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