ozoneformation

Ozone formation refers to the production of ozone (O3) in Earth’s atmosphere, chiefly in the stratosphere where it forms the ozone layer that absorbs a large portion of ultraviolet radiation from the Sun. Ozone is also formed in the troposphere, where it acts as a secondary pollutant that can affect air quality and health.

In the stratosphere, the primary formation mechanism is the Chapman cycle. Ultraviolet light with wavelengths shorter than about 242 nanometers splits molecular oxygen: O2 + hv → 2O. The resulting atomic oxygen reacts with O2 in the presence of a third body M: O + O2 + M → O3 + M. Ozone can be destroyed by photolysis: O3 + hv → O2 + O, and by reactions involving atomic oxygen: O + O3 → 2O2. The overall effect tends to convert O2 into O3, sustaining the ozone layer under suitable atmospheric conditions.

Catalytic destruction cycles also regulate ozone levels. Species such as chlorine and bromine (ClOx, BrOx) and nitrogen oxides (NOx) participate in reactions that destroy ozone and regenerate active catalysts, leading to net ozone loss in certain atmospheric layers. Human-made substances like chlorofluorocarbons (CFCs) and halons have historically increased chlorine and bromine pools, contributing to ozone depletion; regulatory measures have reduced emissions and aided recovery.

Ozone formation at ground level (tropospheric ozone) results from sunlight-driven reactions of NOx and volatile organic compounds, producing a key pollutant that affects air quality and health. Formation rates depend on solar radiation, temperature, humidity, and atmospheric transport, and vary regionally and seasonally.

Ozone observations use units such as Dobson Units and satellite or ground-based measurements. Models of ozone formation and loss are used to understand layer dynamics, assess recovery timelines, and evaluate air-quality implications.