Abstract
Abstract. Methyl iodide (CH3I), bromoform (CHBr3) and dibromomethane (CH2Br2), 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 CH3I, CHBr3 and CH2Br2 was determined along with biological and physical parameters at six 24 h-stations. Low oceanic concentrations of CH3I from 0.1–5.4 pmol L−1 were equally distributed throughout the investigation area. CHBr3 and CH2Br2 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 CH3I, CHBr3, and CH2Br2 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 CH3I 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 CH3I, CHBr3 and CH2Br2 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.
Highlights
IntroductionVolatile halogenated hydrocarbons (halocarbons) occur naturally in the oceans from where they are emitted into the atmosphere
Volatile halogenated hydrocarbons occur naturally in the oceans from where they are emitted into the atmosphere
During DRIVE, oceanic CH3I neither showed a relationship to phytoplankton pigments nor to cyanobacteria, and its distribution appeared mainly as a result of abiotic or indirect biological formation which seemed to be the main driver of the CH3I concentration gradient between sea water and air
Summary
Volatile halogenated hydrocarbons (halocarbons) occur naturally in the oceans from where they are emitted into the atmosphere. Bromine and iodine atoms released from these compounds by photolysis and oxidation can take part in catalytic ozone destroying cycles in both the troposphere and stratosphere (McGivern et al, 2000; Salawitch et al, 2005; Montzka and Reimann, 2011) with iodine participating in aerosol formation (O’Dowd et al, 2002). Halocarbons comprise brominated and iodinated methanes such as bromoform (CHBr3) and dibromomethane (CH2Br2), methyl iodide (CH3I) and diiodomethane, as well as longer chained and mixed halogenated compounds such as iodoethane, chloroiodomethane, and dibromochloromethane. H. Hepach et al.: Drivers of diel and regional variations of halocarbon emissions
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