HALOGEN-CATALYZED METHANE OXIDATION

This paper highlights the importance of halogen-catalyzed methane oxid ation in the upper troposphere and lower stratosphere. The calculated rate of methane oxidation is increased by at least 20% in the upper tr oposphere when halogen catalysis is included. In the lower stratospher e, approximately 25% of methane oxidation can be initiated by chlorine ; the precise fraction is very temperature dependent. Including haloge n-catalyzed methane oxidation increases the HOx and ClOx concentration s and decreases the NOx concentration. The calculated enhancement in t he HOx concentration due to halogen-catalyzed methane oxidation is aro und 10-15% in the lower stratosphere; and around 20% in the upper trop osphere. The decrease in the NOx concentration is around 10% in the up per troposphere. The enhancement in the ClOx concentration is around 7 -10% in the lower stratosphere. The increase in the calculated HOx and ClOx concentrations and the decrease in the NOx concentration lead to a enhancement in the calculated O-3 loss. The additional O-3 loss cal culated is most significant in the upper troposphere where over a 7-da y simulation it was of the order of 0.1-1% for midlatitudes at equinox . As the atmospheric loading of chlorine drops the gross odd-oxygen pr oduction by NO + HO2 will increase, so there will be an accelerated oz one recovery. On a per molecule basis, bromine-catalyzed methane oxida tion is approximately 2 orders of magnitude faster than chlorine catal yzed methane oxidation. In the upper troposphere bromine-catalyzed met hane oxidation destroys ozone at a rate which is approximately one: th ird of that at which nitrogen-catalyzed methane oxidation is producing ozone. Therefore, with the increasing atmospheric bromine loading, br omine-catalyzed methane oxidation is set to become more important. It would be valuable to have kinetic studies of the reaction BrO with CH3 O2 so that the role of bromine-catalyzed methane oxidation can be quan tified more precisely.