CFCs and the Ozone Hole

CFCs (chlorofluorocarbons) were once common in refrigerants and aerosol sprays because they are chemically unreactive at ground level, which is exactly why they last long enough to drift up into the stratosphere. Up there, ultraviolet light finally has enough energy to break the carbon-chlorine bond, releasing a free chlorine atom. That chlorine atom is a catalyst: it rips an oxygen atom off an ozone molecule (O₃) to form chlorine monoxide (ClO) and O₂, then the ClO reacts with a free oxygen atom to release the chlorine atom again, unchanged. Because the chlorine comes back out, one atom can repeat this cycle roughly 100000 times before it is finally removed from the stratosphere, so a tiny amount of CFC does enormous damage.

The hole appears over Antarctica, not over the industrial cities that released the CFCs, because CFCs mix evenly through the atmosphere over a year or two while Antarctic winter creates a unique setup. The extreme cold (below about -78 C) forms polar stratospheric clouds, and chlorine reservoir molecules stick to the ice crystal surfaces on these clouds, where reactions convert them into forms ready to release free chlorine. Nothing happens yet, though, because winter darkness means no UV light to break any bonds. When spring sunlight returns in September and October, it floods the region with UV photons, the chlorine is unleashed all at once, and ozone collapses rapidly, opening the hole.

This sets up the atmospheric lifetime idea used throughout the unit: CFCs decay exponentially once in the stratosphere, with concentration remaining following C = C₀ times e^{−t/lifetime}. It also connects to the Montreal Protocol, the treaty that phased out CFC production and is the reason the hole is slowly closing, showing that a global problem driven by chemistry can be solved by global policy.