Gas-Phase Chemistry in the Stratosphere

The Cl atom released by photolysis of the CFCs reacts in a catalytic chain reaction that leads to the destruction of 03  

Because the concentration of oxygen atoms increases with altitude, the reaction cycle represented by (26) and (27) is important primarily in the middle and upper stratosphere (e.g., Garcia and Solomon, 1994 WMO, 1995). For the lower stratosphere, however, it is only responsible for about 5% of the portion of the total ozone loss that is due to halogens at 15 km and 25% at 21 km (see Fig. 12.8 Wennberg et al., 1994). Most of the 03 loss associated with C10x and BrO at the relatively low altitudes in Fig. 12.8 is due to the following cycle (Solomon et al., 1986 Crutzen and Arnold, 1986)  

This cycle accounts for 30% of the ozone loss due to halogens in the lower stratosphere, and the corresponding cycle for bromine for 20-30% (Wennberg et al., 1994). Reaction of CIO with HOz, reaction (28), produces HOC1 + 02 with a yield 95% at temperatures from 210 to 300 K however, at the lowest end of this temperature range, there is evidence for the produc- 

While more than 90% of reaction (31) at 298 K produces bromine atoms directly, the minor channel producing BrCl is important in the atmosphere under certain conditions (e.g., McKinney et al., 1997). (Measurements of OCIO formed in reaction (31a) in the Antarctic and Arctic stratosphere are discussed later.) For example, McKinney et al. (1997) report large fractions (50-95%) of total bromine in the form of BrO in 

Since BrCl, produced in the minor channel (31c), absorbs strongly in the UV and visible (see Table 4.30), it also ultimately generates atomic bromine. Bromine atoms then react with 03 as well, 

---