[A-AS11] Atmospheric Chemistry

[AAS11-02] Characterization of Pandora direct-sun and sky-scan modes and evaluation of their use in validating satellite NO₂ and HCHO retrievals

*Nguyen Doan Thien Chi^1,2, Hiroshi Tanimoto¹, Satoshi Inomata¹, Tamaki Fujinawa¹, Astrid Müller¹, Takafumi Sugita¹ (1.National Institute for Environmental Studies, Japan, 2.Graduate School of Environmental studies, Nagoya University, Japan)

Keywords:Japan Pandora Network, direct-sun mode, sky-scan mode, horizontal light path length, satellite validation

To ensure the accuracy and long-term stability, satellite data require extensive validations by a reference method. The Pandora instruments, ground-based spectrometers, are now widely used as a reliable source for the satellite validation of nitrogen dioxide (NO₂), formaldehyde (HCHO), and other species. In this study, we explored the horizontal light path length (HLPL) of the Pandora direct-sun mode (DSM) and the sky-scan mode (SSM) for the validation of the TROPOspheric Monitoring Instrument (TROPOMI). Data from these types of observations at the Japan Pandora Network (JPN) sites were examined. Generally, the SSM aligns with the DSM for both NO₂ and HCHO. The direct comparisons with TROPOMI showed a strong correlation in NO₂ TropVCDs for both of the DSM (R = 0.70–0.91, slope = 0.25–0.78) and the SSM (R = 0.69–0.88, slope = 0.41–0.86). Those comparisons in HCHO are weaker. Furthermore, we also applied the HLPL as a criterion to compare TROPOMI with the two Pandora observation modes. The HLPL method probably unable to improve the agreement between TROPOMI and the DSM because the TROPOMI's resolution is coarser than that of HLPL. In contrast, for the TROPOMI and the SSM comparisons, we found that the application of the HLPL significantly improved the agreement. The SSM demonstrated validity comparable to that of the DSM for satellite validation. Our study revealed the importance of considering HLPL and viewing direction as coincidence criteria when using the Pandora instrument for satellite validation. The two Pandora observation modes and the HLPL approach may be employed in the validation efforts of NO₂ and HCHO observations from upcoming satellite missions.