Stratospheric protection

Ever) year our planet is bombarded with enough energy from the Sun to destroy all life. Only the ozone in the stratosphere protects us from that onslaught. The ozone, though, is threatened by modern life styles. Chemicals used as coolants and propellants, such as chlorofluorocarbons (CFCs), and the nitrogen oxides in jet exhausts, have been found to create holes in Earth s protective ozone layer. Because they act as catalysts, even small amounts of these chemicals can cause large changes in the vast reaches of the stratosphere. 

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°. 

U.S. Environmental Protection Agency, List of Substitutes for Ozone-Depleting Substances, last update May 1999, http //www.epa.gov/spdpublc/title6/snap/lists/index.html. Stratospheric Protection Division, Washington, DC, June 1999. 

Why is the ozone layer so important and fragile Ozone in the stratosphere protects life on Earth s surface through its ability to absorb much of the harmful ultraviolet (UV) radiation produced by the sun. As a molecule of ozone absorbs UV it splits into a molecule of conventional oxygen and a highly reactive oxygen radical (an electrically neutral single oxygen atom). 

The outer portion (stratosphere) of the atmosphere plays a significant role in determining the conditions for life at the surface of the Earth. This stratosphere protects the surface from the intense radiation and particles bombarding our planet. Some of the high-energy radiation from the sun acts upon oxygen molecules, O2, in the stratosphere, converting them into ozone, O3. 

As mentioned in Chapter 6, Section 6.2, and illustrated in Figure 6.1, stratospheric ozone, O3, 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 are produced are... 

Paul continued to make major contributions to stratospheric chemistry. For example, he explained how nitric acid clouds cause the Antarctic ozone hole. At the same time, he also turned his attention to the troposphere, which is the air layer that connects with the biosphere and where weather and climate take place. The troposphere is also prone to air pollution, while it is cleaned by oxidation reactions. The self-cleaning capacity relies on the presence of reactive hydroxyl radicals that convert pollutant gases into more soluble compounds that are removed by rain. The primary formation of hydroxyl radicals in turn is from ozone. While most ozone is located in the stratosphere, protecting life on Earth against harmful ultraviolet radiation from the Sun, a small amount is needed in the troposphere to support the self-cleaning capacity. While previous theories had assumed that tropospheric ozone originates in the stratosphere, Paul discovered that much of it is actually chemically formed within the troposphere. The formation mechanism is similar to the creation of ozone pollution in photochemical smog . 

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. 

Depletion of the Ozone Layer. As a constituent of the atmosphere, ozone forms a protective screen by absorbing radiation of wavelengths between 200 and 300 nm, which can damage DNA and be harmful to life. Consequently, a decrease in the stratospheric ozone concentration results in an increase in the uv radiation reaching the earth s surfaces, thus adversely affecting the climate as well as plant and animal life. Pot example, the incidence of skin cancer is related to the amount of exposure to uv radiation. 

The EPA summary (4) for stratospheric ozone and global climate protection lists the basics of the title ... 

OZONE LAYER A thin layer of ozone that lies about 25 kilometres above the earth in the stratosphere. Forms a protective screen against harmful radiation by filtering out ultra-violet rays from the sun. 

Title VI Stratospheric Ozone and Global Climate Protection - The law builds on the market-based structure and requirements currently contained in EPA s regulations to phase out the production of substances that deplete the ozone layer. 

Deals with issues that affect the quality of our air and protection from exposure to harmful radiation. OAR de >el-ops national programs, technical policies, and regulations for controlling air pollution and radiation exposure. Areas of concern to OAR include indoor and outdoor air quality, stationaiy and mobile sources of air pollution, radon, acid rain, stratospheric ozone depletion, radiation protection, and pollution prevention. 

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. 

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. 

The nitrogen oxides are common pollutants generated by internal combustion engines and power plants. They not only contribute to the respiratory distress caused by smog, but if they reach the stratosphere can also threaten the ozone layer that protects Earth from harmful radiation. 

As mentioned previously, N2O plays an important role in stratospheric chemistry by providing the dominant source of NO in the stratosphere (see Section 12.5). What is more difficult to predict is how stratospheric chemistry will change as a result of continued increases in the concentration of atmospheric N2O. Early research suggested that increased N2O would lead to significant reductions in stratospheric O3. However, more current reports suggest that stratospheric NO plays a key role in "protecting" stratospheric O3 from more significant... 

We see that chiorine atoms provide an aitemative mechanism for the reaction of ozone with oxygen atoms. The iower-energy pathway breaks down ozone in the stratosphere at a significantiy faster rate than in the absence of the cataiyst. This disturbs the deiicate baiance among ozone, oxygen atoms, and oxygen molecules in a way that poses a serious threat to the iife-protecting ozone iayer. 

The term CFCs is a general abbreviation for ChloroFluoroCarbons. They have been extensively used since their discovery in the thirties, mainly as refrigerant, foam blowing agent, or solvent because of their unique properties (non toxic, non flammable, cheap). However, after the first warning of Rowland and Molina [1] in 1974 that CFCs could destroy the protective ozone layer, the world has moved rapidly towards a phase-out of CFCs. Because the destruction of stratospheric ozone would lead to an increase of harmful UV-B radiation reaching the earth s surface, the production and use of CFCs is prohibited (since January 1, 1995 in the European Union and since January 1, 1996 worldwide). 

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