is (O3 ) triatomic allotrope of oxygen (O2); unstable blue gas with a pungent, irritating odor. Ozone is formed from oxygen by electric discharge (e.g., lightning) or by ultraviolet radiation (part of the solar spectrum). A layer of ozone in the upper atmosphere protects the earth from excessive exposure to ultraviolet radiation. Ground-level ozone, formed when volatile organic compounds react with nitrogen oxides in the presence of sunlight, is a major component of photochemical smog.