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

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Abstract

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 y (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.

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