Chlorofluorocarbons photolysis

Despite their instability (or perhaps because of it) the oxides of chlorine have been much studied and some (such as CI2O and particularly CIO2) find extensive industrial use. They have also assumed considerable importance in studies of the upper atmosphere because of the vulnerability of ozone in the stratosphere to destruction by the photolysis products of chlorofluorocarbons (p. 848). The compounds to be discussed are ... 

Nitrogen dioxide is about 20 to 50% of the total nitrogen oxides NO, (NO, NOz, HN03, N2Os), while CIO represents about 10 to 15% of the total chlorine species CIO, (Cl, CIO, HCI) at 25 to 30 km. Hence, the rate of ozone removal by CIO, is about equal to that by NO, if the amounts of NO, are equal to those of CIO,. According to a calculation by Turco and Whitten (981), the reduction of ozone in the stratosphere in the year 2022 with a continuous use of chlorofluoromethanes at present levels would be 7%. Rowland and Molina (843) conclude that the ozone depletion level at present is about 1%, but it would increase up to 15 to 20% ifthechlorofluoromethane injection were to continue indefinitely at the present rates. Even if release of chlorofluorocarbons were stopped after a large reduction of ozone were found, it would take 100 or more years for full recovery, since diffusion of chlorofluorocarbons to the stratosphere from the troposphere is a slow process. The only loss mechanism of chlorofluorocarbons is the photolysis in the stratosphere, production of HCI, diffusion back to the troposphere, and rainout. 

The anthropogenic perturbations to the stratosphere include inputs of NOx from nuclear weapons testing, high flying aircraft, and of Cl from the photolysis of chlorofluorocarbons, discussed in detail in a later article, but represented by... 

A major source of anthropogenic chlorine currently results from the photolysis of chlorofluorocarbons and other chlorocarbons in the stratosphere. In the case of CFC13 (also called chlorofluorocarbon-11 or CFC-11) the most likely degradation scheme is the following ... 

The ozone question is complicated by the fact that other chemicals are implicated in its destruction. Chlorofluorocarbons were formerly widely used as propellants in spray cans, and they continue to be used as refrigerants.3 They are extremely stable and long-lived in the environment. However, they too can undergo photolysis in the... 

Large quantities of dichlorodifluoromethane and other chlorofluorocarbons ( Freons ) are used as aerosol propellants, and it has been calculated that there is a danger of reducing the earth s protective ozone layer following photolysis to produce chlorine atoms in the upper atmosphere.184 Milstein and Rowland185 have now measured quantum yields of unity for CF2C12 dissociation [reaction (36)] and for appearance of photo-oxidation products in oxygen. Studies of the... 

At the time of the first tropospheric measurements of man-made halogenated hydrocarbons in the early 1970s, the quantities of the chlorofluorocarbons in the atmosphere were found to be approximately equal to the total amounts ever manufactured. In 1974 Molina and Rowland (Molina and Rowland, 1974 Rowland and Molina, 1975) realized that chlorofluorocarbons (CFCs), manufactured and used by humans in a variety of technological applications from refrigerants to aerosol spray propellants, have no tropospheric sink and persist in the atmosphere until they diffuse high into the stratosphere where the powerful UV light photolyzes them. The photolysis reactions release a chlorine (Cl) atom, for example, for CFCl, (CFC-11) and CF.CU (CFC-12),... 

The first step in the production of ozone, the photolysis of molecular oxygen [reaction (1)], is rate limiting. While ozone production is slow, there are chemical reactions that can rapidly destroy it. One of the major species that is efficient in the removal of ozone is chlorine. The role of chlorine species in the depletion of ozone has been investigated actively since 1974, when Rowland and Molina [2] drew attention to the potential impact of human-made materials known as chlorofluorocarbons (CFCs) on ozone produced in the stratosphere. Chlorofluorocarbons are widely used in our daily life as refrigerants, aerosol propellants, cleaning solvents, and in fire-extinguishing applications. CFCs are stable, chemically inert, and have low toxicity. These properties make CFCs ideal for many applications and account for their wide use. However, the release of chlorine from the photodissociation of chlorofluorocarbons poses a central threat to ozone produced in the stratosphere ... 

Chlorofluorocarbons have unique chemical and thermodynamic properties that make them very attractive for a wide variety of applications, ranging from refrigeration and air conditioning to foam blowing and medical sterilants. Unfortunately, as mounting evidence shows, their inertness allows CFCs to be transported to the stratosphere, where their photolysis releases chlorine atoms that participate in catalytic ozone destruction. Consequently, the manufacture and use of these compounds is being phased out. 

Many chlorine derivatives, including chlorofluorocarbons, can be transformed into a variety of compounds, mostly through oxidation and photolysis by visible solar radiation to produce oxidized species and chlorine radicals respectively. Chlorine radicals are of great interest because they have been specially implicated in the depletion of the ozone layer, as shown with the following equations ... 

As a gas, chlorine dioxide decomposes thermally and in the presence of ultraviolet light to produce the short-lived chlorine oxide radical, CIO. This same radical (a species with an unpaired electron) is also produced by the photolysis of chlorofluorocarbons such as CFCI3 and CF2CI2 and has been implicated in reactions leading to depletion of ozone in the earth s upper atmosphere (Section IX.F). 

The stratospheric region of the atmosphere is located above 15—35 km from the surface of the earth. It contains a deep layer of ozone that acts as a filter of harmful UV radiation of sunlight to reach the earth s surface and thus protects us from hazardous effect of UV radiation. The massive loss of ozone in the stratosphere occurs daily by atmospheric pollutants, UV-induced photolysis of ozone in the presence of man-made chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons (brominated hydrocarbons), CCU, and methylchloroform (CH3CCI3). These halocarbons generate halogen radicals which have active roles for photolysis of ozone. These are also derived from gaseous chlorine and hydrochloric... 

Taketani, F., Takahashi, K., Matsumi, Y. Quantum yields for Cl( Pj) atom formation from the photolysis of chlorofluorocarbons and chlorinated hydrocarbons at 193.3 nm. J. Phys. Chem. A... 

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