In addition, the following HOx/BrOA and Br0t/C10t cycles are important in the lower stratosphere (~ 20 km):
OH + 03 —> H02 + 02 BrO + H02 —> HOBr + 02 HOBr + hv —► OH + Br
BrO + CIO —>■ BrCl + 02 BrCl + hv —► Br + CI
Cl + 03^C10 + 02 BrO + CIO —► ClOO + Br C100 + M-^Cl + 02
5.5.2 Bromine Cycles
While there is a total chlorine compound mixing ratio in the stratosphere of approximately 3400 ppt, that for bromine gases is only — 20 ppt (Figure 5.1). Remarkably, with 150 times less abundance than chlorine, bromine is approximately as important as chlorine in overall ozone destruction. Methyl bromide (CH3Br) constitutes about half the source of bromine in the stratosphere (see Section 2.5). The H-atom-containing bromine compounds, CH3Br, CH2Br2, and CHBr, release their Br almost immediately on entry into the stratosphere; the
halons, CBrFCl2 and CBrF3, release their Br more slowly and therefore at higher altitude. The Brv system is shown in Figure 5.16. In order to understand why bromine compounds are so much more effective in ozone destruction, we need to consider the role of reservoir species in stratospheric ozone destruction cycles.
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