Drivers of diel and regional variations of halocarbon emissions from the tropical North East Atlantic

Abstract

Methyl iodide (CH₃ I), bromoform (CHBr₃ ) and dibromomethane (CH₂ Br₂ ), which are produced naturally in the oceans, take part in ozone chemistry both in the troposphere and the stratosphere. The significance of oceanic upwelling regions for emissions of these trace gases in the global context is still uncertain although they have been identified as important source regions. To better quantify the role of upwelling areas in current and future climate, this paper analyzes major factors that influenced halocarbon emissions from the tropical North East Atlantic including the Mauritanian upwelling during the DRIVE expedition. Diel and regional variability of oceanic and atmospheric CH₃ I, CHBr₃ and CH₂ Br₂ was determined along with biological and physical parameters at six 24 h-stations. Low oceanic concentrations of CH₃ I from 0.1-5.4 pmol L^-1 were equally distributed throughout the investigation area. CHBr₃ and CH₂ Br₂ from 1.0 to 42.4 pmol L^-1 and to 9.4 pmol L^-1 , respectively were measured with maximum concentrations close to the Mauritanian coast. Atmospheric CH₃ I, CHBr₃ , and CH₂ Br₂ of up to 3.3, 8.9, and 3.1 ppt, respectively were detected above the upwelling, as well as up to 1.8, 12.8, and 2.2 ppt at the Cape Verdean coast. While diel variability in CH₃ I emissions could be mainly ascribed to oceanic non-biological production, no main driver was identified for its emissions over the entire study region. In contrast, biological parameters showed the greatest influence on the regional distribution of sea-to-air fluxes of bromocarbons. The diel impact of wind speed on bromocarbon emissions increased with decreasing distance to the coast. The height of the marine atmospheric boundary layer (MABL) influenced halocarbon emissions via its influence on atmospheric mixing ratios. Oceanic and atmospheric halocarbons correlated well in the study region, and in combination with high oceanic CH₃ I, CHBr₃ and CH₂ Br₂ concentrations, local hot spots of atmospheric halocarbons could solely be explained by marine sources. This conclusion is in contrast to previous studies that hypothesized elevated atmospheric halocarbons above the eastern tropical Atlantic to be mainly originated from the West-African continent.

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