On the relation between stratospheric chlorine/bromine loading and short-lived tropospheric source gases

Current methods for estimating the concentrations of inorganic chlorine/bromine species (Cl-y/Br-y) in the stratosphere due to decomposition of tropospheric source gases assume that the Cl-y/Br-y concentration in the stratosphere is determined mainly by the balance between production from in situ oxidation of the source gases in the stratosphere and removal by transport of Cl-y/Br-y out of the stratosphere. The rationale being that for source gases whose lifetimes are of the order of several months or longer the concentration of Cl-y/Br-y in the troposphere is small because they are produced at a relatively slow rate and also removed efficiently by washout processes. As a result of the small concentration, the rate at which Cl-y/Br-y is transported to the stratosphere is expected to be small compared to the in situ stratospheric production. Thus the transport of Cl-y/Br-y from the troposphere contributes little to the stratospheric concentration. In contrast, the origin of stratospheric Cl-y/Br-y from reactive source gases with tropospheric lifetimes comparable to the washout lifetime of Cl-y/Br (of the order of 10-30 days) in the troposphere is distinctly different. The in situ source in the stratosphere is expected to be significantly smaller because only a small portion of the source gas is expected to survive the troposphere to be transported into this region. At the same time these short-lived source gases produce appreciable amounts of Cl-y/Br-y in the troposphere such that transport to the stratosphere offers a larger source for stratospheric Cl-y/Br-y than in situ production. Thus, for reactive source species, simple methods of estimating the concentration of stratospheric Cl-y/Br-y that ignore the tropospheric contribution will seriously underestimate the loading. Therefore estimation of the stratospheric Cl-y/Br-y loading requires not only measurements of tropospheric source gases but also measurements of Cl-y/Br-y at the tropopause. This paper illustrates the mechanism by using results from a two-dimensional chemistry-transport model. However, in view of the importance of tropospheric transport on stratospheric loading the detailed values should be further evaluated using a three-dimensional model with appropriate treatment of convective transport.