The rate coefficients for the reactions of Cl atoms with CH3Br, (k1) a
nd CH2Br2, (k2) were measured as functions of temperature by generatin
g Cl atoms via 308 nm laser photolysis Of Cl2 and measuring their temp
oral profiles via resonance fluorescence detection. The measured rate
coefficients were: k1 = (1.55 +/-0.18) X 10(-11) exp{(-1070 +/- 50)/T}
and k2 = (6.37 +/- 0.55) X 10(-12) exp{(-810 +/- 50)/T} cm3 molecule-
1 s-1. The possible interference of the reaction of CH2Br product with
Cl2 in the measurement of kl was assessed from the temporal profiles
of Cl at high concentrations of Cl2 at 298 K. The rate coefficient at
298 K for the CH2Br + Cl2 reaction was derived to be (5.36 +/- 0.56) X
10(-13) cm3 molecule-1 s-1. Based on the values of k1 and k2, it is d
educed that global atmospheric lifetimes for CH3Br and CH2Br2 are unli
kely to be affected by loss via reaction with Cl atoms. In the marine
boundary layer, the loss via reaction (1) may be significant if the Cl
concentrations are high. If found to be true, the contribution from o
ceans to the overall CH3Br budget may be less than what is currently a
ssumed. (C) 1994 John Wiley & Sons, Inc.