Encyclopedia of Environmental Science and Engineering, Volume I and II

ATMOSPHERIC CHEMISTRY 133

to freeze consumption of CFCs at 1986 levels, effective in

September 1988, and requirements to reduce consumption by

20% by 1992 and by an additional 30% by 1999. In November

1992, the Montreal Protocol on Substances That Deplete the

Ozone Layer revised the phase-out schedule for CFCs to a

complete ban on production by January 1, 1996. In November

1995, additional amendments were adopted to freeze the use

of hydrogen-containing CFCs (HCFCs) and methyl bromide

(CH 3 Br) and eliminate their use by 2020 and 2010, respec-

tively. These agreements were very important steps to address-

ing the problem of CFCs in the atmosphere. This has also led

to major efforts to find environmentally safe alternatives to

these compounds for use in various applications.

Antarctic Ozone

Farman et al. (1985) observed a very significant downward

trend in the total ozone column measured over Halley Bay,

Antarctica (Figure 9). Solomon (1988) has reviewed this

and other data from Antarctica, and has concluded that there

has been a real decrease in the ozone column abundance in

the South Polar region. Other data suggest that the bulk of

the effect on ozone abundance is at lower altitudes in the

stratosphere, between about 12 and 22 km, where the strato-

spheric ozone concentrations decrease quickly and return to

near normal levels as the springtime warms the stratosphere.

The subsequent discussion will outline some of the chemi-

cal explanations for these observations. Some atmospheric

dynamical explanations of the ozone hole have been pro-

posed, but these are not believed to provide an adequate

explanation of the observations.

Figure 10 shows plots of results from flights in the Antarctic

region during August and September 1987 (Anderson et al.,

1991). Ozone- and ClO-measurement instru mentation was

flown into the polar stratosphere on a NASA ER-2 aircraft

(a modified U-2). This figure shows a sharp increase in ClO

concentration as one goes toward the pole and a similar sharp

decrease in stratospheric ozone. On the September 16th flight,

the ClO concentration rose from about 100 to 1200 ppt while

the ozone concentration dropped from about 2500 to 1000

ppb. This strong anticorrelation is consistent with the catalytic

ozone-destruction cycle, reactions (51) and (52).

Solomon (1988) has suggested that polar stratospheric

clouds (PSCs) play an important role in the explanation of

the Antarctic ozone hole. PSCs tend to form when the tem-

perature drops below about 195 K and are generally observed

in the height range from 10 to 25 km. The stratosphere is suf-

ficiently dry that cloud formation does not occur with water-

forming ice crystals alone. At 195 K, nitric acid-trihydrate

will freeze to form cloud particles, and there is inadequate

water alone to form ice, until one goes to an even lower tem-

perature. Significant quantities of nitric acid are in the cloud

particles below 195 K, while they would be in the gas phase

at higher temperatures. PSCs are most intense in the Antarctic

winter and decline in intensity and altitude in the spring, as

the upper regions of the stratosphere begin warming. It was

proposed that HCl(a) ((a)—aerosol phase), absorbed on

the surfaces of PSC particles, and gaseous chlorine nitrate,

ClONO 2 (g), react to release Cl 2 to the gas phase:

ClONO 2 (g)  HCl(a) → Cl 2 (g)  HNO 3 (a) (54)

Subsequent research identified several other gas-surface

reactions on PSCs that also play an important role in polar

stratospheric ozone depletion

ClONO 2 (g)  H 2 O(a) → HOCl(g)  HNO 3 (a,g) (55)

HOCl(g)  HCl(a) → Cl 2 (g)  H 2 O(a) (56)

N 2 O 5 (g)  H 2 O(a) → 2HNO 3 (a,g) (57)

Reactions (55) and (56) have the same net effect as reaction

(54), while reaction (57) removes reactive nitrogen oxides

from the gas phase, reducing the rate of ClO deactivation by

ClO  NO 2 → ClONO 2 (58)

Webster et al. (1993) made the first in situ measurement of

HCl from the ER-2 aircraft. These results suggested that HCl

is not the dominant form of chlorine in the midlatitude lower

stratosphere, as had been believed. These results suggested

that HCl constituted only about 30% of the inorganic chlo-

rine. This has led to the belief that ClONO 2 may be present

at concentrations that exceed that of HCl.

Figure 11 shows a chronology of the polar ozone-

depletion process. As one enters the polar night, ClONO 2

is the dominant inorganic chlorine-containing species, fol-

lowed by HCl and ClO. Due to the lack of sunlight, the

temperature decreases and polar stratospheric clouds form,

permitting reactions (54), (55), and (56) to proceed, produc-

ing gaseous Cl 2. Both HCl and ClONO 2 decrease. As the sun

rises, the Cl 2 is photolyzed, producing Cl atoms that react

1950 1960 1970 1980 1990 2000

Ye a r

100

150

200

250

300

350

Total column ozone (DU)

FIGURE 9 Average total column ozone measured in October

at Halley Bay, Antarctica, from 1957 to 1994. Ten additional

years of data are shown in this plot beyond the period pre-

sented by Farman et al. (1985). From Finlayson-Pitts and Pitts

(2000). With permission.

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