ozone

Ozone (O3) is a triatomic molecule consisting of three oxygen atoms. It is a pale blue gas with a sharp odor and is a highly reactive, comparatively unstable form of oxygen that tends to decompose to diatomic oxygen (O2).

Stratospheric ozone, about 10–50 km up, forms the ozone layer and absorbs ultraviolet radiation, protecting life. Its thickness is commonly measured in Dobson Units. Ozone in the stratosphere is created by photolysis of O2 and subsequently forms from atomic oxygen; it is destroyed by catalytic cycles involving halogens and other species. Human-produced halogens from CFCs and related compounds have caused long-term depletion, notably an annual ozone hole over Antarctica.

Near the surface, ozone forms by sunlight-driven reactions of nitrogen oxides and volatile organic compounds released from vehicles and industry. Surface ozone is a major pollutant in many urban regions, contributing to smog. It irritates the eyes and respiratory tract, worsens asthma, reduces lung function, and harms crops and ecosystems. Because it is a greenhouse gas, it also affects climate, though its lifetime in the atmosphere is relatively short and highly regional.

Ozone is produced commercially by electrical discharge or by ultraviolet irradiation of oxygen. It is used for water treatment, disinfection, and oxidation of organic contaminants. Ozone naturally decomposes to O2 and must be produced and handled with proper precautions due to its strong oxidizing properties.