O O

where Z is either CI or Br. The rate-limiting step is the reaction between HO, and ZO.

The reaction between CIO and BrO has three product channels:

If the reaction follows either reactions (3.22a) or (3.22b), then net loss of Ov

These BrO-CIO cycles are both rate limited by the reaction between BrO and CIO. Cycles (3.23) and (3.24) are important in the formation of the Antarctic ozone hole, which we will discuss in detail in Chapter 7.

If the reaction between BrO and CIO follows reaction (3.22c), then there is no net loss of O,:

There are two more O.-dcstroying reaction sequences that rely on the production of NO and 02 from the photolysis of NO,:

CIO + NO2-^-»CIÔNO2 O, + O, ->■ O, + o2 + 02

NO, + hv NO + 02 NO + O, -> NO, + 02 Cl + O, CIO + o2

The rate-limiting step of both of these reactions is the rate of photolysis of NO, to NO. Note that if NO, is photolyzed to NO, and O. then these cycles lead to no net loss of O,—because the O atom produced will react with 02 to reform Q,. These last two cycles are of limited importance in regulating O,.

3.4 Odd-oxygen Production and Loss Rates

The continuity equation for O, can be written

Production of 0A is almost entirely due to photolysis of 02, so P ~ 2/(,2[0. j. The factor of two accounts lor the fact that each 02 molecule photolyzed produces two Or. Figure 3.5a shows the annually averaged P0 . Production increases with altitude over most of the stratosphere, reaching a maximum over the equator at about 2 hPa. The increase with altitude of P0 occurs because the photolysis rate increases with altitude faster than [02] decreases, so their product increases with altitude. Above 2 hPa, ,/0i ceases to increase rapidly with height, so the decrease in [02] causes their product to decrease with altitude. Also note that P{) at a given pressure is greatest over the equator. This is consistent with the fact that the lower latitudes receive, on average, more sunlight. Figure 3.5b shows the lifetime of O,. with respect to production (xf, = lOJ/P).

The total loss rate of O, is the sum of two times the rates of the rate limiting step of each catalytic cycle plus two times the rate of the reaction between O, + O: L|0, | = 2k, ,);)|CIO||0| + 2ANO)+q(N021[01 + 2AH,)r()lH02][0| + ... + 2*(>vlj|0,||0]. Again, the factor of 2 accounts for the fact that each O, destruction pathway destroys two O,. Figure 3.6a shows the annually averaged loss rate of Oa; Figure 3.6b shows the lifetime of O, with respect to loss (|OJ/(L[OJ) = ML). The hemispheric asymmetry in the lower stratosphere of Figure 3.6 is due to the rapid loss of O, associated with the Antarctic ozone hole.

As with production, Figure 3.6a shows that the loss rate of O, increases rapidly with altitude throughout most of the stratosphere, reflecting the increase with altitude of the abundances of O and the O^destroying reactive radicals. Below about

Latitude
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