The contribution of natural and anthropogenic very short-lived species to stratospheric bromine

Abstract

We have used a global three-dimensional chemical transport model to quantify the impact of the very short-lived substances (VSLS) CHBr₃, CH₂Br₂, CHBr₂Cl, CHBrCl₂, CH₂BrCl and C₂H₅-Br on the bromine budget of the stratosphere. Atmospheric observations of these gases allow constraints on surface mixing ratios that, when incorporated into our model, contribute ~4.9–5.2 parts per trillion (ppt) of inorganic bromine (Br_y) to the stratosphere. Of this total, ~76 % comes from naturally-emitted CHBr₃ and CH₂Br₂. The remaining species individually contribute modest amounts. However, their accumulated total accounts for up to ~1.2 ppt of the supply and thus should not be ignored. We have compared modelled tropical profiles of a range of VSLS with observations from the recent 2009 NSF HIPPO-1 aircraft campaign. Modelled profiles agree reasonably well with observations from the surface to the lower tropical tropopause layer. <br><br> We have also considered the poorly studied anthropogenic VSLS, C₂H₅Br, CH₂BrCH₂Br, <i>n</i>-C₃H₇Br and <i>i</i>-C₃H₇Br. We find the local atmospheric lifetime of these species in the tropical tropopause layer are ~183, 603, 39 and 49 days, respectively. These species, particularly C₂H₅Br and CH₂BrCH₂Br, would thus be important carriers of bromine to the stratosphere if emissions were to increase substantially. Our model shows ~70–73 % and ~80–85 % of bromine from these species in the tropical boundary layer can reach the lower stratosphere.