Ozone: the Air Pollutant
In the stratosphere, ozone molecules play an important role - absorbing ultraviolet radiation from the Sun and shielding Earth from dangerous rays. But in the troposphere, near ground-level, ozone molecules are both air pollutants, threatening the health of living things, and greenhouse gases, trapping heat and contributing to climate change.
A small amount of ozone does occur naturally at ground level. Plants and soil release some. Some migrates down from the stratosphere. But neither of these sources contributes enough ozone to be considered a threat to the health of humans or the environment. Most of the ozone that is found near the ground comes from vehicle exhaust and emissions from factories, power plants, and refineries. Since 1900, the amount of ozone near the Earth's surface has more than doubled due to more automobiles and industry.
Unlike most other air pollutants, ozone is not directly emitted into the air. Tropospheric ozone is formed by the interaction of sunlight, particularly ultraviolet light, with hydrocarbons and nitrogen oxides, which are emitted by automobile tailpipes and smokestacks. In urban areas, high ozone levels usually occur during warm summer months. Typically, ozone levels reach their peak in mid to late afternoon, after exhaust fumes from morning rush hour have had time to react in sunlight. A hot, sunny, still day is the perfect environment for the production of ozone pollution. At the end of the day, as the Sun starts to set, the production of ozone begins to subside. To form, ozone needs sunshine to fuel the chemical reaction.
About Schoenbein Test Strips
Christian Friedrich Schoenbein discovered ozone in 1839 during his tenure as a professor at the University of Basel, Switzerland. He used the reactivity of ozone to measure its presence and demonstrate that it is a naturally occurring component of the atmosphere. He developed a way to measure ozone in the troposphere using a mixture of starch, potassium iodide, and water spread on filter paper. The paper, called Schoenbein paper, changes color when ozone is present. Ozone causes iodide to oxidize into iodine.
Ozone in the air will oxidize the potassium iodide on the test paper to produce iodide. Specifically, potasium iodide (KI), ozone (O3), and water (H2O) react to form potasium hydroxide (KOH), oxygen (O2), and iodide (I2). Here's the equation:
2KI + O3 + H2O ---> 2KOH + O2 + I2
The iodide reacts with starch, staining the paper a shade of purple. The intensity of the purple color depends on the amount of ozone present in the air. The darker the color, the more ozone is present.