Measured
Surface organic gases
NOAA flasks and in-situ gas chromatographs since the late 1970s. Monthly global means. This lab uses those files and nothing modeled on top of them.
MEASUREMENT LEDGER
Chlorine in the stratosphere is mostly a calculation
The source percentages in the ozone budget are not readings from the stratosphere. They are surface flask measurements, multiplied by chlorine atoms, delayed by an assumed age of air, and scaled by a fractional-release factor. The satellites that do look at ozone do not show a hole that follows that calculation from year to year. Papers in the assessment’s own journals say the lower stratosphere is still falling, the cause is not identified, and the reaction the hole model needs has not been observed in the air.
Calculated
The source pie
Age lag plus fractional release. Two published observation-based factor sets, WMO 2011 and Laube 2013 aircraft samples, move equivalent chlorine by more than 20 percent.
Left out
By a rule, not a flux
Inorganic HCl, very-short-lived solvents, feedstock leaks, and the aerosol and water that turn reservoir chlorine into the form that destroys ozone.
1 · Measured
The only long chlorine record is at the ground
These are global monthly means of dry-air mole fraction. The global number is a cosine-weighted average of station monthly means. It is not a stratospheric sample. Methyl chloride, CFC-113, and the HCFCs are not in this extract: NOAA does not publish them in the same combined monthly file, and this lab does not fill the gap with a model.
NOAA GML HATS combined global means, file date 2026-06-11. Parts per trillion, dry air. Cosine-weighted from station flasks and in-situ chromatographs. Surface air.
Every monthly global mean this chart uses. The annual view is the mean of these months. The NOAA files also include each station.
2 · Measured against measured
After chlorine peaked, the ozone hole stopped tracking it
Left axis: NASA Ozone Watch maximum daily area below 220 Dobson units, from TOMS, OMI, and OMPS. Missing days in that series are filled from MERRA, so the area is not pure satellite on every date. Right axis: the NOAA CFC-11 annual mean from the chart above. 1995 has no ozone row.
Annual maximum hole area, minimum total ozone, and the NOAA CFC-11 annual mean on the right axis. NASA’s daily fields are the large record behind each annual row.
1979–1994, CFC-11 vs hole area
r = 0.93
16 years. The rise. r² = 0.87.
1994–2025, same pair
r = 0.40
31 years. r² = 0.16. Most of the annual area is not CFC-11.
Full overlap
r = 0.79
46 years. r² = 0.62. The early rise carries the correlation.
2019’s maximum area was 16.4 million km². 2020 through 2023 were back at 24.8 to 26.5. Surface CFC-11 fell in every one of those years. The year-to-year hole is the polar vortex. A chlorine table that treats that spread as noise is describing a different quantity than the annual measurement.
3 · The inventory assumption, tested on the same file
Reported production was not the emission
The budget assumed that once production of CFC-11 was closed, the atmospheric decline would steepen as the bank emptied. The flask means in this lab do not do that between 2012 and 2018. Rates are the change in the annual mean, divided by the number of years.
| Interval | Start | End | Rate |
|---|---|---|---|
| 1980–1988 | 168.0 ppt | 244.8 ppt | +9.60 ppt/yr |
| 1988–1993 | 244.8 ppt | 268.0 ppt | +4.63 ppt/yr |
| 2002–2012 | 255.8 ppt | 235.2 ppt | -2.06 ppt/yr |
| 2012–2018 | 235.2 ppt | 228.2 ppt | -1.16 ppt/yr |
| 2018–2025 | 228.2 ppt | 211.6 ppt | -2.37 ppt/yr |
The decline slowed from −2.06 ppt per year (2002–2012) to −1.16 (2012–2018), then steepened again to −2.37 (2018–2025). That slowdown is the unreported CFC-11 production NOAA later identified. It is in the measurement. The scenario that omitted bank and feedstock leaks did not contain it. Feedstock release, since revised from observations to about 3.6 percent of production rather than the 0.5 percent the Protocol assumed, is the same class of omission: legal, uncounted, and large enough that scenarios move the recovery date by years.
4 · Do the arithmetic in the open
Move the assumptions. The percentage moves.
Pick a year, a lag, and a release set. The table multiplies the measured surface mixing ratio by chlorine atoms and by the release factor. Nothing else happens. Laube and colleagues (Atmospheric Chemistry and Physics, 2013) derived their factors from aircraft samples and reported a drop of more than 20 percent in equivalent chlorine relative to the WMO 2011 factors. The factors are also not constant in time, which the EESC formula usually assumes they are.
Fractional release
Fractional release at a 3-year mean age, used in the assessment EESC.
| Gas | Surface ppt | × Cl atoms | × release | Contribution | Share of these four |
|---|---|---|---|---|---|
| CFC-11CCl₃F | 231.0 | 3 → 693 | 0.47 | 326 ppt | 43.2% |
| CFC-12CCl₂F₂ | 515.6 | 2 → 1031 | 0.23 | 237 ppt | 31.5% |
| Carbon tetrachlorideCCl₄ | 82.4 | 4 → 330 | 0.56 | 185 ppt | 24.5% |
| Methyl chloroformCH₃CCl₃ | 3.1 | 3 → 9 | 0.67 | 6 ppt | 0.8% |
| Four-gas sum | 754 ppt | 100% |
CFC-1143%
CFC-1231%
Carbon tetrachloride24%
Methyl chloroform1%
Methyl chloride, CFC-113, the HCFCs, very-short-lived chlorine, and inorganic HCl are not in this sum. Changing the lag or the release set changes the shares without any new measurement. That is the whole of the “stratospheric source percentage.”
NOAA’s 2024 mid-latitude table, for comparison
Ozone-Depleting Gas Index, table 2, bromine gases removed. Sum 1047 ppt of equivalent chlorine. Derived the same way: surface gases, age, release factors. Direct inorganic chlorine is not a row.
- CFC-11304 ppt · 29%
- Methyl chloride241 ppt · 23%
- CFC-12222 ppt · 21%
- Carbon tetrachloride164 ppt · 16%
- CFC-11358 ppt · 6%
- HCFCs52 ppt · 5%
- Methyl chloroform2 ppt · 0%
- Minor CFCs4 ppt · 0%
5 · Left out of the table
Omitted by assumption, then measured anyway
Inorganic chlorine
Sea salt and volcanoes put hydrogen chloride into the lower atmosphere. It dissolves. The budget therefore enters inorganic chlorine as a few percent, a residual from campaign closures in the 1990s, and leaves it off the modern equivalent-chlorine table entirely. There is no public year-by-year measurement of the inorganic flux across the tropopause. Volcanic HCl emission estimates for the atmosphere span roughly 0.4 to 11 teragrams of chlorine a year; almost all of that range is tropospheric, and the claim that explosive plumes scrub essentially all of it comes from studied eruptions, not from a continuous flux record.
What those eruptions are measured to change is ozone, without adding a chlorine-source row. Pinatubo’s aerosol activated chlorine that was already in the stratosphere. Hunga Tonga’s water did the same: chlorine activation on humidified volcanic aerosol, and about a 5 percent ozone loss inside the tropical plume in a week (Evan and colleagues, Science, 2023). The chlorine budget does not carry water or aerosol surface area. The ozone measurement does.
Very-short-lived organic chlorine
Dichloromethane and chloroform were left out of the controlled total because their lifetimes are months, under the six-month cutoff. The assumption was that they would not matter in the stratosphere. They are organic, they are mostly industrial, and surface networks measure them rising. The 2022 WMO assessment: if dichloromethane emissions stay at the then-current level, they continue to remove about 1 Dobson unit of global total ozone. That is not the Antarctic hole. It is also not “doesn’t matter,” and it is not a line in the 2024 equivalent-chlorine table used above. This lab has no combined monthly file for those gases, so it does not draw a curve for them.
Methyl chloride is not a pure natural constant
It is the largest natural organic source that does reach the stratosphere, about 23 percent of NOAA’s 2024 mid-latitude equivalent-chlorine row. Part of it is biomass burning and industry. The natural fraction is an inventory split, not a tag on each molecule. It is absent from the four-gas flask chart because the combined monthly product is not published beside CFC-11.
Stratospheric HCl does not name its source
Above about 50 km, ACE-FTS finds hydrogen chloride is 98 to 99 percent of total chlorine. That is the breakdown product of whatever organic gas delivered the atom. You cannot read CFC-11, a volcano, or methyl chloride back out of it without the transport model. Published satellite results are period means, not a daily global source table:
- Aura MLS, Antarctic lower stratosphere: inorganic chlorine in 2013–2016 was 223 ± 93 ppt lower than in 2004–2007 (Strahan and Douglass, 2018). About 0.8 percent a year.
- ACE-FTS, 60°S–60°N, upper stratosphere, 2004–2017: HCl −4.8 ± 0.2 percent per decade (Bernath and Fernando, 2018).
- ACE-FTS chlorine inventory, 2004–2024: global mean total chlorine −9.56 ± 0.28 ppt per year, near 3.3 ppb in 2024 (Raymond and colleagues, 2026). Some altitudes in that inventory are filled with a chemical-transport model and with ground data. It is not a pure stratospheric retrieval at every height.
Aura MLS Level-2 HCl is on the order of 500 megabytes a year of profiles. It is not published as a daily global-mean table. This lab did not average a private extract and it did not draw a smooth line through the trend. A curve of “stratospheric chlorine by source, 1979–2025” does not exist as a measurement.
6 · The literature, not the press release
Decades of data, and the hole is still not explained by the chlorine table
These are the papers behind the three links, plus the laboratory fight over the hole’s key reaction and an earlier aerosol record that was later thrown out. Short quotes are the authors’ claims. The numbers are theirs. None of them is a year-by-year measurement of which source delivered the chlorine.
2023 · Nature Communications
Potential drivers of the recent large Antarctic ozone holes
Kessenich, Seppälä, and Rodger
What was measured. TOMS, OMI, and Aura MLS partial columns, 1979–2022. October total-column trends have R² of 0.09 (September), 0.07 (October), and 0.01 (November). The slope error is larger than the slope.
What it does to the budget. The 2022 assessment said the Antarctic hole was on track to recover by 2066. These authors report another large hole in 2022, like 2020 and 2021. In the ozone layer itself (about 5–50 hPa), October partial column near the pole fell about 26 percent from 2004 to 2022 (75–82°S) and 17 percent over 60–82°S. The upper stratosphere (1–5 hPa) rose a few percent. Recovery aloft is being used to stand in for the layer where the hole actually is.
2018 · Atmospheric Chemistry and Physics
Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery
Ball and colleagues
What was measured. Multiple satellite records of ozone by altitude, 60°S–60°N, after 1998. Not a model.
What it does to the budget. Upper-stratospheric ozone is rising. Lower-stratospheric ozone kept falling, and that fall dominates, so the stratospheric column between 60°S and 60°N trends down. The total column from the ground does not show it, because tropospheric ozone rose and cancelled the stratospheric loss. The authors write that the cause is not clear and that models do not reproduce the trend.
2018 · Geophysical Research Letters
Decline in Antarctic ozone depletion and lower stratospheric chlorine from Aura MLS
Strahan and Douglass
What was measured. Aura MLS ozone and inorganic chlorine inside the Antarctic vortex. The paper opens by stating that unambiguous observational proof of a Montreal Protocol effect on Antarctic ozone was still missing more than 20 years after emissions of the controlled gases had peaked.
What it does to the budget. Their own chlorine estimate in the lower stratosphere did fall. The hole did not follow it. They write that year-to-year ozone loss is largely temperature, not chlorine, and that before this analysis there were no inorganic-chlorine observations in the Antarctic lower stratosphere to check the decrease. NASA’s account of the same work: declining chlorine had not yet caused a recovery of the hole; temperature and winds still drive the annual change; chlorine is not expected to control the area until after the mid-2030s. A 20 percent drop in mid-winter chemical loss from 2005 to 2016 can match a model and still leave the September hole size as weather.
2007 · Nature
Chemists poke holes in ozone theory
Schiermeier, reporting Pope and colleagues on Cl₂O₂
What was measured. A laboratory photolysis spectrum of dichlorine peroxide, the molecule the ozone-hole mechanism uses to return chlorine radicals in dim spring light. Not a measurement inside the hole.
What it does to the budget. The new cross section was almost an order of magnitude below the rate in the model, at the wavelengths the stratosphere actually has. Markus Rex: if the measurements are correct, at least 60 percent of polar ozone destruction is an unknown mechanism, and “we can basically no longer say we understand how ozone holes come into being.” Later laboratory spectra did not confirm that low value, and the assessment put a higher rate back. The correction was another lab spectrum. The reaction has still not been watched in the air. The model’s central step was, for a time, wrong by enough to break the explanation, and the repair was not a stratospheric observation.
2022 · AIP Advances
Observation of large and all-season ozone losses over the tropics
Lu
What was measured. Total ozone, 1980–2020, and a cosmic-ray-driven electron model drawn through it. Their “hole” is a lower-stratospheric loss greater than 25 percent relative to an undisturbed atmosphere, not the 220 Dobson-unit threshold.
What it does to the budget. The paper says a tropical hole has existed since the 1980s, all year, with an area about seven times the Antarctic hole and a comparable depth, and that Antarctic and tropical loss share one mechanism which their electron model fits. Read the axes on their own figure. Antarctic spring total ozone falls from about 340 to near 220 Dobson units. The tropical annual total column falls by about 10 Dobson units, from the low 270s into the low 260s. The dramatic tropical hole is a redefined partial-column percentage, not the quantity used for the Antarctic hole. Two descriptions of the same observations, and they do not agree on what a hole is.
1980 · Monthly Weather Review
Trends in atmospheric transmission at three locations in the United States, 1940–1977
Hoyt, Turner, and Evans
What was measured. Transmission of sunlight at three U.S. stations over 37 years. Aerosol optical depth inferred from that record.
What it does to the budget. They concluded the trend was very small, perhaps nonexistent, and not statistically significant. Using a 6 percent anthropogenic share of aerosol, they got about a 0.02 percent loss of global insolation and “no more than a few hundredths of a degree.” Their sentence: mankind is having no perceptible influence on the global climate through anthropogenic aerosols, and such aerosols “have not been a serious climatic threat in the past nor will they be one in the immediate future.” That was a mainstream measurement paper. The aerosol budget was later rewritten. Particle surface area is also how stratospheric chlorine is supposed to become chemically active. The term was treated as settled, then replaced.