In the lower stratosphere—the region where the PSCs form and chlorine is activated—the concentration of free oxygen atoms is small; few atoms are
produced there on account of the scarcity of the UV-C light that is required to dissociate O2. Furthermore, any atomic oxygen atoms produced in this way immediately collide with the abundant O2 molecules to form ozone, O3. Thus, ozone-destruction mechanisms based upon the O3 O 9: 2 O2 reaction, even when catalyzed, are not important here.
Rather, most of the ozone destruction in the ozone hole occurs via the process called Mechanism II in Chapter 1, with both X and X being atomic chlorine and with the overall reaction being 2 O3 9: 3 O2. Thus the sequence starts with the reaction of chlorine with ozone:
Step 1: Cl O39: ClO O2
Confirmation that ozone destruction occurs by this reaction is evident in Fig- ure 2-4, in which the experimental ClO and O3 concentrations are plotted as a function of latitude for part of the Southern Hemisphere during the spring of 1987. As anticipated if step 1 is the process by which ozone destruction occurs, the two species display opposing trends, i.e., they anticorrelate very closely.
• At sufficient distances away from the South Pole (which is at 90°S), the concentration of ozone is relatively high and that of ClO is low, since chlorine is mainly tied up in inactive forms.
• However, as one travels closer to the Pole and enters the vortex region, the concentration of ClO suddenly becomes high and simultaneously that of O3 falls off sharply (Figure 2-4): most of the chlorine has been activated and most of the ozone has consequently been destroyed. The latitude at which the concentrations both change sharply marks the beginning of the ozone hole, which continues through to the region above the South Pole.
Mechanism I does not create the ozone hole.
1.0
Chlorine monoxide (ppb)
Latitude, approximate
72°S 2.0 1.5 2.5
1.0 0.5 0.5
0 63°S
ClO
O3 ClO
O3
Ozone (ppm)
FIGURE 2-4 Stratospheric ozone and chlorine monoxide concentrations versus latitude near the South Pole (90ºS) on September 16, 1987.
[Source: Reprinted with permission from P. S. Zurer, Chemical and Engineering News (30 May 1988): 16. Copyright 1988 by the American Chemical Society.]
The Chemistry of Ozone Depletion 45
The anticorrelation of ozone and ClO concentrations shown in Figure 2-4 was considered by researchers to be the “smoking gun,” proving that anthro- pogenic chlorine compounds such as CFCs emitted into the atmosphere indeed caused the formation of the ozone hole.
In the next reaction in the Mechanism II sequence, two ClO free radi- cals, produced in two separate step 1 events, combine temporarily to form a nonradical dimer, dichloroperoxide, ClOOCl (or Cl2O2):
Step 2a: 2 ClO 9: Cl!O!O!Cl
The rate of this reaction becomes high, which is important to ozone loss by this mechanism, because the chlorine monoxide concentration has risen steeply due to the activation of the chlorine.
Once the intensity of sunlight has risen appreciably in the Antarctic spring, the dichloroperoxide molecule, ClOOCl, absorbs UV light and splits off one chlorine atom. The resulting ClOO free radical is unstable, and so it subsequently decomposes (in about a day), releasing the other chlorine atom:
Step 2b: ClOOCl UV light 9: ClOO Cl Step 2c: ClOO 9: O2 Cl
Adding steps 2a, 2b, and 2c we see that the net result is the conversion of two ClO molecules to atomic chlorine via the intermediacy of the transient dimer ClOOCl, which corresponds to the second stage of Mechanism II:
Step 2 overall: 2 ClO 9: [ClOOCl] 9: 2 Cl O2
Thus, by these processes, ClO returns to the ozone-destroying form of chlorine, Cl.
If we add the overall reaction step 2 to two times step 1 (the factor of 2 being required to produce the two intermediate ClO species needed in reac- tion 2a so that none remains in the overall equation), we obtain the overall reaction
2 O39: 3 O2
Thus a complete catalytic ozone destruction cycle exists in the lower stratosphere under these special weather conditions, i.e., when a vortex is present. The cycle also requires very cold temperatures, since under warmer conditions ClOOCl is unstable and reverts back to two ClO mol- ecules before it can undergo photolysis, thereby short-circuiting any ozone destruction. Before appreciable sunlight becomes available in the early spring, most of the chlorine exists as ClO and Cl2O2 since step 2b requires fairly intense light levels; such an atmosphere is said to be primed for ozone destruction.
light
Simplified Mechanism II for ozone hole:
Cl O39: ClO O2
2 ClO 9: 2 Cl O2
UV
About three-quarters of the ozone destruction in the Antarctic ozone hole occurs by the mechanism set forth above, in which chlorine is the only catalyst. This ozone-destruction cycle contributes greatly to the creation of the ozone hole. Each chlorine atom destroys about 50 ozone molecules per day during the spring.
The slow step in the mechanism is number 2a, which is the combination of 2 ClO molecules. Since the rate law for step 2a is second order in ClO concentration (i.e., its rate is proportional to the square of the ClO concen- tration), it proceeds at a substantial rate, and the destruction of ozone is significant, only when the ClO concentration is high. The abrupt appear- ance of the ozone hole is consistent with the quadratic rather than linear dependence of ozone destruction upon chlorine concentration by the Cl2O2 mechanism. Let us hope that there are not many more environmental prob- lems whose effects will display such nonlinear behavior and which would similarly surprise us!
PROBLEM 2-1
A minor route for ozone destruction in the ozone hole involves Mechanism II with bromine as X and chlorine as X (or vice-versa). The ClO and BrO free radical molecules produced in these processes then collide with each other and rearrange their atoms to eventually yield O2 and atomic chlorine and bromine. Write out the mechanism for this process, and add up the steps to
determine the overall reaction. ●
PROBLEM 2-2
Suppose that the concentration of chlorine continues to rise in the strato- sphere, but that the relative increase in bromine does not increase propor- tionately. Will the dominant mechanism involving dichloroperoxide or the
“chlorine plus bromine” mechanism of Problem 2-1 become relatively more important or less important as the destroyer of ozone in the Antarctic
spring? ●
PROBLEM 2-3
Why is the mechanism involving dichloroperoxide of negligible importance in the destruction of ozone, compared to the one that proceeds by ClO O,
in the upper levels of the stratosphere? ●
In the lower stratosphere above Antarctica, an ozone destruction rate of about 2% per day occurs each September due to the combined effects of the various catalytic reaction sequences. As a result, by early October almost all
PROBLEM2-1
PROBLEM2-2
PROBLEM2-3
The Chemistry of Ozone Depletion 47
the ozone is wiped out between the altitudes of 15 and 20 km, just the region in which its concentration normally is highest over the Pole. This result is illustrated in Figure 2-5, which shows the measured partial pressure of ozone in October as a function of altitude over the Antarctic in the years before ozone depletion occurred (black curve), in the years when depletion was only partial (dashed curve) and in 2001 (green curve), by which time deple- tion at these levels was total.
In summary, the special vortex weather conditions in the lower strato- sphere above the Antarctic in winter cause denitrification and led to the conversion of inactive chlorine into Cl2 and HOCl. These two compounds produce atomic chlorine when sunlight appears. The chlorine atoms effi- ciently destroy ozone via Mechanism II. Once the vortex disappears in the late spring, the ice particles on which the activation of chlorine com- pounds occurs disappear, the chlorine return to inactive forms, and the hole heals.
The seasonal evolution and decline of the Antarctic ozone hole in 2010 is illustrated in Figure 2-6. Even though stratospheric temperatures were low enough (below 193 K) to produce crystals before July (Figure 2-6c), signifi- cant ozone depletion did not start to occur until August (Figures 2-6a, b), presumably when sunshine first hit the primed chlorine. Maximum depletion and hole size occurred in late September/early October. Stratospheric tem- peratures start to rise appreciably thereafter, and were sufficient to start melt- ing the crystals by mid-October (Figure 2-6c). The hole starts to collapse about a month later.
20 15 25 30
10 5
5 10
Ozone abundance (mPa)
15 0
0
Altitude (kilometers)
FIGURE 2-5 The typical vertical distribution of ozone over Antarctica in mid-spring (October) in 1962–1971 (black curve, before the ozone hole started), in the 1991–2001 period (dashed curve), and in 2001 (green curve).
Ozone partial pressure is in millipascals. [Source: WMO/
UNEP Scientific Assessment of Ozone Depletion 2006.]
Review Questions 2–5 are based on the material in the above sections.
Ozone-hole area (millions of km2)
Jul. 1: 0 (a)
Sep. 25: 22
Dec. 28: 0
Minimum ozone (Dobson units)
Jul. 1: 233 (b)
Dec. 28: 229
Oct. 1: 118
Minimum stratospheric temperature (K)
Aug.
Jul.
Jul. 1: 183 (c)
Jul. 20: 180
Dec. 31: 212
Sep. Oct. Nov. Dec.
FIGURE 2-6 Evolution of the 2010 Antarctic ozone hole. (a) Area covered by the hole, (b) minimum daily amount of overhead ozone, and (c) minimum daily temperature in the lower stratosphere. The dashed line is the temperature at which ice particles form/melt. [Source:
NASA, at http://ozonewatch.
gsfc.nasa.gov/]
Polar Ozone Holes 49
Polar Ozone Holes