Abstract. 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 IASI-A viewing angle in 2015. Our
validation shows that IASI-A 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 (20 %–40 %) due to strong ozone variability and
large total retrievals errors. Compared with ozonesonde data, the IASI-A and
IASI-B O3 TROPO column (defined as the column between 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 IASI-A 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.
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