Stratospheric Chlorine: a Possible Sink for Ozone

TLDR

The study proposes that chlorine oxides (ClOx) could be a significant sink for stratospheric ozone. The authors devise a photochemical scheme with two catalytic cycles for ClOx‑mediated ozone destruction and perform sample calculations for various ClX altitude profiles, including 1 ppb, ground‑level sources, and volcanic injections. The model shows that ClOx destroys ozone more efficiently than analogous NOx species, predicts ClO dominates in the lower and middle stratosphere while HCl dominates aloft, and highlights limitations such as uncertain OH levels, potential ClOO, and the need to couple ClOx with NOx/HOx cycles and consider heterogeneous reactions.

Abstract

This study proposes that the oxides of chlorine, ClO x , may constitute an important sink for stratospheric ozone. A photochemical scheme is devised which includes two catalytic cycles through which ClO x destroys odd oxygen. The individual ClX constituents (HCl, Cl, ClO, and OClO) perform analogously to the respective constituents (HNO 3 , NO, NO 2 , and NO 3 ) in the NO x catalytic cycles, but the ozone destruction efficiency is higher for ClO x . Our photochemical scheme predicts that ClO is the dominant chlorine constituent in the lower and middle stratosphere and HCl dominates in the upper stratosphere. Sample calculations are performed for several ClX altitude profiles: an assumed 1 p.p.b. volume mixing ratio, a ground level source, and direct injection by volcanic explosions. Finally we discuss certain limitations of the present model: uncertainty in stratospheric OH concentrations, the possibility that ClOO exists, the need to couple ClO x cycles with NO x and HO x cycles, and possible heterogeneous reactions.