UV radiation plays a crucial role in the formation of ozone in the Earth’s stratosphere through a process called the ozone-oxygen cycle or Chapman cycle. Here’s how it works:
Step-by-step Role of UV Radiation in Ozone Formation:
- Oxygen Molecule (O₂) Photodissociation:
- High-energy UV radiation, specifically UV-C (wavelengths shorter than about 240 nm), strikes an oxygen molecule (O₂).
- This UV radiation has enough energy to break the O=O bond, splitting O₂ into two separate oxygen atoms (O). This process is called photodissociation.
- Ozone (O₃) Formation:
- Each free oxygen atom (O) then reacts with another oxygen molecule (O₂) in the presence of a third molecule (M, usually nitrogen or oxygen) that carries away excess energy.
- This reaction forms ozone (O₃).
- Ozone Absorbs UV Radiation:
- Ozone molecules absorb UV-B and UV-C radiation, protecting life on Earth by filtering harmful UV rays.
- When ozone absorbs UV, it breaks down into O₂ and a free oxygen atom (O), which can then repeat the cycle.
In short:
- UV radiation initiates ozone formation by breaking O₂ molecules into atomic oxygen.
- Atomic oxygen combines with O₂ to form ozone.
- Ozone absorbs UV, preventing it from reaching the surface, and then can break down back to oxygen — a dynamic balance maintains the ozone layer.
So, UV radiation is both the cause of ozone formation and the reason ozone exists to protect Earth from harmful UV rays. Without UV radiation, this protective ozone layer wouldn’t form!