Ozone characteristics

Ozone measurements can provide three types of information. The first is the total ozone in an atmospheric column. This is measured with the Dobson spectrophotometer, which compares the solar radiation at a wavelength where ozone absorption occurs with that in another wavelength where such effects are absent. Second is the spatial pattern of total ozone. This is determined by satellite sounders such as NASA's Total Ozone Mapping Spectrometer (TOMS) aboard Nimbus-7, Meteor-3 and ADEOS (Advanced Earth Observing Satellite). Third is the vertical distribution of ozone. This can be measured by chemical soundings of the stratosphere, by passive microwave limb sounding from satellites, or calculated at the surface using the Umkehr method. The last method determines the effect of solar zenith angle on the scattering of solar radiation.

Ozone (O3) is concentrated mainly in the stratosphere. Ultraviolet radiation causes the breakup of normal oxygen molecules (O2) into atomic oxygen (O). The atomic oxygen then combines with other normal oxygen molecules to create ozone. Ultraviolet radiation can then split O3 back into O2 and O. This continuing process is known as the ozone-oxygen cycle. The annual cycle of mean ozone column totals is shown in Figure 4.6 for two periods 1964-80 and 1984-93. There is a strong annual cycle in northern high latitudes with values exceeding 400 DU (Dobson units = milli-atmosphere centimeters, or mm column depth at standard temperature and pressure)

Figure 4.6 Mean column ozone totals (Dobson units) versus latitude for 1964-80 and 1984-93 (from Bojkov and Fiolotov, 1995, by permission of AGU).

in late winter and spring. This compares to about 300 DU in September. The late winter to spring maximum in both polar regions is a result of poleward transports of ozone-rich air by the residual meridional plane circulation. The stronger annual cycle in northern high latitudes points to the more dynamic stratospheric circulation.

Of concern is the destruction of ozone by human activities, primarily associated with the release of chlorofluorocarbons (CFCs). Ozone destruction leads to more ultraviolet radiation at the surface. This has health risks, raising the likelihood of skin cancers and increasing ozone production near the surface. Until banned by the Montreal Protocol (signed in 1987 and entered into force in 1989), CFCs found wide use (e.g., as refrigerants and aerosol propellants). Unfortunately, CFCs are very long lived and will influence the ozone layer for decades to come.

CFCs are implicated in the recurrent stratospheric "ozone hole" that develops over Antarctica each spring. In recent years, "mini" ozone holes, analogous to the Antarctic phenomena, have been observed (Solomon, 1999). The full suite of reactions is complex and we summarize only the basic principles here. Briefly, the CFCs slowly work their way into the stratosphere, and are then dissociated by ultraviolet radiation to release chlorine atoms (Cl). The chlorine atoms destroy ozone in a catalytic cycle (Chartrand et al., 1999). A chlorine atom reacts with an ozone molecule to form the compound ClO, leaving an oxygen molecule behind. A free oxygen atom then takes away the oxygen from the ClO. The end result is O2 and Cl. The catalytic cycle of ozone destruction then repeats itself. By this process, a single chlorine atom would keep on destroying ozone forever. While there are reactions that remove Cl by forming "reservoir species", such as hydrochloric acid and chlorine nitrate (deactivating the chlorine), the chlorine atoms can be reactivated in the presence of polar stratospheric clouds (PSCs), allowing them to destroy more ozone. PSCs or nacreous (mother-of-pearl) clouds form at about 20-30 km altitude when temperatures fall below 195 K. The key issue is that hydrochloric acid or chlorine nitrate (stable forms of chlorine) are converted into reactive forms of chlorine on the surface of the PSCs. The role of sunlight explains why the ozone hole develops in spring - although PSCs are most abundant in winter, there is no light to drive the chemistry. In the Arctic stratosphere, the low temperatures associated with PSCs are less frequent than in the Antarctic, but may still occur during about two months of the year (Levi, 1992). As discussed in Chapter 11, links have been proposed between recent trends in the winter Arctic Oscillation and stratosphere ozone loss. One can see a reduction in total column ozone between the earlier and later periods plotted in Figure 4.6.

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