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. This paper assesses the quality of IASI (Infrared Atmospheric Sounding Interferometer)/Metop-A (IASI-A) andIASI/Metop-B (IASI-B) ozone ( O3 ) products (total and partial O3 columns) retrieved with the Fast Optimal Retrievals on Layersfor IASI Ozone (FORLI- O3 ; v20151001) software for 9 years(2008–July 2017) through an extensive intercomparison and validationexercise using independent observations (satellite, ground-based andozonesonde). Compared with the previous version of FORLI-O3 (v20140922),several improvements have been introduced in FORLI- O3 v20151001,including absorbance look-up tables recalculated to cover a larger spectralrange, with additional numerical corrections. This leads to a change of ∼4  % in the total ozone column (TOC) product, which is mainly associatedwith a decrease in the retrieved O3 concentration in the middlestratosphere (above 30 hPa/25 km). IASI-A and IASI-B TOCs are consistent,with a global mean difference of less than 0.3 % for both daytime andnighttime measurements; IASI-A is slightly higher than IASI-B. A globaldifference of less than 2.4 % is found for the tropospheric (TROPO) O3 column product (IASI-A is lower than IASI-B), which is partlydue to a temporary issue related to the IASI-A viewing angle in 2015. Ourvalidation shows that IASI-A and IASI-B TOCs are consistent withGOME-2 (Global Ozone Monitoring Experiment-2), Dobson, Brewer, SAOZ(Systeme d'Analyse par ObservationZenithale) and FTIR (Fourier transform infrared)TOCs, with global mean differences in the range of 0.1 %–2 %depending on the instruments compared. The worst agreement with UV–visretrieved TOC (satellite and ground) is found at the southern high latitudes.The IASI-A and ground-based TOC comparison for the period from 2008 to July2017 shows the long-term stability of IASI-A, with insignificant or small negativedrifts of 1 %–3 % decade −1 . The comparison results of IASI-A and IASI-Bagainst smoothed FTIR and ozonesonde partial O3 columns vary withaltitude and latitude, with the maximum standard deviation being seen for the300–150 hPa column (20 %–40 %) due to strong ozone variability andlarge total retrievals errors. Compared with ozonesonde data, the IASI-A andIASI-B O3 TROPO column (defined as the column between the surfaceand 300 hPa) is positively biased in the high latitudes (4 %–5 %)and negatively biased in the midlatitudes and tropics (11 %–13 % and16 %–19 %, respectively). The IASI-A-to-ozonesonde TROPO comparisonfor the period from 2008 to 2016 shows a significant negative drift in theNorthern Hemisphere of - 8.6 ± 3.4  % decade −1 , which is alsofound in the IASI-A-to-FTIR TROPO comparison. When considering the periodfrom 2011 to 2016, the drift value for the TROPO column decreases and becomesstatistically insignificant. The observed negative drifts of the IASI-A TROPO O3 product (8 %–16 % decade −1 ) over the 2008–2017period might be taken into consideration when deriving trends from thisproduct and this time period.

Validation of the IASI FORLI/EUMETSAT ozone products using satellite (GOME-2), ground-based (Brewer–Dobson, SAOZ, FTIR) and ozonesonde measurements