Mapping stratospheric nitric acid (HNO ₃ ) patterns in the extratropics with nadir-viewing infrared sounders – a retrieval perspective

Abstract. With this paper, we aim to demonstrate how stratospheric HNO3 can be retrieved from nadir hyperspectral infrared (IR) measurements such that it is largely uncorrelated with tropospheric HNO3 and most other interfering signals. This is achieved by decomposing the set of HNO3 sensitive channels into orthogonal vectors that isolate the stratospheric HNO3 signal for use in the retrieval. Such a nadir-IR HNO3 product could add useful information to the monitoring of some stratospheric chemical processes affecting ozone in the extratropics, especially once the state-of-the-art Microwave Limb Sounder (MLS) on Aura is decommissioned in 2026. Nitric acid is typically used as indicator species for heterogeneous chemical processing inside the winter polar stratospheric vortices. The proposed stand-alone stratospheric HNO3 retrieval would be an improvement over the only other nadir-IR HNO3 product available today, namely FORLI (Fast Optimal Retrieval on Layers for IASI), which is a stratosphere + troposphere correlated profile retrieval affected by uncertainty in tropospheric water vapor at the time of measurement. We demonstrate the potential of this new stratospheric nadir-IR HNO3 retrieval strategy using the Community Long-term Infrared Microwave Combined Atmospheric Processing System (CLIMCAPS) as the retrieval framework with measurements from CrIS (Cross-track Infrared Sounder) on the Joint Polar Satellite System 1 (JPSS-1) during the Northern Hemisphere winter of 2019/2020. Future work will focus on optimizing and validating CLIMCAPS HNO3 retrievals for operational deployment.