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

Abstract

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 τc(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 τc provides an accurate representation of the integrated chemical loss during transport. The value of τc 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 τc are similar to the corresponding fractional changes in tropical OH; that is, a 10% increase in tropical OH results in a 10% decrease in τc. This suggests that observation‐based estimates of τc may be used to constrain tropical OH concentrations provided that τc can be determined with sufficient precision.

Evaluating Simulations of Interhemispheric Transport: Interhemispheric Exchange Time Versus SF6 Age

“…In addition to the analysis of the above comprehensive three‐dimensional chemistry‐climate models, we use the idealized Advanced Global Atmospheric Gases Experiment 12‐box model (Rigby et al, ; Yang, Waugh, & Holzer, ) to examine the sensitivity of the SF 6 metrics to difference aspects of the flow. The Advanced Global Atmospheric Gases Experiment model divides the atmosphere into 12 boxes that are vertically separated by stratosphere (S, <200 hPa), upper troposphere (UT, 200–500 hPa), and lower troposphere (LT, 500–1,000 hPa), while meridionally separated by NH extratropics (NE, 30–90°N), NH tropics (NT, 0–30°N), SH tropics (ST, 30–0°S), and SH extratropics (SE, 90–30°S).…”

“…The transport between boxes occurs via advection and diffusion, but is dominated by the diffusive exchange (see Rigby et al, 2013). The transport parameters used are generally similar to those used in Yang, Waugh, and Holzer (2018; see Data Set S1).…”