This article is from the Ozone Depletion: Stratospheric Chlorine and Bromine FAQ, by Robert Parson firstname.lastname@example.org with numerous contributions by others.
Br is present in much smaller quantities than Cl, but it is
much more destructive on a per-atom basis. There is a large
natural source; manmade compounds contribute about 40% of the total.
In the antarctic chlorine is more important than Bromine, but at
middle latitudes their effects are comparable.