Abstract
Abstract. Halocarbons are produced naturally in the oceans by biological and chemical processes. They are emitted from surface seawater into the atmosphere, where they take part in numerous chemical processes such as ozone destruction and the oxidation of mercury and dimethyl sulfide. Here we present oceanic and atmospheric halocarbon data for the Peruvian upwelling zone obtained during the M91 cruise onboard the research vessel METEOR in December 2012. Surface waters during the cruise were characterized by moderate concentrations of bromoform (CHBr3) and dibromomethane (CH2Br2) correlating with diatom biomass derived from marker pigment concentrations, which suggests this phytoplankton group is a likely source. Concentrations measured for the iodinated compounds methyl iodide (CH3I) of up to 35.4 pmol L−1, chloroiodomethane (CH2ClI) of up to 58.1 pmol L−1 and diiodomethane (CH2I2) of up to 32.4 pmol L−1 in water samples were much higher than previously reported for the tropical Atlantic upwelling systems. Iodocarbons also correlated with the diatom biomass and even more significantly with dissolved organic matter (DOM) components measured in the surface water. Our results suggest a biological source of these compounds as a significant driving factor for the observed large iodocarbon concentrations. Elevated atmospheric mixing ratios of CH3I (up to 3.2 ppt), CH2ClI (up to 2.5 ppt) and CH2I2 (3.3 ppt) above the upwelling were correlated with seawater concentrations and high sea-to-air fluxes. During the first part of the cruise, the enhanced iodocarbon production in the Peruvian upwelling contributed significantly to tropospheric iodine levels, while this contribution was considerably smaller during the second part.
Highlights
Brominated and iodinated short-lived halocarbons from the oceans contribute to tropospheric and stratospheric chemistry (von Glasow et al, 2004; Saiz-Lopez et al, 2012b; Carpenter and Reimann, 2014)
While we found the Peruvian upwelling and the adjacent waters to be only a moderate source region for bromocarbons, iodocarbons were observed in high concentrations of 10.9 (0.4–58.1), 9.8 (1.1–35.4) and 7.7 (0.2–32.4) pmol L−1 for CH2ClI, CH3I and CH2I2, respectively (Table 1, Fig. 3a)
CHBr3 and CH2Br2 were significantly correlated with Total chlorophyll a (TChl a) and diatoms, suggesting biological formation of these compounds
Summary
Brominated and iodinated short-lived halocarbons from the oceans contribute to tropospheric and stratospheric chemistry (von Glasow et al, 2004; Saiz-Lopez et al, 2012b; Carpenter and Reimann, 2014) They are significant carriers of iodine and bromine into the marine atmospheric boundary layer (Salawitch, 2006; Jones et al, 2010; Yokouchi et al, 2011; Saiz-Lopez et al, 2012b), where they and their degradation products may be involved in aerosol and ultra-fine particle formation (O’Dowd et al, 2002; Burkholder et al, 2004). Sea-to-air fluxes of these halogenated compounds were derived and their contribution to the tropospheric iodine loading above the tropical eastern Pacific were estimated by combining halocarbon, IO measurements and model calculations
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