Atmospheric Chemistry 1974

Stratospheric Sink for Chlorofluoromethanes

Mario J. Molina & F. Sherwood Rowland

Inert man-made gases drift up and quietly eat the ozone that shields life from the Sun.

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In depth · the introduction

The gases in your grandparents' spray cans were so harmless that nothing on Earth could destroy them — which turned out to be exactly the danger.

The big idea

High above us is a thin layer of ozone, a form of oxygen that absorbs the Sun's most damaging ultraviolet rays before they reach the ground. Without it, sunburn would come in minutes and life on land would be in trouble. Molina and Rowland discovered that an invisible class of man-made gases was quietly eating that shield away.

The culprits were chlorofluorocarbons — CFCs — the safe, cheap chemicals used in refrigerators, air conditioners and aerosol cans. They are so unreactive that nothing in the lower air breaks them down, so they slowly float up to the stratosphere. There, sunlight finally cracks them open and frees chlorine atoms — and a single chlorine atom can go on to destroy about a hundred thousand ozone molecules, one after another, because the chemistry hands the chlorine back unharmed each time.

How it came about

In 1973 Mario Molina, a young Mexican chemist, joined Sherwood Rowland's lab at the University of California, Irvine, and took on what looked like a quiet question: where do CFCs end up? Tracing the chemistry step by step, the two realised with alarm that the answer was the ozone layer, and that the damage could be enormous. They published the warning in Nature in 1974 — and then did something unusual for chemists: they went public, urging a ban on these profitable products. The industry pushed back hard for years. Vindication came slowly, then suddenly, with the discovery of the Antarctic ozone hole in 1985; in 1995 Molina, Rowland and Paul Crutzen shared the Nobel Prize in Chemistry.

Why it mattered

This was the first time science caught a global environmental disaster while it was still only a prediction — and the world acted in time. The 1987 Montreal Protocol phased CFCs out worldwide, and the ozone layer is now slowly healing. It stands as the great proof that humanity can see a planet-sized threat coming and choose, together, to head it off. Every later argument about protecting the atmosphere leans on this example.

A way to picture it

Imagine one vandal with a magic key that opens any lock, snaps it, and reappears in your hand good as new — ready for the next lock. You would not measure the threat by how many keys there are; one is enough to break thousands of locks. A chlorine atom is that key, the ozone molecules are the locks, and ‘catalyst’ is the word for a key that never wears out. In the tool below, run the cycle and watch a single chlorine atom rack up an enormous tally while its own count never rises above one.

Where it sits

The catalytic idea was Paul Crutzen's, who first showed nitrogen oxides eating ozone in 1970; Molina and Rowland aimed it at the man-made CFCs. It belongs to the story of how we learned to read the whole atmosphere as a chemical system — alongside Arrhenius's 1896 work on carbon dioxide and warming and Keeling's CO₂ measurements (both in this Library). And it remains the hopeful counterpart to the climate problem: the one time we found the danger, named it, and fixed it.

The original document

Original source text

M. J. Molina and F. S. Rowland · Nature 249, 810–812 · 28 June 1974

The puzzle of a perfectly stable gas

The chlorofluoromethanes — CFCl₃ and CF₂Cl₂, the workhorse refrigerants, aerosol propellants and solvents of mid-century industry — are prized precisely because they are inert: non-flammable, non-toxic, and unreactive with almost everything at ground level. That virtue is the problem. With no chemical or biological process to remove them, they simply accumulate, and the paper asks the question no one had: if nothing destroys these gases near the surface, where do they finally go?

The only sink: photolysis far overhead

The answer is that they go up. Over years to decades the molecules diffuse into the stratosphere, and only there — above most of the protective ozone, bathed in short-wave ultraviolet that never reaches the ground — are they at last broken apart, releasing free chlorine atoms. The unreactivity that let them survive the troposphere delivers their chlorine intact to the one altitude where it can do the most harm.

The catalytic chain

A chlorine atom then attacks ozone in a two-step chain — Cl + O₃ → ClO + O₂, then ClO + O → Cl + O₂ — whose net effect is O₃ + O → 2 O₂. Crucially the chlorine is handed back at the end of each lap: it is a catalyst, not a reactant consumed. A single atom can therefore run the cycle on the order of a hundred thousand times, destroying ~10⁵ ozone molecules, before it is eventually parked in a longer-lived reservoir such as HCl. The conclusion is stark: continued release of these gases must thin the ozone layer.

What the paper warned — and what it could not yet know

[ … ]

The chemistry was new but not unprecedented: Crutzen (1970) had shown an analogous catalytic destruction of ozone by nitrogen oxides, and Stolarski and Cicerone, the same year, proposed chlorine catalysis from other sources. Molina and Rowland's leap was to name a vast, fast-growing, wholly man-made source — the CFCs — and to follow it to its consequence. The paper could not foresee the speed of what came: the Antarctic ‘ozone hole’ found in 1985 was far deeper than these gas-phase estimates implied, driven by extra reactions on the surfaces of polar stratospheric clouds. The full kinetics and the policy response are downstream of these two pages.

Department of Chemistry, University of California, Irvine · 1974