# Ozone layer <!-- MICROSIMGEN:BEGIN v1.7 — generated by g08_place_microsims.py; three.js first (§15); do not hand-edit inside --> ## Microsims — three.js ### Ozone layer (three.js) <div class="microsim-player"> <iframe src="https://wikitube-3d-microsims.netlify.app/Ozone_layer.html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin" title="Ozone layer — three.js microsim"></iframe> </div> **Open it full-screen:** [Ozone_layer.html](https://wikitube-3d-microsims.netlify.app/Ozone_layer.html) · library `threejs` · route `microsim/threejs/` ### Related microsims Live sims on neighbouring articles: - [[Allotropes_of_oxygen]] - [[Atomic_orbital]] - [[Hemoglobin]] - [[Hydrogen_bond]] - [[Molecular_orbital]] - [[Silicon_dioxide]] *Sim hosted off-article; the article owns the reference, not the runtime (WIKI_RULES §10.4). Placed by `g08_place_microsims.py`.* <!-- MICROSIMGEN:END --> ## Overview The ozone layer is the region of the stratosphere, roughly 15 to 35 kilometres up and peaking near 20 to 25 km, where ozone concentration is highest. It absorbs most of the sun's ultraviolet-B, which is the reason complex life can survive on land. The name misleads in two ways. It is not a layer in any ordinary sense: even at its peak ozone is only a few parts per million of the surrounding air, and if the entire atmospheric column of ozone were brought to sea-level pressure it would form a shell about three millimetres thick. Nor is it a stockpile. Ozone is destroyed and remade continuously, and the layer is simply the balance point of a cycle. That is why a trace contaminant can thin it dramatically, and equally why it can recover. Three millimetres is what stands between the biosphere and the ultraviolet, and the wavelengths matter as much as the amount. Molecular oxygen alone removes essentially everything shorter than about 200 nm, so ozone's contribution begins where O2 gives out. Across the Hartley band, roughly 200 to 310 nm, ozone absorbs strongly, with its largest cross-section near 255 nm; between them O2 and O3 eliminate UV-C, 100 to 280 nm, completely. UV-B, 280 to 315 nm, is the band ozone alone defends, and it defends it steeply: near 300 nm about one photon in a thousand reaches the ground, while near 315 nm most of them do. UV-A, 315 to 400 nm, is barely absorbed and passes through whatever the column does. Because the biological action spectra for DNA damage, sunburn and photokeratitis all rise sharply toward shorter wavelengths, a small fractional change in the column produces a much larger fractional change in the biologically weighted ultraviolet arriving at the surface. That amplification is why a few per cent of column is worth arguing about. The steady state is also fast, faster than the word "balance" suggests. In the upper stratosphere the entire local population of ozone molecules is replaced in a day or less, so what looks like a persistent shell is a standing pattern maintained by two enormous opposing rates. The layer persists not because it is stable but because it is continuously rebuilt. ## The physics The natural cycle was described by Sydney Chapman in 1930 and consists of four reactions: 1. O2 + photon (below 242 nm) -> O + O 2. O + O2 + M -> O3 + M 3. O3 + photon (about 240-310 nm) -> O2 + O 4. O + O3 -> 2 O2 Reactions 2 and 3 are fast and merely shuffle oxygen between atomic O and O3 without changing the total of odd oxygen; reaction 3 is also the ultraviolet absorption that matters for life. Reactions 1 and 4 are slow and set the total. Keeping that distinction straight is the difference between understanding the layer as a rapid equilibrium and mistaking it for a reservoir. The grouping has a name: odd oxygen, Ox = O + O3. Reactions 2 and 3 interconvert the two members of that family thousands of times a day and create or destroy none of it; reaction 1 is the only source and reaction 4 the only sink, so the entire budget of the layer is 1 against 4. Atomic oxygen recombines through reaction 2 in well under a second, so it sits in photochemical equilibrium with ozone at a ratio [O] / [O3] = J3 / (k2 [O2] [M]), where J2 and J3 are the photolysis rates of reactions 1 and 3, k2 and k4 the rate constants of reactions 2 and 4, and M is the third body, in practice N2 or O2, that carries away the energy released when O and O2 combine. Substituting that ratio into the balance between source and sink gives the classical Chapman steady-state relation, [O3] = [O2] sqrt( J2 k2 [M] / ( J3 k4 ) ). That relation also explains where the layer sits, which is less obvious than it first looks. Production needs two ingredients that vary in opposite directions with height. The short-wavelength ultraviolet driving reaction 1 is abundant at the top of the atmosphere and is progressively consumed on the way down; molecular oxygen, the raw material, is scarce at the top and grows exponentially denser downward with a scale height of about 7 km. Their product peaks in between, in the upper stratosphere near 35 to 45 km, and below roughly 25 km almost no ozone is manufactured at all, because the ultraviolet capable of splitting O2 has already been absorbed by the ozone overhead. The density maximum nevertheless sits far lower, near 20 to 25 km at middle latitudes and lower still toward the poles, for two reasons. First, the photochemical lifetime of ozone lengthens enormously downward -- hours near 45 km, days near 35 km, months near 22 km, effectively years below 20 km -- because both loss channels run through atomic oxygen, whose abundance collapses as the air gets denser and recombination speeds up. Second, the Brewer-Dobson circulation, rising in the tropics, drifting poleward through the stratosphere and descending at middle and high latitudes, carries ozone made in the tropical upper stratosphere down into the extratropical lower stratosphere, where it accumulates precisely because it is destroyed so slowly. The layer overhead is largely manufactured somewhere else and delivered. That is also why the total column is lowest over the tropics, where the ozone is made, at typically 250 to 270 Dobson units, and highest over the winter and spring polar regions, where it is stored, at 350 to 400. Chapman's mechanism gets the shape of the profile right and the magnitude badly wrong. Run with laboratory rate constants and no other chemistry, it predicts roughly twice as much ozone as is observed. A factor of two is not a rounding error in a mechanism this simple, and closing it is what forced the search for loss processes Chapman had not written down. Catalytic destruction is what changed the picture. In the chlorine cycle, Cl + O3 -> ClO + O2 ClO + O -> Cl + O2 net: O + O3 -> 2 O2 the chlorine atom is regenerated, so a single atom can destroy many thousands of ozone molecules before it is removed. Mario Molina and F. Sherwood Rowland identified this pathway for chlorofluorocarbons in 1974; the Antarctic hole was reported by Farman, Gardiner and Shanklin in 1985. The disproportion has a precise origin, and it is worth stating carefully because it is the crux of the whole problem. Since atomic oxygen tracks ozone, the Chapman sink of reaction 4 is quadratic in ozone: halve the ozone and that loss falls by a factor of four. A catalyst, by definition, is not consumed, so its concentration does not fall as ozone falls, and the catalytic sink is only linear in ozone: halve the ozone and the loss merely halves. A layer defended by a quadratic loss term is very hard to thin, because it fights back harder the more is taken from it; the same layer attacked through a linear term has lost that defence. A catalyst touches only the destruction side of a balance whose production side cannot adjust to compensate, and that asymmetry, rather than any great abundance of chlorine, is what makes parts per billion matter. Three catalytic families do the work, and chlorine is only the notorious one. NOx, meaning NO and NO2, was identified by Paul Crutzen in 1970 and derives mostly from nitrous oxide released by soils and oceans; it dominates loss through the middle stratosphere, between about 25 and 40 km. HOx, meaning OH and HO2, dominates above roughly 45 km and again in the lowest stratosphere. ClOx, meaning Cl and ClO, is the anthropogenic one. Richard Stolarski and Ralph Cicerone showed in 1974 that stratospheric chlorine would be an efficient ozone sink; Molina and Rowland, later that same year, supplied the source that turned a laboratory curiosity into an industrial problem. The chlorofluorocarbons were chosen as refrigerants and propellants precisely because they are inert, non-toxic and insoluble, which means nothing in the troposphere removes them. They survive to be lifted into the stratosphere, where ultraviolet finally photolyses them and liberates chlorine atoms. The properties that made them ideal industrial fluids are exactly the properties that delivered them intact to the one place they could do harm. Once there, most stratospheric chlorine is normally locked into the reservoir species hydrogen chloride and chlorine nitrate, ClONO2, neither of which attacks ozone, and under ordinary gas-phase conditions that sequestration holds mid-latitude depletion to a few per cent of the column. Undoing it is what makes the poles different. Column ozone is measured in Dobson units, where 1 DU corresponds to a layer 0.01 mm thick at standard temperature and pressure. A typical global column is near 300 DU, and an ozone hole is defined as a column below 220 DU. Stated properly, the Dobson unit is the thickness the whole column would occupy if it were brought to standard temperature and pressure, counted in hundredths of a millimetre; equivalently, 1 DU is 2.687 x 10^16 molecules per square centimetre. A global mean near 300 DU is therefore the same statement as the three millimetres above, and the 220 DU hole threshold is a shade over two millimetres of gas. The unit is coarse enough to be read at a glance and fine enough that the natural latitudinal spread, from roughly 250 DU in the tropics to 400 DU at high latitudes in spring, is obvious in the numbers, which is part of why it has outlived a century of changes in instrumentation. ## The polar ozone hole Nothing in the gas-phase chemistry above predicts what Farman, Gardiner and Shanklin found in the Halley Bay records. Antarctic total ozone each October had fallen by more than a third since the 1970s, and both the magnitude and the sharp seasonality were incompatible with every model then in use. The resolution is that the winter Antarctic stratosphere stops behaving like ordinary air. The polar vortex, a ring of strong circumpolar winds, seals the air over the continent off from mid-latitude exchange. Radiative cooling through the long polar night drives temperatures in the 12 to 24 km layer below about 195 K, and polar stratospheric clouds condense out of air that is normally far too dry to form cloud at all: first particles of nitric acid trihydrate, then, below about 188 K, water ice. Their surfaces change what chemistry is possible. Reactions that are impossibly slow between gas molecules run readily on a solid or liquid surface, and two of them matter: ClONO2 + HCl -> Cl2 + HNO3 ClONO2 + H2O -> HOCl + HNO3 Both convert the inert reservoirs into chlorine compounds that fall apart in the first sunlight of spring, and both survive the dark until it arrives. Meanwhile the nitric acid stays behind in the particles, and where those particles grow large enough to sediment out, nitrogen is removed from the layer altogether -- denitrification -- so the NO2 that would normally recapture ClO back into chlorine nitrate is simply gone. By late winter most of the available chlorine is sitting as ClO, waiting for light. When the sun returns in September it triggers a cycle that needs no atomic oxygen at all, which is the crucial point: atomic oxygen is scarce in the cold, dim lower stratosphere, so the ordinary ClOx pair would stall there. The ClO dimer mechanism, characterised by Luisa and Mario Molina in 1987, is rate-limited by the self-reaction of ClO: ClO + ClO + M -> Cl2O2 + M Cl2O2 + photon -> Cl + ClOO ClOO + M -> Cl + O2 + M 2 x ( Cl + O3 -> ClO + O2 ) net: 2 O3 -> 3 O2 Because the rate-limiting step is second order in ClO, the loss depends very nonlinearly on chlorine loading, and a parallel ClO plus BrO cycle contributes a further fifth or so. Ozone between roughly 14 and 22 km is then destroyed almost completely over a few weeks, and the column falls from a normal 300 DU or more to values below 150 DU; the lowest ozonesonde reading over the South Pole, in October 1994, was about 73 DU. Aircraft campaigns led by James Anderson confirmed the mechanism directly, measuring the sharp anticorrelation between ClO and O3 across the edge of the vortex. The Arctic does not behave the same way, and the asymmetry is geographical rather than chemical. Its vortex is weaker, warmer and more often disrupted by planetary waves breaking against the northern hemisphere's mountains and land-sea contrasts, so widespread polar stratospheric clouds form only in unusually cold winters. Arctic springs show real but far more variable losses; the exceptional winters of 2011 and 2020 approached Antarctic-like chemistry, and most come nowhere near it. ## Measurement and the Montreal Protocol The record that made the hole visible exists because of an instrument. G. M. B. Dobson designed a spectrophotometer that measures column ozone by comparing solar intensity at paired ultraviolet wavelengths, exploiting the steepness of the absorption curve so that the ratio depends on ozone and very little else. The network built out from that design, expanded during the International Geophysical Year of 1957, is the reason the Antarctic series was long enough for a one-third decline to be recognised as a trend rather than an instrument fault. Vertical information came first from the Umkehr technique, inverting the ratio of zenith-sky brightness as the sun sets, and then from balloon-borne ozonesondes, which still supply the profile record. Satellite mapping began with the backscatter ultraviolet instrument on Nimbus-4 in 1970 and has continued through TOMS, SBUV, OMI and OMPS. The policy response was unusually fast, and it moved ahead of certainty rather than waiting for it. The Vienna Convention was agreed in 1985, the year of the Farman paper; the Montreal Protocol on Substances that Deplete the Ozone Layer was adopted on 16 September 1987 and entered into force in 1989, before the polar mechanism was fully established. Successive amendments -- London 1990, Copenhagen 1992, Montreal 1997, Beijing 1999 -- tightened the phase-out schedules and widened the coverage, and the Kigali Amendment of 2016 addresses hydrofluorocarbons, which do not deplete ozone but are potent greenhouse gases. Every UN member state has ratified it. What has actually been observed since then deserves care, because the treaty is often described as a finished success and it is not one yet. Effective stratospheric chlorine peaked in the late 1990s and has been declining slowly, limited by CFC atmospheric lifetimes of fifty to a hundred years. Upper-stratospheric ozone between 35 and 45 km, where the chlorine signal is cleanest, has been increasing since about 2000 at roughly two per cent per decade, and that is a genuine recovery which can be attributed to the Protocol. The Antarctic hole shows emerging signs of healing in September column amount and areal extent since 2000, but interannual variability is large, and several recent years have produced deep, long-lived holes under the influence of stratospheric dynamics, wildfire smoke and volcanic water vapour. Mid-latitude total column ozone has not yet shown a statistically significant increase in most analyses, because a continued decline in the lower stratosphere at those latitudes -- for reasons still debated, and probably dynamical rather than chemical -- has so far largely cancelled the upper-stratospheric gain. WMO and UNEP assessments project a return to 1980 values around 2066 for the Antarctic, 2045 for the Arctic and roughly 2040 for mid-latitudes, assuming continued compliance. The honest summary is that recovery has been detected and attributed but is nowhere near complete: the layer is on a mid-century trajectory, not restored. ## Controls -> what each maps to | Control | Maps to | Range / values | Physical meaning | |---|---|---|---| | Stratospheric chlorine | Cl loading | 0.6 - 3.5 ppb | Pre-industrial to the 1990s peak; drives catalytic loss and opens the polar hole | | Solar ultraviolet | photon flux multiplier | 0.4 - 1.6 | Scales reactions 1 and 3; the real 11-year cycle is far smaller than this range | | Show photons | -- | on / off | Incoming ultraviolet, absorbed at the altitude where ozone stops it | | Show altitude profile | -- | on / off | Ozone concentration against height, mid-latitude versus polar | | Spin the globe | -- | on / off | Rotation only; disabled under prefers-reduced-motion | ## Learning objective After playing, a learner can explain why the ozone layer is a steady state rather than a reservoir, describe how a catalytic cycle lets a trace species destroy ozone far out of proportion to its abundance, and interpret a column measurement in Dobson units against the 220 DU hole threshold. A learner who has followed the argument further can also say why the density peak sits near 20 to 25 km when production peaks nearer 40 km, which requires holding together the opposing altitude dependence of ultraviolet and of oxygen density along with the downward lengthening of ozone's chemical lifetime and the delivery of ozone by the Brewer-Dobson circulation. The same learner should be able to say why the damage appeared first over Antarctica in spring rather than over the industrialised mid-latitudes that emitted the chlorine. ## Limits and connections This is a box model of the Chapman cycle plus catalytic loss, not a radiative-transfer or chemical-transport model. Photolysis prefactors are tuned rather than spectrally integrated. Two features are prescribed rather than simulated: the Brewer-Dobson circulation's pole-to-equator gradient and the lower-stratospheric transport reservoir; without them a purely local steady state puts the column maximum over the tropics, which is backwards. The polar chemistry also includes the ClO dimer cycle, because the gas-phase chlorine pair above is limited by atomic oxygen and cannot by itself produce a polar hole -- which was the actual puzzle of 1985. The altitude scale in the rendering is exaggerated by a large factor; the real stratosphere is a thin skin on the globe. The allotrope chemistry itself is in [[Allotropes_of_oxygen]]. Several things this article describes are outside the model's scope rather than merely coarse within it. The NOx and HOx catalytic families, which between them dominate natural loss through most of the stratosphere, are not represented separately; the chlorine control stands in for catalytic loss in general. Bromine, which supplies roughly a fifth of real polar loss, is folded into the same term rather than tracked. Denitrification appears as a consequence of the polar chemistry, not as an adjustable quantity. And because the chlorine control is a loading rather than a trajectory, there is no time axis along which the Montreal Protocol can play out: the sim can show the difference between 0.6 and 3.5 ppb, but not the fifty-to-a-hundred-year atmospheric lifetimes that make crossing back between them take until mid-century. Column values in Dobson units are a diagnostic read off the model, not a simulated Dobson spectrophotometer, so they should be compared with observations for magnitude and shape rather than treated as a retrieval. ## References Anderson, J. G., Brune, W. H., and Proffitt, M. H. "Ozone Destruction by Chlorine Radicals within the Antarctic Vortex: The Spatial and Temporal Evolution of ClO-O3 Anticorrelation Based on In Situ ER-2 Data." *Journal of Geophysical Research* 94, no. D9 (1989): 11465-11479. Ball, W. T., Alsing, J., Mortlock, D. J., Staehelin, J., Haigh, J. D., Peter, T., and others. "Evidence for a Continuous Decline in Lower Stratospheric Ozone Offsetting Large Recovery in the Upper Stratosphere." *Atmospheric Chemistry and Physics* 18, no. 2 (2018): 1379-1394. Brewer, A. W. "Evidence for a World Circulation Provided by the Measurements of Helium and Water Vapour Distribution in the Stratosphere." *Quarterly Journal of the Royal Meteorological Society* 75, no. 326 (1949): 351-363. Chapman, Sydney. "A Theory of Upper-Atmospheric Ozone." *Memoirs of the Royal Meteorological Society* 3, no. 26 (1930): 103-125. Crutzen, Paul J. "The Influence of Nitrogen Oxides on the Atmospheric Ozone Content." *Quarterly Journal of the Royal Meteorological Society* 96, no. 408 (1970): 320-325. Dobson, G. M. B. "Forty Years' Research on Atmospheric Ozone at Oxford: A History." *Applied Optics* 7, no. 3 (1968): 387-405. Farman, J. C., Gardiner, B. G., and Shanklin, J. D. "Large Losses of Total Ozone in Antarctica Reveal Seasonal ClOx/NOx Interaction." *Nature* 315 (1985): 207-210. Molina, L. T., and Molina, M. J. "Production of Cl2O2 from the Self-Reaction of the ClO Radical." *Journal of Physical Chemistry* 91, no. 2 (1987): 433-436. Molina, Mario J., and Rowland, F. Sherwood. "Stratospheric Sink for Chlorofluoromethanes: Chlorine Atom-Catalysed Destruction of Ozone." *Nature* 249 (1974): 810-812. Solomon, Susan. "Stratospheric Ozone Depletion: A Review of Concepts and History." *Reviews of Geophysics* 37, no. 3 (1999): 275-316. Solomon, S., Garcia, R. R., Rowland, F. S., and Wuebbles, D. J. "On the Depletion of Antarctic Ozone." *Nature* 321 (1986): 755-758. Solomon, S., Ivy, D. J., Kinnison, D., Mills, M. J., Neely, R. R., and Schmidt, A. "Emergence of Healing in the Antarctic Ozone Layer." *Science* 353, no. 6296 (2016): 269-274. Stolarski, R. S., and Cicerone, R. J. "Stratospheric Chlorine: A Possible Sink for Ozone." *Canadian Journal of Chemistry* 52, no. 8 (1974): 1610-1615. World Meteorological Organization and United Nations Environment Programme. *Scientific Assessment of Ozone Depletion: 2022*. GAW Report No. 278. Geneva: World Meteorological Organization, 2022. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Ozone_layer) : [Wikitube](https://en.wikitube.io/wiki/Ozone_layer) ## Previous hub tags Tree parent: [[Oxygen]]. Legacy hubs: `REACTION`. --- *Created 2026-08-05 - append-only - hand-authored to WIKI_REPOPULATION_PROTOCOL v1.0 section 5 - 0 deletions*