ozonelayer

The ozone layer is a region of Earth’s stratosphere that contains a relatively high concentration of ozone (O3). It absorbs a large portion of the Sun’s ultraviolet radiation, especially UV-B and UV-C, helping protect living organisms from DNA damage, skin cancer, and cataracts, and reducing surface-level photochemical smog. The layer lies roughly 10 to 50 kilometers above the surface, with the greatest ozone abundance typically around 20 to 30 kilometers. The total amount of ozone in a column is commonly expressed in Dobson Units, with around 300 DU representing roughly a 3 millimeter-thick layer at standard conditions.

Ozone is formed when ultraviolet light splits molecular oxygen (O2) to produce atomic oxygen, which then combines with O2 to form O3. It is destroyed through catalytic cycles involving chlorine and bromine radicals released from human-made halogenated compounds, as well as nitrogen oxides and hydrogen oxides. These destruction processes are most pronounced in the polar stratosphere during spring, where polar stratospheric clouds enable reactions that lead to pronounced ozone loss, producing the seasonal ozone hole over Antarctica and, to a lesser extent, over the Arctic.

Human activity led to accelerated ozone depletion beginning in the late 20th century. The discovery of the Antarctic ozone hole in 1985 spurred international action, culminating in the Montreal Protocol (1987) and subsequent amendments that phased out many ozone-depleting substances. Observations indicate the atmosphere is slowly recovering, with recovery projections suggesting regional differences and a full restoration timeline extending into the mid- to late 21st century. Measurements come from satellites and ground-based instruments that monitor total and profile ozone.