Ozone Reactions: Physics and Chemistry of the Stratosphere

CFCs: Double Danger

The CFCs present a double danger to the global environment because they have the potential for both ozone depletion and global warming, since they also are greenhouse gases. Figure 1 shows the energy absorbing potential (band strength) for various gases at different wavelengths.

Infrared radiation from the earth has substantial energy at wavelengths between 7 and 10 micrometers. The graph shows that the CFCs (underlined in blue) have moderate to high absorbing capabilities at these wavelengths and thereby contribute to global warming.

Figure 2 gives the combined global warming and ozone depleting effects of the chlorinated compounds.

Molecules located in the upper right part of the chart have large detrimental effects for both reasons and are candidates to be replaced with more benign molecules located in the lower left part of the chart. The process of making an HCFC from a CFC reduces the environmental impact of the molecule but it negates one of the reasons the CFCs were created in the first place, namely because they are so non-reactive. The result is that when HCFCs are used in, say, a refrigerator they also interact with the lubricating oil added to the working fluid. This means that the lubricating capability is diminished, which reduces the lifetime of the refrigerator's compressor. Also, the compressor may require a higher operating pressure which demands more energy and puts more strain on seals and other components, making the refrigerator less energy efficient and subject to malfunction. The CFCs are very convenient chemicals, except for their environmental hazard, and several negative factors that must be considered in the search for suitable replacements. The Upper Atmosphere Research Satellite (UARS) measures temperature, pressure, wind variability and gas species concentrations in the stratosphere at altitude ranges from 10 to 100 km.

Atmospheric Concentrations

Recent trends in atmospheric concentration are shown in Figure 3, and current values are given by the Carbon Dioxide Information Analysis Center

Note that concentrations are given in parts per trillion per year (ppt/yr). The most recent data reveal that the concentrations are still increasing but less rapidly than in the past (Figure 4).

Concentrations will likely increase for some time for several reasons. Although the CFCs are not manufactured in the US, they are manufactured in some countries and are even the substance of a developing black market for international trade. Also, CFCs used before the manufacturing ban remain in air conditioners, in automobiles, homes and commercial and industrial buildings as well as in junkyards and landfills. As equipment containing CFCs deteriorate, seals age and containers rupture releasing CFCs to the atmosphere for some time. CFCs have been used in the past for manufacturing foams that are used for insulating materials. These foams have millions of tiny bubbles containing CFCs, which do not readily diffuse through the foam, to help retain their insulating property. Eventually, however, they will be released to the atmosphere, creating a source of atmospheric CFCs long after they are no longer manufactured.

There is some encouraging news however, on atmospheric CFC concentrations: even though measurements are consistent in showing continued increases in CFC concentrations, they are increasing at a decreasing rate, as shown in Figure 5 for Alaska, Colorado, Hawaii, Samoa, and the South Pole.

For example CFC 11 presently is increasing at only about four parts per trillion per year as compared with about 10 to 12 ppt just a few years ago. The 1989 Montreal Protocol to Reduce Substances that Deplete the Ozone Layer (and subsequent amendments) called for the elimination of CFC-11, -12, -113, -114, and -115 and methyl chloroform (CH₃C_l3), carbon tetrachloride (CCl₄) and the halons (H-1211, - -1301, -2402) by the end of the 20^thcentury. This is widely regarded as the stimulating motivation for major international agreement on a significant environmental issue. The CFC numbering scheme was devised to label these molecules without revealing their molecular structure. See also the "numbers game."

Stratospheric Ozone and Human Health

In the unit on human health, we will discuss in more detail the reason stratospheric ozone is directly important to humans, but at this point we will briefly discuss the relationship of stratospheric ozone to ultraviolet (UV) light.

Wavelengths of light in the visible part of the spectrum of solar energy extend from about 0.7 to about 0.4 nanometers. Radiant energy having wavelengths shorter than about 0.4 nanometers is classified as ultraviolet light: "near UV" or UV-A for wavelength of 400 to 320 nanometers, mid UV or UV-B for wavelengths of 320 to 290 nanometers, and UV-C for wavelengths of 290 to 250 nanometers. Stratospheric ozone is very efficient at absorbing solar energy with wavelengths of less than 290 nanometers, but its efficiency drops off in the UV-B range (the range of wavelengths that give a sunburn) (Figure 6).

As stratospheric ozone is depleted, more energy in the UV-B range is allowed to pass though the stratosphere and troposphere and enter the biosphere - the sphere of plant, animal, marine, and soil organisms near the earth's surface (Figure 7).

Some evidence from Canada in Figure 8 shows UV radiation at the earth's surface increasing differently at different wavelengths, but the largest observed increase is at those wavelengths where ozone absorption is large.

Fortunately the increase is less in the summer, when humans have more skin exposed to the sun, than it is in the winter. The NOAA/EPA UV index provides a measure of skin damage caused by UV radiation based on weather forecasts.

Additional data from Montreal shows that the average daily flux of UV-B radiation is somewhat higher in 1993 than 1989 for both summer and winter (Figure 9). Updates on current ozone measurements from Antarctica can be obtained from the stations at Halley, Rothera and Vernadsky/Faraday. Canadian monthly and annual graphs of ozone levels demonstrate that ozone levels over several Canadian cities fall typically 5-10% below normal levels in the winter months. NASA also provides current maps of global ozone.

Regional average ozone decreases for other areas from 1960 to 1990 show weakly declining values over North America, Europe, and the Far East (Figure 10).

High latitudes generally are experiencing more rapid decreases, and decreases in winter are larger than in summer.

Ozone depletion is related to increases in skin cancer. A depletion of 2% total ozone is expected to lead to about one-half million additional cases of skin cancer and additional 9,300 deaths. Measured values of ozone in the latitude range of the United States currently are about 6-7% below the natural levels.

Future Concentrations of Atmospheric Chlorine

There is some encouraging news in the long term as can be seen in Figure 11, which shows the atmospheric concentration of chlorine from 1960 to the present and projected into the future.

The first strong evidence for the existence of the Antarctic ozone hole was in the 1970's when chlorine concentrations reached about 2 ppb. When the CFCs were implicated as the source of the chlorine leading to ozone destruction, the 1987 Montreal Protocol was adopted. As confirming evidence continued to accumulate, the 1990 and 1992 revisions put increased restrictions on CFC production. The graph shows that with the present restrictions in place, ozone levels may return to 1970 levels by the year 2060. DuPont Chemical which was a major producer of CFCs, and the one that invented them in the first place, was very quick to abandon their production of CFCs (Figure 12). Despite overall progress in reducing ozone-depleting chemicals, year-to-year fluctuations may depart from the long-term trend. The 2006 hole was the largest ever measured. The cause of the large hole in 2006 seems to be related to the very cold temperatures in the South Polar stratosphere.

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