How is ozone formed in the atmosphere?
Ozone is not emitted directly into the air. Instead, it is created through chemical reactions involving solar radiation and precursor gases. The process differs significantly depending on the atmospheric layer.
1. Stratospheric Ozone (The Natural "Good" Ozone)
In the stratosphere (10–50 km above Earth), ozone is formed through the Chapman Cycle. This process requires high-energy UV-C radiation.
- Step 1: Photolysis – UV light hits an oxygen molecule ($O_2$), splitting it into two individual oxygen atoms ($O$).
- Step 2: Synthesis – These highly reactive single oxygen atoms quickly collide and bond with another $O_2$ molecule to form Ozone ($O_3$).
$O_2 + \text{UV-C} \rightarrow O + O$
$O + O_2 + M \rightarrow O_3 + M$
*(Note: 'M' represents a third molecule like $N_2$ that absorbs excess energy to stabilize the reaction)*
2. Tropospheric Ozone (The "Bad" Ground-Level Ozone)
At ground level, the high-energy UV-C is already filtered out. Ozone here is a secondary pollutant formed by the reaction of man-made chemicals in the presence of sunlight.
- Precursors: Nitrogen Oxides ($NO_x$) from car exhaust and Volatile Organic Compounds (VOCs) from industrial solvents.
- The Reaction: Sunlight breaks a nitrogen dioxide ($NO_2$) molecule into nitric oxide ($NO$) and a free oxygen atom ($O$). This atom then joins $O_2$ to create ozone.
$NO_2 + \text{Sunlight} \rightarrow NO + O$
$O + O_2 \rightarrow O_3$
$O + O_2 \rightarrow O_3$
Comparison Summary
| Feature | Stratospheric Ozone | Tropospheric Ozone |
|---|---|---|
| Altitude | 10–50 km | 0–10 km (Surface) |
| Primary Energy | High-energy UV-C | Visible/Near-UV Sunlight |
| Source | Natural Photochemistry | Pollution ($NO_x$ + VOCs) |
| Effect | Protects against UV | Respiratory Irritant / Smog |