A region of the atmosphere from about 15 to 60 km (roughly the extent of the stratosphere) that contains large concentrations of ozone. The ozone concentration peaks at about 10¹³ molecules per cubic centimeter near 20 km, while the mixing ratio peaks at slightly higher altitude (about 9–10 ppm at 30 km). Ozone in this region is produced from photolysis of O₂ molecules and is destroyed by reactions involving the oxides of nitrogen, chlorine, and hydrogen. Because of the strong UV absorption spectrum of ozone, the ozone layer effectively limits penetration of UV radiation to the earth's surface to wavelengths longer than 290 nm.