Chemistry
The Chemistry of Atmospheric Ozone Depletion by Chlorofluorocarbons
Quick fact
A single chlorine atom can destroy up to 100,000 ozone molecules before it is removed from the stratosphere.
Why this is interesting
You’ve probably heard that CFCs destroy the ozone layer. But how can a chemical that is so stable and harmless at ground level become so destructive high above?
Read the full explanation
Understanding The Chemistry of Atmospheric Ozone Depletion by Chlorofluorocarbons
Imagine a quiet, inert molecule like a CFC – it doesn’t react easily. That stability is why CFCs were so popular in refrigerators and spray cans. But when these molecules drift up into the stratosphere, intense ultraviolet (UV) radiation from the sun has enough energy to break them apart. This releases a chlorine atom, which is highly reactive. The chlorine atom then sets off a chain reaction: it steals an oxygen atom from ozone, forming chlorine monoxide and leaving ordinary oxygen. Then the chlorine monoxide reacts with a free oxygen atom, releasing the chlorine atom again. The chlorine acts like a catalytic nanoparticle – it is not consumed, so it can repeat the cycle, destroying ozone molecules one after another. This is why even tiny amounts of CFCs can have massive effects.
A deeper explanation
The stratospheric ozone depletion cycle begins when UV radiation (wavelength < 240 nm) photolyzes a CFC, such as CCl₃F, releasing a chlorine atom (Cl). The chlorine atom then reacts with ozone (O₃): Cl + O₃ → ClO + O₂. The chlorine monoxide (ClO) then reacts with a free oxygen atom (O) from photolysis of O₂, producing Cl and O₂: ClO + O → Cl + O₂. The net reaction is O₃ + O → 2O₂, with Cl acting as a catalyst. This cycle is efficient because the free oxygen atom is continuously generated by solar UV photolysis of O₂. Without chlorine, O₃ and O would naturally recombine to form O₂, but the chlorine cycle short-circuits that. The chain reaction can destroy thousands of ozone molecules per chlorine atom before the chlorine is removed by forming HCl or ClONO₂, which are temporary reservoirs. This mechanism explains the 'ozone hole' and highlights how human-made chemicals can disrupt a delicate atmospheric balance.