A new approach for constraining tropical OH based on the path-dependent lifetime of multiple trace gases

Hydroxyl radical (OH) is the dominant "cleansing" oxidant of the troposphere, which determines the chemical lifetime of many trace species. Here, we propose a new approach based on the concept of the path-dependent lifetime tc for constraining OH in the tropics, where OH is most abundant. tc(r) was introduced by Holzer and Waugh (2015) and parameterizes the integrated chemical loss during transport from the Northern-Hemisphere mid-latitude source to a given location r. We estimate tc(r) for a suite of CFC replacement gases using a simple 12-box model. We show that, for trace species with global lifetimes from 0.5 years to 50 years, tc provides an accurate representation of the integrated chemical loss. Furthermore, fractional changes in Southern-Hemisphere extratropical tc are similar to the corresponding fractional changes in tropical OH, i.e., a 10% increase in tropical OH results in about a 10% decrease in tc. The slope of this linear relationship between tc and tropical OH is almost identical for all tracers considered. This suggests that observationally based estimates of tc for a suite of tracers may be used to constrain tropical OH concentrations, provided tc can be determined with sufficient precision.