Oceanic distributions and emissions of short-lived halocarbons

Abstract

Using data from seven cruises over a 10-year span, we report marine boundary layer mixing ratios (i.e., dry mole fractions as pmol mol-1 or ppt), degrees of surface seawater saturation, and air-sea fluxes of three short-lived halocarbons that are significant in tropospheric and potentially stratospheric chemistry. CHBr3, CH2Br2, and CH3I were all highly supersaturated almost everywhere, all the time. Highest saturations of the two polybrominated gases were observed in coastal waters and areas of upwelling, such as those near the equator and along ocean fronts. CH3I distributions reflected the different chemistry and cycling of this gas in both the water and the atmosphere. Seasonal variations in fluxes were apparent where cruises overlapped and were consistent among oceans. Undersaturations of these gases were noted at some locations in the Southern Ocean, owing to mixing of surface and subsurface waters, not necessarily biological or chemical sinks. The Pacific Ocean appears to be a much stronger source of CHBr3 to the marine boundary layer than the Atlantic. The high supersaturations, fluxes, and marine boundary layer mixing ratios in the tropics are consistent with the suggestion that tropical convection could deliver some portion of these gases and their breakdown products to the upper troposphere and lower stratosphere. Copyright 2007 by the American Geophysical Union.