Trends in Free Tropospheric Ozone from Homogenized Ground-based and Profile Datasets (1995-2020): The TOAR II/HEGIFTOM Project

Free tropospheric ozone (FTO₃) is an important greenhouse gas forcer. Satellite estimates of FTO₃ are uneven in temporal coverage and their profiling capability is limited. However, the ozonesonde network, tropospheric ozone lidars and commercial aircraft projects provide accurate high-resolution FTO₃ profiles over dozens of locations globally. In addition, networks of spectrometric instruments provide high-quality FTO₃ estimates with consistent calibration. In the framework of the second phase of the IGAC (International Global Atmospheric Chemistry Project) Tropospheric Ozone Assessment Report (TOAR-II), it was recognized that it is essential to put all the network measurements on the same basis with respect to absolute references and processing methods. The relevant procedures are referred to as “harmonization” or “homogenization”. The TOAR II working group, “HEGIFTOM” (Harmonization and Evaluation of Ground-based Instruments for Free-Tropospheric Ozone Measurements), has carried out harmonized data from five types of instruments (mid-1990s to 2020): ozonesondes, commercial aircraft IAGOS landing/takeoff profiles, Fourier-Transform Infrared spectrometer (FTIR), tropospheric Lidar, and Brewer/Dobson Umkehr. This paper briefly summarizes the homogenization effort for each network and describes the resulting quality-assessed ozone profile or segment (partial column) data sets. We present FTO₃ trends over the past ~25 years period for stations from pole-to-pole based on the HEGIFTOM datasets. For stations with multiple instruments, the consistency of the calculated trends among different techniques is evaluated.