How Ozone Forms and Breaks Down in the Stratosphere

The stratosphere, located roughly 10 to 50 kilometres above Earth’s surface, is home to a thin but critical shield of ozone gas. This shield absorbs the majority of the Sun’s harmful ultraviolet radiation before it can reach the ground. But how exactly does ozone form up there, and what causes it to break apart? The answers lie in a continuous chemical cycle driven entirely by sunlight – a cycle that keeps life on Earth protected, yet remains surprisingly delicate.

What is ozone, and where is it found?
Formation of ozone in the atmosphere
Step 1: splitting of molecular oxygen
Step 2: ozone is created
Where does most ozone production occur?
Ozone dissociation: how ozone breaks down
Photodissociation of ozone
Recombination reaction
The dynamic equilibrium
Why the ozone-oxygen cycle matters for life on Earth
Role of Dobson units in measuring ozone
What exactly is a Dobson unit?
Who developed this unit?
How do scientists use Dobson units?
Natural versus human-caused ozone changes
Key takeaways

What is ozone, and where is it found?

Ozone (O₃) is a molecule made up of three oxygen atoms. While regular oxygen gas (O₂) – the kind we breathe – contains two oxygen atoms, ozone has an extra one, making it chemically reactive and unstable. About 90% of all atmospheric ozone resides in the stratosphere, forming what we commonly call the “ozone layer.” This layer sits approximately 15 to 35 kilometres above Earth’s surface.

Despite its importance, the ozone layer is remarkably thin. If all the ozone in the atmosphere were compressed to sea-level pressure, it would form a layer only about 3 millimetres thick – roughly the height of two pennies stacked together. Yet this slender layer is responsible for absorbing 97 to 99 percent of the Sun’s medium-frequency ultraviolet light.

Formation of ozone in the atmosphere

Ozone formation in the stratosphere is driven by ultraviolet radiation from the Sun. The process was first described in 1930 by British scientist Sydney Chapman, and the set of reactions he proposed is now known as the Chapman cycle. It remains the foundational model for understanding how ozone is naturally produced and maintained.

Step 1: splitting of molecular oxygen

The process begins when high-energy ultraviolet radiation (specifically UV-C, with wavelengths shorter than 240 nm) strikes a molecule of molecular oxygen (O₂) in the upper stratosphere. This energy is enough to break the bond between the two oxygen atoms, splitting the molecule into two individual oxygen atoms (O). This reaction is called photodissociation or photolysis.

The reaction looks like this:
O₂ + UV radiation → O + O

Step 2: ozone is created

Each free oxygen atom (O) released in Step 1 is highly reactive. It quickly collides with a nearby molecular oxygen molecule (O₂) and bonds with it, forming ozone (O₃). However, this reaction requires a third molecule – often nitrogen (N₂) or another O₂ molecule – to absorb the excess energy released during the bonding process. Without this third body (denoted as “M”), the newly formed ozone would be too unstable to hold together.

The reaction is:
O + O₂ + M → O₃ + M

The basic ingredients for ozone formation are UV radiation and O₂, which is why the ozone layer exists at a specific altitude range. Too high in the atmosphere (in the mesosphere), there is plenty of UV but not enough O₂ because air pressure is extremely low. Too low (in the troposphere), there is abundant O₂ but not enough UV because it has already been absorbed by oxygen and ozone higher up. The stratosphere provides the right balance of both.

Where does most ozone production occur?

Most ozone production takes place in the tropical upper stratosphere, where solar UV radiation is most intense. The total mass of ozone generated each day across the globe is approximately 400 million metric tons. However, the global mass of ozone remains relatively constant at around 3 billion metric tons, because ozone is simultaneously being destroyed at nearly the same rate it is being created.

Once formed in the tropics, ozone-rich air does not stay put. Atmospheric circulation patterns – known as the Brewer-Dobson circulation – transport ozone from the tropics toward the poles. This is why ozone concentrations are often higher at mid and high latitudes than at the equator, even though production is greatest in the tropics.

Ozone dissociation: how ozone breaks down

Just as sunlight creates ozone, it also destroys it. Ozone molecules are inherently unstable and readily absorb ultraviolet radiation, which triggers their breakdown. This process of ozone dissociation is the other half of the Chapman cycle, and it is just as important as formation in maintaining the balance of the ozone layer.

Photodissociation of ozone

When an ozone molecule absorbs UV-B or UV-C radiation (wavelengths up to about 320 nm), it splits into one molecule of molecular oxygen (O₂) and one free oxygen atom (O):

O₃ + UV radiation → O₂ + O

The free oxygen atom can then react with another O₂ molecule to regenerate ozone, continuing the cycle. This continuous loop of formation and destruction converts UV energy into heat without any net loss of ozone. It is this heat generation that actually causes the stratosphere to warm with increasing altitude – a defining characteristic of that atmospheric layer.

Recombination reaction

There is also a less common reaction where a free oxygen atom collides directly with an ozone molecule, producing two molecules of molecular oxygen:

O + O₃ → 2O₂

This is a natural part of the Chapman cycle and has been occurring for millions of years. Under normal conditions, this reaction is relatively uncommon in the stratosphere because the concentration of free oxygen atoms is much lower than that of O₂ molecules.

The dynamic equilibrium

Together, these formation and destruction reactions establish a dynamic equilibrium. Ozone is constantly being created and broken apart, but the total amount stays roughly stable over time. This balance is what sustains the protective ozone layer. The rate of each reaction depends on factors such as sunlight intensity, air density, temperature, and altitude.

However, this equilibrium is not immune to disruption. Human-made chemicals, particularly chlorofluorocarbons (CFCs), have introduced chlorine and bromine into the stratosphere, which act as catalysts that accelerate ozone destruction far beyond what the Chapman cycle alone would predict. A single chlorine atom can destroy over 100,000 ozone molecules before being removed from the stratosphere.

Why the ozone-oxygen cycle matters for life on Earth

The constant cycling of ozone formation and dissociation serves a dual purpose. First, it creates and maintains the ozone layer itself. Second, and perhaps more importantly, the process converts harmful UV radiation into heat, effectively acting as a sunscreen for the entire planet.

The ozone layer absorbs UV radiation with wavelengths up to about 310 nm, which includes all of the dangerous UV-C and most of the UV-B radiation emitted by the Sun. Without this absorption, life on Earth’s surface would be exposed to intense radiation capable of damaging DNA, causing skin cancers and cataracts, suppressing immune systems, and harming ecosystems ranging from agricultural crops to marine plankton.

Role of Dobson units in measuring ozone

Scientists need a reliable way to measure the total amount of ozone in the atmosphere. Since ozone is dispersed across the entire stratosphere rather than concentrated in a single layer, researchers measure the total column ozone – that is, the total amount of ozone in a vertical column extending from the ground to the edge of space. The standard unit for this measurement is the Dobson Unit (DU).

What exactly is a Dobson unit?

One Dobson Unit represents the number of ozone molecules needed to form a layer of pure ozone 0.01 millimetres thick at standard temperature (0°C) and pressure (1 atmosphere). Put differently, one DU corresponds to roughly 2.69 × 10¹⁶ ozone molecules per square centimetre of the column’s base.

The global average thickness of the ozone layer is about 300 DU, which translates to a compressed pure-ozone layer just 3 mm thick. To put that in perspective, if you compressed all the air in a vertical column of the atmosphere to sea level, that column would be about 8 kilometres thick. The ozone within it would account for just 3 mm of that total.

Who developed this unit?

The Dobson Unit is named after Gordon Dobson, a researcher at the University of Oxford who built the first instrument to systematically measure total ozone from the ground in the 1920s. His instrument – the Dobson spectrophotometer – became the backbone of global ozone monitoring and remains in use at nearly 100 observatories around the world today.

How do scientists use Dobson units?

Dobson Units allow scientists to track ozone levels over time and across regions, identify trends in ozone depletion, and assess the effectiveness of international agreements like the Montreal Protocol. NASA uses a baseline value of 220 DU for the Antarctic ozone hole, since values below this threshold were not observed before 1979 and indicate ozone loss caused by chlorine and bromine compounds.

Modern ozone monitoring combines ground-based Dobson and Brewer spectrophotometers with satellite instruments such as the Total Ozone Mapping Spectrometer (TOMS). The World Meteorological Organization (WMO) has coordinated a global ozone observing system since the International Geophysical Year of 1957, ensuring standardized, high-quality measurements across all observing stations.

Natural versus human-caused ozone changes

Ozone concentrations naturally fluctuate due to sunspot cycles, seasonal changes, and latitude. These variations are predictable, and each natural dip in ozone has historically been followed by recovery. The Chapman cycle, under normal conditions, keeps the ozone layer in a self-sustaining balance.

The problem arose in the 1970s when scientists discovered that human-made chemicals – especially CFCs used in refrigerators, aerosol sprays, and air conditioners – were releasing chlorine atoms into the stratosphere. These atoms catalytically destroy ozone at rates far exceeding natural breakdown. The discovery led to the Montreal Protocol, which has been largely successful in reducing the production of ozone-depleting substances. Thanks to this agreement, an estimated 443 million cases of skin cancer and 2.3 million skin cancer deaths are expected to be prevented among people born between 1890 and 2100 in the United States alone.

Key takeaways

The formation and dissociation of ozone in the stratosphere is a continuous, sunlight-driven process described by the Chapman cycle. Molecular oxygen is split by high-energy UV radiation into free oxygen atoms, which then combine with other O₂ molecules to form ozone. Ozone, in turn, absorbs UV radiation and breaks apart – only to reform again. This dynamic equilibrium sustains the protective ozone layer that shields all life on Earth from dangerous ultraviolet radiation. Scientists monitor the health of this layer using Dobson Units, a measurement system that has been in place for nearly a century and continues to guide global environmental policy.

What do you think? Given that the ozone layer is just 3 millimetres thick when compressed, how does knowing its fragility change the way you view everyday products that may still contribute to atmospheric pollution? And with the Montreal Protocol showing real success, could its cooperative model be applied to other global environmental challenges like climate change?

References

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Table of Contents

What is ozone, and where is it found? 🔗

Formation of ozone in the atmosphere 🔗

Step 1: splitting of molecular oxygen 🔗

Step 2: ozone is created 🔗

Where does most ozone production occur? 🔗

Ozone dissociation: how ozone breaks down 🔗

Photodissociation of ozone 🔗

Recombination reaction 🔗

The dynamic equilibrium 🔗

Why the ozone-oxygen cycle matters for life on Earth 🔗

Role of Dobson units in measuring ozone 🔗

What exactly is a Dobson unit? 🔗

Who developed this unit? 🔗

How do scientists use Dobson units? 🔗

Natural versus human-caused ozone changes 🔗

Key takeaways 🔗