Volatile halocarbon measurements in the marine boundary layer at Cape Point, South Africa

Abstract Volatile organic halocarbons play a significant role in the biogeochemical cycling of halogens in the atmosphere. High frequency, in situ measurements of methyl iodide (CH 3 I), bromoform (CHBr 3 ), dibromomethane (CH 2 Br 2 ) and methyl bromide (CH 3 Br) were measured in the marine boundary layer at Cape Point, South Africa by an automated Absorption Desorption System-Gas Chromatography-Mass Spectrometer (ADS-GC-MS) between January and November 2017. These are the first multi-halocarbon measurements recorded at Cape Point and represent the longest time series to date. Baseline wind conditions were established using both wind direction and radon ( 222 Rn) concentration as markers of clean air. The baseline mixing ratios observed were 0.61 ± 0.29, 2.02 ± 0.89, 1.08 ± 0.17 and 6.09 ± 0.50 ppt, for CH 3 I, CHBr 3 , CH 2 Br 2 , and CH 3 Br, respectively. A statistically significant difference in short-lived halocarbon occurrence was established between anthropogenically affected and clean marine air masses for CH 3 Br at the Cape Point site. The CHBr 3 and CH 2 Br 2 mixing ratios were not statistically different (p > 0.05) when comparing the anthropogenically affected and clean marine air masses. The mixing ratios of CH 3 I suggested a strong seasonal variation with higher production in austral Summer-Autumn months than in the austral Winter-Spring months. A general chemical decay line was calculated for baseline CH 2 Br 2 and CHBr 3 measurements at Cape Point. An analysis of the ln([CH 2 Br 2 ]/[CHBr 3 ]):ln([CHBr 3 ]) ratio for the Cape Point dataset exhibited a slight deflection to the right of the general chemical decay line, suggesting the influence of greater [CHBr 3 ]:[CH 2 Br 2 ] source ratios rather than their dilution.