Finding the missing stratospheric Br_y: a global modeling study of CHBr₃ and CH₂Br₂

Abstract. Recent in situ and satellite measurements suggest a contribution of ~5 pptv to stratospheric inorganic bromine from short-lived bromocarbons. We conduct a modeling study of the two most important short-lived bromocarbons, bromoform (CHBr₃) and dibromomethane (CH₂Br₂), with the Goddard Earth Observing System Chemistry Climate Model (GEOS CCM) to account for this missing stratospheric bromine. We derive a "top-down" emission estimate of CHBr₃ and CH₂Br₂ using airborne measurements in the Pacific and North American troposphere and lower stratosphere obtained during previous NASA aircraft campaigns. Our emission estimate suggests that to reproduce the observed concentrations in the free troposphere, a global oceanic emission of 425 Gg Br yr⁻¹ for CHBr₃ and 57 Gg Br yr⁻¹ for CH₂Br₂ is needed, with 60% of emissions from open ocean and 40% from coastal regions. Although our simple emission scheme assumes no seasonal variations, the model reproduces the observed seasonal variations of the short-lived bromocarbons with high concentrations in winter and low concentrations in summer. This indicates that the seasonality of short-lived bromocarbons is largely due to seasonality in their chemical loss and transport. The inclusion of CHBr₃ and CH₂Br₂ contributes ~5 pptv bromine throughout the stratosphere. Both the source gases and inorganic bromine produced from source gas degradation (Br_y^VSLS) in the troposphere are transported into the stratosphere, and are equally important. Inorganic bromine accounts for half (2.5 pptv) of the bromine from the inclusion of CHBr₃ and CH₂Br₂ near the tropical tropopause and its contribution rapidly increases to ~100% as altitude increases. More than 85% of the wet scavenging of Br_y^VSLS occurs in large-scale precipitation below 500 hPa. Our sensitivity study with wet scavenging in convective updrafts switched off suggests that Br_y^VSLS in the stratosphere is not sensitive to convection. Convective scavenging only accounts for ~0.2 pptv (4%) difference in inorganic bromine delivered to the stratosphere.