CI o

and similarly for the case where bromine replaces chlorine. The reaction sequence involving collision of CIO with BrO discussed for the Antarctic ozone hole is also operative here.

PROBLEM 1

Deduce the overall reaction equation for the reaction sequence shown at left.

This mechanism explains why, in the current high-chlorine lower stratosphere, large volcanic eruptions can deplete mid-latitude stratospheric ozone for a few years, but it does not account for the overall trend of decreasing ozone in the 1980s. Some of the decrease is probably due to the mechanism operating on the background concentration of sulfuric acid particles in the lower stratosphere; its magnitude would have increased continuously in this time period since the chlorine levels were continuously increasing. Chlorine and bromine increases combined resulted in about a 4% decline in mid-latitude ozone levels in the 1979-1995 period. However, much of the gradual decline over mid-latitudes is believed to be due to other factors, such as springtime dilution of ozone-depleted polar air and its transport out of the polar regions, changes in the solar cycle, and both natural and anthropogenic changes in the pattern of atmospheric transport and temperatures.

New Zealand. Calculations indicate that the extent of UV increases since the 1980s over mid- and high-latitude regions amounts to 6-14%. The most definitive experimental evidence comes from New Zealand, where long-term summertime increases in UV-B, but as expected not in UV-A, amounted to 12% by 1998-1999. The situation over mid-latitudes is complicated by the facts that some UV-B is absorbed by the ground-level ozone produced by pollution reactions (as explained in Chapter 3), thereby masking any changes in UV-B due to small amounts of stratospheric ozone depletion, and that records of UV received at the Earth's surface were started only in the 1990s.

Continue reading here: The Chemicals That Cause Ozone Destruction

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