2 x 1015molecule cm-,) is a significant fraction (2-10%) of that of the initial NO2 concentration, and deviations from pseudo-fist-order kinetics may be expected. However, no such deviations were discernible within the experimental data scatter. Assuming that all F atoms either react directly with NO2 or produce species that react with NO,, then the average NO2 concentration in a given experiments is [N021av= [NO210 [F]d2. This expression was used to correct the data shown in Figure 7B. Corrections applied were in the range 1-5%. Linear least-squares analysis of the data in Figure 7B gives k7 = (6.5 f0.4) x 10-l2 cm3 molecule-' s-l. The y-axis intercept in Figure 7B is (-0.3 f 0.8) x lo4 s-l and is not statistically significant. RO2 NO2 reactions are pressure dependent and are generally at, or near, the high pressure limit at 1 atm total pressure of SF6 diluent." This value of k7 is in accordance with rate constants previously determined for RO;! NO;! reactions.l1 Kinetics of the Reactions of CI Atoms with CCl3CH3, CCbCHO, and CC13CHzOH. Prior to investigatingthe atmospheric fate of CCl3CH20 radicals, a series of relative rate experiments was performed using the FTIR system to investigate the kinetics of the reactions of C1 atoms with CC13CH3, CC13CH0, and CCl3CH20H. The techniques used are described in detail elsewhere." Photolysis of molecular chlorine was used as a source of C1 atoms.
+
C1,
C1+ CCl,CH, C1+ CC1,CHO
(14) (15)
As discussed p r e v i o ~ s l y ,under ~ ~ . ~the ~ present experimental conditions with [CC13CH3] = 10 mbar and [02] = 20 mbar, reactions 13 and 14 will have a negligible influence.
-
CCl,CH, -I-HC1
-
CC1,CO
C1+ CCl,CH,OH
-
+ HC1
products
(16) (17) (18)
The kinetics of reaction 16 were measured relative to reaction 19; reactions 17 and 18 were studied relative to (20):
-
(13)
+ hv - 2C1
C1+ CD, C1+ C2H,C1
-
+ DC1 C,H4C1 + HC1 CD,
(19) (20)
The observed losses of CCl3CH3 versus CD4, CCl3CHO versus CzH5C1, and CCl3CH20H versus CzH5C1 following the UV irradiation of CC13CH3/CDdClz, CC~~CHO/CZHSC~/C~Z, and C C ~ ~ C H ~ O W C ~ H Smixtures, C Y C ~ ~ respectively, in 700 Torr total pressure of N2, or air,diluent are shown in Figure 8. There was no discernible difference between data obtained in N2, or air, diluent. Linear least-squares analysis gives kldk19 = (1.62 f 0.05>,k17/k20 = 0.75 f0.06, and klg/kzo = 0.37 f 0.03. Using 25 gives k16 = (9.9 k19 = 6.1 x 24 and k20 = 8.04 x f 0.3) x 10-15, k17 = (6.0 f 0.5) x 10-l2, and k18 = (3.0 f 0.2) x cm3 molecule-' s-l, respectively. We estimate that potential systematic errors associated with uncertainties in the reference rate constants could add an additional 20% to the uncertainty range. Propagating this additional 20% uncertainty gives k16 = (9.9 f 2.0) x k17 = (6.0 f 1.3) x lo-',, and kls = (3.0 & 0.6) x cm3 molecule-' S-1.
The kinetics of reaction 16 have been studied previously by Wine et aLZ6and Tschuikow-Roux et al.27 Using an
6576 J. Phys. Chem., Vol. 99, No. 17, 1995
Platz et al. A
I 0.05
0.00
20
0.15
I
c
n
10
+ L VI
.-
L
0 T)
3
2! K
0 0.0
0.5
1 .o [NO] (mbor)
1.5
2.0 R
0.4
1
0.4
c
30
D +,-
e
iY)
L
t
u
20
c
.-E
fl\
I :::4 1
o
0.2
0.3
P C 10
1700
1725
, 1750
1775
1825
1800
4 \ E=COC12
1850
1875
1900
L
0 U 3
2
h
WAVENUMBER ( c m - l )
0
0.0
1 .o
0.5 "21
1.5
2.0
(mbor)
Figure 7. (A) Plot of the pseudo-first-order rate constant for the formation of NO2 as a function of the NO concentration. (B) Plot of the pseudo-first-order rate constant for the decay of NO2 as a function of the NO2 concentration. 2.2
0.0
I
/ '
0.5
i.a
1.5
2.0
2.5
Ln ([Reference],,/[Reference],)
Figure 8. Results of the C1 atom relative rate experiments: observed loss of CC13CH3 versus CD4 (circles), CCl3CHO versus C2H5Cl (triangles),and CCl3CH20H versus C2HsCl (diamonds). Filled symbols
were acquired in 700 Torr of air; open symbols were acquired in 700 TOITof N2. absolute rate technique Wine et aLZ6measured an upper limit of k16 3.7 x at room temperature while TschuikowRoux et al.27used a relative rate method to derive k16 = (1.6 f 0.4) x cm3 molecule-' s-l. The kinetics of reaction 17 have been studied by Scollard et aLZ8who report k17 = (7.1 k 0.5) x cm3 molecule-' s-l. Within the cited experimental uncertainties the results from the present work are consistent with the previous studies of kif, and k17. There have been no studied of klg with which to compare the present results.
Figure 9. Infrared spectra acquired before (A) and after (B) a 1 min irradiation of a mixture of 740 mTon of CClsCH3 and 330 mTorr of F2 in 700 Torr of air diluent (experiment 3; see Table 2). Panels D and E show reference spectra of CC13CHO and COC12, respectively.
Study of the Atmospheric Fate of CCbCH20 Radicals. To determine the atmospheric fate of the alkoxy radical CCl3CH20, experiments were performed in which F2/CC13CH3/02 mixtures at a total pressure of 700 Torr made up with N2 diluent were irradiated in the FTIR smog chamber system. The loss of CC13CH3 and the formation of products were monitored by FTIR spectroscopy. Figure 9 shows IR spectra acquired before (A) and after (B) a l min irradiation of a mixture of 740 mTorr of CCl3CH3 and 330 mTorr of F2 in 700 Torr of air diluent (expt 3; see Table 2). Panel C (Figure 9) shows the product spectrum derived by subtracting A from B. Comparison of the product spectrum with reference spectra of CCl3CHO and COC12 shown in panels D and E shows the formation of these products. In addition to CCl3CHO and COC12, CO was detected as a product. Observed product yields are listed in Table 2. IR features attributable to HCHO were searched for but not found, and an upper limit of 0.1 mTorr was established for the yield of HCHO. Within the experimental errors, the observed products of CCl3CHO and COCl2 account for 100% of the loss of CCl3CH3. No evidence was observed for the presence of CC13CH20H in the product spectra, and an upper limit of 7% for the molar yield (A[CCl3CH2OH]/(A[CC13CHO] A[COC12])) of this species was established. By analogy to the behavior of other peroxy radicals,6,28it is expected that the self-reaction of CC13CH202 radicals proceeds via two possible channels:
+
CC13CH,02
+ CC13CH20,
-
+
+
+
+ 0, (5b)
CC13CH20 CC13CH20 0, (5a) CC13CH20,
+ CC13CH,0,
CC13CH20H CC1,CHO
The absence of IR features attributable to CC13CHzOH product
J. Phys. Chem., Vol. 99, No. 17, 1995 6577
Atmospheric Chemistry of l , l , 1-Trichloroethane
TABLE 2: Product Yield@Following the Irradiation of CCbCH.dFd02 Mixtures in 700 Torr of Diluent 0.11
0.1
147
2 4
6 8 0.75 0.74 0.85 0.88 0.79 0.89 0.88 0.88
0.3 0.3 0.3 0.2 0.3 0.3 0.3 0.6
700 147 48 17 10.5 10.1 4.5 3.4
10 1 1 1 1 1 1 1 1
ndd
0.66
2.2 3.6 4.4 5.5 nd
1.09 1.09 1.05 1.oo 1.65 2.11 1.45 1.12 1.32 1.12 0.53 0.46
nd nd nd nd nd nd nd
0.14 0.84 1.63 3.4 4.1 0.19 0.37 0.56 1.02 1.11 1.21 1.12 1.40
0.54 1.33 2.31 3.1 4.3 0.34 0.78 0.54 0.94 1.08 1.03 1.35 1.40
a Concentrationsin units of mTorr, with no corrections of any kind applied to data. Units of Torr. W lamps were not equipped with preignition coils; hence the exact irradiation times may vary. Consumption of CC13CH3 was not discernible.
suggests that channel 5a is dominant and channel 5b is, at most, of minor importance. This observation is consistent with the observed behavior of the analogous species CH2C102 radicals where the molecular channel of the self-reaction is of negligible (
