Ozone Layer Depletion: Chapman Cycle to Hole

A recurring trap on this chapter asks a candidate to explain why the ozone hole forms over Antarctica rather than the Arctic, or over India, even though CFCs mix through the entire global stratosphere. The answer is not about where the chemicals are, it is about temperature and darkness, and a question phrased around polar stratospheric clouds or the polar vortex is testing exactly this mechanism rather than the Montreal Protocol dates.

Renewable Energy and Climate Mitigation Tech covers the Montreal Protocol, the Kigali Amendment and India's HFC phase-down schedule in depth. This note covers the underlying atmospheric science instead: how ozone actually forms and is destroyed, why the hole sits over Antarctica, and what increased UV-B does at the surface.

How ozone forms and is destroyed

Roughly 90% of atmospheric ozone sits in the stratosphere, between about 10 and 50 kilometres above the surface, with concentration peaking at around 30 to 35 km, the layer loosely called the "ozone layer". This matters because incoming solar ultraviolet radiation splits into three bands by wavelength: UV-A (315 to 400 nm) passes through the ozone layer largely unaffected and reaches the surface in full; UV-C (below 280 nm), the most damaging, is almost entirely absorbed by ozone and never reaches the ground; UV-B (280 to 315 nm) sits in between, strongly but not fully screened, so how much reaches the surface depends directly on how much ozone stands in the way.

Stratospheric ozone is created and destroyed continuously by a natural process called the Chapman cycle. Ultraviolet radiation with a wavelength below about 242 nanometres splits an oxygen molecule (O2) into two free oxygen atoms. Each free atom then collides with an intact O2 molecule to form ozone (O3). Ozone itself absorbs a different, longer band of UV radiation (below about 320 nm), which breaks it back apart into O2 and a free oxygen atom, releasing heat in the process, this absorption is what actually warms the stratosphere and shields the surface. Under natural conditions, formation and destruction balance out to a roughly steady background concentration.

Chlorine and bromine radicals from CFCs disrupt this balance by adding a much faster, human-caused destruction pathway. UV radiation breaks a carbon-chlorine bond inside a CFC molecule, releasing a free chlorine atom. That chlorine atom reacts with an ozone molecule, stripping away one oxygen atom to form chlorine monoxide (ClO) and leaving ordinary O2 behind. The ClO then reacts with a free oxygen atom, releasing the chlorine atom again, unchanged, ready to repeat the cycle. Because the chlorine is regenerated rather than consumed, it acts as a catalyst rather than a reactant. A single chlorine atom can destroy on the order of a thousand ozone molecules before it is eventually locked into a stable, unreactive compound. Bromine radicals, mainly from halons, run the same catalytic cycle and are considerably more efficient per atom, though present in far smaller quantities.

This difference in potency is exactly what Ozone Depletion Potential (ODP) measures: the ratio of a substance's ozone-destroying impact to that of an equal mass of CFC-11, whose ODP is fixed by definition at 1.0. Per the US EPA, CFCs and HCFCs range from about 0.01 to 1.0, while bromine-based halons, used historically in fire extinguishers, range up to 10, reflecting bromine's greater efficiency at the same catalytic cycle. ODP is a distinct scale from Global Warming Potential (GWP), the metric that governs the Kigali Amendment's HFC phase-down covered in the renewable energy and climate mitigation note; a substance can score high on one and near zero on the other, which is precisely why HFCs replaced CFCs in the first place, and precisely why that replacement then became a warming problem of its own.

The Antarctic ozone hole

The ozone hole is a seasonal, not permanent, feature: it opens over Antarctica each southern spring (roughly August to November) and closes up again by early summer. Two Antarctica-specific conditions drive this, neither of which is really about local chlorine concentration.

First, the polar vortex, a band of fast, circling winds that forms over Antarctica in the southern winter, isolates the air mass above the continent from the rest of the stratosphere for months. Nothing mixes in or out. Second, this isolation lets stratospheric temperatures inside the vortex fall low enough, well below minus 78 degrees Celsius, for polar stratospheric clouds (PSCs) to form, ice and nitric-acid particles suspended in the otherwise near-empty stratosphere. Reactions on the surfaces of these PSC particles convert stable, unreactive chlorine reservoir compounds (chlorine nitrate and hydrogen chloride) into the active, ozone-destroying forms, chiefly ClO. Through the dark polar winter this activated chlorine simply accumulates, because the catalytic destruction cycle needs sunlight to run.

When sunlight returns with the arrival of spring, that stored-up reactive chlorine begins destroying ozone rapidly, and because the polar vortex is still holding the air mass in isolation, the depleted air cannot be replenished by ozone-rich air from lower latitudes. The result is the sharp seasonal collapse in ozone concentration recorded as the hole. The Arctic lacks an equivalently cold, stable, long-lived vortex (Northern Hemisphere land masses and mountains disturb the stratospheric flow), so it does not develop a hole of comparable size or reliability.

Ozone amount is measured in Dobson Units (DU): one DU is the thickness a column of pure ozone would have if compressed to standard temperature and pressure, equal to a 0.01 mm layer. A typical mid-latitude column reads around 300 DU; over Antarctica in spring, NASA's Ozone Watch records the depleted region averaging down to about 100 DU. The phenomenon was first measured, not merely theorised, in 1985: British Antarctic Survey scientists J. C. Farman, B. G. Gardiner and J. D. Shanklin published ground-based Dobson spectrophotometer readings from Halley Research Station, taken since 1956, showing a springtime collapse in total column ozone over Antarctica, in the journal Nature. That measurement followed, and confirmed, a theoretical prediction: in 1974, Mario Molina and F. Sherwood Rowland had proposed in Nature that CFCs would migrate to the stratosphere and catalytically destroy ozone, work for which they shared the 1995 Nobel Prize in Chemistry with Paul Crutzen.

The hole's size is genuinely dynamic and worth citing with a date. NASA and NOAA recorded the 2025 Antarctic ozone hole reaching a maximum single-day extent of 8.83 million square miles (22.86 million square kilometres) on 9 September 2025, with an average extent of 7.23 million square miles (18.71 million square kilometres) over the peak depletion window of 7 September to 13 October. That ranked as the fifth smallest since 1992, the year Montreal Protocol controls began taking effect, and it closed nearly three weeks earlier than the past decade's average. Scientists attribute the shrinking trend to the ongoing decline of ozone-depleting chemicals in the atmosphere. The 2022 WMO/UNEP Scientific Assessment of Ozone Depletion projects full recovery to 1980 levels at around 2040 for near-global (60 degrees South to 60 degrees North) column ozone, around 2045 for the Arctic, and around 2065 for the slower-recovering Antarctic, under a middle-of-the-road emissions scenario.

UV-B radiation: health and ecological effects

Ozone's absorption of UV-B is the entire reason its depletion matters at the surface. Per the US EPA, excessive UV-B exposure raises the risk of non-melanoma skin cancer directly and plays a major role in the development of malignant melanoma, the more dangerous skin cancer, which the EPA notes already causes nearly 8,500 deaths a year in the United States. UV-B exposure is also linked to cataracts, a clouding of the eye's lens, and to suppression of the human immune system. Beyond human health, direct reductions in phytoplankton production from UVB have been demonstrated; phytoplankton sit at the base of the marine food web and are a major carbon sink, so a reduction there has knock-on ecological effects. UVB also damages terrestrial plant life directly and accelerates the breakdown of outdoor materials such as plastics.

Exam angle

This chapter is tested less on dates and more on mechanism, which is why statement questions favour distractor pairs like: ozone is formed by UV splitting O2 (not O3) into atoms that then recombine with O2; chlorine is a catalyst in the destruction cycle, not a reactant consumed by it, so a single atom destroys many ozone molecules over time; the Antarctic hole forms because of the polar vortex and polar stratospheric clouds trapping and activating chlorine through winter darkness, not because CFCs concentrate there; and the hole is seasonal (opens in southern spring, closes by summer) rather than a permanent hole. A question naming polar stratospheric clouds or the polar vortex is testing the Antarctic-specific mechanism above, while a question naming HFCs as ozone-depleting is testing the Kigali distinction covered in the renewable energy and climate mitigation note instead. One more pairing worth locking in: Ozone Depletion Potential (a substance's ozone-destroying power relative to CFC-11) and Global Warming Potential (its heat-trapping power relative to CO2) are different scales entirely, a high score on one says nothing about the other, and that gap is the whole reason the Montreal-to-Kigali story needed two separate treaties.

Quick revision points

• Ozone layer: about 90% of atmospheric ozone lies in the stratosphere, roughly 10 to 50 km up, peaking around 30 to 35 km. UV-A largely passes through, UV-C is almost fully absorbed, UV-B is partially screened, so the amount reaching the surface depends on ozone concentration.
• Chapman cycle: UV below 242 nm splits O2 into atoms that combine with O2 to form O3; UV below 320 nm splits O3 back apart, this absorption warms the stratosphere and shields the surface.
• Catalytic destruction: chlorine (from CFCs) and bromine (from halons) radicals destroy ozone in a cycle that regenerates the radical each time, so one chlorine atom can destroy roughly a thousand ozone molecules.
• Dobson Unit: measures ozone column thickness; typical mid-latitude reading is about 300 DU, the depleted Antarctic region averages down to about 100 DU.
• Ozone Depletion Potential (ODP): a substance's ozone-destroying impact relative to CFC-11 (fixed at 1.0); CFCs and HCFCs range roughly 0.01 to 1.0, bromine-based halons run up to 10. A distinct scale from Global Warming Potential, which governs the Kigali HFC phase-down.
• Discovery: Molina and Rowland predicted CFC-driven ozone destruction in 1974 (Nobel Prize in Chemistry, 1995, shared with Crutzen); Farman, Gardiner and Shanklin measured the actual springtime collapse at Halley Research Station, published in Nature in 1985.
• Antarctic ozone hole: forms in southern spring because the polar vortex isolates stratospheric air through winter, letting it get cold enough for polar stratospheric clouds to activate stored chlorine, which then destroys ozone rapidly once sunlight returns.
• 2025 status: peak extent 8.83 million sq mi (22.86 million sq km) on 9 September 2025, fifth smallest since 1992, closed nearly three weeks early; full recovery to 1980 levels projected around 2040 (near-global), 2045 (Arctic), 2065 (Antarctic) per the 2022 WMO/UNEP assessment.
• UV-B effects: raises non-melanoma skin cancer risk and melanoma development, linked to cataracts and immune suppression, reduces phytoplankton production and damages terrestrial plants and outdoor materials.

Try a few questions now to see how the mechanism and the current numbers get tested together.