As we noted in the earlier section, ozone is generally in a balance between photochemical production and loss, as determined by naturally occuring concentrations of nitrogen, chlorine, bromine, and hydrogen compounds in the stratosphere. The increase of these naturally occuring compounds, or the introduction of new compounds can increase ozone loss, whereas ozone production is basically fixed by the intensity of solar UV output. It is now recognized that most of the chlorine in the stratosphere is human produced, and it has upset the natural production-loss balance in the stratosphere.
The chlorine found in the stratosphere principally comes from chlorofluorocarbons (CFCs, see chapter 10). In 1928, Thomas Midgely produced a compound called CFC-12 (dichlorodifluoromethane). Midgely developed CFC-12 as a replacement refrigerant for highly toxic and flammable refrigerants such as ammonia. Midgely showed that CFC-12 was non-toxic, non-flammable, and had thermal properties that made it an excellent refrigerant. In 1930, Dupont and General Motors began to market CFCs under the trade name Freon. Since then, other CFCs have been synthesized and produced in large quantities. CFCs have been extensively used in refrigerators and as propellants in aerosol cans. CFCs are composed of fluorine, carbon, and chlorine. Vast amounts of chlorine are found on Earth in other forms, particularly salt (NaCl). Fortunately, salt is water soluble, so this form of chlorine is essentially completely rained out before getting to the stratosphere. CFCs are not water soluble, and are very nonreactive (a wonderful property that makes CFCs nontoxic and adds to their "good guy" image). The stability of CFC molecules means that CFCs can only be destroyed by the extremely energetic UV (photolysis) above most of the ozone layer.
As a consequence of their use, CFCs are eventually released into the atmosphere. They become well mixed throughout the troposphere over about one year after being released (see further description of transport processes in Chapter 6). The CFCs enter the stratosphere through the tropical upper troposphere region. Figure 1.07 shows an averaged distribution of CFC-12, measured by the Cryogenic Limb Array Etalon Spectrometer (CLAES) on the Upper Atmosphere Research Satellite (UARS) during the June-July 1992 period. The units are parts per trillion by volume (pptv), indicating that the CFC-12 molecule comprises a miniscule fraction of a specific piece of air.
Superimposed on this figure are blue arrows that show the Brewer-Dobson circulation pattern for the June-July period (northern summer). After entering the lower stratosphere, the CFCs can either be mixed to higher latitudes (i.e. closer to the poles) or slowly carried into the upper stratosphere where they are broken down by the sun's UV radiation. Because CFCs are broken down by UV radiation at higher altitudes, CFC-12 concentrations decrease with altitude. It takes about a full year for the average CFC molecule to get to the upper stratosphere from the tropical upper troposphere. However, most of this air entering the stratosphere gets recycled back into the troposphere before reaching the upper stratosphere. It takes a few decades or more to cycle all of the air in the troposphere through the upper stratosphere. This slow circulation of CFCs through the upper stratosphere means that it will take decades to cycle all of the CFCs through the upper stratosphere.
This cycling of air through the stratosphere and the destruction of the CFCs by UV can be likened to the ability of a small air cleaner to clean all of the air inside a domed stadium. First, the air has to be wafted near the air-cleaner (i.e., carried from the troposphere, through the tropical tropopause into the upper stratosphere, see Figures 1.06 and 1.07). The air is then cycled through the air cleaner (i.e., the CFC is broken up by the UV radiation). The "smallness" of the air cleaner compared to the stadium is analogous to the fact that there are far fewer air molecules in the stratosphere versus the troposphere. While the air cleaner produces clean air, the byproduct of moving CFCs through the upper stratosphere is the production of chlorine molecules. The small air cleaner can eventually cycle all of the air inside the stadium, but it will take quite a long time. Similarly, it will take a long time to cycle all of the CFCs through the upper stratosphere.
Once chlorine is liberated from the CFC by the UV photolysis, it can catalytically destroy ozone. Eventually, the Cl atoms react with methane to form HCl (hydrocloric acid). This HCl byproduct of CFC photolysis can be used to assess the conversion of non-reactive CFCs to reactive Cl. Thus, it allows us to determine how much free chlorine has been liberated from the CFC species and is able to destroy ozone. HCl levels in the stratosphere are being measured by the Halogen Occultation Experiment (HALOE) aboard UARS.
Near the bottom of the stratosphere at 16 km, relative HCl levels are quite low, while near the top of the stratosphere at 60 km, these HCl levels are quite high. As the CFCs rise in the stratosphere to high altitudes, they are broken apart and the chlorine is liberated, the Cl atoms catalytically destroy ozone, and eventually react with methane (CH4) to form HCl. Hence, high HCl and low CFC values are found near 50 km, while low HCl and high CFC values are found at low altitudes. Figure 1.08 is a plot of HALOE HCl data. Concentrations are in parts per billion for a six year period (1992-1998) averaged over the 55°S to 55°N latitude range, so it is essentially a global average.
The HALOE data also show that HCl concentrations have been increasing since the early 1990's. This HCl increase is consistent with the trend in concentrations of chlorine observed in the troposphere due to the release of manmade CFCs. These increased chlorine concentrations are evidence of the impact of CFCs on the stratosphere, and they have directly lead to greater ozone loss.
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