Seasonal variation of tropospheric methyl bromide concentrations: Constraints on anthropogenic input

Although removal of tropospheric methyl bromide (CH 3 Br) is dominated by the reaction with the seasonally varying hydroxyl (HO) radical concentration, the anticipated corresponding seasonal dependence of CH 3 Br, as found for other gases with major HO sinks, has been sought previously without success [WMO, 1995]. Our observations of northern hemispheric boundary layer CH 3 Br concentrations do reveal substantial seasonal changes. The high latitude CH 3 Br North/South interhemispheric concentration ratio (IHR) varies from a maximum of 1.35±0.04 (1σ) in March–April to 1.10±0.04 in September, with an equal area and seasonally (EAS) weighted average IHR of 1.21±0.03. These observations suggest northern hemispheric emissions are about 15 kilotons/year less than when an IHR of 1.3 is considered [WMO, 1995]. The observed seasonality also partially explains the differences in the IHR reported by several research groups [WMO, 1995] and places needed constraints on the magnitude and seasonality of sources and sinks of CH 3 Br.