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Publication - Professor Simon O'Doherty

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

    Citation

    Kuyper, B, Lesch, T, Labuschagne, C, Martin, D, Young, D, Khan, MAH, Williams, AG, O'Doherty, S, Davies-Coleman, MT & Shallcross, DE, 2019, ‘Volatile halocarbon measurements in the marine boundary layer at Cape Point, South Africa’. Atmospheric Environment, vol 214.

    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 (CH3I), bromoform (CHBr3), dibromomethane (CH2Br2) and methyl bromide (CH3Br) 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 (222Rn) 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 CH3I, CHBr3, CH2Br2, and CH3Br, respectively. A statistically significant difference in short-lived halocarbon occurrence was established between anthropogenically affected and clean marine air masses for CH3Br at the Cape Point site. The CHBr3 and CH2Br2 mixing ratios were not statistically different (p > 0.05) when comparing the anthropogenically affected and clean marine air masses. The mixing ratios of CH3I 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 CH2Br2 and CHBr3 measurements at Cape Point. An analysis of the ln([CH2Br2]/[CHBr3]):ln([CHBr3]) ratio for the Cape Point dataset exhibited a slight deflection to the right of the general chemical decay line, suggesting the influence of greater [CHBr3]:[CH2Br2] source ratios rather than their dilution.

    Full details in the University publications repository