Stratospheric Chlorine: a Possible Sink for Ozone

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.