R8 2 x CI 03 CIO

The second reaction implies that the ozone destruction rate depends on the square of the CIO radical concentration. If we also consider that the formation of chemically active chlorine (CI and CIO) by reaction R13 involves a reaction between two chlorine-containing species - ClONO? and HC1 - we note that the rate of ozone decomposition could be proportional to between the second and fourth power of the stratospheric chlorine content. With this increasing by 4% per year, as it has been doing until the beginning of the 1990s, the ozone destruction rate could increase by between 8% and 17% per year. The current stratospheric chlorine abundance is about six times greater than that of the natural background that is provided by QI3CI, implying at least 36 times faster anthropogenic than natural ozone destruction by the CI and CIO radicals. In situ observations on a stratospheric research aircraft validated the above explanation for the origin of the ozone hole (Anderson et aL, 1989). Precisely in the polar areas, where the stratosphere gets very cold in winter and remains cold during early spring, measurements show high concentrations of CIO radicals and simultaneously rapid ozone destruction (Figure 1,4). It is also important to note here that because of the strong ozone loss, heating of the stratosphere in the ozone-poor air does not take place, leading to lower temperatures and thus enhancing ice or supercooled liquid particle formation, chlorine activation, and ozone depletion, producing a series of positive feedbacks.

In the meantime, it was found that also in the Northern Hemisphere during late winter and early spring, ozone is being increasingly destroyed, although to a lesser extent lhan over Antarctica because stratospheric temperatures are generally about 10 C higher than over Antarctica, thus causing less-efficient particle formation and chlorine activation. During the 1980s, ozone depiction was most evident between January and

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