Ozone is the triatomic form of oxygen, 03, and is generally regarded as the most important species that determines the oxidizing capacity of the troposphere. The word ozone comes from the Greek word ozein, which means "to smell." Probably the name of this gas originated from early laboratory studies when ozone was first discovered because of its distinctive acrid odor. The German scientist Christian Friedrich Schonbein is credited with ozone's discovery in 1839, while he was a professor at the University of Basel in Switzerland.

One of the goals of Schonbein's research was to show that ozone is a permanent and natural component of the atmosphere. He devised a method to measure ozone in the atmosphere that was capable of measuring very low levels simply and easily. The method used soon became known as Schonbein paper and involved the simple process of saturating a strip of paper with potassium iodide (KI) and then allowing it to dry. In the presence of ozone, the potassium iodide oxidized and is converted to potassium iodate (KI03). In the process of this conversion the paper changes color to various hues of blue. More ozone present in the atmosphere resulted in the paper becoming a deeper shade of blue. Schonbein calibrated the amount of color change into a measurement standard called Schonbein units, which allowed scientists to put out a new piece of Schonbein paper each day and measure the relative amount of ozone in the atmosphere.

Although the methods of measurement have been modified over the years, scientists continued to use KI to measure ozone for more than a century. One modification involved pumping ambient air through a KI solution and measuring the amount of iodide being converted to iodate since an electrical current is created in the solution

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.

as the conversion takes place. The reaction took place within a matter of seconds, and the amount of electric current was easily quantifiable. This method, known as the wet method, was the predominant way ozone was measured until the 1960s, when other methods using newer optical technology became available and increased the accuracy of the measurements. One problem with the wet method was that other chemicals in the atmosphere interfered with the chemical reaction. The most common of these interfering trace gases is sulfur dioxide (S02), a pollutant that is primarily a by-product of coal combustion.

In the early part of the twentieth century, ground-based and balloon-borne measurements discovered that most of the atmosphere's ozone is located in the stratosphere with highest concentrations located between 15 and 30 km. For a long time, it was believed that tropospheric ozone originated from the stratosphere and that most of it was destroyed by contact with Earth's surface. Ozone was known to be produced by the photodissociation of molecular oxygen, 02, a process that can only occur at wavelengths shorter than 242 nm. The atomic oxygen formed as a product of this photodissociation would then recombine with another oxygen molecule to make ozone. Because such short-wavelength radiation is present only in the stratosphere, no tropospheric ozone production is possible by this mechanism. In the 1940s, however, it became obvious that production of ozone was also taking place in the troposphere. The overall reaction mechanism was eventually identified by Arie Haagen-Smit of the California Institute of Technology located in highly polluted southern California. The smog chemistry hypothesized by Haagen-Smit was still thought to be a relatively small source on the global scale since ~90% of the ozone was located in the stratosphere, creating a ubiquitous source of tropospheric ozone as stratosphere air was transported into the troposphere. It was not until the 1970s that this viewpoint was challenged when Paul Crutzen (Crutzen, 1974) and other scientists at the time showed that consideration of "smog chemistry" in the background troposphere could produce a sizable source of tropospheric ozone and must be included in the global tropospheric ozone budget. Crutzen's pioneering work on tropospheric ozone was noted when he received the Nobel Prize for Chemistry in 1995.

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