1849
J. Phys. Chem. 1982, 86. 1849-1853
for the aquo complex hemin and cyt0chrome.'~7'~As previously suggested, it seems that these reactions proceed via an outer-sphere electron transfer pro~ess.~' The superoxide radical reduction reaction with its 2-3 orders magnitude change between the aquo and the other complexes seems to suggest a change in the reaction mechanism. It is suggested that, because of the high rate of ligand exchange and consequently high lability of the sixth water molecule, the reduction of the Fe'I'TMPyP aquo complex proceeds via the formation of the intermediate FemTMPyP-02- complex. The half-life of this intermediate (which converts to FenTMPyP) is long enough (-1 ms) so that the oxidation reaction by another superoxide radical may react with the intermediate. The Fen*TMPyP complexes investigated in this study are diliganded complexes with relatively high stability constants," which makes a ligand exchange process unlikely. We therefore have to assume in accord with our experimental results that no intermediate (of the FemTMPyP(L)(02-) complex) is formed and the reduction reaction proceeds via an outer-sphere mechanism with rate conThe stants similar to those found in heme difference in the reoxidation reaction by 0; is accounted for by the fact that in the diliganded complexes this oxidation reaction is with the fully reduced FenTMPyP(L)z. The difference in the action of the aquo complex as compared to the other complex is further illustrated by the rate of oxidation of the reduced iron(1I) porphyrin. Here again, the relatively rapid reaction of the aquo complex seems (47) Goff, H.;Simic, M.Biochim. Biophys. Acta 1976, 392, 201.
to proceed via rapid formation of the intermediate Fe"TMPyP(02) which again probably results from the high lability of the aquo complex. The oxidation of the diliganded highly stabilised complexes must proceed via a different mechanism. The nature of this mechanism is still unknown, and further experiments are conducted to clarify the mechanism. One possibility considered is that the oxidation reactions proceed via the minute amount of the aquo complex which may be present in the reduced solution. Conclusion This paper presents results showing that the Fe"TMPyP aquo complex is an excellent electron acceptor. Its high reactivity toward 02-(k = 2 X lo9 M-' s-' ) seems to be related to its high ligand exchange rate. This provides the porphyrin with a site through which the superoxide radical is bound (inner-sphere mechanism). It seems also that the FemTMPyP-02- complex is a relatively long-life intermediate and is readily oxidized by another 02-(k = 2.3 X 109 M-' s-' at p = M). The mechanism of the reaction of Fe(II1) with 02-and the rate constants for the individual reactions in this mechanism are similar to those found for the superoxide dismutase. Consequently, this iron porphyrin should show similar catalytic activity toward the superoxide radical. Acknowledgment. We thank Professor T. Kuwana for the donation of the Fe'I'TMPyP, Dr. A. Bettelheim for valuable discussions, Mr. D. Weinraub for his most useful help, and Mr. Y. Ogdan for the effective efforts in maintainiig the linear accelerator and the electronic equipment.
Thermai Decomposition Kinetics of CH302N02 Abraha Bahta, R. SlmonaHls, and Jullan Helcklen' Department ot Chemishy and Ionosphere Research Laboratory. The Pennsylvania State Universliy, Universlv Park, Pennsylvania 16802 (Receh&: August 17, 1981; I n Final F m : Januaty 15, 1982)
The thermal decomposition rate of CH3OZNOzwas studied over the temperature range 256-268 K at -350 torr total pressure and over the pressure range 50-720 torr at 263 K by perturbation of the equilibrium CH302 + NO2 (+M) * CH302N02(+M) (3, -3) with NO, CH302+ NO CH30 + NOz (4). The CH3OZNOzwas generated in situ by the photolysis of Clz in the presence of Oz, CHI, and NOz. The decomposition kinetics were monitored in the presence of NO by the change in ultraviolet absorption at 250 nm. The Arrhenius expression obtained for the thermal decomposition is k9 = 6 X 1015exp(-(21100 f 1500)lRT)s-l at -350 torr total pressure (mostly CHI) where R = 1.987 cal/(mol K). The uncertainty in the Arrhenius parameters can be greatly reduced by combining this expression with data for k3 and thermodynamics data to give k3= (6 f 3) X 1015 exp(-(21300 f 300)/RT) s-l at -350 torr total pressure. Computations based on the pressure dependence of the forward reaction give k-3m = 2.1 X 10l6expi-(21700 f 300)lRT) s-l and k-30 = 3.3 X exp(-(20150 f 300)IRTJcm3s-l. At 263.3 K the equilibrium constant K3,-3(263.3K) is determined to be (2.68 f 0.26) X cm3. In the stratosphere the CH3OZNO2lifetime will be controlled by photodissociation. Approximate calculations indicate that CH302N02may play a role in the NO, budget of the lower stratosphere.
-
Introduction Peroxynitrates are potentially important intermediates in the atmospheric oxidation of a variety of organic compounds. Their atmospheric importance depends upon their thermal and photolytic stability. Methyl peroxynitrate, CH302N02,is expected to be an atmospheric transient formed from the oxidation of CHI. In this paper we report our study of the thermal decomposition of 0022-3654/82/2086-1849801.25/0
CH3O2NOZwhich evidently has not been studied before. Experimental Section Apparatus and Procedure. The ultraviolet spectrometer and vacuum system were essentially as described bef0re.l (1) 0. Morel, R. Simonaitis, and J. Heicklen, Chem. Phys. Lett., 73, 38 (1980).
0 1982 American Chemical Society
1850
The Journal of Physical Chemistry, Vol. 86, No. 10, 1982
0 L . J
4.
I20
'
240
v
0'
'
m '
o '
6 '
0'
w '
'
'
IM
m
TIME (SECONDS1
Flgwe 1. Absorption vs. time profile for the run at 208 K with k-, = 3.18 X IO-* s-'.
The Pyrex reaction cell was 20.3 cm long and 5 cm in diameter with quartz windows and with inner and outer jackets for circulating a refrigerant (ethanol) and for insulation, respectively. The temperature was uniform and stable over the course of a run to within 0.5 "C. The inlet to the cell was equipped with jets spaced along the length of the cell for introducing the reactants into the cell with uniformity throughout. The time required for complete mixing was tested by adding pure Nz into a mixture containing NOz. As the N2 is added the absorption is perturbed, but it returns to its original value in a matter of seconds. Irradiation was from two Hanovia Type SH mediumpressure lamps passed through a Corning CS 7-37 glass filter to isolate the 366-nm Hg emission line. The methyl peroxynitrate was prepared in situ from the photolysis of C12 at 366 nm in the presence of NO2, 02,and CHI. Ultraviolet absorption at 250 nm was measured as a function of time. An increase in absorption due to CH3OzNOzproduction occurs during irradiation. When theNOz is consumed, production of CH3O2NOZ ceases, and the absorption remains constant. At this point irradiation is terminated, NO is added to the reaction cell, and the total absorption is monitored as a function of time. Upon addition of NO the absorption again increases, primarily due to formation of CH30N0, to a limiting value. A typical absorption vs. time profile is shown in Figure 1. This figure shows about a 7-s induction time after the NO is added (due to mixing) before the absorption rises sharply. Materials. Clz (Matheson, research purity) was purified by passage over KOH in order to remove HC1 and vacuum distilled. The NOz and NO (Matheson) were vacuum distilled. The white color of the respective solids indicated that they were free of each other. The O2(Matheson, extra dry) and CHI (Matheson, ultrahigh purity) were used without further purification.
Results The in situ production of CH302N02from the photolysis of C12 at 366 nm in the presence of Oz/NOz/CH4occurs via the following reactions: C12 + hu (366 nm) 2Cl rate = I,
+
C1+ CHI
the right under our experimental conditions. Addition of NO, upon the termination of photolysis, provides a sink for CH3O2 radicals, thus the equilibrium 3, -3 is perturbed leading to the thermal decomposition of CH302N02.The pertiment reactions are CH3O2 + NO CH,O + NO2 (4) CH30N0 (+M) CH,O + NO (+MI (5) CH30 + NO2 (+M) CH30NOz (+M) (6) Reactions 1-4 are all well-known and have been reviewed recently.2 Reactions 5 and 6 are also known and their rate Coefficients have been meas~red."~In the determination of k-, reactions 3 and 4 enter into the analysis but only in a minor way (see below); thus the uncertainty in their rate coefficients affects the determination of k-, to a negligible extent. The rate coefficients k5 and k6 do not enter into the computation at all. The kinetics of the thermal decomposition of CH30aNOa were followed by monitoring the total absorbance change due to the removal of CH3OZNO2and the production of CH30N0 and CH30NOZafter the addition of NO. Based on the mechanism consisting of reactions 3-6, the total absorbance, A,, is the sum of the absorbances of CH3OzNOz, NOa, CH,ONO, and CH30NOzwhich have respective absorption cross sections of 3.5 X 10-19,21.86 X 1.34 X 10-l8,5and 3.71 X cm2.5 The variation of A, as a function of time is independent of the individual species absorbances and is given by CY-' In [(A, - At)/(A, - A,)] = k-,t (1)
--
f I
Bahta et ai.
+ HC1 CH302+ M
CH,
(1)
CH3 + O2 M (2) CH30a + NO2 (+M) e CH302N02 (+M) (3, -3) The photolysis of NOz is about -2% of that for C12and, in any event, is irrelevant because the 0 atom produced adds to 0, which immediately reacts with the NO produced to regenerate NOz + 02.In the absence of significant loss processes for CH302,CH3OZNO2is relatively stable, since the equilibrium in reaction 3 is shifted far to
where A, is the final absorption, 4 is the initial absorption (see Figure l),and a 1 k4[NO]/(k3[NOa]+ k4[NO]). A plot of the left-hand side of eq I vs. time will be linear with a slope of k,. a can be computed from the now reasonably well-known values of k, and k4 The recommended value of k4 = 7.4 X cm3s-l is independent of temperature. We have recently determined k, = 2.1 X exp[(380 f 250)/T] cm3s-'.~ At room temperature the values agree almost exactly and the small variation with temperature over the small temperature range has no significant effect upon the determination of kv3. k3 is both pressure and temperature dependent, but a reasonably good interpolation formula,2 eq TI, allows k3 to be computed for the log k{M,TJ= log (1
+ k2k;E;kJTJ
conditions of interest, where the subscripts 0 and refer respectively to the low- and high-pressure limiting rate coefficients. The respective rate coefficients are2 ko(TJ= (1.5 f 0.8) X 10-30(T/300)-n cm6 s-'
k , ( n = (6.5 f 3.2) X 10-12(T/300)-mcm3 s-' where n = 4.0 f 2.0 and m = 2.0 f 2.0. Actually, 01 is a weak function of the time; typically it dropped from 0.95 at the beginning of the run to 0.90 at the end. In evaluating the left-hand side of eq I, CY was computed point by point. (2).'Chemical Kinetic and Photochemical Data for Use in Stratospheric Modelling", National Aeronautics and Space Administration,Jet Propulsion Laboratory Pubication 81-3, 1981. (3) L. Batt, h t . J. Chem. Kinet., 11, 977 (1979). (4) L. Batt and G. N. Rattray, Znt. J. Chem. Kinet., 11,1183 (1979). (5) J. G . Calvert and J. N. Pith, Jr., 'Photochemistry", Wiley, New York, 1966. (6) R. Simonaitis and J. Heicklen, J. Phys. Chem., 85, 2946 (1981).
The Journal of Physical Chemistry. Vol. 86, No. 10, 1982 1851
Thermal Decomposition of CHBOINOP
501 \
40
\
'0
15
30 45 60 Time, seconds
75
365
TABLE I: Temperature Dependence of the Thermal Decomposition of CH,O,NO,
1.20 1.24 1.23 1.27 1.24 1.24 1.21 1.23 1.23 1.23 1.24 1.37 1.28 1.25 1.35 1.19 1.33 1.28 1.17 1.31
2.13 2.54 2.20 2.20 2.35 2.28 2.28 2.04 2.85 2.12 2.52 2.88 3.10 3.14 2.96 2.20 2.13 2.56 2.09 2.67
-
0.94 1.18 1.33 1.33 1.33 1.18 0.97 1.21 1.49 1.35 1.21 1.94 1.48 1.30 1.57 1.16 1.12 1.42 1.16 1.57
7.64 9.08 3.50 3.46 3.70 8.20 8.15 3.24 4.50 3.35 3.14 3.70 4.00 4.92 3.83 3.45 3.38 3.31 3.31 3.46
0.52 0.58 0.84 0.83 0.78 0.76 0.82 1.40 1.55 1.58 1.69 2.42 2.55 2.56 2.75 3.21 2.76 2.86 3.08 3.18
[CH,] = 300 torr; [N,] 5 0 torr; [Cl,] = 5 * 1 torr. [CH,O,NO,], is the initial CH,O,NO, concentration based on the concentration of NO, originally used. a
375
380
IOOO/T
Figure 2. Plot of a-' In ((A - A,)/(A - A,)] vs. time (eq I) for the run at 267 K with k , = 2.75 X 10-2s-'.
256.8 256.8 257.8 258.3 258.8 259.3 259.3 263.3 263.3 263.3 264.3 265.3 265.8 266.8 266.8 266.8 267.3 267.3 267.3 267.8
370
A typical plot of eq I is shown in Figure 2. The results of the kinetic measurements for the temmperature de0 are presented in Table I. pendence study at ~ 3 5 torr Unfortunately, due to experimental constraints, the temperature range covered is not large, and variations in [CH3O2NOZlo and [NO] were negligible. The highest value possible of [CH302N0210was used to give the maximum absorbance change when NO was added. This upper limiting value was determined by the maximum NOz concentration that could be used and still not have the NOz compete with Ozfor CH3 radicals during the production
,
385
390
395
O K - '
Flgure 3. Arrhenlus plot of k-3.
of CH3O2NO2.The [NO] was kept fixed at its optimum level to minimize ita reaction with Ozand to keep a as close to 1.0 as possible. However, the change in k-, is substantial and the measurements are of good precision. Table I1 presents the data obtained from the pressure study at 263 K. The pressure range covered was from 50 to 720 torr. In both studies the NO concentration was adjusted to minimize the reaction with O2 2N0
+02
-
+
2N02
-
(7)
and to maintain [NO]/[NO,] 4-5 in order to keep a 1. The maximum correction for k-, due to reaction 7 was 4 % at a 20.90. The O2pressure and the [NOZ]/[O2]used were controlled by the need to minimize reaction 8. CH3 + NO2 (+M)
-
CH3N02 (+M)
(8)
For the temperature dependence study CH4was the only diluent used. CHI was also used for almost all runs in the experiments where the total pressure of the system was varied. The exceptions were the high-pressure runs (-720 torr total pressure). Here two sets of runs were performed, one with CHI as a diluent and the other with N2 as a diluent. These experimens were done to determine if Nz and CHI are very different in efficiency in their role as diluents. At -720 torr total pressure, Table I1 shows that, within the limits of experimental error, k-, is essentially the same, regardless of the diluent used. This is not surprising, since the relative efficiency of SF, and Nz, two gases of very different complexity, is only ~ 1 . 2 . ' An Arrhenius plot of k, at a total concentration of 1.3 X 1019cm-, is shown in Figure 3. The line is the leastsqures fit to the data. The Arrhenius expression based on the least-squares line is k-, = 6 X 1015 expi-21100 + 1500/RTJ s-l at -350 torr total pressure (mostly CHI), where R = 1.987 cal/mol. The stated uncertainty in the (7) S. P. Sander and R. T. Watson, J.Phys. Chem., 84, 1664 (1980).
The Journal of Physical Chemistty, Vol. 86, No. 10, 1982
1852
Bahta et al.
TABLE 11: Pressure Dependence of the Thermal Decomposition of CH30,N0, at 263.3 K 10-15.
lo-''
lo,*
102k-,, s-'
10-l9 [ 0,1, cm-3 2.08 2.73 2.66 2.57
exptl 0.61 0.72 0.70 0.85
1.27 1.26 1.32
3.73 2.99 3.80
1.01 0.99 1.11
1.18 1.07 1.20 av 1.15 i: 0.07
1.81 1.87 1.96 1.87
1.46 1.32 1.50 1.38
2.87 2.98 3.12 2.98
1.13 1.19 1.27 1.14
1.43 1.19 1.37 1.23 av 1.31 i: 0.12
1.23 1.23 1.23 1.24 1.37
2.04 2.85 2.12 2.52 2.88
1.21 1.49 1.35 1.21 1.94
3.24 4.50 3.35 3.14 3.70
1.40 1.55 1.58 1.69 2.42
1.40 1.55 1.58 1.56 1.78 av 1.57 + O.lgd
262.6 263.3 262.8 262.3
2.64 2.60 2.64 2.63
2.17 2.27 2.44 2.52
1.11 1.07 1.07 1.08
3.46 3.62 3.88 4.02
1.85 2.27 1.76 1.86
2.06 2.27 1.90 2.17 av 2.10 * 0.19
263.8 263.3 262.8 262.3 262.3 263.3
2.50* 2.60* 2.68* 2.81* 2.66* 2.96*
2.77 2.91 2.38 2.01 2.03 2.11
1.54 1.47 1.53 1.53 1.56 1.57
3.98 4.17 3.43 2.91 2.95 3.07
2.47 2.54 1.98 1.96 1.83 2.39
2.29 2.54 2.14 2.29 2.14 2.39 av 2.29
T, K 262.8 262.3 263.3 263.3
[MI ," [ CH,O,NO, cm-' cm-3 0.20 1.32 0.20 1.73 0.20 1.65 0.19 1.61
262.3 262.8 262.8
0.38 0.38 0.37
2.35 1.87 2.38
261.8 263.3 262.8 262.8
0.75 0.74 0.75 0.74
263.3 263.3 263.3 264.3 265.3
-
[NO], cm-, 1.32 1.20 1.20 1.08
normalizedC 0.66 0.84 0.70 0.85 av 0.76i: 0.10
-
Mostly methane, except when marked (asterisk), where [MI is mostly N,. [Cl,] = (1.8 f 0.3) X initial concentration of CH,O,NO, based on NO, concentration originally used. 21300 cal K-' mol-'. ized to 263.3 K on the basis E, Taken from Table I. a
* [CH,O,NO,],
i:
lo''
~ m - ~ . k-,{263.3 K} normal-
TABLE 111: Data for the Evaluation of K,
10-l9 [ M ] )(I 101'k3, cm-) cm3 s-' 0.20 1.93 0.38 2.70 0.75 3.71 1.23 4.51 2.64 5.93
" [MI
0
0
W
08
1
I
12
I6
20
24
28
32
1 6 ' ' [MI , cm-'
Flgure 4. Plot of k , vs. [MI at 263.3 K (data points). The solid line represents the values for k , computed from eq 11.
activation energy is an estimate based on the scatter of points in Figure 3. A plot of k3vs. [MI at 263.3 K is shown in Figure 4. The points, along with their error bars, are the experimental values of k3; the solid line gives the pressure dependence of reaction 3 calculated from eq 11. Figure 4 shows that the pressure falloff of k, is consistent with that of k3 as required by the principle of microscopic reversibility. With the experimentally determined k3and the appropriate computed k,, the equilibrium constant, K3,-,(263.3K},is calculated to be (2.68 f 0.26) X cm3.
-
0.20
- %
at 263.3 K
102k-3 , S-
0.76 1.15 1.31 1.57 2.10
1010K3.-,,cm3 2.54 2.35 2.83 2.87 2.82 av 2.68
f
0.26
mostly CH,.
Individual values for K3,&63.3 K} at various pressures are listed in Table 111. The pressure dependence work was done only at one temperature; it would have been interesting to measure the Arrhenius parameters for the reverse reaction at high and low pressures at all temperatures. However, this was not done because the precision of the measurements was probably not sufficient to detect with certainty a =1 kcal/mol change in the activation energy. Instead the high- and low-pressure Arrhenius parameters for the reverse reaction were computed from the pressure dependence of the forward reaction.7p0 Discussion The Arrhenius parameters determined in this study have considerable uncertainty because of the small temperature range employed. However, the Arrhenius parameters may (8)A. R.Ravishanha, F. L.Eisele, and P.H.Wine, J. Chem. Phys., 73,3743 (1980).
The Journal of Physical Chemistty, Vol. 86, No. 10, 1982 1853
Thermal DecomposRlon of CH302N02
TABLE V: Computed Values of Ao-, and Eo-,
TABLE IV: Computed Values of A"-, and E"-,
m n 6 4
m
4
2
0
n
4
2.2 X 10l6 21700 1.3 X lot6 21400
3 . 1 X loi6 21900 2.1 X 10l6 21700
3.4 X 10l6 22000 3.1 X loi6 21900
6
1.3 X loi6 2 . 2 X 10l6 6.8 X lo" 21000 21400 21700 A"-, in units of s-', E S 3 in units of cal mol-'.
be better defined by considering data for the forward reaction and the expected entropy change for reaction 3. Experimentally the entropy and enthalpy changes (AS, and AH,) for reaction 3 may be obtained from the Van't Hoff equation In K = AS,/R - AH,/RT (111) since the equilibrium constant K3,-, = k3/k-, may be computed from the data for the forward reaction and the present data for the reverse reaction. Unfortunately, this method leads to imprecise values of AHrand AS,, because of the small temperature range employed in this study. A better approach is to estimate the best value of AS, and with the excellent data for the forward reacti~n&~a compute the preexponential factor A-,. Once A-, is determined in this way, E-3 can be computed from the measured rate constants for the reverse reaction. AS', for reaction 3 must be nearly the same as for reaction 9 for which good thermodynamic properties are HO2 + NO2 (+MI F? H02N02 (+MI (9, -9) available. For reaction 9, AS, = 40.5 eu for ASP298{H02} = 54.73 eu? hS,0,(N02J = 57.34 eu,'O and ASPm(H02N02} = 71.6 f 1 eu." The value for H02NOzis an estimated value. A low value of 69 eu is inconsistent with the pressure dependence of reaction 9, whereas a higher value is considered unlikely.ll A_, may be computed from In (A3/A-,)= AS',/R by using A3 obtained from the data for the forward reaction and AS,. Values of A, under our conditions were computed by using the Troe interpolation formulas given before., Most of our measurements were done in CHI as the diluent, whereas the forward reaction was studied with SF6,N,, and Ar as d i l ~ e n t s . ~In? computing ~ k3 under our conditions no efficiency corrections were made, since we have shown that the efficiencies of CHI and N2 are similar. Using AS,{265 KJ = 40 f 1eu and A3 = 5.73 X lo-', cm3 we obtained A-3 = (8.9 f 3) X 10l5 s-l. With k-, obtained from the least-squares line of Figure 3 and A-, as determined above, E-, = 21300 f 300 cal/mol is obtained. These Arrhenius parameters are in excellent agreement with those determined from the least-squares Arrhenius plot, but the uncertainty in A , and E-3 has been significantly reduced. Values of A-3 and E-3 at the high- and low-pressure limits were computed from the pressure dependence of the forward reaction as computed from eq I1 for all possible combinations of the matrix formed by m = 4,2, and 0 and n = 6, 4 , and 2. The results for k-3mare shown in Table IV. For the recommended values of m = 2 and n = 4, k," = 2.1 X 10l6 exp((-21700 f 300)lRT) s-l. The values of A" for reactions -3 and -9 should be nearly the same and ~
~~~
0
3.3 X 20150
8.2 X lo-' 19300
1.7 X lo-' 18400
1.1 x 10-3 20900
3.3 x 10-4 20150
8.2 x 10-5 19300 2 2.9 x 10-3 1.1 x 104 3.3 x 10-4 21500 20900 20150 a A'-, in units of cm3 s-'; Eo-, in units of cal/mol. 4
2
~
2
~~
(9)C.J. Howard, J. Am. Chem. SOC.,102,6937(1980). (10)JANAF Thermochemical Tables, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No.37 (1971). (11)A. C.Baldwin and D. M. Golden, J. Phys. Chem., 82,644(1978).
TABLE VI: Thermal Lifetimes of CH,O,NO, altitude, km
T,K
press., torr
0 15 25
291 211 227
760 85 20
A?-;' Is 4 2 days 13 days
this is in fact the case, since it has been estimated that A+" = 2.5 X 10l6 A glance at Table IV shows that the set m = 2 and n = 4 does not give a unique value of k-,"; the values along the diagonal of the matrix are the same. The table also shows that the computed high-pressure A factor is insensitive to m and n values and that all values in the upper right of the matrix are close to the expected value of 2.5 X 10l6s-l. The combinations in the lower left, however, give factors which are probably too low. The corresponding calculations for k-2 are given in Table V. For the recommended values of m = 2 and n = 4, k-2 = 3.3 X lo4 exp{(-20150 f 350)/Rg cm3 s-l. Again unique values for kO , are not obtained for a given value of m and n. The thermal lifetimes, k-3-1, of CH302N02for several altitudes are presented in Table VI. These range from 1 s at the surface of the earth to 42 days at 15 km. The actual atmospheric lifetimes are also influenced by photodissociation. The photodissociation rate coefficient of CH302N02,J, is not known with certainty, since the absorption spectrum has been measured for wavelengths only below 290 nm,' but it is very likely that in the critical 300-340-nm range it is very close to that for HOzNOzfor which J = 1 X s-l (J-l 1day) in the altitude range of 0-25 km.12 Thus, in the lower stratosphere the lifetime of CH302N02is determined primarily by photolysis. Unlike H02N02,CH3O2NO2is not expected to react rapidly with OH radicals. Therefore, in the lower stratosphere the photostationary state concentration of CH30 2 N 0 2is given by [CH302N021 kio[CH.d [OH1(k3[N021/Jk4[NOl) (IV) where klo is the rate coefficient for the reaction OH + CH4 H20 CH, (10) With approximate daytime constituent concentrations at the tropopause, which are needed to compute [CH302N02] from eq IV, of [CH,] = 7 X 10l2cm-,, [OH] (2-3) X 106 ~ m -and ~ , [NO]/[NO,] 1.5, [CH3O2NO2] (5-10) X 108 is obtained. This value is of the same order of magnitude as the daytime [NO,] implying that CH302N02 may play a significant role in the NO, budget of the lower stratosphere. Acknowledgment. This work was supported by the National Science Foundation under Grant No. ATM7909169 and the National Aeronautics and Space Administration under Grant No. NGL 39-009-003for which we are grateful.
-
4
+
-
(12)L. T.Molina and M. J. Molina, J. Photochem., 15, 97 (1981).
