Intercomparison between Surrogate, Explicit and Full Treatments of VSL Bromine Chemistry within the CAM-Chem Chemistry-Climate Model

Many Chemistry Climate Models (CCMs) include a simplified treatment of brominated very short-lived (VSLBr) species by assuming long-lived methyl bromide (CH3Br) as a surrogate for VSLBr. However, given that VSLBr (i.e., bromoform CHBr3 and dibromomethane CH2Br2) decompose more rapidly than CH3Br, their impact on upper tropospheric chemistry and lowermost stratospheric ozone cannot be neglected. Thus, a mistreatment of VSLBr in CCMs may yield an unrealistic representation of their associated impacts. Here, we present a comprehensive intercomparison between various VSLBr chemical approaches with increasing degrees of complexity (i.e., surrogate, explicit, and full), and quantify the global impacts of these natural bromocarbons on tropospheric and stratospheric ozone, as well as on other oxidizing agents.  Differences between chemical schemes maximize in the lowermost stratosphere and mid-latitude free troposphere, resulting in a latitudinally dependent reduction of ~1−7 DU in total ozone column and a ~5−15 % decrease of the OH/HO2 ratio, for full compared to surrogate. These bromine-driven changes in HOx abundances are expected to slow-down the oxidative processing of greenhouse gases (i.e., to increase the CH4 lifetime) in a region where these long-lived species have a final chance to undergo tropospheric degradation before injection to the stratosphere. Given the negligible additional computational cost and chemical complexity, we encourage all CCMs oriented to projecting the coupled evolution of stratospheric ozone within a changing climate to include a complete tropospheric representation of VSLBr sources and chemistry in the troposphere and stratosphere.