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A brief note about ozone formation and how it is helpful for the living organisms
Ozone is a gas found in the atmosphere in very trace amounts. Depending on where it is located, ozone can be beneficial ("good ozone") or detrimental ("bad ozone"). On average, every ten million air molecules contains only about three molecules of ozone. Indeed, if all the ozone in the atmosphere were collected in a layer at Earth's surface, that layer would only have the thickness of three dimes. But despite its scarcity, ozone plays very significant roles in the atmosphere.
Chemically, the ozone molecule consists of three atoms of oxygen arranged in the shape of a wide V. Its formula is O3 (the more familiar form of oxygen that one breathes has only two atoms of oxygen and a chemical formula of O2). Gaseous ozone is bluish in color and has a pungent, distinctive smell. In fact, the name ozone is derived from the Greek word ozein, meaning "to smell or greek." The smell of ozone can often be noticed near electrical transformers or nearby lightning strikes. It is formed in these instances when an electrical discharge breaks an oxygen molecule (O2) into free oxygen atoms (O), which then combine with O2 in the air to make O3. In addition to its roles in the atmosphere, ozone is a chemically reactive oxidizing agent that is used as an air purifier, a water sterilizer, and a bleaching agent.
Ozone is mainly found in the two regions of the atmosphere that are closest to the earth's surface. About 10 percent of the atmosphere's ozone is in the lowest-lying atmospheric region, the troposphere. This ozone is formed in a series of chemical reactions that involve the interaction of nitrogen oxides, volatile organic compounds, and sunlight. Most ozone (about 90%) resides in the next atmospheric layer, the stratosphere. The stratosphere begins between 8 and 18 kilometers (5 and 11 miles) above the earth's surface and extends up to about 50 kilometers (30 miles). The ozone in this region is commonly known as the ozone layer. Stratospheric ozone is formed when the sun's ultraviolet (UV) radiation breaks apart molecular oxygen (O2) to form O atoms, which then combine with O2 to make ozone.
In the upper atmosphere, stratospheric ozone plays a beneficial role by absorbing most of the sun's biologically damaging ultraviolet sunlight (called UV-B), allowing only a small amount to reach the earth's surface. The absorption of ultraviolet radiation by ozone creates a source of heat, which actually defines the stratosphere (a region in which the temperature rises as one goes to higher altitudes). Ozone thus plays a key role in the temperature structure of the earth's atmosphere. Without the filtering action of the ozone layer, more of the sun's UV-B radiation would penetrate the atmosphere and reach the earth's surface. Many experimental studies of plants and animals and clinical studies of humans have shown that excessive exposure to UV-B radiation has harmful effects. Serious long-term effects can include skin cancers and eye damage. The UV-absorbing role of stratospheric ozone is what lies behind the expression that ozone is "good up high."

In the troposphere, ozone comes into direct contact with life-forms. Although some amount of ozone is naturally present in the lower atmosphere, excessive amounts of this lower-atmospheric ozone are undesirable (or bad ozone). This is because ozone reacts strongly with other molecules, including molecules that make up the tissues of plants and animals. Several studies have documented the harmful effects of excessive ozone on crop production, forest growth, and human health. For example, people with asthma are particularly vulnerable to the adverse effects of ozone. Thus, ozone is "bad nearby."


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