KINETICS OF THE REACTIONS OF OH WITH METHYL CHLOROFORM AND METHANE - IMPLICATIONS FOR GLOBAL TROPOSPHERIC OH AND THE METHANE BUDGET

The hydroxyl radical plays a central role in the oxidation of methane, larger organics including methyl chloroform, and CO in the troposphere. Hence a knowledge of the kinetics of such reactions is essential in estimating atmospheric lifetimes of these organics as well as global OH concentrations. We report here fast flow discharge studies of the absolute rate constants for both the OH-CH3CC13 and OH-CH4 reactions from 278-378 K in He at 1.0 +/- 0.05 Torr pressure. For CH3CCl3, k(CH3CCl3) = (9.1(-3.7)+4.6 X 10(-13))exp [-(1337 +/- 150)/T cm3 molecule-1 s-1 (2-sigma). The rate constants at 298 K and 277 K, 1.0 and 0.73 x 10(-14) cm3 molecule-1 s-1 respectively, are approximately 15% and 5% below the values now used to estimate global OH concentrations. The use of these lower rate constants in computations of global OH based on CH3CCl3 emissions and ambient measurements will increase the estimated OH concentrations, particularly at higher temperatures found in warmer equatorial regions. For CH4, k(CH4) = (4.0(-1.3)+1.6 x 10(-12))exp[-(1944 +/- 114)/T] cm3 molecule-1 s-1 (2-sigma). At 298 K and 277 K respectively, this gives values which are approximately 23% and 28% below the values which have generally been used in tropospheric models. Our results therefore support the recent suggestion that the methane flux may have been overestimated in the past.