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 195K, 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-ofatmosphere 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.