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At an altitude of roughly 32 km (20 mi) above Earth’s surface, the stratosphere hosts a stable concentration of about 8 parts per million of ozone. This thin, invisible shield plays a vital role by absorbing ultraviolet (UV) radiation that would otherwise create hostile conditions for life.
Ozone (O₃) forms when a free oxygen atom collides with an oxygen molecule (O₂), a process that is facilitated by a third molecule to carry away excess energy. The reaction can be summarized as:
O + O₂ + M → O₃ + M
When UV photons strike O₃, the molecule splits into O₂ and a free oxygen atom. At the right pressure, the liberated oxygen atom quickly recombines with another O₂ molecule, sustaining the ozone layer where formation balances destruction.
UV radiation is a form of electromagnetic energy just beyond visible light, beginning at about 400 nm and extending down to 100 nm. It is divided into three bands:
Short‑wave UV carries enough energy to ionise atoms and molecules, making UV‑C the most harmful, followed by UV‑B and then UV‑A.
Because the electronic energy levels of ozone align with the UV spectrum, the layer is exceptionally effective at filtering harmful radiation. Typical absorption rates are:
These figures depend on the local ozone density. Emissions of chlorofluorocarbons (CFCs) and other ozone‑depleting substances shift the balance toward destruction, thinning the layer. NASA warns that a continued decline could “sterilize the Earth’s surface” by allowing excessive UV to reach the planet.
Understanding how the ozone layer mitigates UV exposure underscores its essential role in safeguarding ecosystems, human health, and the planet’s overall habitability.
