Ozone Catalytic Cycles

There are three important catalytic cycles or three ways to lose your ozone 4 

4 With apologies to Paul Simon s song Fifty Ways to Leave Your Lover.  

The N0/N02 pathway. One source of NO is the reaction of N20 with excited oxygen atoms O  

In this case, N20 (called nitrous oxide or laughing gas) has natural sources, such as emissions from swamps and other oxygen-free ( anoxic ) waters and soils. The oxygen atoms in this reaction can come from several tropospheric photolytic reactions involving OH or OOH. Another source of NO is the thermal reaction between N2 and 02  

This reaction requires very high temperatures, and thus, it occurs mostly in combustion systems such as automobile and jet engines. This reaction is also likely to occur in a thermonuclear explosion, although the production of NO from such an event would be the least of our problems. 

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Perfluorinated ethers and perfluorinated tertiary amines do not contribute to the formation of ground level ozone and are exempt from VOC regulations (32). The commercial compounds discussed above have an ozone depletion potential of zero because they do not contain either chlorine or bromine which take part in catalytic cycles that destroy stratospheric ozone (33). 

Reactions 5 and 6 constitute a catalytic cycle because the radical NO that attacks O3 is regenerated by the reaction of NO2 with an O-atom. The net effect is the removal of one O3 molecule and one O-atom. Thus, although the concentration of NO and NO2 (or NOx) in the stratosphere is small, each NO molecule can destroy thousands of ozone molecules before being scavenged by a reaction such as the following ... 

The catalytic cycle described earlier (reactions 8 and 9) cannot explain the rapid depletion of ozone over the South Pole, because reaction 9 requires free oxygen atoms, which are too scarce in the polar stratosphere to react at any appreciable rate with QO. Several catalytic cycles that do not require oxygen atoms have been suggested as being at work over Antarctica. 

It is interesting to compare the rate constants of the oxygen-only ozone destruction reaction with those of the catalytic ozone destruction cycle. The rate constants for reactions 4-6 at 30 km are given below in units of cm molecules s . 

Halogen oxide radicals such as CIO and BrO are important reactive intermediates in the catalytic cycles of ozone destruction in the middle and upper stratosphere. The first absorption band CIO(/l211 <— X2 I) starts from 318 nm and has a series of vibronic bands that converge to a broad continuum at wavelengths shorter than 264nm (Fig. 8).98-101 In this continuum region four dissociation pathways are thermodynamically possible,33... 

The chlorofluorocarbon effect on the ozone layer illustrates another chemical concern—the special problem that can arise when materials released into the environment are able to act as catalysts. If every chlorine atom generated in the upper atmosphere simply destroyed one ozone molecule, the effect would be minimal. But chemists have elucidated the catalytic cycle by which each chlorine atom destroys thousands of ozone molecules. It is particularly important for chemists to study and understand which substances can have such catalytic effects— and to learn how to prevent the release of such substances into the environment. 

In the late 1960s, direct observations of substantial amounts (3ppb) of nitric acid vapor in the stratosphere were reported. Crutzen [118] reasoned that if HN03 vapor is present in the stratosphere, it could be broken down to a degree to the active oxides of nitrogen NO (NO and N02) and that these oxides could form a catalytic cycle (or the destruction of the ozone). Johnston and Whitten [119] first realized that if this were so, then supersonic aircraft flying in the stratosphere could wreak harm to the ozone balance in the stratosphere. Much of what appears in this section is drawn from an excellent review by Johnston and Whitten [119]. The most pertinent of the possible NO reactions in the atmosphere are... 

The rate of reaction (8.165) is known accurately only at room temperature, and extrapolation to stratospheric temperature is uncertain nevertheless, the extrapolated values indicate that the N03 catalytic cycle [reactions (8.165) and (8.166)] destroys ozone faster than the N02 cycle below 22 km and in the region where the temperature is at least 220 K. 

It is possible to similarly estimate the effect of the various cycles upon ozone destruction. The results can be summarized as follows between 15 and 20 km, the N03 catalytic cycle dominates between 20 and 40 km, the N02 cycle dominates between 40 and 45km, the N02, HO, and O mechanisms are about equal and above 45 km, the HO reactions are the controlling reactions. 

In 1974, Cicerone and Stolarski suggested that if there were sources of atomic chlorine in the stratosphere, the following catalytic ozone destruction cycle... 

The involvement of reactive nitrogen, reactive hydrogen, and reactive chlorine in catalytic cycles that destroy ozone has been known for about 20 years. These cycles have the form... 

Figure 7. Calculated ozone production and loss rates for two different conditions from the AER two-dimensional model. Production and loss rates above 20 km are diurnally averaged loss rates for the spring equinox at 30°N. Midday loss rates are approximately two times larger. Production and loss rates for midday below 20 km are calculated for the chemically perturbed region over Antarctica on September 16,1987. The catalytic cycles responsible for the loss are explained in the text. Although ozone loss occurs at higher altitudes over Antarctica, in situ observations extend only to 19 km.

In the weak sunlight of polar spring, these gas-phase chlorine species release their chlorine atoms, which attack ozone almost exclusively. The catalytic cycle that requires a reaction between CIO and O is not very effective, because few oxygen atoms exist in these cold, relatively dark regions. Instead, CIO reacts with another CIO molecule, forming Cl202, which can then be easily photolyzed by the weak visible sunlight that penetrates the atmosphere. 

A second catalytic cycle involves the CIO and BrO radicals, which react some form Cl and Br atoms, which can then react with ozone to form CIO and BrO again. 

Approximately one-half of the total reaction of CIO and BrO results in the destruction of ozone. Other mechanisms exist, such as a catalytic cycles that are rate-limited by the reaction between CIO and O and between CIO and H02 (25), but the contribution from these reactions is small in the polar regions. 

In the preceding section it was indicated that sunlight is the primary initiator for the buildup of ozone and other oxidants as a result of a rapid conversion of NO to N02 by catalytic cycles involving H02 and R02 radicals. 

Pollution in the stratosphere may induce the reduction of ozone without participation of sunlight in the case of NO injected directly into the stratosphere by SST s. while in the case of chlorofluoromethanes, their photodissociation by sunlight to produce Cl atoms is required for the reduction of ozone by a catalytic cycle involving Cl and CIO. The time scale required... 

Since kU)i 1 (),()(X) x kliH) at 230 K, an approximate temperature in the stratosphere, even a very small fraction of N02, 0.01% of 03, present in the stratosphere is as eflective as destroying ozone by (VI-100). Besides the NO- NOj cycle, another effective catalytic cycle is a Cl-CIO chain [Molina and Rowland, (711) Crutzen (252)]. 

Minor species observed in the stratosphere are shown in Fig. VIII-11. Of these it is now believed that nitric oxide is the most effective agent to destroy ozone by a catalytic cycle... 

The destruction of ozone by another catalytic cycle (an HO cycle) is. n mated to be about 10% of the NO cycle... 

Ozone reductions in the stratosphere resulting from the increases in NOy and H2O are due to enhanced ozone loss through the HOx and NOx catalytic cycles. 

Johnston (543) and others have proposed that the most important catalytic cycle responsible for ozone destruction is a N0-N02 cycle... 

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