Decoupling the Effects of Transport and Chemical Loss on Tropospheric Composition: A Model Study of Path‐Dependent Lifetimes

The distribution of most trace species within the troposphere depends on a complex interplay between chemistry and transport. Holzer and Waugh (2015) introduced the concept of a path‐dependent lifetime τ c ( r ), that parameterizes the integrated chemical loss during transport from the source to a given location r . Here we examine whether this parameterization provides a new approach for decoupling transport and chemistry. We present calculations of path‐dependent lifetimes for a suite of chlorofluorocarbon replacement gases in a simple 12‐box model and show that for trace species with global lifetimes from 0.5 years to 50 years τ c provides an accurate representation of the integrated chemical loss during transport. The value of τ c is shown to be sensitive to tropical hydroxyl radical (OH) concentrations, with much weaker sensitivity to extratropical OH or transport. Furthermore, fractional changes in Southern Hemisphere extratropical τ c are similar to the corresponding fractional changes in tropical OH; that is, a 10% increase in tropical OH results in a 10% decrease in τ c . This suggests that observation‐based estimates of τ c may be used to constrain tropical OH concentrations provided that τ c can be determined with sufficient precision.