Nitric acid retrieval in the Antarctic atmosphere with all-sky IASI spectra and comparison with AURA-MLS observations

Nitric acid is a key gas for understanding the processes leading to ozone depletion in Antarctica. The Antarctica ozone hole is a cyclic phenomenon that begins in early September when the region exits the polar night and fully develops in October-November. Nitric acid is the primary constituent of stratospheric aerosol. In the Antarctic region, when the temperature gets below 195 K, it condenses from the gas phase to NAT or nitric acid trihydrate (HNO3·3H2O) in the form of ice crystals. Recently, we have developed a new forward/inverse model capable of computing the top-of- atmosphere infrared spectral radiance in all-sky conditions (clear/cloudy, day/night) and surface type (land or ocean). The new forward/inverse system has been applied to IASI (infrared Atmospheric Sounder interferometer) to retrieve O3 and HNO3 simultaneously. We have analyzed data for September 2021 and 2023, and HNO3 column retrievals have been compared to those observed with the MLS (Microwave Limb Sounder) instrument to check the capability of IASI to estimate HNO3. The paper also addresses the relationship between HNO3 and O3, especially at the onset of the ozone hole in the early spring. It will be shown that ozone depletion is paralleled by a consistent diminishing of HNO3 from the gas phase.