HBr, in the stratosphere, ozone is continuously being formed via the Chapman mechanism. However, halogen radicals such as bromine and chlorine (released from anthropogenic and some natural sources) catalyze the breakdown of ozone which results in a net ozone loss:
- O3 + Br----> BrO + O2
A reservoir species is a compound whose formation breaks the destruction cycle by taking the halogen radical out of the cycle. Two examples are HCl and HBr. Unfortunately, the success of HBr as a reservoir species is very limited due to the fact that it photodissociates quickly and easily. Thus, the ozone depleting effects of bromine are, atom for atom, more severe than those of chlorine.
[Graedel, T.D. and Paul Crutzen. Atmospheric Change: an Earth system perspective; 1993; Freeman Press.]
Source: Atmospheric Chemistry Glossary