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 (NO2) 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 NO2 and HCHO, with correlations of 0.80–0.91 for NO2 and 0.72–0.95 for HCHO. Comparisons with TROPOMI showed strong correlations for NO2 in both DSM (R = 0.70–0.91, slope = 0.25–0.78) and SSM (R = 0.69–0.88, slope = 0.41–0.86), while correlations for HCHO were weaker. The HLPL was determined to be 1.62 ± 0.96 km for DSM and 12.0 ± 1.8 km for SSM. This study demonstrates that considering HLPL as coincidence criteria is essential when using Pandora instruments for satellite NO2 validation, particularly in urban areas with heterogeneous pollution distributions. Importantly, SSM observations, which have been underutilized for satellite validation, can provide comparable results to DSM when properly accounting for spatial representation, offering potential for expanded validation capabilities in atmospheric monitoring networks.