Reservoir species, ozone depletion

The chlorine-containing product species (HCl, CIONO2, HOCl) are "inert reservoirs" because they are not directly involved in ozone depletion however, they eventually break down by absorbing solar radiation or by reaction with other free radicals, returning chlorine to its catalytically active form. Ozone is formed fastest in the upper stratosphere at tropical latitudes (by reactions 1 and 2), and in those regions a few percent of the chlorine is in its active "free radical" form the rest is in the "inert reservoir" form (see Figure 3). 

Several authors (8,9) suggested that PSCs could play a major role in the depletion of ozone over Antarctica by promoting the release of active chlorine from its reservoir species, mainly by the following reaction ... 

The Airborne Submillimeter SIS Radiometer (ASUR), operated on-board the German research aircraft FALCON, measures thermal emission lines of stratospheric trace gases at submillimeter wavelength. Measurement campaigns with respect to ozone depletion in the Arctic winter stratosphere were carried out in yearly intervals from 1992-97 to investigate the distributions of the radical chlorine monoxide (CIO), the reservoir species hydrochloric acid (HC1), the chemically inert tracer nitrous oxide (N20), and ozone (O3). The high sensitivity of the receiver allowed to take spatially well resolved measurements inside, at the edge, and outside of the Arctic polar vortex. This paper focuses on the results obtained for CIO from... 

Reactions (1), (2) and (4) convert stable chlorine reservoir species, CIONO, and HC1, into the more easily photolyzable species Cl, HOC1, and C1NO, (nitryl chloride), respectively. This unique chemistry of CIONO, and N,0, on the cold surfaces of the PSC-surfaces is taking place due to the low temperatures of 180 to 200 K encountered in the lower stratosphere at altitudes between 15 and 25 km in the polar vortex. At sunrise, after the polar winter, these photolabile species release Cl atoms that initiate the chain destruction of ozone according to the mechanism, which is responsible for the fast ozone depletion event occuring within a few days to several weeks [34,35] ... 

Bromine containing species, introduced from Man s release of halons, are also believed to play a significant role in the polar ozone depletion, despite the fact that the total inorganic bromine concentration in the stratosphere is typically two orders of magnitude lower than the inorganic chlorine. This is manifested in the presence of Br0N02 and HOBr, which however are less stable than the chlorine reservoirs, so that relatively more BrO, is in the active fotm [36,37]. There is a synergism between the chlorine and bromine species the oxides radicals GO and BrO react with each other to produce a series of products, G, Br, BrCl and OCIO. The latter compound is an indicator of the elevated levels of both BrO and GO [38]. 

Any process that even modestly shifts the balance away from the reservoir species to CIO can have a large impact on ozone depletion. 

Volcanic injection of large quantities of sulfate aerosol into the stratosphere offers the opportunity to examine the sensitivity of ozone depletion and species concentrations to a major perturbation in aerosol surface area (Hofmann and Solomon 1989 Johnston et al. 1992 Prather 1992 Mills et al. 1993). The increase in stratospheric aerosol surface area resulting from a major volcanic eruption can lead to profound effects on C10 -induced ozone depletion chemistry. Because the heterogeneous reaction of N205 and water on the surface of stratospheric aerosols effectively removes N02 from the active reaction system, less N02 is available to react with CIO to form the reservoir species C10N02. As a result, more CIO is present in active CIO cycles. Therefore an increase in stratospheric aerosol surface area, as from a volcanic eruption, can serve to make the chlorine present more effective at ozone depletion, even if no increases in chlorine are occurring. 

FIGURE 4.10 Simplified schematic of the CIO, ozone depletion cycle. Cycling between Cl and CIO continually converts odd oxygen to even oxygen. The reservoir species CIONO., HCl. and HOCI sequester activ e chlorine and diminish the effectiveness of the CIO, cycle. 

The existence of reservoir species is central to the ozone depletion cycles. In every cycle a reactive free radical can be temporarily sequestered as a relatively unreactive reservoir species. In fact, HCI and CIONO2 together store as much as 99% of the active chlorine. Thus only a small change in the abundance of reservoir species can have a profound effect on the catalytic efficiency of a cycle. The importance of relative concentrations in determining the predominance of different reactions in the ozone depletion cycles can be illustrated in the case of CIO, cycles. Above 20 km, CIO, Cycle 1 is a dominant contributor to ozone loss. At lower altitudes where atomic oxygen levels are significantly lower, other cycles, which involve coupling with HO, and NO, become important ... 

Gas-phase chemistry associated with the ClOj, and NO cycles is not capable of explaining the polar ozone hole phenomenon. Heterogeneous reactions occurring on PSCs play the pivotal role in polar ozone depletion (McElroy et al., 1986 Solomon et al., 1986 Molina, 1991). The ozone hole is sharply defined between about 12 and 24 km altitude. Polar stratospheric clouds occur in the altitude range 10 to 25 km. Ordinarily, liberation of active chlorine from the reservoir species HCl and CIONO2 is rather slow, but the PSCs promote... 

Halogens are most important for ozone depletion. Below we will see that NOj, and halogens produce condensed reservoir species such as CIONO2 and BrON02, which play a role in the ozone hole (Fig. 182). Precursors, such as halogenated organic compounds, are photolyzed but also react with the oxygen atoms X = H, Br, F and Cl. 

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