FTIR Study of CH3O2 + CH3O2 and CH3O2 + O3
J. Phys. Chem. A, Vol. 102, No. 15, 1998 2553
peroxy radical during the night with [CH3O2]/[HO2] > 60. The
nighttime overall peroxy radical decay could be described using
a simple model with two reactions: second-order loss of CH3O2
via self-reaction using the rate coefficient given in the 1994
the effects of reactions 7, 8, and 10 cannot simply be separated,
in contrast to the conclusions of Monks et al. The analysis
employed by Monks et al. is thus too simplistic, and their upper
limit for k10 should not be considered reliable.
-
6
NASA compilation and a small first-order loss of 3.3 × 10
For the nighttime conditions reported by Monks et al. of
-1
-13
s . Attributing the first-order loss solely to reaction with O3
[CH3O2] ) 1 ppt and [O3] ) 16 ppb with k7 ) 3.5 × 10 and
-
18
-17
3
-1 -1
(
present at 16 ppb) gives an upper limit of k10 < 8 × 10
k10 ) 1.0 × 10 cm molecule s , the instantaneous pseudo-
first-order loss rate of CH3O2 radicals via reaction with O3 is
approximately 20% of that of the CH3O2 self-reaction. Hence,
the reaction of CH3O2 radicals with O3 may play a minor role
in the nighttime decay of CH3O2 radicals in very clean air. Under
continental conditions such as those studied by Cantrell et al.
3
-1 -1
cm molecule s . As described in the previous section, the
rate coefficient in the 1994 NASA compilation is an overesti-
mate of the “true” bimolecular rate constant, k7, since it has
not been corrected for secondary loss of CH3O2 via reaction
with HO2 radicals.
4
In the atmosphere, the concentration of CH3O2 radicals is
low and the fate of the HO2 depends critically on the ratio O3:
CH3O2, which governs whether HO2 leads to regeneration of
CH3O2 (via reactions 6 and 25) or to radical loss by reaction 8.
in the southeast United States, the influence of terrestrial
nighttime NO sources is probably important and it is unlikely
that reaction 10 is of any significance.
Acknowledgment. The authors thank John Orlando and
Chris Cantrell of NCAR for their careful reading of the
manuscript. NCAR is sponsored by the National Science
Foundation. The work was partly supported by the Global
Tropospheric Chemistry Program.
CH O + CH O f CH O + CH O + O
2
(7a)
(9)
3
2
3
2
3
3
CH O + O f HCHO + HO
2
3
2
HO + O f OH + 2O
2
(6)
2
3
References and Notes
OH + CH f H O + CH
3
(25)
(14)
(8)
(1) Ridley, B. A.; Madronich, S.; Chatfield, R. B.; Walega, J. G.;
Shetter, R. E.; Carroll, M. A.; Montzka, D. D. J. Geophys. Res. 1992, 97,
4
2
1
0375.
CH + O + M f CH O + M
3
2
3
2
(2) Liu, S. C.; Trainer, M.; Carroll, M. A.; H u¨ bler, G.; Montzka, D.
D.; Norton, R. B.; Ridley, B. A.; Walega, J. G.; Atlas, E. A.; Heikes, B.
G.; Huebert, B. J.; Warren, W. J. Geophys. Res. 1992, 97, 10463.
CH O + HO f CH OOH + O
2
3
2
2
3
(
3) Hauglustaine, D. A.; Madronich, S.; Ridley, B. A.; Walega, J. G.;
Cantrell, C. A.; Shetter, R. E.; H u¨ bler, G. J. Geophys. Res. 1996, 101, 14681.
4) Cantrell, C. A.; Lind, J. A.; Shetter, R. E.; Calvert, J. G.; Goldan,
(
OH + CO (+ O ) f HO + CO
2
(26)
2
2
P. D.; Kuster, W.; Fehsenfeld, F. C.; Montzka, S. A.; Parrish, D. D.;
Williams, E. J.; Buhr, M. P.; Westberg, H. H.; Allwine, G.; Martin, R. J.
Geophys. Res. 1992, 97, 20671.
When O3 is high, reaction 7a does not lead to a net loss of
radicals; however, if the ozone mixing ratio is relatively low,
reaction 7a leads to the loss of two radicals. Thus there is no
a priori reason to expect the apparent rate coefficient for RO2
loss in the atmosphere to be k7a + k7b. To a good approximation,
the apparent second-order rate coefficient for loss of RO2 (the
sum of CH3O2 and HO2) should be equal to k7b + k8([HO2]/
(
5) Cantrell, C. A.; Shetter, R. E.; Gilpin, T. M.; Calvert, J. G. J.
Geophys. Res. 1996, 101, 14643.
6) Cantrell, C. A.; Shetter, R. E.; Calvert, J. G.; Eisele, F. L.; Williams,
(
E.; Baumann, K.; Brune, W. H.; Stevens, P. S.; Mather, J. H. J. Geophys.
Res. 1997, 102, 6369.
(
7) Monks, P. S.; Carpenter, L. J.; Penkett, S. J.; Ayers, G. P. Geophys.
Res. Lett. 1996, 23, 535.
8) Penkett, S. A.; Monks, P. S.; Carpenter, L. J.; Clemitshaw, K. C.;
Ayers, G. P.; Gillett, R. W.; Galbally, I. E.; Meyer, C. P. J. Geophys. Res.
997, 102, 12805.
9) Cantrell, C. A.; Shetter, R. E.; Calvert, J. G.; Eisele, F. L.; Tanner,
(
[
CH3O2]). A reaction between CH3O2 and O3 would also
1
convert CH3O2 to HO2 and would tend to increase the loss rate
of RO2 if the HO2 reacts with CH3O2 and not with O3. Thus,
even though HO2 is the minor radical at night, its rapid reaction
with CH3O2 can impact the loss of both species (and their sum,
which is measured by the chemical amplifier). During daytime
hours, higher levels of NO are present and higher HO2:CH3O2
(
D. J. J. Geophys. Res. 1997, 102, 15899.
(10) Sander, S. P.; Watson, R. T. J. Phys. Chem. 1981, 85, 2960.
(11) McAdam, K.; Veyret, B.; Lesclaux, R. Chem. Phys. Lett. 1987,
1
33, 39.
(
(
12) Kurylo, M. J.; Wallington, T. J. Chem. Phys. Lett. 1987, 138, 543.
13) Lightfoot, P. D.; Lesclaux, R.; Veyret, B. J. Phys. Chem. 1990,
ratios occur, so reaction 7 is not as important.
94, 700.
(14) Jenkin, M. E.; Cox, R. A.; Hayman, G. D.; Whyte, L. J. J. Chem.
Soc., Faraday Trans 2 1988, 84, 913.
15) Cox, R. A.; Tyndall, G. S. J. Chem. Soc., Faraday Trans. 2 1980,
76, 153.
(16) Simon, F. G.; Schneider, W.; Moortgat, G. K. Int. J. Chem. Kinet.
990, 22, 791.
17) Parkes, D. A. Proceedings of the 15th International Symposium
on Combustion, Tokyo, 1974; The Combustion Institute: Pittsburgh, PA,
1975; p 795.
(18) Weaver, J.; Meagher, J.; Shortridge, R.; Heicklen, J. J. Photochem.
1975, 4, 341.
A box model34 was used to simulate the effects of HO2
cycling and reaction 10 on nighttime radical concentrations. The
conditions were chosen to be similar to those encountered during
(
7
the measurements of Monks et al.: RO2 2 ppt, O3 16 ppb, and
1
zero NO. The decay of the total RO2 concentration was
simulated with k10 equal to the extremes of the reported values,
(
-
17
3
-1 -1
zero and 3 × 10
apparent rate coefficient for RO2 loss (3.2 × 10
molecule s ) is given by the empirical relationship katm )
.8k7a + k7b; i.e., most of the HO2 radicals react with O3 and
simply recycle RO2. The inclusion of reaction 10 with a rate
cm molecule s . In the first case, the
-
13
3
cm
-
1
-1
(
19) Kan, C. S.; Calvert, J. G.; Shaw, J. H. J. Phys. Chem. 1980, 84,
411.
(20) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Phys.
Chem. 1981, 85, 877.
21) Horie, O.; Crowley, J. N.; Moortgat, G. K. J. Phys. Chem. 1990,
0
3
-17
3
-1 -1
coefficient of 3 × 10 cm molecule
s
leads to an increase
(
of approximately 20% in the apparent rate coefficient for loss
of RO2. The increase expressed as a pseudo-first-order loss
9
4, 8198.
(22) Anastasi, C.; Couzens, P. J.; Waddington, D. J.; Brown, M. J.;
Smith, D. B. 10th International Symposium on Gas Kinetics, Swansea, U.K.,
-
6
-1
for RO2 is 3 × 10 s , which is very close to the “residual
loss” of RO2 radicals discussed by Monks et al. However, the
actual first-order rate coefficient for CH3O2 reacting with O3 is
.2 × 10 s , due to the recycling of HO2 radicals. Thus,
1
988.
(23) Simonaitis, R.; Heicklen, J. J. Phys. Chem. 1975, 79, 298.
24) Wallington, T. J.; Japar, S. M. J. Atmos. Chem. 1989, 10, 399.
(
-
5
-1
1
(25) Renaud, R.; Leitch, L. C. Can. J. Chem. 1954, 32, 545.