Definition: Molecule composed of three atoms of oxygen (O3) and as a result is a very strong oxidizer. It is photoreactive at the Earth's surface, meaning it forms when nitrogen oxides and volatile organic compounds are exposed to sunlight. In high doses for long periods, it is harmful to plants, and causes respiratory problems in humans. It is the same chemical which blocks dangerous UV radiation from the sun high in the atmosphere, but at the surface, it is a principal component of urban smog. Ozone concentrations are measured in parts per billion (ppb), referring to the proportion of ozone in the air to the total volume of air.
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