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

This paper assesses the quality of IASI (Infrared Atmospheric Sounding Interferometer)/Metop-A (IASI-A) and IASI/Metop-B (IASI-B) ozone (O₃) products (total and partial O₃ columns) retrieved with the Fast Optimal Retrievals on Layers for IASI Ozone (FORLI-O₃; v20151001) software for 9 years (2008-July 2017) through an extensive intercomparison and validation exercise using independent observations (satellite, ground-based and ozonesonde). 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 spectral range, with additional numerical corrections. This leads to a change of ~ 4% in the total ozone column (TOC) product, which is mainly associated with a decrease in the retrieved O3 concentration in the middle stratosphere (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 and nighttime measurements; IASI-A is slightly higher than IASI-B. A global difference of less than 2.4% is found for the tropospheric (TROPO) O3 column product (IASI-A is lower than IASI-B), which is partly due to a temporary issue related to the IASIA viewing angle in 2015. Our validation shows that IASIA and IASI-B TOCs are consistent with GOME-2 (Global Ozone Monitoring Experiment-2), Dobson, Brewer, SAOZ (Système d'Analyse par Observation Zénithale) 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-vis retrieved 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 July 2017 shows the long-term stability of IASI-A, with insignificant or small negative drifts of 1%-3%decade^-1. The comparison results of IASI-A and IASI-B against smoothed FTIR and ozonesonde partial O3 columns vary with altitude and latitude, with the maximum standard deviation being seen for the 300-150 hPa column (2%-40%) due to strong ozone variability and large total retrievals errors. Compared with ozonesonde data, the IASIA and IASI-B O3 TROPO column (defined as the columnbetween the surface and 300 hPa) is positively biased in the high latitudes (4%-5%) and negatively biased in the midlatitudes and tropics (11%-13% and 16%-19%, respectively). The IASI-A-to-ozonesonde TROPO comparison for the period from 2008 to 2016 shows a significant negative drift in the Northern Hemisphere of -8:6±3:4% decade^-1, which is also found in the IASI-A-to-FTIR TROPO comparison. When considering the period from 2011 to 2016, the drift value for the TROPO column decreases and becomes statistically insignificant. The observed negative drifts of the IASIA TROPO O₃ product (8%-16% decade^-1) over the 2008- 2017 period might be taken into consideration when deriving trends from this product and this time period. [ABSTRACT FROM AUTHOR]