Catalytic destruction of ozone

Fig. 11-3. Stratospheric ozone and CIO concentrations at an altitude of 18 km measured by aircraft flying south over Antarctica on September 27,1987. The dramatic decrease in ozone at a latitude of 71 degrees is attributed to the role of CIO in catalytic destruction of ozone. Adapted from Anderson et al. (13).

Fig. 3.9. Photochemical formation and non-catalytic destruction of ozone. UV-C radiation (200-280 nm wavelength) UV-B radiation (280-320 nm wavelength). Note how high-quality energy (UV radiation) is converted into lower quality energy (heat). Catalysts such as freons or nitrogen oxides can destroy ozone (e.g. Cl + 03 —> CIO + 02).

Recent discussions of stratospheric chemistry have dealt with the effect of freons on ozone balance through a Cl/ClO catalytic destruction of ozone. The fundamental absorption band of CIO is measured to be at 11 /xm. Isotopically substituted CO2 laser based OA absorption measurement technique should allow us to carry out fundamental measurements on CIO and its diurnal variation in the stratosphere to provide yet another important parameter (in addition to NO above) in the stratospheric ozone chemistry. 

What role do chlorofluorocarbons play in the catalytic destruction of ozone ... 

The atmospheric chemistry of the organobromides is similar to that of the organochlo-rides degradation ultimately produces bromine atoms which may participate in catalytic destruction of ozone through a BrOx catalytic cycle (reactions 12 and 13). 

Stratospheric ozone Emission of ozone-depleting compounds (CFCs, Halons) Chemical reaction release of C1 and Br in stratosphere Catalytic destruction of ozone in stratosphere Skin and crop damage, damage to materials Ozone Depletion Potential (ODP)... 

Similar chain reactions can be written for reactions involving R02- In contrast, when relatively little NO is present, as in the remote atmosphere, the following cycle can dominate over ozone production leading to the catalytic destruction of ozone, viz ... 

Potentially, the most important effect of reactive halogen species maybe that their chemistry may lead to the catalytic destruction of ozone via two distinct cycles Cycle I ... 

Definitely yes. In the troposphere, to which I think you may be referring, N2O is relatively inert. It is in the stratosphere where reaction (18) plays such an important role releasing the nitrogen oxides which dominate the catalytic destruction of ozone. 

We close this section by summarizing the chemistry of chlorine and ozone in the Earth s stratosphere. There are three essential elements in the Rowland-Molina mechanism linking the release of fluorocarbons to the gas phase catalytic destruction of ozone in the stratosphere ... 

The catalytic destruction of ozone by NO/NO2 also illustrates rate-limiting steps. At 30 km altitude, the rate constants for three bimolecular ozone destruction reactions are given below. The units are all cm /molecs ... 

The catalytic destruction of ozone by NO is the most important process that occurs in the middle and upper stratosphere. We should stress here that this process is possible even in unpolluted atmosphere since small amounts of nitrous oxide, N2O, from biological denitrification have always been present in the stratosphere, which is the precursor of nitric oxide, NO, in reaction with atomic oxygen (reaction (11)). Most collisions with atomic oxygen form N2 and O2, but a few form NO (see also Section 4)... 

The catalytic destruction of ozone occurs via a two-step mechanism, where X can be any of several species ... 

The catalytic destruction of ozone in the stratosphere involves reactions between gases there, so it is an example of homogeneous catalysis. The most important catalyst for this process is chlorine. Much of the chlorine present in the stratosphere comes from CFC molecules that were released in the troposphere and slowly migrated to the stratosphere. (Because they are very unreactive at ground level, nearly all CFCs that are released into the atmosphere eventually find their way to the stratosphere.) Upon absorption of UV light, the CFCs initiate a catalyzed reaction mechanism ... 

However, e addition of Cl (from synthetic chlorofluorocarbons) to the upper atmosphere has resulted in another pathway by which O3 can be destroyed. The first step in this pathway—called the catalytic destruction of ozone—is the reaction of Cl with O3 to form CIO and O2. 

In the case of the catalytic destruction of ozone, the catalyst speeds up a reaction that we do not want to happen. Most of the time, however, catalysts are used to speed up reactions that we do want to happen. For example, your car most likely has a catalytic converter in its exhaust system. The catalytic converter contains a catalyst that converts exhaust pollutants (such as carbon monoxide) into less harmful substances (such as carbon dioxide). These reactions occur only with the help of a catalyst because they are too slow to occur otherwise. 

In the catalytic destruction of ozone (red), the activation barrier for the rate-limiting step is much lower than in the nncatalyzed process (blue). 

In this reaction, an ozone molecule collides with an oxygen atom to form two oxygen molecules in a single elementary step. The reason that Earth has a protective ozone layer in the upper atmosphere is that the activation energy for this reaction is fairly high and the reaction, therefore, proceeds at a fairly slow rate the ozone layer does not rapidly decompose into O2. However, the addition of Cl atoms (which come from the photodissociation of man-made chlorofluorocarlxms) to the upper atmosphere makes available another pathway by which O3 can be destroyed. The first step in this pathway—called the catalytic destruction of ozone—is the reaction of Cl with O3 to form CIO and O2 ... 

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