Mapping seasonal nitric acid (HNO₃) patterns in the extratropics with nadir-viewing infrared sounders – a retrieval perspective

Abstract. With this paper, we aim to shed light on the extent to which nadir-viewing hyperspectral infrared (IR) sounders can support the study of stratospheric chemical processes and ozone loss in the extratropics. We use CLIMCAPS (Community Long-term Infrared Microwave Combined Atmospheric Processing System) retrievals from JPSS-1 (Joint Polar Satellite System) CrIS (Cross-track Infrared Sounder) measurements as the baseline case. CLIMCAPS retrieves a suite of Earth system variables that includes atmospheric temperature (T_air), water vapor (H₂O_vap), carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), ozone (O₃), and nitric acid (HNO₃). Unlike the Rodgers (2000) Optimal Estimation (OE) retrieval approach, CLIMCAPS regularizes its Bayesian inverse solution dynamically using singular value decomposition (SVD) at run-time to separate measurement signal from noise. We illustrate how the CLIMCAPS approach enables stable retrievals of stratospheric HNO₃ under highly variable conditions, allowing characterization of seasonal patterns. Nitric acid is typically used as an indicator species for heterogeneous chemical processing inside the winter stratospheric polar vortices. This paper summarizes our diagnostic evaluation of CLIMCAPS observing capability during the Northern Hemisphere winter of 2019/2020. We contrast CLIMCAPS HNO₃ retrievals with those from the limb-viewing MLS (Microwave Limb Sounder) to illustrate the capability of this retrieval approach and lay the foundation for in-depth validation studies in future.