Retrieving Optical Depths of Nitric Acid Trihydrate Particles From IASI Measurements

Abstract. The ozone hole is formed as a result of physicochemical processes that take place in the unique environment of the polar night, especially in the Antarctic. The polar vortex is responsible for extremely cold stratospheric temperatures, which allow polar stratospheric clouds (PSCs) to form. The present study focuses on particles of nitric acid trihydrate (NAT, HNO₃ · 3H₂O), which is one of the key components of PSCs. We present a method to retrieve optical depths (ODs) of NAT particles from the hyperspectral measurements of the Infrared Atmospheric Sounding Interferometer (IASI) on board the Metop satellite platforms. The retrieval process is based on the construction of lookup tables using forward radiative transfer simulations for different thermal contrast situations, and is applied to 18 years of IASI spectra measured over the South Pole. The optical depth being proportional to the number of particles for a fixed particle size distribution, this results in a coherent quantitative record of Antarctic NAT particles. We show the monthly mean distributions of NAT ODs over the South Pole throughout the period 2008–2025, revealing a strong correlation with stratospheric temperatures. The distributions are more homogeneous and relevant than those derived from the previously published semi-quantitative index, as the thermal contrast is accounted for during the retrieval. In order to generalize the method, we also show its applicability over the North Pole using IASI spectra measured in January. The results reveal that IASI measurements allow the identification of events characterized by a high abundance of NAT particles on a daily basis, in both polar regions.