Kinetics of the reactions of OH with methyl chloroform and methane: Implications for global tropospheric OH and the methane budget

Abstract

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‐CH 3 CCl 3 and OH‐CH 4 reactions from 278–378 K in He at 1.05±0.05 Torr pressure. For CH 3 CCl 3 , . The rate constants at 298 K and 277 K, 1.0 and 0.73×10 −14 cm 3 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 CH 3 CCl 3 emissions and ambient measurements will increase the estimated OH concentrations, particularly at higher temperatures found in warmer equatorial regions. For CH 4 , . At 298 K and 277 K respectively, this gives values which are ∼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.