Stratospheric Ozone Variability
4.4.1 CFC impact on upper stratospheric ozone -- The most important influence on the long-term variability of stratospheric ozone is the introduction of significant amounts of chlorine into the upper stratosphere from industrially produced chlorofluorocarbon (CFC) compounds (see Chapter 10 for more detailed information). The initial predictions of effects of chlorine from CFCs indicated that the change in ozone would be a maximum in the upper stratosphere near 40 km altitude. This change was predicted to result from a relatively simple set of reactions that form the chlorine, bromine, nitrogen, and hydrogen catalytic cycles for ozone destruction. Observations over the past 20 years indicate that these decreases are indeed occurring (see Chapter 9).
5.4.1 CFC impact on lower stratospheric ozone -- Chlorofluorocarbons (or CFCs) were developed in 1928 as a benign and inert chemical compound for refrigeration, replacing toxic and flammable refrigerants like ammonia. CFCs quickly gained enormous usage over the next several decades as coolants and spray propellants. Seventy years later, manmade CFCs today represent the most important influence on the long term variability of lower stratospheric ozone. They also represent the most serious threat to the the ozone layer, which shields the Earth's surface from biologically destructive ultraviolet light emitted by the sun. Chapters 1 and 10 give additional background on CFCs, while Chapter 11 discusses in great depth the "ozone hole" problem that has developed over Antarctica as a result of CFCs.
How do CFC compounds impact lower stratospheric ozone? In brief, the stable CFC molecule rises into the lower stratosphere, where intense UV light breaks down the molecule, liberating chlorine. This chlorine is then free to react in catalytic cycles to destroy ozone, especially in the presence of polar stratospheric clouds (PSCs). These catalytic cycles represent more complicated photochemistry than the simple oxygen chemistry discussed so far. Chapters 5 and 11 detail these reactions.
Consequences on lower stratospheric ozone have been dramatic at certain times of the year over Antarctica. It is there that exceptionally cold winter conditions permit widespread PSC formation. Total column amounts of ozone in spring over the Antarctic have fallen by over 50 percent (see Chapter 1). Overall in the lower stratosphere, ozone concentrations have declined by smaller amounts over the last 20 years (see Chapter 9).
The primary driver for expected photochemical change is chlorine from CFC's. They were calculated to induce a nearly linear downward trend in the total global ozone amount starting in 1970 and continuing until about 1995. Because the provisions of the Montreal Protocol (see Chapter 10) have begun to take effect, this linear decrease in ozone is now predicted to flatten out and eventually turn around. Figure 8.09 shows a model calculation for the effect on ozone of increasing chlorine, the solar cycle, and volcanic activity. The model calculations project into the future using assumptions about the release of CFCs and other source gases such as methane and nitrous oxide.
In nearly all studies of long-term ozone trends it has been assumed that a linear trend is synonymous with the photochemical trend. This was a reasonable assumption for the period 1970 through the present, but it will no longer be true for future analyses, as the models predict that the recovery process should now be in its early stages. One analysis question that will now be asked is, "Do we see any evidence for the beginning of the recovery?".