17 How does bromine affect ozone?
Description
This article is from the Ozone Depletion: Stratospheric Chlorine and Bromine FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
17 How does bromine affect ozone?
Bromine concentrations in the stratosphere are ~150 times smaller
than chlorine concentrations. However, atom-for-atom Br is 10-100
times as effective as Cl in destroying ozone. (The reason for this
is that there is no stable 'reservoir' for Br in the stratosphere
- HBr and BrONO2 are very easily photolyzed so that nearly all of
the Br is in a form that can react with ozone. Contrariwise, F is
innocuous in the stratosphere because its reservoir, HF, is
extremely stable.) So, while Br is less important than Cl, it must
still be taken into account. Interestingly, one principal
pathway by which Br destroys ozone also involves Cl:
BrO + ClO -> BrCl + O2
BrCl + hv -> Br + Cl
Br + O3 -> BrO + O2
Cl + O3 -> ClO + O2
-----------------------
Net: 2 O3 -> 3 O2
[Wayne p. 164] [Solomon]
so reducing stratospheric chlorine concentrations will, as a
side-effect, slow down the bromine pathways as well.
Another important mechanism combines Br with HOx radicals:
BrO + HO2 -> HOBr
HOBr + hv -> Br + OH
Br + O3 -> BrO + O2
OH + O3 -> HO2 + O2
-----------------------
Net: 2 O3 -> 3 O2
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