Cycles such as HO* cycles 1 and 2, NO* cycle 1, and CIO* cycle 1 would appear to go on destroying 03 forever. Actually, the cycle can be interrupted when the reactive species, OH, N02, CI, and CIO, become tied up in relatively stable species so that they are not available to act as catalysts in the cycles. Knowing the competing reactions that can occur, it is possible to estimate the average number of times each catalytic 03 destruction cycle will proceed before one of the reactants participates in a competing reaction and terminates the cycle. For example, at current stratospheric concentrations, CIO* Cycle 1, once initiated, loops, on average, 105 times before the CI atom or the CIO molecule reacts with some other species to terminate the cycle. This means that, on average, one chlorine atom can destroy 100,000 molecules of ozone before it is otherwise removed. The frequency of such cycle termination reactions is thus critical to the overall efficiency of a cycle. The removal of a reactive species can be permanent if the product actually leaves the stratosphere by eventually migrating down to the troposphere, where it is removed from the atmosphere altogether. Examples of cycle-interrupting reactions that can lead to ultimate removal from the atmosphere are
Both nitric acid (HN03) and hydrogen chloride (HC1) are relatively stable in the stratosphere and some fraction of each migrates back to and is removed from the troposphere by precipitation.
A reactive species can also be temporarily removed from a catalytic cycle and be stored in a reservoir species, which itself is relatively unreactive but is not actually removed from the atmosphere. One of the most important reservoir species in the stratosphere is chlorine nitrate (C10N02), formed by
C10N02 + M
Chlorine nitrate can photolyze
thereby releasing CI or CIO back into the active CIO* reservoir. Chlorine nitrate is an especially important reservoir species because it stores two catalytic agents, N02 and CIO.
Partitioning of chlorine between reactive (e.g., CI and CIO) and reservoir forms (e.g., HQ and C10N02) depends on temperature, altitude, and latitude history of an air parcel. For the midlatitude, lower stratosphere, HC1 and C10N02 are the dominant reservoir species for chlorine, constituting over 90% of the total inorganic chlorine (Figure 5.14). From the Clv family in Figure 5.15, CI can be bled off the internal cycle by reacting with methane to produce HC1, and the chlorine atom in HQ can be returned to active CI if HC1 reacts with OH. The abundances of CH4 and OH will control the amount of CI sequestered as HC1. CIO can be temporarily removed from active participation in the CIO, catalytic cycle by reacting with N02 to form ClONO, or with H02 to form HOCl. Both C10N02 and HOCl can photolyze and release active chlorine back into the cycle. The importance of C10N02 and HOCl as reservoir species will depend on the abundances of N02 and H02 relative to atomic oxygen.
Lifetime of the Reservoir Species CI0N02 and HC1 Chlorine nitrate (C10N02) photodissociates back to CIO; its lifetime against photolysis is tciono: — JaotKh-the mid stratosphere (15-30 km), /cionoi — 5 x 10-5s-1, so Tciono2 ^ 5 h.
The lifetime of HC1 against reaction with OH is thci = (^oh+hci[OH])-1, where *oh+hci =2.6 x 10"l2 exp (-350/7). At 35 km (237K), ¿oh+hci = 6x l0"ncm3 molecule 1 s_i, and ÍOH] = 107 molecules cm-3, so t^ci — 2 days. At lower altitudes, where OH levels are lower, the HC1 lifetime increases to several weeks.
From Figure 5.15 we note the amounts of C10N02 and HC1 relative to those of CI and CIO. N02 and CH4 are responsible for shifting most of the active chlorine into these reservoir species. Typically
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