Validation of all S5P ozone products (total columns, tropospheric columns and profiles) with a single reference network.

Ground-based Fourier Transform Infrared (FTIR) instruments from the Network for the Detection of Atmospheric Composition Change (NDACC) provide long-term and continuous measurements of many atmospheric trace gases at more that 20 stations. This network is already used in the S5P validation of CO and CH4 (Sha et al., 2021) as well as HCHO (Vigouroux et al., 2020), and NO2 (validation reports https://mpc-vdaf.tropomi.eu/).

Ozone is one of the major FTIR NDACC target gas and its retrieval strategy is harmonized within the network (Vigouroux et al. 2015). Nonetheless, while ozone data from FTIR measurements are contributing to many ozone trend studies (e.g. Vigouroux et al., 2015, Godin-Beekman, et al., 2022, Van Malderen et al. ,2025), they have been poorly used for satellite validation. The reasons are partly historical (Brewen/Dobson are traditionally used for total column validation) and partly scientific (FTIR has low vertical resolution so the ozone sondes and lidars are preferred for profile validation of Limb satellites).

However, the ground-based FTIR ozone products are well suited for the validation of Nadir sounding satellites such as S5P: FTIR retrievals provide both ozone total column with a high precision better than 2% and ozone profiles with low vertical resolution (approximately 4 degrees of freedom for signal - DOFS), which is similar to the S5P ozone profile products (about 5 DOFS). The strength of FTIR data compared to ozone sondes measurements is that they have sensitivity up to about 45 km, allowing the validation of S5P profiles above the 30 km limit reached by the sondes. These higher altitudes can be reached by Lidar data, but the stations equipped with such instruments are sparse and therefore provide lower representativeness of the validation. In addition, FTIR has also a good sensitivity in the troposphere (1 DOFS) which allows for the validation of the specific S5P tropospheric column product as well, although only 3 FTIR stations are located within the 20°S – 20°N band for which the S5P ozone tropospheric product is provided.

We will show validation results for the three different S5P products: total columns,  tropospheric columns, and profiles. The profile validation will be made by comparing ozone from both instruments in 4 vertical layers following the FTIR averaging kernels and DOFS. The effect of the different a priori information and vertical sensitivities of both S5P and FTIR will be investigated. Our results (accuracy and precision of the S5P ozone products) will be put in perspective with the past S5P ozone validation studies (Garane et al., 2019; Hubert et al., 2021, Keppens et al., 2024).

This validation exercise is the first step towards the future use of FTIR for the validation of geostationary satellites such as S4/S5 (ESA project CHEOPS S4/5) or TEMPO. One of the major advantage of this reference network is to enable the evaluation of the ozone diurnal cycle, as FTIR acquisitions are made throughout the day in clear sky conditions.