AP Syllabus focus:
‘Stratospheric ozone depletion can be caused by human-made chemicals such as CFCs and by natural processes such as springtime melting of Antarctic ice crystals in the atmosphere.’
Stratospheric ozone depletion happens when reactions in the upper atmosphere destroy ozone faster than it forms. The main drivers are human-made ozone-depleting substances and polar spring conditions over Antarctica that activate chlorine chemistry.
What is being depleted (and where)
Most atmospheric ozone (O₃) that matters for protection is in the stratosphere (about 15–35 km up). Depletion refers to a reduction in stratospheric ozone concentration, especially dramatic in the Antarctic “ozone hole.”
False-color satellite/map visualization of total-column ozone over Antarctica (Dobson units), showing the low-ozone region commonly referred to as the “ozone hole.” The labeled scale makes clear that depletion is quantified as reduced ozone abundance, not a literal gap in the atmosphere. Source
Ozone layer: A region of the stratosphere with elevated ozone concentrations that absorbs much of the Sun’s incoming ultraviolet (UV) radiation.
Ozone is continuously created and destroyed naturally, but depletion occurs when additional catalysts greatly accelerate destruction.
Human-made causes: ozone-depleting substances (ODS)
Why CFCs are especially important
Chlorofluorocarbons (CFCs) were widely used in refrigerants, aerosol propellants, and foam-blowing because they are nonreactive in the lower atmosphere. That stability allows them to persist long enough to reach the stratosphere.
Chlorofluorocarbons (CFCs): Human-made compounds containing chlorine, fluorine, and carbon that are chemically stable in the troposphere but can release chlorine in the stratosphere.
In the stratosphere, higher-energy UV light breaks CFCs apart, releasing chlorine atoms/radicals. Chlorine participates in catalytic cycles: it helps convert ozone to ordinary oxygen, while the chlorine is regenerated and can repeat the process many times. This is why a small amount of CFC-derived chlorine can destroy a large amount of ozone.
Other human-made contributors (same basic mechanism)
While the specification highlights CFCs, APES students should recognise that other chlorine- or bromine-containing industrial chemicals can also deplete ozone because they can release halogen radicals in the stratosphere.
Halons (contain bromine) are very effective ozone destroyers per molecule.
Other legacy ODS include certain solvents and industrial chemicals that transport halogens aloft.
The unifying cause is the delivery of reactive halogens (Cl and Br) to the stratosphere, where they catalyse ozone loss.
Natural processes: Antarctic springtime ice crystals
Polar stratospheric clouds (PSCs) and “activation” of chlorine
The specification’s natural process refers to springtime melting of Antarctic ice crystals—more precisely, chemical reactions that occur on the surfaces of ice crystals in polar stratospheric clouds, followed by rapid ozone loss when sunlight returns.
Polar stratospheric clouds (PSCs): High-altitude clouds made of ice crystals that form in the extremely cold polar stratosphere and provide surfaces for reactions that promote ozone destruction.
Key idea: much stratospheric chlorine exists in relatively “safe” reservoir forms (not actively destroying ozone).
Two-panel schematic illustrating why PSCs accelerate ozone loss: reservoir chlorine species are converted on ice-crystal surfaces into forms that sunlight can rapidly turn into reactive chlorine. The figure links surface (heterogeneous) chemistry to catalytic ozone destruction in spring, clarifying why Antarctic conditions produce intense seasonal depletion. Source
In polar winter, PSCs form and enable heterogeneous reactions (reactions on a surface rather than in open air) that convert reservoir chlorine into forms that can quickly become reactive when illuminated.
Why the ozone hole is strongest over Antarctica (timing + conditions)
Several Antarctic conditions combine to make spring ozone depletion severe:
Extreme cold promotes PSC formation, increasing reactive surface area for chlorine activation.
The polar vortex (strong, isolated circulation) traps air over Antarctica, allowing activated chlorine to accumulate rather than mixing away.
In spring, increasing sunlight drives photochemical reactions that generate highly reactive chlorine, triggering rapid ozone destruction.
As temperatures rise (and ice crystals “melt”/PSCs disappear), the intense depletion period ends, but significant ozone loss has already occurred.
This is why the Antarctic ozone hole is highly seasonal: the chemistry depends on wintertime PSCs plus springtime sunlight.
How to connect the causes (what to remember for APES)
Human-made chemicals such as CFCs supply long-lived chlorine sources that reach the stratosphere.
Antarctic ice crystals in spring (PSCs and their seasonal breakup) enable reactions that convert chlorine into ozone-destroying forms, producing rapid, localized depletion.
The core mechanism is catalytic destruction: reactive halogens are reused, allowing repeated ozone loss from a single released atom.
FAQ
In mid-winter there is little/no sunlight, limiting photochemical steps that generate highly reactive chlorine.
In spring, sunlight returns while the stratosphere can still be cold enough for recent PSC-driven “activation,” so destruction accelerates.
Bromine radicals can participate in catalytic cycles that destroy ozone efficiently.
Molecule-for-molecule, bromine chemistry can be more ozone-destructive than chlorine chemistry under similar conditions.
They measure altitude-specific ozone using balloons, satellites, and ground-based instruments.
Stratospheric depletion is a reduction high in the atmosphere; urban ozone is a near-surface pollutant formed from NOx and VOCs in sunlight.
Antarctica typically has a colder, more stable polar vortex, promoting more frequent PSC formation and stronger isolation.
The Arctic stratosphere is often warmer and more variable, so the same chemistry is less consistently intense.
Many ODS have long atmospheric lifetimes, so previously emitted compounds persist and keep supplying halogens to the stratosphere.
Transport to the stratosphere and slow removal mean recovery takes decades even after controls.
Practice Questions
State two causes of stratospheric ozone depletion. (2 marks)
Identifies human-made chemicals such as CFCs (1)
Identifies natural polar process involving Antarctic ice crystals/PSCs in spring (1)
Explain how CFCs and springtime Antarctic ice crystals together lead to severe seasonal ozone depletion over Antarctica. (6 marks)
CFCs are stable enough to reach the stratosphere (1)
UV radiation breaks CFCs, releasing reactive chlorine (1)
Chlorine destroys ozone via catalytic cycles (chlorine regenerated) (1)
Very low Antarctic stratospheric temperatures form polar stratospheric clouds/ice crystals (1)
Reactions on ice surfaces convert chlorine into forms that become highly reactive when sunlight returns in spring (1)
Polar vortex isolates air, allowing strong local depletion/“ozone hole” (1)
