Ozone Depletion & Ozone-Depleting Substances

1. The ozone layer

• About 90% of atmospheric ozone (O₃) lies in the stratosphere, mainly 15–35 km above the surface.
• It absorbs most of the sun's harmful ultraviolet-B (UV-B) and all UV-C radiation.
• Formed when UV splits O₂ into O atoms, which combine with O₂: ${\text{O}}_2\stackrel{UV}{\to }2\text{O}$; $\text{O}+{\text{O}}_2\to {\text{O}}_3$.
• Thickness is measured in Dobson units (DU); the global average is about 300 DU. An area below 220 DU is called an ozone hole.
• Ground-level (tropospheric) ozone is a harmful pollutant formed from NOₓ and VOCs in sunlight (photochemical smog) — "good up high, bad nearby".