How Pollutants Destroy the Ozone Layer: A Chemical Breakdown

The stratospheric ozone layer, sitting roughly 15 to 35 kilometres above Earth’s surface, is the planet’s primary shield against harmful solar ultraviolet radiation. Yet for decades, human-produced chemicals have been steadily eroding it through a set of well-understood chemical reactions. What makes ozone destruction particularly alarming is not just that it happens – but how efficiently it happens. A single pollutant molecule can trigger a chain reaction that wipes out thousands of ozone molecules. Understanding the chemistry behind this destruction is essential to appreciating both the scale of the problem and why international action has been so critical.

Role of chlorine and bromine radicals in ozone destruction
The chlorine catalytic cycle (Cycle 1)
Polar cycles: ClO-ClO and ClO-BrO reactions (Cycles 2 and 3)
Effects of polar stratospheric clouds (PSCs) on ozone depletion
Denitrification and prolonged destruction
The role of the polar vortex
Global ozone loss and its consequences
UV-B radiation and human health
Impacts on ecosystems
Signs of recovery – and remaining risks

Role of chlorine and bromine radicals in ozone destruction

The primary drivers of stratospheric ozone depletion are halogen radicals – specifically, reactive forms of chlorine and bromine. These originate largely from human-made compounds such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and halons, which drift up into the stratosphere where intense UV radiation breaks them apart, releasing free chlorine (Cl) and bromine (Br) atoms.

Once released, these atoms destroy ozone through catalytic cycles – sequences of reactions in which the halogen atom acts as a catalyst, meaning it is regenerated after each cycle and can go on to destroy ozone again and again. According to NOAA’s Scientific Assessment of Ozone Depletion, a single chlorine or bromine atom can destroy many thousands of ozone molecules before it finally leaves the stratosphere.

The chlorine catalytic cycle (Cycle 1)

The most fundamental chlorine-driven mechanism – known as Cycle 1 – involves two linked reactions. First, a chlorine atom reacts with an ozone molecule (O₃) to produce chlorine monoxide (ClO) and molecular oxygen (O₂). In the second step, ClO reacts with an oxygen atom to regenerate the chlorine atom and release another oxygen molecule. The net result: one ozone molecule and one oxygen atom are converted into two ordinary oxygen molecules, with chlorine playing no net role other than as a catalyst. This cycle is most active in the upper stratosphere at tropical and mid-latitudes, where both atomic oxygen and UV radiation are plentiful.

The reactions can be summarised as:

Cl + O₃ → ClO + O₂
ClO + O → Cl + O₂
Net: O₃ + O → 2O₂

As Chemistry LibreTexts explains, this mechanism was first identified in 1974 by Sherwood Rowland and Mario Molina, who discovered that CFCs could be photolysed in the stratosphere to release chlorine radicals – a discovery that eventually earned them the Nobel Prize in Chemistry.

Polar cycles: ClO-ClO and ClO-BrO reactions (Cycles 2 and 3)

In polar regions, a different set of reactions becomes dominant. Here, the concentration of ClO builds up to extremely high levels, making the standard Cycle 1 less relevant (since atomic oxygen is scarce at lower altitudes). Instead, Cycle 2 is initiated when two ClO molecules react with each other to form a chlorine peroxide dimer (Cl₂O₂), which then breaks apart in sunlight to release two chlorine atoms. Both chlorine atoms then each destroy an ozone molecule, resulting in the net destruction of two ozone molecules per cycle.

Cycle 3 involves the cross-reaction between ClO and bromine monoxide (BrO). Despite bromine being far less abundant than chlorine in the stratosphere, research published in Atmospheric Chemistry and Physics notes that bromine is significantly more reactive on a per-atom basis – its inorganic reservoir compounds are less stable, meaning a higher fraction of bromine exists in active, ozone-destroying form at any given time. The ClO + BrO cycle can proceed through two pathways, both regenerating Cl and Br atoms that continue the destruction process. Together, Cycles 2 and 3 account for the majority of ozone loss observed in both the Arctic and Antarctic stratospheres.

Effects of polar stratospheric clouds (PSCs) on ozone depletion

While the catalytic cycles above explain how ozone is destroyed, a separate set of processes explains why polar regions suffer such disproportionate losses. The key lies in Polar Stratospheric Clouds (PSCs) – clouds that form in the extreme cold of the polar winter stratosphere, typically at altitudes of 15 to 25 kilometres when temperatures drop below approximately −78°C.

During most of the year, the bulk of stratospheric chlorine is locked in stable “reservoir” compounds – primarily chlorine nitrate (ClONO₂) and hydrogen chloride (HCl) – which do not react with ozone. PSCs change this entirely. According to a review published in Reviews of Geophysics and summarised by AGU’s EOS journal, PSC particles host surface reactions that transform these inert reservoir compounds into reactive chlorine forms, particularly ClO. The key heterogeneous reactions include:

HCl + ClONO₂ → Cl₂ + HNO₃ (on PSC surfaces)
N₂O₅ + H₂O → 2HNO₃ (on PSC surfaces)

The molecular chlorine (Cl₂) produced here is rapidly photolysed by springtime sunlight into two free Cl atoms, which then enter the catalytic destruction cycles. As a result, NOAA’s ozone assessment reports that ClO levels in polar regions can increase from a small fraction of available chlorine to nearly all of the reactive chlorine present – a dramatic activation that drives rapid, severe ozone loss.

Denitrification and prolonged destruction

PSCs do more than just activate chlorine – they also extend the destruction period through a process called denitrification. Large PSC particles, particularly those containing nitric acid trihydrate (NAT), sediment downward under gravity, physically removing nitric acid (HNO₃) from the stratosphere. This matters because HNO₃ is a source of nitrogen oxides (NOₓ) that would normally convert reactive ClO back into the inert reservoir compound ClONO₂. With NOₓ removed, ClO stays active for longer, prolonging the ozone-destruction cycle well into the polar spring. Research from the British Antarctic Survey confirms that PSCs prolong ozone depletion by delaying this chlorine deactivation.

The role of the polar vortex

The damage is further concentrated by the polar vortex – a tight circulation of stratospheric winds that forms over the poles in winter, isolating the cold air mass within. This containment prevents ozone-rich air from lower latitudes from mixing in and replenishing what is lost. The Antarctic vortex is particularly strong and stable, which is why the Antarctic ozone hole is more severe and consistent than Arctic depletion. When spring arrives, sunlight photolyses the activated chlorine compounds and PSCs begin to melt, releasing a burst of reactive chlorine that drives the seasonal peak in ozone destruction.

Global ozone loss and its consequences

The combined effect of halogen-catalysed chemistry, PSC-driven chlorine activation, and polar vortex dynamics has produced measurable, well-documented ozone loss on a global scale. The most visible manifestation is the Antarctic ozone hole – a seasonal depletion of stratospheric ozone over Antarctica each September to November. Total column ozone over Antarctica has dropped to as low as 33 percent of pre-1975 values in the most severely affected areas, with over 50 percent of lower stratospheric ozone destroyed within the polar vortex during peak depletion years.

Beyond Antarctica, mid-latitude ozone has also declined, though less severely. The Arctic has experienced significant springtime ozone losses as well, particularly in cold years with a strong polar vortex – 2020 saw the largest Arctic ozone loss on record, with average total ozone columns over the Arctic polar cap reaching 100 Dobson Units below the 1979-2019 mean, according to the UNEP Environmental Effects Assessment Panel.

UV-B radiation and human health

The most direct consequence of ozone loss is the increased penetration of UV-B radiation (wavelengths 280-315 nm) to Earth’s surface. The ozone layer normally absorbs most incoming UV-B, but each percentage drop in ozone allows a proportional increase in surface UV-B. The US EPA notes that excessive UV-B exposure raises the risk of skin cancers, cataracts, and immune system suppression in humans. The agency estimates that full implementation of the Montreal Protocol is expected to prevent approximately 443 million cases of skin cancer and 2.3 million skin cancer deaths among people born between 1890 and 2100 in the United States alone.

Impacts on ecosystems

The effects extend well beyond human health. A review in the scientific literature highlights that elevated UV-B affects animal, plant, and marine life significantly – with concerns that ozone depletion could threaten many plant species and disrupt global food security. In aquatic ecosystems, UV-B reduces the productivity of phytoplankton, the microscopic organisms that form the base of marine food webs and play a critical role in carbon cycling. Terrestrial plants experience disrupted developmental processes, altered nutrient distribution, and reduced growth under elevated UV conditions.

Signs of recovery – and remaining risks

The global response to ozone depletion – primarily through the 1987 Montreal Protocol, which phased out the production of CFCs and other ozone-depleting substances – has had measurable success. The World Meteorological Organization reported in 2024 that the Antarctic ozone hole was smaller than in recent years and that the ozone layer is now on track to recover to 1980s levels by the middle of this century. To date, the Montreal Protocol has led to the phase-out of over 99% of controlled ozone-depleting substances. However, full recovery is still decades away, and the interaction between ozone chemistry and a changing climate introduces new uncertainties. Climate change can cool the stratosphere, potentially sustaining PSC formation for longer periods and complicating the recovery trajectory.

The chemistry of ozone destruction – from the elegantly simple catalytic cycle of a single chlorine atom to the amplifying machinery of polar stratospheric clouds – demonstrates how small quantities of pollutants can produce outsized, global consequences. It is also a powerful demonstration of what science-based international policy can achieve when the evidence is clear and the will is collective.

What do you think? Given that the ozone layer is recovering but won’t return to pre-depletion levels until around mid-century, do you think the pace of international environmental action has been fast enough – or does it reveal a deeper challenge in how the world responds to slow-moving environmental crises? And as climate change now interacts with ozone chemistry in complex ways, how should scientific uncertainty factor into policy decisions when the stakes are this high?

References

PDF 📄

Table of Contents

Role of chlorine and bromine radicals in ozone destruction 🔗

The chlorine catalytic cycle (Cycle 1) 🔗

Polar cycles: ClO-ClO and ClO-BrO reactions (Cycles 2 and 3) 🔗

Effects of polar stratospheric clouds (PSCs) on ozone depletion 🔗

Denitrification and prolonged destruction 🔗

The role of the polar vortex 🔗

Global ozone loss and its consequences 🔗

UV-B radiation and human health 🔗

Impacts on ecosystems 🔗

Signs of recovery – and remaining risks 🔗