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

Total column ozone (TCO) has been observed since the 1920s, with a systematic monitoring network established during the International Geophysical Year (1957–1958). We compile and assess TCO records from six major ground-based instrument types: Dobson and Brewer spectrophotometers, Filter ozonometers, zenith-sky DOAS (UVVIS), Fourier-transform infrared (FTIR) spectroscopy, and Pandora spectrometers. Data are drawn from the World Ozone and Ultraviolet Radiation Data Centre, Network for the Detection of Atmospheric Composition Change, Pandonia Global Network, and European Brewer Network. Using harmonized statistical criteria and daily comparisons with multiple satellite products and four reanalysis datasets, we evaluate site-level performance in 5-year intervals from 1940 to 2024. Metrics include mean bias, variability of daily and monthly differences, seasonal amplitude, and the range of annual means, with percentile-based thresholds used to classify data quality. Ground-based annual means generally agree with satellite and reanalysis benchmarks within ± 2 %, with typical variability near 2 %. Larger discrepancies occur in the pre-satellite era, where reanalyses show biases of up to −5 % relative to Dobson observations. Network-wide distributions of daily mean differences to those from reanalysis and satellite data indicate comparable internal consistency for Brewer, Dobson, FTIR, and Pandora (standard deviations generally around 2 %), while Filter and UVVIS exhibit slightly broader spreads (< 3 %), especially at high latitudes. Network capacity has evolved substantially since the 2000s, with a decline in Dobson sites and expansion of Brewer and Pandora observations. By providing station-level flags and thresholds, this assessment helps network operators prioritize calibration and reprocessing efforts and helps users identify robust records, thereby improving confidence in long-term ozone trend detection and satellite validation.