Global ground-based tropospheric ozone measurements: reference data and individual site trends (2000–2022) from the TOAR-II/HEGIFTOM project

Tropospheric ozone trends from models and satellites are found to diverge. Ground-based (GB) ob-
servations are used to reference models and satellites, but GB data themselves might display station biases and
discontinuities. Reprocessing with uniform procedures, the TOAR-II working group Harmonization and Eval-
uation of Ground-based Instruments for Free-Tropospheric Ozone Measurements (HEGIFTOM) homogenized
public data from five networks: ozonesondes, In-service Aircraft for a Global Observing System (IAGOS) pro-
files, solar absorption Fourier transform infrared (FTIR) spectrometer measurements, lidar observations, and
Dobson Umkehr data. Amounts and uncertainties for total tropospheric ozone (TrOC; surface to 300 hPa), as
well as free- and lower-tropospheric ozone, are calculated for each network. We report trends (2000 to 2022)
for these segments using quantile regression (QR) and multiple linear regression (MLR) for 55 datasets, includ-
ing six multi-instrument stations. The findings are that (1) median TrOC trends computed with QR and MLR
trends are essentially the same; (2) pole-to-pole, across all longitudes, TrOC trends fall within +3 to −3 ppbv
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per decade, equivalent to (−4 % to +8 %) per decade depending on site; (3) the greatest fractional increases
occur over most tropical and subtropical sites, with decreases at northern high latitudes, but these patterns are
not uniform; (4) post-COVID trends are smaller than pre-COVID trends for Northern Hemisphere mid-latitude
sites. In summary, this analysis conducted in the frame of TOAR-II/HEGIFTOM shows that high-quality, multi-
instrument, harmonized data over a wide range of ground sites provide clear standard references for TOAR-II
models and evolving tropospheric ozone satellite products for 2000–2022.