Long-term halocarbon observations from a coastal and an inland site in Sabah, Malaysian Borneo

Abstract

Short-lived halocarbons are believed to have important sources in the tropics, where rapid vertical transport could provide a significant source to the stratosphere. In this study, quasi-continuous measurements of short-lived halocarbons are reported for two tropical sites in Sabah (Malaysian Borneo), one coastal and one inland (rainforest). We present the observations for C₂ Cl₄ , CHBr₃ , CH₂ Br₂^* (actually ~80% CH₂ Br₂ and ~20% CHBrCl₂ ) and CH₃ I from November 2008 to January 2010 made using our μDirac gas chromatographs with electron capture detection (GC-ECD). We focus on the first 15 months of observations, showing over one annual cycle for each compound and therefore adding significantly to the few limited-duration observational studies that have been conducted thus far in southeast Asia. The main feature in the C₂ Cl₄ behaviour at both sites is its annual cycle, with the winter months being influenced by northerly flow with higher concentrations, typical of the Northern Hemisphere, and with the summer months influenced by southerly flow and lower concentrations representative of the Southern Hemisphere. No such clear annual cycle is seen for CHBr₃ , CH₂ Br₂^* or CH₃ I. The baseline values for CHBr₃ and CH₂ Br₂^* are similar at the coastal (overall median: CHBr₃ 1.7 ppt, CH₂ Br₂^* 1.4 ppt) and inland sites (CHBr₃ 1.6 ppt, CH₂ Br₂^* 1.1 ppt), but periods with elevated values are seen at the coast (overall 95th percentile: CHBr₃ 4.4 ppt, CH₂ Br₂^ast 1.9 ppt), presumably resulting from the stronger influence of coastal emissions. Overall median bromine values from [CHBr₃ × 3] + [CH₂ Br₂^* × 2] are 8.0 ppt at the coast and 6.8 ppt inland. The median values reported here are largely consistent with other limited tropical data and imply that southeast Asia generally is not, as has been suggested, a hot spot for emissions of these compounds. These baseline values are consistent with the most recent emissions found for southeast Asia using the p-TOMCAT (Toulouse Off-line Model of Chemistry And Transport) model. CH₃ I, which is only observed at the coastal site, is the shortest-lived compound measured in this study, and the observed atmospheric variations reflect this, with high variability throughout the study period.

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