Atmospheric trends of the halon gases from polar firn air

Abstract. Four halons (H-1301, H-1211, H-2402 and H-1202) have been measured in air samples collected from polar firn from Dome Concordia (Dome C), Antarctica, from Devon Island, Canada and the North Greenland Ice Core Project (NGRIP) site, Greenland. H-2402 and H-1202 are reported for the first time in firn air. The depth profiles show the concentrations of all four halons to be zero at the base of the firn thus demonstrating their entirely anthropogenic origin. This is the first evidence of this for H-2402 and H-1202. A 2-D atmospheric model was run to produce historical trends in the atmospheric concentrations at the firn sites, which were then input into a firn diffusion model to produce concentration depth profiles for comparison with the measurements. The firn measurements provide constraints on the atmospheric concentrations in both hemispheres which allow the global emission rates and their latitudinal distribution in the atmospheric model to be evaluated. Global emission trends previously determined from measurements at Cape Grim are found to be consistent with the firn data. Further emissions of H-1202 in recent years (late 1980s onwards) are likely to have come from latitudes mostly south of 40° N, but emissions prior to that may have come from further north. Emissions of H-1211 may also have shifted to latitudes south of 40° N during the late 1980s. The firn data is used to derive atmospheric trends in total organic bromine in the form of halons for both polar regions.