HOBr

Figure 4.32 Schematic of the Br family. Arrows show conversion padiways between species.

Note that BrCl production is just one product channel of the reaction between CIO and BrO; the other products are ClOO + Br and OCIO + Br (reactions (3.22b) and (3.22c)).

Formation of Brr from reservoirs BrO NO, is converted back to Br( through the photolysis reaction

About 29% of the BrONO, molecules photolyze to produce Br and NO,, with the remaining 71% producing BrO and NO, (see DeMore et al. |5|, p. 199). Loss of Br0N02 via oxidation is not important.

HBr is converted back to Br, in the reaction

In the upper stratosphere, the reaction between HBr and O atoms becomes important:

The primary pathway for conversion of HOBr back to Br, is photolysis below -25 km,

and oxidation by O atoms at higher altitudes,

BrCl is rapidly photolyzed and destroyed by oxidation (t!trt:l < I min) [1311 to reform Br,. In general, under normal sunlit conditions the abundance of BrCl is negligible. At night in the upper stratosphere, BrCl can become an important Br, reservoir.

Interconversion among B>\ reservoirs The heterogeneous hydrolysis of BrONO, on aerosol surfaces,

is an important reaction for bromine partitioning in the lower stratosphere [133]. The sticking coefficient y for this reaction is -0.8, independent of temperature [102].

There is an analogous hydrolysis reaction involving ClONO, on sulfate aerosol surfaces. The sticking coefficient y for this ClONO, reaction, however, shows a strong temperature dependence, and the reaction turns out to be important only at extremely low temperatures (< 200 K) or at high SADs such as those found after volcanic eruptions. These hydrolysis reactions will be discussed more thoroughly in Chapters 6 and 7.

There are relatively few measurements of bromine species in the stratosphere. Those measurements that exist, however, generally show reasonable agreement with models [132,134]. Figures 4.33-4.36 show cross-sections of Brv and its important components, Br,, BrONO,, and HOBr.

Brv (Figure 4.33) increases rapidly with altitude in the lower and mid-stratosphere, with no increase above -30 km. This occurs because organic bromine molecules are reasonably weak, and so are destroyed in the lower and mid-stratosphere—by the time an air parcel reaches the mid-stratosphere all of the organic bromine molecules have been destroyed. This can be compared to Cly (see Figure 4.7), which increases with altitude throughout the stratosphere owing to the stronger bonds and therefore slower destruction rates of organic chlorine molecules.

Figure 4.34 shows a model simulation of Br, mixing ratio. As one can see, the abundance of Br, increases rapidly with altitude due to an increase in both Brv and in the ratio [Brt|/[Br. [ with altitude. Figure 4.35 shows a cross-section of the abundance of BrONO,. The distribution of BrONO: is similar to that of ClONO,: in the lower stratosphere the rapid increase in Bry with altitude causes the abundance of

Latitude

Figure 4.33 Contours of Brv abundance (pptv) Cor December. Values are from the Goddard two-dimensional climatologica! circulation model [73]. Based on a model tun to steady slate using CFC and halon emission levels for 1990.

Latitude

Figure 4.33 Contours of Brv abundance (pptv) Cor December. Values are from the Goddard two-dimensional climatologica! circulation model [73]. Based on a model tun to steady slate using CFC and halon emission levels for 1990.

Latitude

Figure 4.34 Contours of Br,, abundance (pptv) for December. Data from the Goddard two-dimensional climatological circulation model 1731. Based on a model run to steady state using CFC and halon emission levels for 1990.

Latitude

Figure 4.34 Contours of Br,, abundance (pptv) for December. Data from the Goddard two-dimensional climatological circulation model 1731. Based on a model run to steady state using CFC and halon emission levels for 1990.

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