Comprehensive Analysis of Bias in TEMPO NO₂ Column Densities Through Pandora Observations

Tropospheric Emissions: Monitoring of Pollution (TEMPO) is the first geostationary satellite instrument to monitor air pollutants across North America. This study uses Pandora observations to analyze the bias in TEMPO Level-3 total column density of NO₂ (TOTNO₂) from August 2023 to December 2024. TEMPO achieves high accuracy at 5% cloud-filtering threshold: correlation coefficient (R) of 0.86, index of agreement (IOA) of 0.91, mean absolute bias (MAB) of 1.423 × 10¹⁵ molecules/cm², and a percentage MAB (MABP) of 23.1%, corresponding to a 12% underestimation. Accuracy decreases when pixels with greater cloud-cover are included. Solar zenith angle (SZA) of 10-20° yields the highest accuracy (R: 0.87, MABP: 22.7%), whereas SZAs of 70-80° yield the lowest (R: 0.71, MABP: 35.2%). Consequently, early-morning or near-sunset observations are less reliable than midday. This discrepancy could stem from inaccurate simulation of diurnal variations in the boundary-layer height in the a-priori, and from larger uncertainties in radiative transfer at high SZAs. TEMPO overestimates TOTNO₂ at low NO₂ levels and underestimates at high levels, with maximum biases of +16% (low) and - 31% (high), respectively. Station-to-station performance varies considerably, with R ranging from 0.29 to 0.84 and MABP from 14.9% to 49.3%. Stations situated at higher altitudes relative to the ground show reduced agreement with TEMPO, as Pandora cannot detect NO₂ below the instrument's altitude, whereas TEMPO retrieves the full column. Validation of TEMPO TOTNO₂ at TROPOMI overpass time indicates that TEMPO's performance relative to Pandora (IOA: 0.93, MABP: 22.3%) closely matches that of TROPOMI (IOA: 0.92, MABP: 20.1%).