Halogen‐catalyzed methane oxidation

Abstract

This paper highlights the importance of halogen‐catalyzed methane oxidation in the upper troposphere and lower stratosphere. The calculated rate of methane oxidation is increased by at least 20% in the upper troposphere when halogen catalysis is included. In the lower stratosphere, approximately 25% of methane oxidation can be initiated by chlorine; the precise fraction is very temperature dependent. Including halogen‐catalyzed methane oxidation increases the HOx and ClOx concentrations and decreases the NOx concentration. The calculated enhancement in the HOx concentration due to halogen‐catalyzed methane oxidation is around 10–15% in the lower stratosphere; and around 20% in the upper troposphere. The decrease in the NOx concentration is around 10% in the upper 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 O3 loss. The additional O3 loss calculated is most significant in the upper troposphere where over a 7‐day 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 production by NO + HO2 will increase, so there will be an accelerated ozone recovery. On a per molecule basis, bromine‐catalyzed methane oxidation is approximately 2 orders of magnitude faster than chlorine catalyzed methane oxidation. In the upper troposphere bromine‐catalyzed methane oxidation destroys ozone at a rate which is approximately one third of that at which nitrogen‐catalyzed methane oxidation is producing ozone. Therefore, with the increasing atmospheric bromine loading, bromine‐catalyzed methane oxidation is set to become more important. It would be valuable to have kinetic studies of the reaction BrO with CH3O2 so that the role of bromine‐catalyzed methane oxidation can be quantified more precisely.