Ozone Near and Far
The presence of ozone in the air can be beneficial or harmful, depending on where it is. Ozone may in the future also become important in sterilizing the potable water supply. No wonder it sometimes becomes confusing. Ozone is an allotropic form of molecular oxygen. Ordinary oxygen as it is found in the atmosphere consists of two oxygen atoms in each molecule. In this form it is stable enough to remain for the most part unchanged in the environment. On the other hand, it is reactive enough to participate in the metabolic processes of animals and to oxidize various substances—the rusting of iron, for example, or the degradation of plastics. Unlike ordinary oxygen, ozone consists of three atoms of oxygen per molecule, a structure which is much less stable than molecular oxygen, and therefore much more reactive. It is this enhanced reactivity which is the key to understanding the various roles of ozone in the atmosphere. Miles above the atmosphere in which we live is the stratosphere, a thick layer of mostly empty space, but with significant amounts of various gases, including molecular oxygen. As high-energy radiation enters this layer, some oxygen molecules are split into their two constituent oxygen atoms, a form of free radicals, highly unstable and reactive. These atoms readily attack other oxygen molecules to form ozone. As a result of the operation of this mechanism over a few billion years, a moderate concentration of ozone has accumulated in the stratosphere. The system has come to an equilibrium such that the rate of destruction of ozone through its reaction with other atmospheric gases is balanced by the rate of new ozone being formed. Ozone has the ability to absorb ultraviolet radiation, particularly the high-energy forms which can be so damaging to animal tissue. This radiation can cause genetic damage to future offspring as well as immediate damage in the form of cancer. Numerous other effects include such things as blinding certain animals and making them incapable of finding food. We human beings are most fortunate to have come into being on a planet where we are protected by an ozone layer. Unfortunately, technological advances have made our position less secure. Certain chlorinated and fluorinated hydrocarbons, generally known as freons, are highly stable, and this stability has made them useful as propellants in spray cans and refrigerants in air conditioners and refrigerators. Escaping into the atmosphere after use, freon resists attack as it rises slowly, and so enters the stratosphere unchanged. Here it encounters ozone, a much stronger oxidizing agent than anything in the lower atmosphere, and a reaction takes place as a result of which ozone is converted into oxygen. The more freon that enters the stratosphere, therefore, the less ozone there is to protect us. Drastic reduction in the use of freons is necessary to allow the ozone layer to build up again to its previous equilibrium level. Ozone is also involved in ground-level smog. Smog (a coined expression derived from the words “smoke” and “fog”), in its most destructive form, is known as “classical smog.” Coal-burning furnaces releasing soot and oxides of sulfur into a foggy atmosphere produce a fine mist of particulates soaked with dilute sulfuric acid. Breathed into the lungs, the toxic particles become lodged in the alveoli, where they can erode lung tissue and do permanent damage. Classical smog (also known as “London smog”) episodes have resulted in thousands of deaths within a few days. Sulfur oxides from fossil fuels are also responsible for acid rain, which can be damaging to many forms of life. Fortunately, air quality regulations have significantly reduced the incidence of classical smog. More familiar to us is photochemical smog, formed under completely different conditions. Exhaust from automobiles contains uncombusted hydrocarbons, oxides of nitrogen, and carbon monoxide. In a warm sunny climate, the solar radiation catalyzes a series of chain reactions in which nitrogen-containing hydrocarbon oxides, severely irritating to the eyes and mucous membranes, are produced. These extremely reactive compounds combine with molecular oxygen to produce ozone, an irritant in itself. Even though ozone is produced merely as a side reaction in the photochemical reactions, it is often taken as an indication of air quality on a given day. The invigorating feeling one gets on bright sunny days in the mountains or at the seashore is largely due to somewhat higher concentrations of ozone. In the case of photochemical smog, invigoration gives way to irritation. The health effects of photochemical smog are not nearly so severe as classical smog, but its presence is much more widespread and affects many more people. Ozone is also used in a variety ways in the chemical industry. It is interesting to note that one does not buy drums of ozone. When ozone is needed, it is generated on the spot using an ozonizer, which subjects atmospheric oxygen to an electric discharge to produce ozone. Ozone may also have a future as a replacement for chlorine in purifying potable water. Chlorine over the decades has been a safe and effective sterilizer for drinking water and sometimes even for sewage treatment. Perhaps its chief advantage is that a certain amount of chlorine remains dissolved in the water after it leaves the plant, and this excess protects against subsequent microbial contamination. As a result, a simple test for the presence of chlorine can give an assurance of the sterility of the water. In the last few decades water sources have been accumulating small concentrations of hydrocarbons, in themselves relatively harmless. Using chlorine in these circumstances produces chlorinated hydrocarbons, considered to be carcinogenic. However effective chlorine may be against microbes, it does not seem much of a bargain if its use increases the incidence of cancer. Here ozone may come to the rescue. Sterilizing with ozone is as effective as with chlorine, and although it does not leave that reassuring excess to protect against future contamination, neither does it produce carcinogenic byproducts. So ozone is good in the stratosphere, bad at ground level, and possibly useful in a chemical plant. Like so much of life, everything depends upon the circumstances. Dom
Roberti |