Century-Long Ground Ozone Records Put to the Test in First Global Network Audit

The ozone layer is one of the most closely watched shields in the solar system, and now the instruments that have watched it for nearly a century have been audited on a global scale for the first time. A team led by Xiaoyi Zhao of Environment and Climate Change Canada has compiled and evaluated total column ozone records from six major ground-based instrument networks, spanning observations from 1940 to 2024, and graded every station against a battery of satellites and four atmospheric reanalysis datasets. The verdict, published in Atmospheric Chemistry and Physics, is largely reassuring: the world’s reference instruments agree with independent benchmarks to within about two percent, and the flagship networks to within one percent in recent years.

The stakes of this audit are higher than they might appear. Ground-based ozone records are the backbone of nearly everything we know about the ozone layer: they underpin the detection of long-term trends, they measure whether the Montreal Protocol is actually healing the stratosphere, and they serve as the yardstick against which every satellite retrieval and reanalysis product is validated. If those yardsticks drift, the entire edifice of ozone recovery science wobbles with them. Yet until now, the most comprehensive performance assessment of the global network dated from 2008, before a wave of new satellite missions, new reanalysis products, and the explosive growth of the Pandonia Global Network reshaped the observing system.

The study covers six instrument types with very different personalities. The Dobson spectrophotometer, invented in the 1920s, measures ozone by comparing ultraviolet light at wavelength pairs that ozone absorbs differently, using a double monochromator to suppress stray light. The Brewer spectrophotometer, designed in the 1970s to automate the process, uses a diffraction grating to measure four wavelengths between 306 and 320 nanometers and can operate unattended in direct sun, moonlight, and zenith-sky modes. Together these are the World Meteorological Organization’s reference instruments, with accuracy better than one percent, traceable through a strict calibration hierarchy from world primary standards down to field instruments. Filter ozonometers, compact devices using fixed interference filters near 306 and 328 nanometers, offer operational simplicity at lower precision. Zenith-sky DOAS instruments, including the SAOZ systems that dominate polar monitoring, retrieve ozone from scattered twilight sunlight and carry a typical accuracy around five percent. Fourier-transform infrared spectrometers retrieve ozone from solar absorption spectra alongside a dozen other gases, and the newest network, Pandora, records direct-sun spectra from 280 to 525 nanometers with central processing through the Pandonia Global Network.

To grade these instruments fairly, the team built a harmonized statistical framework. For every station and every five-year window, they calculated the daily percentage difference between ground-based observations and comparison data from satellite overpasses and four reanalyses: ERA5, MERRA-2, JRA-3Q, and MSR2. Five metrics captured different failure modes: the mean bias, the standard deviation of daily and monthly differences, the amplitude of the seasonal cycle in the differences, and the range of annual means. A well-calibrated instrument should show a stable seasonal signature against fixed benchmarks; drifts in that signature can betray errors in extraterrestrial constants, stray light, or the effective temperature of the ozone layer itself.

The methodological innovation lies in how stations are classified. Rather than imposing one-size-fits-all limits, the team derived percentile-based thresholds from the internal statistics of each network against each benchmark. This matters because the networks differ in precision: the Brewer and Dobson networks show standard deviations of daily differences around two percent, while Filter and UVVIS instruments spread slightly wider, under three percent, especially at high latitudes. Applying Brewer-grade criteria to a UVVIS instrument would artificially condemn data that is perfectly acceptable for its class. The thresholds label each station-period as high quality, medium quality with minor issues, or not assured with major issues, and in most five-year periods fewer than twenty percent of stations worldwide fell into the not-assured category.

The headline numbers are striking. Ground-based annual means generally agree with satellite and reanalysis benchmarks within plus or minus two percent, with typical variability near two percent. The two WMO reference networks agree with each other and with the benchmarks mostly within one percent in recent decades. The world reference Dobson at Boulder and the world reference Brewer in Toronto, along with the European regional reference at Izaña, all show clean long-term records, confirming that the anchor points of the global system are holding steady. The Pandora network, despite being the youngest, shows statistical consistency comparable to the reference networks, with a standard deviation of daily mean differences of 1.83 percent, actually slightly better than Brewer’s 1.97 and Dobson’s 2.04, albeit over a shorter record.

Reanalysis datasets proved both a blessing and a caution. Because most reanalyses assimilate satellite ozone but not ground-based data, they serve as genuinely independent referees, and they allowed the assessment to extend back before the satellite era began in the 1970s. But the pre-satellite portions of ERA5 and JRA-3Q, where ozone is only indirectly constrained by meteorological observations, show biases of up to minus five percent relative to Dobson measurements. The team also caught a sudden two percent shift in MERRA-2 around 2005, when the assimilation system switched its satellite inputs from TOMS and SBUV to OMI and MLS, a reminder that reanalyses carry artifacts of their own. To handle conflicting verdicts from the four reanalyses, the researchers merged their assessments using the Dempster-Shafer evidence theory, which penalizes datasets that repeatedly disagree with their peers.

The audit also exposed the shifting geography of ozone monitoring. The Dobson network peaked at 76 active sites during 1985 to 1994 and has since declined to 51, raising concerns about long-term continuity even as the remaining stations perform superbly. The Brewer network has grown to 87 active sites, making it the largest reference network. Filter ozonometers have contracted from 52 sites to just 20, mostly in Russia. The NDACC FTIR and UVVIS networks report ozone at 22 and 17 sites respectively, while the Pandonia Global Network has surged since 2015 to 138 active sites. Notably, the study found and corrected a data-reporting error in the Filter network archive affecting observations from 2007 to 2024, and documented a confirmed hydrogen lamp failure at the Princess Elisabeth Station in Antarctica that degraded Brewer data in the 2022-23 and 2023-24 seasons before the instrument was recalibrated and returned to service in December 2024.

The practical payoff is a quality-assessed, station-level dataset with flags that data centres can use immediately. The World Ozone and Ultraviolet Radiation Data Centre and its counterparts can now steer users toward the most reliable records for climatologies, trend analyses, and satellite validation, identify sites and periods needing reprocessing, and prioritize calibration campaigns where they matter most. The audit also flags structural vulnerabilities: seventeen percent of Pandora sites are run by university researchers, whose instruments often retire with them, and vast regions including Africa and parts of South America remain thinly covered. A companion paper will turn the tables, using these quality-assured ground records to evaluate the satellites and reanalyses themselves. For a planet betting its ultraviolet protection on a handful of aging spectrophotometers and a rising generation of new instruments, knowing exactly which numbers to trust is the quiet foundation on which the ozone recovery story rests.

Subject of Research: Performance assessment of global ground-based total column ozone monitoring networks from 1940 to 2024

Article Title: Measurement report: Global Total Ozone Records – Part 1: Ground-based monitoring networks performance assessment and status review

Article References: Zhao, X., Fioletov, V., Petropavlovskikh, I., Velazco, V., Redondas, A., Solomatnikova, A., Vigouroux, C., Strong, K., Van Roozendael, M., Pazmino, A., Hanisco, T. F., Cede, A., Tiefengraber, M., Labow, G., Fujiwara, M., van der A, R., Griffin, D., McLinden, C., Kralidis, T., … Lee, S. C. (2026). Measurement report: Global Total Ozone Records – Part 1: Ground-based monitoring networks performance assessment and status review. Atmospheric Chemistry and Physics, 26(19), 13693-13720. https://doi.org/10.5194/acp-26-13693-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-13693-2026

Keywords: ozone layer, total column ozone, Dobson spectrophotometer, Brewer spectrophotometer, Pandora, satellite validation, reanalysis, Montreal Protocol, WOUDC, NDACC, atmospheric monitoring, data quality