The discovery, explained · Chemist no. 86 of 350

Paul Crutzen and the Discovery That Nitrogen Oxides Destroy the Ozone Layer

By Aniket Bhardwaj · 24 Oct 2026 · 7 min read · Physical chemistry

The ozone layer is a thin shield of O₃ molecules, mostly 15 to 35 km above the Earth, that absorbs most of the harmful ultraviolet-B radiation from the Sun. If it were compressed to ordinary air pressure at sea level, it would be only about 3 mm thick. In 1970 a young Dutch scientist, Paul Crutzen, then working at Stockholm University, showed that the natural ozone layer is controlled by tiny amounts of nitrogen oxides, NO and NO₂, acting as catalysts. His discovery explained why the Chapman theory of ozone was incomplete and opened the way to understanding how human activities, from supersonic aircraft to fertilisers, could change the ozone layer. For this, Crutzen shared the 1995 Nobel Prize in Chemistry with Mario Molina and F. Sherwood Rowland.

The Scientific Problem Before the Discovery

In 1930 the British scientist Sydney Chapman proposed a simple set of reactions for the formation and destruction of ozone in the stratosphere, now called the Chapman cycle:

1)  O2 + hν (λ less than 242 nm) → O + O
2)  O + O2 + M → O3 + M
3)  O3 + hν (λ about 200 to 320 nm) → O2 + O
4)  O + O3 → 2 O2

Reactions 1 and 2 produce ozone; reactions 3 and 4 remove it. Step 3 is not a net loss, because the oxygen atom produced quickly regenerates O₃ by step 2. The real loss is step 4. M is a third body (N₂ or O₂) that carries away excess energy.

The problem was that when scientists put the measured rate constants into the Chapman model, it predicted too much ozone, about twice the amount measured in the real atmosphere. Something else was destroying ozone. Researchers suspected that hydrogen radicals (HO_x) took part, but the numbers still did not match. Crutzen looked at a different family of molecules.

The Discovery / Experiment — Step by Step

Crutzen's work was a combination of laboratory kinetic data, measurements of trace gases and careful modelling. The logic can be followed in steps:

Nitrous oxide at the surface. N₂O is made by soil bacteria in nitrification and denitrification. It is very unreactive in the lower atmosphere, so it survives long enough to rise into the stratosphere.
Conversion to NO. In the stratosphere a small fraction of N₂O reacts with excited oxygen atoms: N2O + O(¹D) → 2 NO.
Catalytic cycle. NO destroys ozone in a chain that regenerates NO:

NO + O3 → NO2 + O2
NO2 + O → NO + O2
-------------------------
Net:   O3 + O → 2 O2

Because NO is regenerated, one molecule can destroy thousands of ozone molecules before it is removed. This is why the effect is so large even when NO_x is present in parts per billion. In 1970 Crutzen published the calculation, showing that when the NO_x cycle is added to the Chapman scheme, the predicted ozone amount agrees with observation. He also proposed that anything that raises stratospheric NO_x would reduce ozone.

The first warnings

In 1971 Crutzen, and independently Harold Johnston in the United States, pointed out that fleets of supersonic transport aircraft, flying in the stratosphere and emitting NO from their engines, could cause measurable ozone loss. This was an early example of atmospheric science affecting technology policy. Crutzen later studied the effect of nuclear war on the atmosphere (the "nuclear winter" idea, 1982, with John Birks) and the global impact of biomass burning. In 2000, with Eugene Stoermer, he popularised the term Anthropocene for a new geological age shaped by human activities.

The Chemistry Behind It

What makes a good catalyst in the atmosphere?

A catalytic cycle needs a species that reacts with O₃, and a second step that regenerates the catalyst using O atoms. A general form is:

X + O3 → XO + O2
XO + O → X + O2
Net: O3 + O → 2 O2     (X = NO, Cl, Br, OH, H)

The same scheme with X = Cl was proposed in 1974 by Molina and Rowland for chlorofluorocarbons. So Crutzen's NO_x cycle is the first member of a family of ozone-destroying catalytic cycles. The importance of each catalyst varies with altitude. Near 40 km, HO_x and Cl_x matter; around 25 km, NO_x is the main catalyst of natural ozone loss.

Catalyst X	Main source	Notes
NO (NO_x)	N₂O from soils, aircraft, fertilisers	Crutzen, 1970
Cl (Cl_x)	CFCs and other chlorine compounds	Molina and Rowland, 1974
OH, H (HO_x)	Water vapour and methane	Studied in the late 1960s and 1970s
Br (Br_x)	Halons, methyl bromide	Very efficient per atom

Applications Today

Policy: the knowledge of catalytic cycles supported the Montreal Protocol (1987), which phased out ozone-depleting substances.
Air quality: Crutzen showed that in the lower atmosphere (troposphere), NO_x actually produces ozone in the presence of sunlight and hydrocarbons, which is the basis of photochemical smog in cities such as Delhi.
Agriculture: N₂O from fertilised soils is both a greenhouse gas and a source of stratospheric NO_x, so nitrogen management matters for climate and ozone.
Aviation and rockets: engine emissions in the stratosphere are assessed with models that include the NO_x cycle.
Climate science: the idea of the Anthropocene and discussion of geoengineering, such as stratospheric sulfate injection, are linked to Crutzen's later writings.

Exam Corner — 5 Likely Points

Chapman cycle: O₂ photolysis (below 242 nm), O + O₂ + M → O₃ + M, O₃ photolysis, and O + O₃ → 2O₂.
NO_x cycle: NO + O₃ → NO₂ + O₂; NO₂ + O → NO + O₂.
Catalyst NO is not consumed; one NO can destroy many O₃ molecules.
Source of stratospheric NO: N₂O + O(¹D) → 2NO.
Nobel Prize 1995: Crutzen, Molina, Rowland, for work on formation and decomposition of ozone.

Common Misconceptions

"Ozone in the stratosphere and ozone near the ground are the same problem." Same molecule, opposite roles: stratospheric ozone protects, tropospheric ozone is a pollutant.
"Only CFCs destroy ozone." Natural NO_x and HO_x cycles also remove ozone, and CFCs add to this loss.
"The ozone hole is the same as global warming." They are different problems, although some gases (like CFCs and N₂O) play a role in both.

Quick Revision Box

Chapman (1930) overestimated stratospheric ozone.
Crutzen (1970): NO and NO₂ catalyse ozone destruction.
Net: O₃ + O → 2O₂, with NO regenerated.
Source of NO: N₂O from soil microbes reacting with O(¹D).
Supersonic aircraft warning in 1971 by Crutzen and by Johnston.
Nobel Prize in Chemistry 1995 shared with Molina and Rowland.

FAQ

Is the ozone layer made only of ozone?

No. It is a region of the stratosphere where ozone is relatively concentrated, but even there ozone is only a few parts per million of the air.

What is a Dobson unit?

One Dobson unit is the amount of ozone that would make a layer 0.01 mm thick at 0 °C and 1 atm. The global average is about 300 DU.

Why does a catalyst matter so much for ozone?

A catalyst is regenerated in each cycle, so a tiny amount of NO or Cl can destroy a very large number of ozone molecules.

Did Crutzen discover the ozone hole?

No. The Antarctic ozone hole was reported in 1985 by Farman, Gardiner and Shanklin. Crutzen's 1970 work gave part of the chemical background.

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