O3xo oX 03X0

FIGURE 1-10 Summary of X0 + 0->X + 0, XO + X'O —»-*X + X' + 02

catalytic ozone destruction by Mechanisms I and II. O, + O —»2 O, overall 2 03 —»3 O, overall catalyst, the steady-state concentration of ozone must then decrease to a new, lower value at which the rates of formation and destruction are again equal. However, it should be clear from the discussion above that due to its constant re-formation reactions, atmospheric ozone cannot be permanently and totally destroyed, no matter how great the level of catalyst. It should also be realized that any decrease in the concentration of ozone at higher altitudes allows more UV penetration to lower altitudes, which produces more ozone there; thus there is some "self-healing" of total ozone loss.

Atomic Chlorine and Bromine as X Catalysts

The decomposition of synthetic chlorine-containing gases in the stratosphere over the last few decades has generated a substantial amount of atomic chlorine, CI, in this region. As the stratospheric chlorine concentration increases, so does the potential for ozone destruction, since the free radical CI is an efficient X catalyst.

However, synthetic gases are not the only suppliers of chlorine to the ozone layer. There always has been some chlorine in the stratosphere as a result of the slow upward migration of the methyl chloride gas, CH3C1 (also called chloromethane), produced at the Earth's surface—mainly in the oceans as a result of the interaction of chloride ion with decaying vegetation. Recently another large source of methyl chloride, from tropical plants, has been discovered; this may be the missing source of the compound for which scientists have been searching.

Only a portion of the methyl chloride molecules are destroyed in the troposphere. When intact molecules of it reach the stratosphere, they are pho-tochemically decomposed by UV-C or attacked by OH radicals. In either case, atomic chlorine, CI, is eventually produced:

Chlorine atoms are efficient X catalysts for ozone destruction by Mechanism I:

Continue reading here: ClO Oc o2

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