Impact of Very Short-Lived Substances on Stratospheric Bromine Loading

Abstract

Recent studies have shown the importance of very short-lived substances (VSLS) for the abundance of stratospheric bromine. In this work, the transport of bromine VSLS into the stratosphere is investigated with a three-dimensional chemistry transport model. The novelty of this approach is the explicit treatment of convective transport in a purely isentropic model, a key prerequisite for the realistic reproduction of the complex interplay of horizontal advection, local deep convection and large-scale diabatic heating in the tropical tropopause layer. Comparisons with observations show that the model is generally able to produce realistic distributions of the two major bromine VSLS, bromoform (CHBr3) and dibromomethane (CH2Br2). In addition, an analysis of the regional transport efficiency suggests that the Western Pacific is the most important source area for VSLS into the stratosphere; approximately 50% of the total amount of bromine VSLS in the TTL is contributed by this region. Another important question is how dehydration in the tropical tropopause impacts on stratospheric bromine loading. An idealized modeling approach assuming total solubility for inorganic bromine predicts that about 60% of bromine originated from VSLS is able to reach the stratosphere, which is consistent with earlier modeling approaches that use a comparable simple dehydration mechanism. However, when applying a more complete chemistry scheme the model results show that virtually the entire amount of bromine contributed by VSLS enters the stratosphere, rendering the impact of dehydration and scavenging on inorganic bromine insignificant in the TTL. This discrepancy is mainly caused by the low fraction of actually soluble inorganic bromine, the small available particle surface area density that restricts adsorption and finally heterogeneous reactions which are able to release adsorbed species into gas phase. Long-term calculations of VSLS injection into the stratosphere reveal a robust correlation between sea surface temperature, convective activity and the amount of short-lived source gases in the TTL, which becomes especially clear during the perturbations induced by El Nino seasons. Finally, the impact of additional bromine originated from VSLS on stratospheric ozone depletion is analyzed. The model predicts that for 5 parts per trillion by volume (pptv) of bromine contributed by VSLS on average about 1.3% of global total column ozone is destroyed.