Ozone catalytic destruction

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).

Two-level diffuser contactors, which involve application of ozone-rich gas to the lower chamber. Lower chamber off-gases are applied to the upper chamber. Offgas treatment from contactors is an important consideration. Methods employed for off-gas treatment include dilution, destruction via granular activated carbon, thermal or catalytic destruction, and recycling. 

Stratospheric ozone production is balanced by various catalytic destruction sequences ... 

In order to calculate the steady-state concentration of ozone in the stratosphere, we need to balance the rate of production of odd oxygen with its rate of destruction. Chapman originally thought that the destruction was due to the reaction O + 03 —> 2O2, but we now know that this pathway is a minor sink compared to the catalytic destruction of 03 by the trace species OH, NO, and Cl. The former two of these are natural constituents of the atmosphere, formed primarily in the photodissociation of water or nitric oxide, respectively. The Cl atoms are produced as the result of manmade chlorofluorocarbons, which are photodissociated by sunlight in the stratosphere to produce free chlorine atoms. It was Rowland and Molina who proposed in 1974 that the reactions Cl + 03 —> CIO + O2 followed by CIO + O —> Cl + O2 could act to reduce the concentration of stratospheric ozone.10 The net result of ah of these catalytic reactions is 2O3 — 3O2. 

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).

These reactions emphasize the importance of solar radiation in NO/NO2 catalytic destruction cycle of ozone. One can immediately see that to provide any reliable observational basis for importance of NO/NO2 in ozone balance, we must have not only NO/NO2 concentration but also its diurnal variation which provides proper check on the time constants for the reactions described in Eqs. (7)-(9). 

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 ... 

Catalytic Destruction There are several different mechanisms of ozone destruction that predominate... 

Ozone depletion Destruction of the stratospheric ozone layer that protects the Earth from harmful effects of ultraviolet radiation. Depletion of the ozone layer is due to the breakdown of certain chlorine- or bromine-containing compounds (chlorofluorocarbons or halons), which break down when they reach the stratosphere and then catalytically destroy ozone molecules. 

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 ... 

Oxidizer Chemical substance that causes oxygen to combine with another chemical substance examples include oxygen and hydrogen peroxide Ozone depletion Destruction of the stratospheric ozone layer that protects the Earth from harmful effects of ultraviolet radiation. Depletion of ozone layer is due to the breakdown of certain chlorine- and/or bromine-containing compounds (chlorofluorocarbons or halons), which break down when they reach the stratosphere and then catalytically destroy ozone molecules Ozone layer Protective layer in the atmosphere, about 15 miles above the ground. The ozone layer absorbs some of the sun s ultraviolet rays, thereby reducing the amount of potentially harmful radiation that reaches the Earth s surface PAHs Polycyclic aromatic hydrocarbons... 

These reactions constitute a source of OH in upper stratosphere, which ultimately controls the upper boundary of the ozone layer through the OH/HO2 catalytic destruction cycles [5] getting rid of about 10% of the existing ozone [6]. The decrease of the ozone layer is a well known problem, see e,g, [3,7-9]. 

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 ... 

Once released in the middle atmosphere, fluorine, chlorine and bromine atoms react rapidly with ozone to form FO, CIO and BrO. Further reactions, which will be presented in the following sections, lead to efficient catalytic destruction of odd oxygen via CIO and BrO. 

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 NO produced through reaction (41) allows the chain of reactions leading to catalytic destruction of stratospheric ozone to be initiated. N2O is the main source of NO in the stratosphere and is, therefore, the important natural regulator of stratospheric ozone. An increase in N2O most probably affects stratospheric ozone concentrations. At present, there is too much uncertainty to predict the extent of this destruction but in any case, the role of the Asian countries in magnification of this process is significant. 

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 ... 

Catalytic Destruction of Stratospheric Ozone, CFCs and Stratospheric Ozone... 

The Catalytic Destruction of Stratospheric Ozone movie eChapter 18.3) d ows how NO lowers the concentration of ozone by catalyzing its decomposition. Based on the movie, describe how you think a higher NO2 concentration in the stratosphere would affect ozone concentration. Explain your reasoning, and use equations where necessary. 

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. 

---