refers to ozone (O3) in the region of the atmosphere that extends from Earth's surface to about 7 miles up. As opposed to stratospheric ozone (the "good ozone" that protects us from excess ultraviolet (UV) radiation from the sun), tropospheric ozone or "bad ozone," results from the interaction of nitrogen oxides (NOx), volatile organic compounds (VOCs), and sunlight. Most of the pollutants which lead to the formation of tropospheric ozone come from automobiles, power plants, and other human activities. In many cities, ozone is a significant health problem; 98 U.S. cities have higher concentrations of ozone than the U.S. Environmental Protection Agency finds acceptable. Ozone also causes $3 to 5 billion a year in lost crop production and significant losses in forest products. Tropospheric ozone is also a significant greenhouse gas.

Ozone (O3) is a key constituent of the troposphere (it is also an important constituent of certain regions of the stratosphere commonly known as the Ozone layer). Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night. At abnormally high concentrations brought about by human activities (largely the combustion of fossil fuel), it is a pollutant, a constituent of smog.