Ultraviolet radiation ozone, stratospheric

We begin our exploration of delocalized bonds with ozone, O3. As described in Chapter 7, ozone in the upper stratosphere protects plants and animals from hazardous ultraviolet radiation. Ozone has 18 valence electrons and a Lewis stmcture that appears in Figure 10-36a. Experimental measurements show that ozone is a bent molecule with a bond angle of 118°. 

In recent years, there has been considerable concern over reductions in stratospheric ozone concentrations resulting from human activities. Since stratospheric ozone is the primary screen of solar ultraviolet radiation, ozone reduction would increase ultraviolet radiation (UV-B, wavelengths between 280 and 320 nm) reaching the earth s surface. 

Ozone, produced in the stratosphere by the action of ultraviolet light on 02, helps shield the surface of the earth from harmful ultraviolet radiation. Ozone is slowly decomposed by reaction with oxygen atoms according to the following equation ... 

Above the troposphere, temperatures increase with altitude, reaching a maximum of nearly 2°C at about 50 km. This region of the atmosphere is called the stratosphere. The stratosphere contains a layer of ozone, a gas that helps shield Earth s surface from the Sun s harmful ultraviolet radiation. Ozone protects Earth by absorbing solar radiation, which raises the temperature of the stratosphere in the process. You read about the ozone layer in Chapter 1 as you began your study of chemistry. 

Convection ceases at the tropopause level, and the temperature in the stratosphere and mesosphere is determined strictly by radiation balance. At altitudes above 20 km the absorption of solar ultraviolet radiation becomes increasingly important. The temperature peak at the stratopause has its origin in the absorption of near-ultraviolet radiation by stratospheric ozone. In fact, the existence of the ozone layer is in itself a consequence of the ultraviolet (UV) irradiation of the atmosphere. The enormous temperature increase in the thermosphere is due to the absorption of extremely shortwaved and thus energetic radiation coupled with the tenuity of the atmosphere, which prevents an effective removal of heat by thermal radiation. Instead, the heat must be carried downward by conduction toward denser layers of the atmosphere, where H20 and C02 are sufficiently abundant to permit the excess energy to be radiated into space. 

Ozone, known for its beneficial role as a protective screen against ultraviolet radiation in the stratosphere, is a major pollutant at low altitudes (from 0 to 2000 m) affecting plants, animals and human beings. Ozone can be formed by a succession of photochemical reactions that preferentially involve hydrocarbons and nitrogen oxides emitted by the different combustion systems such as engines and furnaces. 

Measurements of ozone (O3) concentrations in the atmosphere are of particular importance. Ozone absorbs strongly in the ultraviolet region and it is this absorption which protects us from a dangerously high dose of ultraviolet radiation from the sun. The vitally important ozone layer lies in the stratosphere and is typically about 10 km thick with a maximum concentration about 25 km above the surface of the earth. Extreme depletion of ozone in a localised part of the atmosphere creates what is known as an ozone hole. 

Ozone, which occurs in the stratosphere (15—50 km) in concentrations of 1—10 ppm, is formed by the action of solar radiation on molecular oxygen. It absorbs biologically damaging ultraviolet radiation (200—300 nm), prevents the radiation from reaching the surface of the earth, and contributes to thermal equiHbrium on earth. 

An important effect of air pollution on the atmosphere is change in spectral transmission. The spectral regions of greatest concern are the ultraviolet and the visible. Changes in ultraviolet radiation have demonstrable adverse effects e.g., a decrease in the stratospheric ozone layer permits harmful UV radiation to penetrate to the surface of the earth. Excessive exposure to UV radiation results in increases in skin cancer and cataracts. The worldwide effort to reduce the release of stratospheric ozone-depleting chemicals such as chlorofluorocarbons is directed toward reducing this increased risk of skin cancer and cataracts for future generations. 

A particularly important property of ozone is its strong absorption in the ultraviolet region of the spectrum between 220-290 nm ( max255.3nm) this protects the surface of the earth and its inhabitants from the intense ultraviolet radiation of the sun. Indeed, it is this absorption of energy, and the consequent rise in temperature, which is the main cause for the existence of the stratosphere in the first place. 

About 51 percent of solar energy incident at the top of the atmosphere reaches Earth s surface. Energetic solar ultraviolet radiation affects the chemistry of the atmosphere, especially the stratosphere where, through a series of photochemical reactions, it is responsible for the creation of ozone (O,). Ozone in the stratosphere absorbs most of the short-wave solar ultraviolet (UV) radiation, and some long-wave infrared radiation. Water vapor and carbon dioxide in the troposphere also absorb infrared radiation. 

H. 10 An important role of stratospheric ozone, Os, is to remove damaging ultraviolet radiation from sunlight. One result is the eventual dissociation of gaseous ozone into molecular oxygen gas. Write a balanced equation for the dissociation reaction. 

I Stratospheric ozone, O, protects life on Farth from harmful ultraviolet radiation from the Sun. Suggest two Lewis structures that contribute to the resonance structure for the 02 molecule. Experimental data show that the two bond lengths are the same. 

Susan Solomon and James Anderson showed that CFCs produce chlorine atoms and chlorine oxide under the conditions of the ozone layer and identified the CFCs emanating from everyday objects, such as cans of hair spray, refrigerators, and air conditioners, as the primary culprits in the destruction of stratospheric ozone. The CFC molecules are not very polar, and so they do not dissolve in rain or the oceans. Instead, they rise to the stratosphere, where they are exposed to ultraviolet radiation from the Sun. They readily dissociate in the presence of this radiation and form chlorine atoms, which destroy ozone by various mechanisms, one of which is... 

A representation of the stratospheric system that shields terrestrial life from excessive solar ultraviolet radiation is presented in Figure 4. Our primary concern is the decrease of stratospheric ozone, most striking in the Antarctic, which has been linked to increases in CFCs from the troposphere, and the possible increased transport of these compounds between the stratosphere and the troposphere by increased temperature driven circulation. 

Reactions 1 to 4 are known collectively as the Chapman mechanism (first outlined by Sidney Chapman (1) in 1930. They basically explain how ozone can exist in the stratosphere in a dynamic balance it is continuously being produced by the action of solar ultraviolet radiation on oxygen molecules and destroyed by several natural chemical processes in the atmosphere. 

The Antarctic ozone hole is the result of anthropogenic release of trace gases into the atmosphere (CFCs in particular), causing a decrease in stratospheric ozone and a subsequent increase in solar ultraviolet radiation reaching the earth s surface. 

Recently, there have also been some concerns over possible problems related to hydrogen gas leakage as the molecular hydrogen leaks from most containment vessels. It has been hypothesized that if significant amounts of H2 escape to stratosphere, FT free radicals can be formed due to ultraviolet radiation, which in turn can enhance the ozone depletion. However, the effect of these leakage problems may not be significant as the amount of hydrogen that leaks presently is much lower (by a factor of 10-100) than the hypothesized 10-20%. 

Ultraviolet radiation (UVR) is a natural fraction of the solar radiation, and therefore has always influenced life in aquatic ecosystems. The development of oxygenic photosynthesis 2.5-2 J billion years ago (Holland 1984) led to drastic chemical changes in the Earth s oceans and atmosphere. The gradual increase in photosyn-thetically produced oxygen over millions of years was accompanied by a strong enrichment of it in the atmosphere, which ultimately acted as precursor for the ozone (03) layer in the stratosphere. 

Located several kilometres above the Earth s surface is the stratosphere. Here the ozone layer acts as a filter, protecting life on Earth from harmful low-wavelength ultraviolet radiation known as UV-C, which damages biological macromolecules such as proteins and DNA. In order to understand the effects of anthropogenic input into the stratosphere, the production and destruction of the ozone layer has been studied by a variety of photochemical models and experimental methods. 

Ozone (O3) is formed in the tropical stratosphere, around 12 to 30 miles above the ground, where solar radiation is intense it then migrates to the polar regions. The O3 concentration can be as high as 10 ppm in the stratosphere there, it absorbs a large part of the harmfirl ultraviolet radiation from the sun, thereby protecting life on Earth. CFCs are volatile and persist in the lower atmosphere (the troposphere) because of their inert nature they resist chemical degradation reactions. It is estimated that... 

The concentration of ozone near the Earth s surface is very low, typically in the range of 15-45 pphv (parts per billion by volume). In contrast, ozone is more abundant in the Earth s stratosphere, where it is formed by the action of ultraviolet radiation on molecules of dioxygen. A distinction is sometimes made between stratospheric ozone ("good ozone") and tropospheric (low-level or surface ozone "bad ozone"). This distinction arises from the fact that stratospheric ozone reduces the amount of ultraviolet radiation that reaches the Earth, reducing the rate of skin cancer and other medical problems... 

The role of CFCs in the destruction of ozone in the stratosphere was something of a surprise to some researchers because those compounds are normally quite stable. In fact, their stability is one of their most desirable properties for many industrial and commercial applications. But, when CFCs escape into the atmosphere and drift upward, they are exposed to ultraviolet radiation in sunlight and, as is oxygen itself, are dissociated by that radiation. In the case of Freon-12 (CCI2F2), photodissociation results in the formation of free chlorine atoms ... 

F. Sherwood Rowland (1927-) and Mario Molina (1943-) predicted the destruction of stratospheric ozone in 1974. Rowland and Molina theorized that inert CFCs could drift into the stratosphere, where they would be broken down by ultraviolet radiation. Once in the stratosphere, the CFCs would become a source of ozone-depleting chlorine. The destruction of ozone by CFCs can be represented by the following series of reactions ... 

It is clear from the data presented in this chapter that the effects of control strategies developed for CFCs and halons are already measurable. Although loss of stratospheric ozone with accompanying increases in ultraviolet radiation in some locations have clearly occurred, the tropospheric concentrations of CFCs are not increasing nearly as fast as in the past. Indeed, the concentrations of CFC-11 and CFC-113 appear to have peaked and have started to decline. The equivalent effective stratospheric chlorine concentrations are predicted to have peaked about 1997 and to return to levels found around 1980 at about the year 2050 (World Meteorological Organization, 1995). The significance of the 1980 level is that these levels resulted in detectable Antarctic ozone depletion. 

Lubin, D and E. H. Jensen, Effects of Clouds and Stratospheric Ozone Depletion on Ultraviolet Radiation Trends, Nature, 377, 710-713 (1995). 

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