ClOOct o2

Inactivation of chlorine

Activation of chlorine on particle surfaces

FIGURE 2-7 A summary of the main ozone destruction reaction cycles operating in the Antarctic ozone hole.

there is some sunshine at that time) and in the summer as well as the spring, and, indeed, there is now some persistence of the depletion from one year to the next,

• The vertical region over which almost total ozone depletion occurs, 12-22 km, has not increased since the mid-1990s.

A review of the possible signs of recovery of the ozone layer published in 2006 pointed out that natural variations, such as the solar cycle and polar temperatures, could mask any trends in stratospheric ozone-hole recovery of the magnitude expected to date and indeed for the next few decades.

The various reactions that lead to catalytic ozone destruction by atomic chlorine by various mechanisms are summarized in Figure 2-7.

Stratospheric Ozone Destruction over the Arctic Region

Given the similarity in climate, it may seem surprising that an ozone hole above the Arctic did not start to form at the same time as one occurred in the Antarctic. Episodes of partial springtime ozone depletion over the Arctic region have occurred several times since the mid-1990s. The phenomenon is less severe than in Antarctica because the stratospheric temperature over the Arctic does not fall as low for as long and air circulation to surrounding areas is not as limited. The flow of tropospheric air over mid-latitude mountain ranges (Himalayas, Rockies) in the Northern Hemisphere creates waves of air that can mix with polar air, warming the Arctic stratosphere. Because the air is generally not as cold, polar stratospheric clouds form less frequently over the Arctic than over the Antarctic and do not last as long. In the past, only small crystals were formed; these are not large enough to fall out of the stratosphere and thereby denitrify it. However, during the extended polar night, the chlorine nitrate and hydrogen chloride do react on the surface of the small particles to produce molecular chlorine, which then dissociates to atomic chlorine and by reaction with an ozone molecule becomes chlorine monoxide, as illustrated in Figure 2-8. Notice that, although HC1 is converted completely in the PSCs, the C10N02, which is present in excess, is not completely eliminated in the stratosphere above the North Pole. Once the PSCs disappear as air temperatures rise, chlorine nitrate initially dominates since it forms rapidly from CIO and nitrogen dioxide. The reaction of atomic chlorine with methane is a slower process, and consequently the HC1 concentration is slower to rise.

FIGURE 2-8 The evolution of stratospheric chlorine chemistry with time above the Arctic in winter and spring. [Source: Redrawn from C. R. Webster et al., Science 261 (1993): 1130.

Polar night

Sunlight

Recovery

HN03 ^NOx C10 + N02^>C10N02 NO + CIO CI + N02 CH4 + CI —>HC\ +CH3

Polar night

Sunlight

Recovery

HN03 ^NOx C10 + N02^>C10N02 NO + CIO CI + N02 CH4 + CI —>HC\ +CH3

FIGURE 2-8 The evolution of stratospheric chlorine chemistry with time above the Arctic in winter and spring. [Source: Redrawn from C. R. Webster et al., Science 261 (1993): 1130.

Time

Before the mid-1990s, the vortex containing the cold air mass above the Arctic broke up by late winter; therefore N02-containing air mixed with vortex air before much sunlight returned to the polar region in the spring. Since the stratospheric air temperature usually rose above -80°C by early March, the nitric acid in the particles was converted back to gaseous nitrogen dioxide before the intense spring sunlight could drive the CI2O2 mechanism. Due to increases in NOz from both these sources, the activated chlorine was mostly transformed back to C10N02 before it could destroy much ozone (Figure 2-8). Thus the total extent of ozone destruction over the Arctic area was much less than that over the Antarctic in the past.

Unfortunately, there have been ominous signs in recent decades that springtime conditions above the Arctic have been changing for the worse, with the result that ozone depletion there accelerated in the lower stratosphere in some years. The Arctic vortex in the winter and spring of 1995-1996 was exceptionally cold and persistent, resulting in significant chlorine-catalyzed losses of ozone as late as mid-April. Large, nitric acid-containing particles were formed, and persisted long enough to fall out of the stratosphere, thereby denitrifying certain regions. In addition, the often-irregular shape of the Arctic vortex means that there are frequent occasions when an "arm" of it passes over a sunlit area in late winter (before the bulk of the vortex is illuminated); temporary ozone depletion occurs within such arms. For example, a portion of the vortex passed across Great Britain during March 1996, producing record lows of 195 DU in northern Scotland.

However, the extent of winter-spring ozone loss over Arctic regions has been very inconsistent, with almost no depletion in some recent winters but significant depletion in others, as indicated in Figure 2-9. The amount of ozone loss correlated linearly with the area associated with polar stratospheric clouds (Figure 2-9). Both the maximum extent of ozone depletion and the maximum vortex area appear to be increasing with time, although these extremes are achieved only every few years when the vortex of cold air above the Arctic remains stable into the late winter and early spring. The greatest ozone depletion over the Arctic observed so far, about 135 DU, occurred in the very cold winter of 2004-2005; that for 2005-2006 was considerably less since the temperatures were not as cold.

For reasons that will be explained in Chapter 6, both the depletion of ozone and the increase in carbon dioxide levels cool the stratosphere, which will lead to even more depletion if cooling occurs in the springtime and thereby extends the period in which PSCs remain. Some scientists predict that recovery from ozone depletion will be slower in the Arctic than in the Antarctic because of the cooling effects of C02 and o3. Scientists do not yet know whether or not the abrupt cooling in the winter of 2004-2005 that produced record ozone depletion was due largely to the effects of increased CO?.

FIGURE 2-9 Loss of overhead ozone over the Artie versus the size of the polar stratospheric cloud in recent years. [Source: Redrawn from M. Rex el a!., "Arctic Winter 2005: Implications for Stratospheric Ozone Loss and Climate Change," Geophysical Research Letters 31 (2006): L04116.J

FIGURE 2-9 Loss of overhead ozone over the Artie versus the size of the polar stratospheric cloud in recent years. [Source: Redrawn from M. Rex el a!., "Arctic Winter 2005: Implications for Stratospheric Ozone Loss and Climate Change," Geophysical Research Letters 31 (2006): L04116.J

Ozone Depletion Definition

Because the magnitude of ozone depletion above the Arctic in some recent winters was about the same as that observed over the South Pole in the early 1980s, some atmospheric scientists have stated that an Arctic ozone hole now forms in some years. Since depletion of overhead ozone is never 100% complete, the definition of what conditions constitute a hole is somewhat arbitrary.

The chemistry underlying mid-latitude losses in stratospheric ozone is discussed in Box 2-1. A systematic view of the various atmospheric chemical reactions discussed in this chapter is given in Chapter 5, after the corresponding reactions in the troposphere have been discussed.

Increases in UV at Ground Level

Experimentally, the amount of UV-B from sunlight (see Chapter 1) reaching ground level increases by a factor of 3 to 6 in the Antarctic during the early part of the spring because of the appearance of the ozone hole. Biologically, the most dangerous UV doses under hole conditions occur in the late spring (November and December), when the Sun is higher in the sky than in earlier months and low overhead ozone values still prevail. Abnormally high UV levels have also been detected in southern Argentina when ozone-depleted stratospheric air from the Antarctic traveled over the area.

Increases in ground-level UV-B intensity have also been measured in the spring months in mid-latitude regions in North America, Europe, and

BOX 2-1

The Chemistry Behind Mid-Latitude Decreases in Stratospheric Ozone

As noted earlier, there was a worldwide decrease of several percent in the steady-state ozone concentration in the stratosphere over nonpolar areas during the 1980s and an additional short-term major decrease from 1992 to 1994. The extent of depletion closely mirrored the total ozone concentration for any given month; the greatest depletion occurred in the March-April period and the least in the early fall.

Scientists have had a harder time tracking down the source of the mid-latitude ozone depletion than that over polar regions. As in Antarctica, almost all the ozone loss in nonpolar regions occurs in the lower stratosphere. Some scientists have speculated that reactions leading to ozone destruction could occur not only on ice crystals but also on the surfaces of other particles present in the lower stratosphere. They suggested that the reactions could occur on cold liquid droplets consisting mainly of sulfuric acid that occur naturally in the lower stratosphere at all latitudes. The liquid droplets would have to be cold enough for them to take up significant amounts of gaseous HC1, or no net reaction would take place. There always exists a small background amount of the acid, due to the oxidation of the naturally occurring gas carbonyl sulfide, COS, some of which survives long enough to reach the stratosphere. However, the dominant though erratic source of the H2S04 at these altitudes is direct injection into the stratosphere of sulfur dioxide gas emitted from volcanoes, followed by its oxidation to the acid. Indeed, the steep decline in ozone in 1992-1993 followed the June 1991 massive eruption of Mt. Pinatubo in the Phillipines, and measurable ozone depletion was noted for several years after the eruption of El Chichon in Mexico in 1982. There were dips significantly below the trend for the ozone levels— both of these periods temporarily increased the concentration of sulfuric acid droplets in the lower stratosphere.

The other relevant reaction that takes place on the surface of the sulfuric acid droplets results in some denitrification of stratospheric air. In the gas-phase steps of the sequence, ozone itself converts some nitrogen dioxide, N02, to nitrogen trioxide, NO3, which then combines with other NG2 molecules to form dinitrogen pentoxide, N205:

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