Characterization of Pandora Sky-Scan Modes from the Japan Pandora Network and Evaluation of Their Use in Validating Satellite NO 2 and HCHO Retrievals

Satellite observations provide essential global information on atmospheric nitrogen dioxide (NO 2 ) and formaldehyde (HCHO), but require robust validation to ensure accuracy, regional consistency, and long-term stability. This study evaluates the horizontal light path length (HLPL) of Pandora ground-based spectrometers operating in direct-sun mode (DSM) and sky-scan mode (SSM), and assesses their effectiveness in validating TROPOspheric Monitoring Instrument (TROPOMI) retrievals. The Pandora observations from the Japan Pandora Network (JPN) sites, comprising seven stations located between 33° and 43°N latitude, are examined. We found good agreement between SSM and DSM observations for both NO 2 and HCHO, with correlations of 0.80–0.91 for NO 2 and 0.72–0.95 for HCHO. Comparisons with TROPOMI showed strong correlations for NO 2 in both DSM ( R = 0.70–0.91, slope = 0.25–0.78) and SSM ( R =…