Ozone in the enviromnent
The ozone smog
Ozone naturally occurs in the environment. It is constantly formed in the earth's atmosphere by action of ultraviolet light emitted by the sun on oxygen molecules. Ozone thus constitutes in the stratosphere a shield that prevents the ultraviolet radiation, which causes biological damage (skin cancer) from passing to the earth's surface. Without ozone in the upper atmosphere, life on earth would not have evolved and could not exist today.
The hole in the ozone layer
About 90 % of ozone in the earth's atmosphere is in the stratosphere (12 to 50 km of altitude). It forms a layer where its concentration is higher than anywhere but low ranging from 1 to 20 ppm compared with the oxygen concentration of about 210000 ppm. The ozone layer is what makes the sky blue. Ozone concentration in this layer depends on the latitude, the season, the weather conditions and the pollutants released from human activity (for example, CFC). The latter can involve complex reactions that decompose ozone. This explains the hole in the ozone layer over the poles.
The other 10% of the ozone found in the earth's atmosphere are in the troposphere (earth's surface to 12 km up). Ozone is naturally created during thunder and lightning storms but mainly results from hydrocarbons and nitrogen oxides released by automobiles and industry. Ozone concentration reaches excessive levels (0,3 ppm) when these emissions, sunlight, moisture and temperature combine the right conditions for smog. Ozone in troposphere at such concentration can cause damage to human health like eyes irritation, coughing, difficulty in breathing. The characteristic sharp, irritating odor is readily detectable by human nose at low concentration (0,02 ppm). However, ozone in the troposphere is one of the greenhouse gases that make earth habitable for life. Ozone purifies the air that we breathe by reacting with noxious compounds responsible for or issued from its formation. Ozone monitoring in large cities during smog days lead to the general confusion: ozone is smog. Ozone is a key ingredient in smog and a powerful and natural greenhouse gas.
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