Ozone is a molecule of three oxygen atoms bound together (O₃). It is unstable and highly reactive. Ozone is used as a bleach, a deodorizing agent, and a sterilization agent for air and drinking water. At low concentrations, it is toxic.

Ozone is found naturally in small concentrations in the stratosphere, a layer of Earth's upper atmosphere. In this upper atmosphere, ozone is made when ultraviolet light from the sun splits an oxygen molecule (O₂), forming two single oxygen atoms. Each single oxygen atom then binds to an oxygen molecule to form ozone. Stratospheric ozone has been called "good" ozone because it protects the Earth's surface from dangerous ultraviolet light.

 

Photo courtesy NIEHS/NIH Ozone production from NOx pollutants. Oxygen atoms freed from nitrogen dioxide by the action of sunlight attack oxygen molecules to make ozone. Nitrogen oxide can combine with ozone to reforem nitrogen dioxide and the cycle repeats.

Ozone can also be found in the troposphere, the lowest layer of the atmosphere. Tropospheric ozone (often termed "bad" ozone) is man-made, a result of air pollution from internal combustion engines and power plants. Automobile exhausts and industrial emissions release a family of nitrogen oxide gases (NOx) and volatile organic compounds (VOC), by-products of burning gasoline and coal. NOx and VOC combine chemically with oxygen to form ozone during sunny, high-temperature conditions of late spring, summer and early fall. High levels of ozone are usually formed in the heat of the afternoon and early evening, dissipating during the cooler nights.

 

Photo courtesy NASA Worldwide seasonal changes in tropospheric ozone. Tropospheric ozone increases during summers in the northern and southern hemispheres when the climate is hot. The most is observed during summer in the northern hemisphere.

Although ozone pollution is formed mainly in urban and suburban areas, it ends up in rural areas as well, carried by prevailing winds or resulting from cars and trucks that travel into rural areas. Significant levels of ozone pollution can be detected in rural areas as far as 250 miles (150 km) downwind from urban industrial zones.

 

Photo courtesy NIEHS/NIH Ozone pollution can travel from urban to rural areas.

 

Make Your Own Ozone Detector You can make ozone test strips to detect and monitor ozone levels in your own backyard or around your school. You will need: corn starch filter paper (coffee filters work well) potassium iodide (can be ordered from a science education supplier such as Carolina Biological Supply or Fisher Scientific) Basically, you make a paste from water, corn starch and potassium-iodide and paint this paste on strips of filter paper. You then expose the strips to the air for 8 hours. Ozone in the air will react with the potassium iodide to change the color of the strip. You will need to know the relative humidity, which you can get from a newspaper, weather broadcast or home weather station.

 How Ozone Works ???