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Bromine-containing organic compounds (bromocarbons) are ubiquitous in the oceans, and they are mainly formed by macro- and microalgae1. Of these naturally produced substances, bromoform (CHBr3) is the best known, since it is the most abundant brominated organic. It is formed in enzymatic processes developed to protect algal cells of reactive oxygen species, such as hydrogen peroxide, formed during photosynthesis1,2. In addition, several other bromocarbons are formed, such as dibromomethane (CH2Br2), dibromochloromethane (CHBr2Cl), and bromodichloromethane (CHBrCl2), either directly through the same enzymatic pathway, or by nucleophilic substitution of CHBr3. In the atmosphere, bromocarbons are photochemically degraded to reactive bromine which initiate the depletion of ozone and mercury, a contaminant of global concern3,4,5. BrO, formed by the reaction of atmospheric Br atoms with O3, has an impact on the sulfur cycle through the oxidation of dimethyl sulfide (DMS), leading to a 18% reduction in the DMS burden and lifetime6. The Br atom is the dominant oxidizer in mercury depletion events in the polar atmosphere7. Reactive bromine from the decomposition of bromocarbons also contributes to the depletion of ozone in the lower stratosphere8. The interplay between the oceanic sources of bromocarbons and their atmospheric impacts has led to the suggestion that these compounds are a link between climate change and atmospheric ozone9.
In this work, we report profile measurements of bromocarbons in snow, sea ice, and air during Antarctic winter that represent a new source of atmospheric bromine during the polar night. We combine the observations with a state-of-the-art global chemistry-climate model. The results provide strong evidence that Antarctic winter sea ice is a significant source of bromocarbons, which spread and contribute to the burden of atmospheric bromine throughout the southern hemisphere.
Subsequently, a number of different organic compounds such as ketones, phenols, alkenes etc., can react with HOBr to form bromocarbons28. Therefore, the differences in organo-bromine concentrations found in our ice cores can be due to differences in composition of the organic compounds being brominated. The similarities in depth profiles within the sea ice between the individual bromocarbon indicate similar formation mechanisms for all four investigated bromocarbons.
Micro-organisms could also be responsible for the bromocarbon production, as they have been shown to be active in sea ice during winter29,30. For instance, if the enzymatic pathways are active, the stress caused by the measured high salinity at the ice interface, low temperatures, and/or inclusion in the ice lattice could increase the production, and contribute to the high concentrations of bromocarbons found at the snow/ice interface. Hence, although little is known about how bromocarbons form under darkness; our results provide a strong case, for the first time, that first year Antarctic sea ice has extremely high bromocarbon concentrations at a period with no sunlight.
Modeled distribution of monthly averaged tropospheric (CHBr3, CH2Br2, CHBr2Cl, and CHBrCl2) over the Southern Hemisphere. This spatial distribution is the result of sea ice emissions of bromocarbons during the Antarctic winter using the averaged fluxes of bromocarbons from Table 1 (flux in stations with snow)
In summary, this study underscores that a large source of bromine from winter sea ice is currently neglected in climate models. The relatively long atmospheric lifetime of Antarctic winter sea ice-emitted bromocarbons allows their transport to lower latitudes and even to the stratosphere, adding to the global atmospheric bromine burden. Consideration of this new source of polar bromine requires re-assessment of the bromine-mediated impacts on the global tropospheric ozone budget and mercury deposition.
For presentation of the vertical distribution of halocarbons in sea ice, halocarbon concentrations in bulk ice were divided by the brine volume calculated for each sample. This brine normalized concentration represents the estimated concentration of halocarbons in the sea ice brine.
K.A., A.G., and M.A. conceived the experiments, participated in the cruise, and sampled and interpreted all halocarbon data. C.A.C. and A.S.-L. performed the atmospheric modeling. All authors interpreted the results and contributed to writing the manuscript.
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