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Knowing the spatial and seasonal distributions of nitric acid(HNO3) around the globe is of great interest and allows us to comprehend theprocesses regulating stratospheric ozone, especially in the polar regions.Due to its unprecedented spatial and temporal sampling, the nadir-viewingInfrared Atmospheric Sounding Interferometer (IASI) is capable of sounding theatmosphere twice a day globally, with good spectral resolution and lownoise. With the Fast Optimal Retrievals on Layers for IASI (FORLI)algorithm, we are retrieving, in near real time, columns as well as verticalprofiles of several atmospheric species, among which is HNO3. Wepresent in this paper the first characterization of the FORLI-HNO3profile products, in terms of vertical sensitivity and error budgets. Weshow that the sensitivity of IASI to HNO3 is highest in the lowerstratosphere (10–20 km), where the largest amounts of HNO3 are found,but that the vertical sensitivity of IASI only allows one level ofinformation on the profile (degrees of freedom for signal, DOFS; ∼  1). The sensitivity near the surface is negligible in mostcases, and for this reason, a partial column (5–35 km) is used for theanalyses. Both vertical profiles and partial columns are compared to FTIRground-based measurements from the Network for the Detection of AtmosphericComposition Change (NDACC) to characterize the accuracy and precision of theFORLI-HNO3 product. The profile validation is conducted through thesmoothing of the raw FTIR profiles by the IASI averaging kernels and givesgood results, with a slight overestimation of IASI measurements in the uppertroposphere/lower stratosphere (UTLS) at the six chosen stations (Thule,Kiruna, Jungfraujoch, Izaña, Lauder and Arrival Heights). The validationof the partial columns (5–35 km) is also conclusive with a mean correlationof 0.93 between IASI and the FTIR measurements. An initial survey of theHNO3 spatial and seasonal variabilities obtained from IASI measurementsfor a 1-year (2011) data set shows that the expected latitudinal gradientof concentrations from low to high latitudes and the large seasonalvariability in polar regions (cycle amplitude around 30 % of the seasonalsignal, peak to peak) are well represented by IASI data

First characterization and validation of FORLI-HNO&lt;sub&gt;3&lt;/sub&gt; vertical profiles retrieved from IASI/Metop

First characterization and validation of FORLI-HNO<sub>3</sub> vertical profiles retrieved from IASI/Metop