BROMINE-CHLORINE COUPLING IN THE ANTARCTIC OZONE HOLE

The contribution from the chlorine and bromine species in the formatio n of the Antarctic ozone hole is evaluated. Since chlorine and bromine compounds are of different industrial origin, it is desirable, from a policy point of view, to be able Go attribute chlorine-catalyzed loss of ozone with those reactions directly involving chlorine species, an d likewise for bromine-catalyzed loss. In the stratosphere, however,mo st of the chemical families are highly coupled, and, for example, chan ges in the chlorine abundance will alter the partitioning in other fam ilies and thus the rate of ozone loss. This modeling study examines fo rmation of the antarctic ozone hole for a wide range of bromine concen trations (5 - 25 pptv) and for chlorine concentrations typical of the last two decades (1.5, 2.5 and 3.5 ppbv). We follow the photochemical evolution of a single parcel of air,typical of the inner Antarctic vor tex (50 mbar, 70 degrees S, NOy = 2 ppbv, with polar stratospheric clo uds(PSC)) from August I to November 1. For all of these ranges of chlo rine and bromine loading, we would predict a substantial ozone hole (l ocal depletion greater than 90%) within the de-nitrified, PSC-perturbe d vortex. The contributions of the different catalytic cycles responsi ble for ozone loss are tabulated. The deep minimum in ozone is driven primarily by the chlorine abundance. As bromine levels decrease, the m agnitude of the chlorine-catalyzed ozone loss increases to take up the slack. This is because bromine suppresses ClO by accelerating the con version of ClO and Cl2O2 back to HCl. For this range of conditions the local relative efficiency of ozone destruction per bromine atom to th at per chlorine atom (alpha-factor) ranges from 33 to 55, decreasing w ith increase of bromine.