Abstract

Halogens play a major role in catalytic cycles leading to the destruction of ozone not only in the stratosphere but also in the troposphere and marine environments. In low NO_x conditions, indicative of that in the marine boundary layer, inclusion of halogen oxides in the methane oxidation cycle not only competes with ozone-producing NO_x catalysis of peroxy radicals, but the products formed from the reaction of XO with RO₂ deplete ozone further.

Limited data are available for peroxy radicals reacting with halogen oxides. Temperature dependent kinetics and product studies were carried out for the reaction of ClO and BrO with CH₃O₂ using a turbulent flow tube coupled to a chemical ionisation mass spectrometer (CIMS).

An Arrhenius expression was obtained for the overall rate coefficient of CH₃O₂ + ClO reaction:

k(T) = 1.96 (+0.28 −0.24) x 10^-11 exp[(-626 ± 35) / T] cm³ molecule^-1 s^-1. Over a range of pressure (100– 200 Torr) and temperature (298–223 K) no pressure dependence is observed. The smaller rate coefficients measured at lower temperatures compared with both previous low temperature studies are believed to arise through the reduction of secondary chemistry and greater sensitivity in terms of reactant detection (hence much lower initial concentrations were employed). These new data reduce the effectiveness of ozone loss cycles involving reaction of CH₃O₂ + ClO in the polar stratosphere by around a factor of 1.5 and restrict the importance of the reaction to the tropical and extra-tropical clean marine environments in the troposphere.

For the reaction of BrO + CH3O2 an Arrhenius expression was determined to be k(T) = 2 .42 (+1.02 −0.72)x10 ^-14 exp[(1617 ± 94) / T] cm³ molecule^-1 s^-1 over a temperature range of 243 – 296 K. No pressure dependence was observed for this reaction. A more pronounced apparent negative activation energy, double that retrieved in a previous study, was observed. HOBr was found to be the major product with a yield of 0.8 ± 0.1 at 296 K. HOO Br was found to be a minor product with an upper limit of 0.1. According to modelling studies, the increased apparent negative activation energy would lead to a significant source of HOx and a non-negligible source of HC(O)OH in the upper troposphere.