Ozone Vertical Profiles

Ol M

M Dh

5 10 20

50 100 200

10000 100 200 \ Ozone Partial Pressure, nbar

South Pole (Historical) October y-,\ Mean

TOMS Oct. 5, 1987

Ozone Partial Pressure, nbar

McMurdo (Seasonal)

jf Aug. 29 1987

0 100 200 Ozone Partial Pressure, nbar

0 100 200 Ozone Partial Pressure, nbar

s-1987 Avg. Other Sept./Oct>S°undin8s 1987 *— "

s-1987 Avg. Other Sept./Oct>S°undin8s 1987 *— "

Dakshin Gangotri (Seasonal)

40 80 120 160 Ozone Partial Pressure, nbar

500 0

Syowa -(Historical)

Syowa -(Historical)

25 20 15 10

100 200 Ozone Partial Pressure, nbar

0 100 200 Ozone Partial Pressure, nbar

FIGURE 5.18 Observations of the change in October total ozone profiles over Antarctica (WMO 1994). Historical data at South Pole and Syowa show changes in October mean profiles measured in the 1960s and 1970s as compared to more recent observations. Changes in seasonal vertical profiles are shown at the other stations. Isopleths mapped onto Antartica represent TOMS ozone column measurements on Oct. 5, 1987.

depletion resulting from the CIO* catalytic cycle would manifest itself predominantly at middle and lower latitudes and at altitudes between 35 and 45km. Figure 5.18 shows historical changes in October ozone profiles over Antarctica.

Although the Antarctic has some of the Earth's highest ozone concentrations during much of the year, most of its ozone is actually made in the tropics and delivered, along with the molecular reservoirs of chlorine, to the Antarctic by large-scale air movement. Arctic ozone is similar. The Antarctic stratosphere is deficient in atomic oxygen because of the absence of the intense UV radiation. As air cools during the Antarctic winter, it descends and develops a westerly circulation. This polar vortex develops a core of very cold air. In the winter and early spring the vortex is extremely stable, effectively sealing off air in the vortex from that outside. The polar vortex serves to keep high levels of the imported ozone trapped over Antarctica for the several-month period each year. As the sun returns in September at the end of the long polar night, the temperature rises and the vortex weakens, eventually breaking down in November. Normally the amount of ozone in the polar vortex begins to decrease somewhat as the Antarctic emerges from the months-long austral night in late August and early September. It levels off in October and increases in November. The discovery of the Antarctic ozone hole represented a significant change in historic patterns; the springtime ozone levels decreased to unprecedented levels, with each succeeding year being, more or less, worse than the year before.

It was initially suggested that the Antarctic ozone hole could be explained on the basis of solar cycles or purely atmospheric dynamics. Neither explanation was consistent with observed features of the ozone hole. Chemical explanations based on the gas-phase catalytic cycles described above were advanced. As noted, little ozone is produced in the polar stratosphere as the low Sun elevation (large solar zenith angle) results in essentially no photodissociation of 02. Thus catalytic cycles that require oxygen atoms were not able to explain the massive ozone depletion. Moreover, CFCs and halons would be most effective in ozone depletion in the Antarctic stratosphere at an altitude of about 40 km, whereas the ozone hole is sharply defined between 12 and 24 km altitude. Also, existing levels of CFCs and halons could lead at most to an 03 depletion at 40 km of 5-10%, far below that observed.

Molina and Molina (1987) proposed that a mechanism involving the CIO dimer, C1202, might be involved:

(Bimolecular reactions of CIO and CIO are slow and can be neglected. The termolecular reaction 1 is facilitated at higher pressures, that is, larger M, and low temperature.) C1202 has been shown to have the symmetric structure ClOOCl (McGrath et al. 1990). Photolysis of ClOOCl has two possible channels:

Only reaction 4a can lead to an ozone-destroying cycle. Indeed, reaction 4a is the main photolysis path (Molina et al., 1990). The ClOO product rapidly decomposes to yield a CI atom and 02,

Thus reaction 2 is seen to be a composite of reactions 4a and 5, leading to the release of both chlorine atoms from C1202.

Two similar cycles involving both CIO, and BrO, cycles also can take place:

CIO + BrO —* BrCl + 02 BrCl + hv —> Br + CI Cl + 03^C10 + 02 Br + 03 —> BrO + 02 Net: 203—> 3 02

CIO + BrO —► ClOO + Br ClOO + M —> CI + 02 + M Cl + 03^C10 + 02 Br + 03 —> BrO + 02 Net: 203 —> 3 02

Note that atomic oxygen is not required in either cycle.

If sufficient concentrations of CIO and BrO could be generated, then the three cycles shown above could lead to substantial 03 depletion. However, gas-phase chemistry alone does not produce the necessary CIO and BrO concentrations needed to sustain these cycles.

5.7.1 Polar Stratospheric Clouds

The stratosphere is very dry and generally cloudless. The long polar night, however, produces temperatures as low as 183 K (—90°C) at heights of 15-20 km. At these temperatures even the small amount of water vapor condenses to form polar stratospheric clouds (PSCs), seen as wispy pink or green clouds in the twilight sky over polar regions.

The conceptual breakthrough in explaining the Antarctic ozone hole occurred when it was realized that PSCs provide the surfaces on which halogen-containing reservoir species are converted to active catalytic species. Then, intense interest ensued: what are PSCs made of—pure ice or ice mixed with other species? Nitrate was detected in PSCs, and both laboratory and theoretical studies showed that nitric acid trihydrate, HN03-3H20, denoted NAT, is the thermodynamically stable form of HN03 in ice at polar stratospheric temperature (Peter 1996). It was also discovered that some PSCs are liquid particles composed of supercooled ternary solutions of H2S04, HN03, and H20.

An important implication of the fact that PSCs contain nitrate is that, if the particles are sufficiently large, they can fall out of the stratosphere and thereby permanently remove nitrogen from the stratosphere. The removal of nitrogen from the stratosphere is termed denitrification. If nitrate-containing PSCs sediment out of the stratosphere, then that could lead to an appreciably lower supply of nitrate for possible return to NO, (and conversion of CIO to C10N02).

Stratospheric NAT particles were first detected in situ in the 1999-2000 SAGE III "ozone loss and validation experiment," carried out in the stratosphere over northern Sweden (Voight et al. 2000; Fahey et al. 2001). The particles identified were large enough (I -20 |im in diameter) to be able to fall a substantial distance before evaporating. The fall of nitrate-containing PSC particles was therefore established as the mechanism by which the stratosphere is denitrified.

5.7.2 PSCs and the Ozone Hole

Gas-phase chemistry associated with the CIO* and NO* cycles is not capable of explaining the polar ozone hole phenomenon. The ozone hole is sharply defined between about 12 and 24 km altitude. Polar stratospheric clouds occur in the altitude range 10-25 km. Ordinarily, liberation of active chlorine from the reservoir species HC1 and C10N02 is rather slow, but the PSCs promote the conversion of the major chlorine reservoirs, HC1 and C10N02, to photolytically active chlorine. The first step in the process, absorption of gaseous HC1 by PSCs, occurs very efficiently. This step is followed by the heterogeneous reaction of gaseous C10N02 with the particle, where (s) denotes a species on the surface of the ice, with the liberation of molecular chlorine as a gas and the retention of nitric acid in the particles. This is the most important chlorine-activating reaction in the polar stratosphere. (The gas-phase reaction between HC1 and C10N02 is extremely slow.) The solubility of HC1 in normal stratospheric aerosol, 50-80 wt% H2S04 solutions, is low. When stratospheric temperatures drop below 200 K, the stratospheric particles absorb water and allow HC1 to be absorbed, setting the stage for reaction 1.

Gaseous Cl2 released from the PSCs in reaction 1 rapidly photolyzes, producing free CI atoms, while the other product, HN03, remains in the ice, leading to the overall removal of nitrogen oxides from the gas phase. This trapping of HN03 further facilitates catalytic 03 destruction by removing NO* from the system, which might otherwise react with CIO to form C10N02. The net result of reaction 1 is

Net: HCl(s) + N02 + 2 03 —>C10 + HN03(s) + 2 02

The reaction between C10N02 and H20(s), which is very slow in the gas phase, also can occur:

The gaseous HOC1 rapidly photolyzes to yield a free chlorine atom. HOC1 itself can undergo a subsequent heterogeneous reaction (Abbatt and Molina 1992)

(reaction 1)

HCl(s) + ClONOz —► Cl2 + HN03(s) Cl2 +hv^2C\ 2[Cl + 03 ^ CIO + 02] CIO + N02 + M C10N02 + M

(reaction 1) (reaction 2) (reaction 3) (reaction 4)

(reaction 5)

(reaction 6)

NOj.

Polar Vortex Antarctic

FIGURE 5.19 Catalytic cycles in 03 depletion involving polar stratospheric clouds (PSCs).

The mechanism of ozone destruction in the polar stratosphere is thus as follows. Two ingredients are necessary: cold temperatures and sunlight. The absence of either one of these precludes establishing the destruction mechanism. Cold temperatures are needed to form polar stratospheric clouds to provide the surfaces on which the heterogeneous reactions take place. The reservoir species C10N02 and N205 react heterogeneously with PSCs on which HC1 has been absorbed to produce gaseous Cl2, HOC1, and C1N02. Sunlight is then required to photolyze the gaseous Cl2, HOC1, and C1N02 that are produced as a result of the heterogeneous reactions. At sunrise, the Cl2, C1N02, and HOC1 are photolyzed, releasing free CI atoms that then react with 03 by reaction 3. Figure 5.19 depicts the catalytic cycles and the role of PSCs. At first, the CIO just accumulates (recall that the O atoms normally needed to complete the cycle by CIO + O are essentially absent in the polar stratosphere). However, once the CIO concentrations are sufficiently large, the three catalytic cycles presented in the beginning of this section occur. The ClO-CIO cycle accounts for ~ 60% of the Antarctic ozone loss, and the ClO-BrO cycles account for ~ 40% of the removal.

Furthermore, since much of the NO, is tied up as HN03 in PSCs, the normally moderating effect of NO,, through formation of C10N02, is absent. In fact, massive ozone depletion requires that the abundance of gaseous HN03 be very low. The major process removing HN03 from the gas phase at temperatures below about 195 K is the formation of NAT PSCs. Of course, HN03 is also removed by HCl(s) + C10N02, but that is only the NO, associated with C10N02, and removal of HN03 by this reaction alone would not be sufficient to accomplish the large-scale denitrification that is required; that requires the formation of PSCs and the removal of the nitrogen associated with them. PSCs exhibit a bimodal size distribution in the Antarctic stratosphere, with most of the nitrate concentrated in particles with radii > 1pm. The bimodal size distribution sets the stage for efficient denitrification, with nitrate particles either falling on their own or serving as

Inactive surface Active _ gas-phase_ Inactive

' chlorine reactions chlorine

i \

reactions

i i hcl \j

hcl

A ! (2cu+ci0 +20,0,) 7

= J\

i t ^^^ i

:iono2

Fall early - Winter- late

Spring

Time

Denitrification & dehydration^

Surface processing

Chlorine catalyzed_

ozone destruction

Inactive chlorine

Chlorine catalyzed_

ozone destruction

Inactive chlorine

FIGURE 5.20 Schematic of photochemical and dynamical features of polar ozone depletion [WMO (1994), as adapted from Webster et al. (1993)]. Upper panel shows the conversion of chlorine from inactive reservoir forms, C10N02 and HC1, to active forms, CI and CIO, in the winter in the lower stratosphere, followed by reestablishment of the inactive forms in spring. Corresponding stages of the polar vortex are indicated in the lower panel, where the temperature scale represents changes in the minimum temperatures in the lower polar stratosphere.

FIGURE 5.20 Schematic of photochemical and dynamical features of polar ozone depletion [WMO (1994), as adapted from Webster et al. (1993)]. Upper panel shows the conversion of chlorine from inactive reservoir forms, C10N02 and HC1, to active forms, CI and CIO, in the winter in the lower stratosphere, followed by reestablishment of the inactive forms in spring. Corresponding stages of the polar vortex are indicated in the lower panel, where the temperature scale represents changes in the minimum temperatures in the lower polar stratosphere.

nuclei for condensation of ice (Salawitch et al. 1989). Figure 5.20 is a schematic of the time evolution of the polar stratospheric chlorine chemistry.

In the Antarctic winter vortex, vertical transport within the vortex as well as horizontal transport across the boundaries of the vortex is slower than the characteristic time for the ozone-depleting reactions. The local rate of loss of ozone can be approximated as that of the rate-determining step:

CIO levels are typically elevated by a factor of 100 over their normal levels in the 12 to 24 km altitude range. With CIO mixing ratios in the range of 1 to 1.3 ppb, the above rate predicts complete 03 removal in about 60 days.

Eventually, as the polar air mass warms through breakup of the polar vortex and by absorption of sunlight, the PSCs evaporate, releasing HN03. The nitric acid vapor photolyzes and reacts with OH to restore gas-phase NO*:

Gaseous N02 reacts with CIO to again tie up active chlorine as C10N02. The C10/HC1 ratio is indicative of the course of the ozone destruction process. Most of the atomic chlorine in the stratosphere reacts either with 03, or with CH4. The ratio of rate constants, &ci+o3 Aci+cu,. is about 900 at 200 K. The CI + CH4 reaction is the principal source of stratospheric HC1, and this reaction governs the recovery rate of HC1 following its loss from the PSC-catalyzed reaction HCl(s) + C10N02. Once PSC chlorine conversion has ceased, HC1 recovers to its original amount with a characteristic time of A [HC1]/^ci+cut [C1][CH4]. This recovery time is estimated to be about ninety 12-h days, assuming a mean temperature of 200 K, a mean [CI] of 0.015 ppt, and a mean [CH4] of 0.8 ppm during the recovery process (Webster et al. 1993). Because an important contribution to stratospheric warming is solar absorption by 03, and because 03 levels have been depleted, the usual warmup is delayed, prolonging the duration of the ozone hole.

After discovery of the Antarctic ozone hole a number of field campaigns were mounted to measure concentrations of important species in the ozone depletion cycle. The key active chlorine species in the polar ozone-destroying catalytic cycle is CIO. Simultaneous measurement of CIO and 03, as shown in Figure 5.21, provided conclusive evidence linking CIO generation to ozone loss. At an altitude near 20 km, CIO mixing ratios reached 1 ppb, several orders of magnitude higher than those in the midlatitude stratosphere, indicating almost total conversion of chlorine to active species within the polar vortex (Anderson et al. 1991). Significant reductions in the column abundances of HC1, C10N02, and N02 are equally important evidence as elevated CIO in verifying the mechanism of catalytic ozone destruction.

FIGURE 5.21 Simultaneous measurements of 03 and CIO made aboard the NASA ER-2 aircraft on a flight from Punta Arenas, Chile (53-72°S), on September 16, 1987 (Anderson et al. 1989). As the plane entered the polar vortex, concentrations of CIO increased to about 500 times normal levels while 03 plummeted.

FIGURE 5.21 Simultaneous measurements of 03 and CIO made aboard the NASA ER-2 aircraft on a flight from Punta Arenas, Chile (53-72°S), on September 16, 1987 (Anderson et al. 1989). As the plane entered the polar vortex, concentrations of CIO increased to about 500 times normal levels while 03 plummeted.

ANTARCTIC (S.H.) Powr Vortex Minimum Temperature, 20 km

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