Unit 11: Atmospheric Pollution // Section 10: Stratospheric Ozone
Earth's stratospheric ozone layer, which contains about 90 percent of the ozone in the atmosphere, makes the planet habitable by absorbing harmful solar ultraviolet (UV) radiation before it reaches the planet's surface. UV radiation damages cells and causes sunburn and premature skin aging in low doses. At higher levels, it can cause skin cancer and immune system suppression. Earth's stratospheric ozone layer absorbs 99 percent of incoming solar UV radiation.
Scientists have worked to understand the chemistry of the ozone layer since its discovery in the 1920s. In 1930 British geophysicist Sydney Chapman described a process in which strong UV photons photolyze oxygen molecules (O2) into highly reactive oxygen atoms. These atoms rapidly combine with O2 to form ozone (O3) (Fig. 16). This process is still recognized as the only significant source of ozone to the stratosphere. Research and controversy have focused on identifying stratospheric ozone sinks.
Figure 16. Ozone production
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Source: Courtesy National Aeronatics and Space Administration.
Ozone is produced by different processes in the stratosphere, where it is beneficial, and near the Earth's surface in the troposphere, where it is harmful. The mechanism for stratospheric ozone formation, photolysis of O2, does not take place in the troposphere because the strong UV photons needed for this photolysis have been totally absorbed by O2 and ozone in the stratosphere. In the troposphere, by contrast, abundance of VOCs promotes ozone formation by the mechanism described above in Section 4. Ozone levels in the stratosphere are 10 to 100 times higher than what one observes at Earth's surface in the worst smog events. Fortunately we are not there to breathe it, though exposure of passengers in jet aircraft to stratospheric ozone has emerged recently as a matter of public health concern.
To explain observed stratospheric ozone concentrations, we need to balance ozone production and loss. Formation of ozone in the stratosphere is simple to understand, but the mechanisms for ozone loss are considerably more complicated. Ozone photolyzes to release O2 and O, but this is not an actual sink since O2 and O can just recombine to ozone. The main mechanism for ozone loss in the natural stratosphere is a catalytic cycle involving NOx radicals, which speed up ozone loss by cycling between NO and NO2 but are not consumed in the process.
The main source of NOx in the troposphere is combustion; in contrast, the main source in the stratosphere is oxidation of nitrous oxide (N2O), which is emitted ubiquitously by bacteria at the Earth's surface. Nitrous oxide is inert in the troposphere and can therefore be transported up to the stratosphere, where much stronger UV radiation enables its oxidation. Nitrous oxide emissions have increased over the past century due to agriculture, but the rise has been relatively modest (from 285 to 310 parts per million by volume) and of little consequence for the ozone layer.
In 1974 chemists Sherwood Roland and Mario Molina identified a major threat to the ozone layer: rising atmospheric concentrations of manmade industrial chemicals called chlorofluorocarbons (CFCs), which at the time were widely used as refrigerants, in aerosol sprays, and in manufacturing plastic foams. CFC molecules are inert in the troposphere, so they are transported to the stratosphere, where they photolyze and release chlorine (Cl) atoms. Chlorine atoms cause catalytic ozone loss by cycling with ClO (Fig. 17).
Figure 17. Chlorine-catalyzed ozone depletion mechanism
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Eventually chlorine radicals (Cl and ClO) are converted to the stable nonradical chlorine reservoirs of hydrogen chloride (HCl) and chlorine nitrate (ClNO3). These reservoirs slowly "leak" by oxidation and photolysis to regenerate chlorine radicals. Chlorine is finally removed when it is transported to the troposphere and washed out through deposition. However, this transport process is slow. Concern over chlorine-catalyzed ozone loss through the mechanism shown in Figure 17 led in the 1980s to the first measures to regulate production of CFCs.
In 1985 scientists from the British Arctic Survey reported that springtime stratospheric ozone levels over their station at Halley Bay had fallen sharply since the 1970s. Global satellite data soon showed that stratospheric ozone levels were decreasing over most of the southern polar latitudes. This pattern, widely referred to as the "ozone hole" (more accurately, ozone thinning), proved to be caused by high chlorine radical concentrations, as well as by bromine radicals (Br), which also trigger catalytic cycles with chlorine to consume ozone.
The source of the high chlorine radicals was found to be a fast reaction of the chlorine reservoirs HCl and ClNO3 at the surface of icy particles formed at the very cold temperatures of the Antarctic wintertime stratosphere and called polar stratospheric clouds (PSCs). HCl and ClNO3 react on PSC surfaces to produce molecular chlorine (Cl2) and nitric acid. Cl2 then rapidly photolyzes in spring to release chlorine atoms and trigger ozone loss.
Ozone depletion has worsened since 1985. Today springtime ozone levels over Antarctica are less than half of levels recorded in the 1960s, and the 2006 Antarctic ozone hole covered 29 million square kilometers, tying the largest value previously recorded in 2000 (Fig. 18). In the 1990s ozone loss by the same mechanism was discovered in the Arctic springtime stratosphere, although Arctic ozone depletion is not as extensive as in Antarctica because temperatures are not as consistently cold.
Figure 18. Antarctic ozone hole, October 4, 2004
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Source: Courtesy National Aeronautics and Space Administration.
Rowland and Molina's warnings about CFCs and ozone depletion, followed by the discovery of the ozone hole, spurred the negotiation of several international agreements to protect the ozone layer, leading eventually to a worldwide ban on CFC production in 1996 (for details see Section 12, "Major Laws and Treaties," below). CFCs have lifetimes in the atmosphere of 50-100 years, so it will take that long for past damage to the ozone layer to be undone.
The Antarctic ozone hole is expected to gradually heal over the next several decades, but the effects of climate change pose major uncertainties. Greenhouse gases are well known to cool the stratosphere (although they warm the Earth's surface), and gradual decrease in stratospheric temperatures has been observed over the past decades. Cooling of the polar stratosphere promotes the formation of PSCs and thus the release of chlorine radicals from chlorine reservoirs. The question now is whether the rate of decrease of stratospheric chlorine over the next decades will be sufficiently fast to stay ahead of the cooling caused by increasing greenhouse gases. This situation is being closely watched by atmospheric scientists both in Antarctica and the Arctic.