Ozone in the Environment
Ozone naturally occurs in the environment and forms a vital protective layer high above us. However, it can also appear at ground level as a component of smog. Let's explore how ozone behaves in our atmosphere—and why it matters to life on Earth.
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 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 layerStratosphere
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 still 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 pollutants released from human activity (e.g., CFC). These pollutants can trigger complex reactions that decompose ozone, explaining the hole in the ozone layer over the poles.
Ozone in the TroposphereTroposphere
The other 10% of the ozone found in the earth's atmosphere is in the troposphere (surface to 12 km up). Ozone is naturally created during thunder and lightning storms but is also formed from hydrocarbons and nitrogen oxides released by automobiles and industry. Under the right conditions, ozone in the troposphere can reach excessive levels (0.3 ppm), creating smog harmful to human health. Symptoms include eye irritation, coughing, and difficulty breathing. The sharp, irritating odor of ozone is detectable at concentrations as low as 0.02 ppm.
Despite this, tropospheric ozone is one of the greenhouse gases that help make earth habitable. It purifies the air by reacting with noxious compounds. Ozone monitoring in large cities during smog days can cause confusion: "ozone is smog." In fact, ozone is a key ingredient in smog but also a powerful, natural greenhouse gas.