II.

Composition of the Ozone Layer

Ozone is formed in the atmosphere when ultraviolet radiation from the Sun splits one oxygen molecule into two oxygen atoms (O₂). The atomic oxygen then combines with another oxygen molecule to form ozone (O₃). Most ozone found in the Earth’s atmosphere occurs in one layer in the stratosphere, between altitudes of around 20 to 50 km (12 to 30 mi). Early in the Earth’s history, the gradual build-up of this layer from oxygen released by marine life allowed life to develop on land. Today, the ozone layer helps to produce the observed vertical structure of the atmosphere, and absorbs harmful ultraviolet radiation that would otherwise damage plant and animal life (also causing skin cancer) on the Earth’s surface. In contrast, ozone close to the Earth’s surface is a health hazard, as it is one of the major constituents of photochemical smog.