OZONE LAYER DESTRUCTION

14.4. OZONE LAYER DESTRUCTION

Recall from Section 9.9 that stratospheric ozone, O 3 , serves as a shield to absorb harmful ultraviolet radiation in the stratosphere, protecting living beings on the earth from the effects of excessive amounts of such radiation. The two reactions by which stratospheric ozone is produced are,

(14.4.1) O + O 2 + M

( λ < 242.4 nm)

(14.4.2) and it is destroyed by photodissociation,

→ O 3 + M (energy-absorbing N 2 or O 2 )

(14.4.3) and a series of reactions from which the net result is the following: O + O 3 → 2O 2

( λ < 325 nm)

The concentration of ozone in the stratosphere is a steady-state concentration resulting from the balance of ozone production and destruction by the above processes. The quantities of ozone involved are interesting. A total of about 350,000 metric tons of ozone are formed and destroyed daily. Ozone never makes up more than a small fraction of the gases in the ozone layer. In fact, if all the atmosphere’s ozone were in a layer at 273 K and 1 atm, it would be only 3 mm thick!

Ozone absorbs ultraviolet radiation very strongly in the region 220-330 nm. Therefore, it is effective in filtering out dangerous UV-B radiation, 290 nm < λ <

320 nm. (UV-A radiation, 320 nm-400 nm, is relatively less harmful and UV-C radi- ation, < 290 nm does not penetrate to the troposphere.) If UV-B were not absorbed by ozone, severe damage would result to exposed forms of life on the earth. Absorption of electromagnetic radiation by ozone converts the radiation’s energy to heat and is responsible for the temperature maximum encountered at the boundary between the stratosphere and the mesosphere at an altitude of approximately 50 km. The reason that the temperature maximum occurs at a higher altitude than that of the maximum ozone concentration arises from the fact that ozone is such an effective absorber of ultraviolet light, so that most of this radiation is absorbed in the upper stratosphere where it generates heat and only a small fraction reaches the lower altitudes, which remain relatively cool.

Increased intensities of ground-level ultraviolet radiation caused by stratospheric ozone destruction would have some significant adverse consequences. One major effect would be on plants, including crops used for food. The destruction of micro- scopic plants that are the basis of the ocean’s food chain (phytoplankton) could severely reduce the productivity of the world’s seas. Human exposure would result in an increased incidence of cataracts. The effect of most concern to humans is the elevated occurrence of skin cancer in individuals exposed to ultraviolet radiation. This is because UV-B radiation is absorbed by cellular DNA (see Chapter 22) resulting in photochemical reactions that alter the function of DNA so that the genetic code is improperly translated during cell division. This can result in uncontrolled cell division leading to skin cancer. People with light complexions lack protective melanin, which absorbs UV-B radiation, and are especially susceptible to its effects. The most common type of skin cancer resulting from ultraviolet exposure is squamous cell carcinoma, which forms lesions that are readily removed and has little tendency to spread (metastasize). Readily metastasized malignant melanoma caused by absorption of UV-B radiation is often fatal. Fortunately, this form of skin cancer is relatively uncommon.

The major culprit in ozone depletion consists of chlorofluorocarbon (CFC) compounds, commonly known as “Freons.” These volatile compounds have been used and released to a very large extent in recent decades. The major use associated with CFCs is as refrigerant fluids. Other applications have included solvents, aerosol propellants, and blowing agents in the fabrication of foam plastics. The same extreme chemical stability that makes CFCs nontoxic enables them to persist for years in the atmosphere and to enter the stratosphere. In the stratosphere, as discussed in Section 12.7, the photochemical dissociation of CFCs by intense ultraviolet radiation,

(14.4.5) yields chlorine atoms, each of which can go through chain reactions, particularly the

CF 2 Cl 2 + h ν → Cl • + CClF 2•

following: Cl • + O 3 → ClO • + O 2 (14.4.6) ClO • + O → Cl • + O 2

The net effect of these reactions is catalysis of the destruction of several thousand molecules of O 3 for each Cl atom produced. Because of their widespread use and persistency, the two CFCs of most concern in ozone destruction are CFC-11 and CFC-12, CFCl 3 , and CF 2 Cl 2 , respectively. Even in the intense ultraviolet radiation of the stratosphere the most persistent chlorofluorcarbons have lifetimes of the order of 100 years.

The most prominent instance of ozone layer destruction is the so-called “Antarctic ozone hole” that has shown up in recent years. This phenomenon is mani- fested by the appearance during the Antarctic’s late winter and early spring of severely depleted stratospheric ozone (up to 50%) over the polar region. The reasons

why this occurs are related to the normal effect of NO 2 in limiting Cl-atom-catalyzed destruction of ozone by combining with ClO,

ClO + NO 2 → ClONO 2 (14.4.8) In the polar regions, particularly Antarctica, NO x gases are removed along with

water by freezing in polar stratospheric clouds at temperatures below -70˚C as compounds such as ClONO 2 and HNO 3 • 3H 2 O. During the Antarctic winter, HOCl and Cl 2 are generated and accumulate at the surfaces of the solid cloud particles by the reactions,

ClONO 2 + H 2 O → HOCl + HNO 3 (14.4.9) ClONO 2 + HCl → Cl 2 + HNO 3 (14.4.10) where the HCl comes primarily from the reaction of stratospheric methane, CH 4 ,

with Cl • atoms produced from chlorofluorocarbons. The preceding reactions are aided by the tendency of the HNO 3 product to become hydrogen-bonded with water in the cloud particles. The result of these processes is that over the winter months photoreactive Cl 2 and HOCl accumulate in the Antarctic stratospheric region in the absence of sunlight then undergo a burst of photochemical activity when spring arrives as shown by the following reactions:

HOCl + h ν → HO • + Cl • (14.4.11) Cl 2 + h ν → Cl • + Cl •

(14.4.12) The Cl atoms react to destroy ozone according to Reaction 14.4.6. Under conditions

of Antarctic spring, not enough O • atoms are available to regenerate Cl atoms from ClO • by Reaction 14.4.7. It is now known that ClO • forms the ClO-OCl dimer, which regenerates Cl • by the following reactions:

ClOOCl + h ν → ClOO • + Cl • (14.4.13)

ClOO • + h ν → O 2 + Cl •

Chlorofluorocarbon Substitutes and Ozone Depletion

Currently, the substitutes for ozone-destroying chlorofluorocarbon compounds are hydrohaloalkanes, compounds that contain at least one H atom. Each molecule of this class of compounds has an H-C bond that is susceptible to attack by HO • radical in the troposphere, thereby eliminating the compound with its potential to produce ozone-depleting Cl atoms before it reaches the stratosphere. The tropo- spheric chemistry involved in the destruction of hydrohaloalkanes is discussed briefly in Section 12.7. The substitutes are either hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs). The compounds used or proposed for use include

HCFC-22 (CHClF 2 ), HCFC-123 (CHCl 2 CF 3 ), HCFC-141b (CH 3 CCl 2 F), HCFC-124 (CHClFCF 3 ), HCFC-225ca, HCFC-225cb, HCFC-142b (CH 3 CClF 2 ), HFC-134a

(CH 2 FCF 3 ), and HFC-152a (CH 3 CHF 2 ).

The tropospheric lifetimes of hydrohalocarbons depend upon a number of factors. These are molar mass, number of hydrogen atoms (particularly important because of the H-C bond that is vulnerable to attack by HO • ), number of carbon atoms, number of F atoms β to hydrogen, number of chlorine atoms α and β to hydrogen (where α and β designate positions on the same and on adjacent carbon atoms, respectively), rate of reaction with HO • radical, and photolytic cross section (tendency to undergo photolysis). Ozone depletion potentials of HCFCs and HFCs are compiled to express potential likelihood for the destruction of stratospheric ozone relative to a value of 1.0 for CFC-11, a non-hydrogen-containing chlorofluor-

ocarbon with a formula of CFCl 3 . The ozone-depletion potentials of some of the substitutes mentioned above are HCFC-22, 0.030, HCFC-123, 0.013, HCFC-141b,

0.10, HCFC-124, 0.035, and HCFC-142b, 0.038. Low ozone depletion potential correlates with short tropospheric lifetime, which means that the compound is destroyed in the troposphere before migrating to the stratosphere.