Tropospheric ozone is an important greenhouse gas, is detrimental to human health and crop and ecosystem productivity, and controls the oxidizing capacity of the troposphere. Tropospheric ozone is extremely difficult to monitor on the global scale due to its highly variable distribution and interannual variability, and the ongoing shift of ozone precursor emissions from high latitudes to low latitudes. In contrast to the regular assessment of stratospheric ozone since 1985 by the WMO/UNEP Scientific Assessments of Ozone Depletion, an up-to-date scientific assessment of tropospheric ozone’s global distribution and trends from the surface to the tropopause has only been initiated by the Tropospheric Ozone Assessment Report (TOAR) Activity in 2014. As of February 2020, TOAR has entered its second phase (TOAR-II), the first phase having been completed in 2019. The TOAR mission continues, with TOAR-II providing updated and extended information on tropospheric ozone.

Well-established techniques to measure the vertical distribution of ozone in the free troposphere and tropopause region include ozonesondes, UV-photometers onboard commercial & research aircraft, Lidars, Dobson Umkehrs, Brewer Mark IV Umkehrs, and FTIRs. Recently, new ground-based UV-visible spectrometers (e.g. MAX-DOAS, Pandora) and satellite instruments (e.g. OMI, TROPOMI), which are based on remote sensing techniques, play an increasing role in the characterization of tropospheric ozone. TOAR I (Gaudel et al., 2018, Tarasick et al., 2019) showed that besides clear regional differences, the distribution and trends of ozone in the troposphere and tropopause region are not always consistent between different datasets obtained from the different standard ozone observing techniques.

Satellite biases range between -10% and +20% with standard deviations of about 10-30%, versus 5-10% for sondes, aircraft instruments, lidar and ground-based FTIR (Tarasick et al., 2019). The ground-based ozone datasets serve an important role as reference data for the satellite measurements. For example, in TOAR II, the Satellite Ozone Focus Working Group aims to reconcile differences between satellite-based ozone retrievals by applying a common methodology for validating trends, using the long-term ozonesonde record.

The ground-based free tropospheric ozone working group, named HEGIFTOM (Harmonization and Evaluation of Ground-based Instruments for Free Tropospheric Ozone Measurements) will evaluate and harmonize the tropospheric ozone data obtained from the different observing networks of ground-based instruments, in order to

• reconcile the differences in ozone distribution and trends between the different ground-based platforms, and
• provide other TOAR-II working groups harmonized and evaluated datasets of the vertical ozone distributions that can be used either for validation studies of satellite retrievals and numerical models or for other scientific studies within TOAR-II (e.g. trends assessment, process studies).