Global analysis of nitrogen dioxide and formaldehyde column densities from the Pandora global network: Variability and implications for satellite validation

Abstract

This study harnesses quality-assured global Pandora observations (2019-2023) from the Pandonia Global Network (PGN) to investigate diurnal and seasonal variations of NO2 and HCHO-key proxies for tropospheric O3-and to evaluate TROPOMI satellite observations. NO2 vertical column densities (VCDs) at Polluted Urban stations peak in winter and gradually increase throughout the day, but show a decrease in afternoons in summer due to photochemical loss. Conversely, Rural/Background stations exhibit summer maxima with monotonic daytime increases across seasons, driven by stratospheric NO2 variability. HCHO VCDs are higher in summer at most sites, with a morning increase followed by elevated concentrations throughout the afternoon. The spatial representativeness mismatch between satellite and Pandora observations results in negative biases in TROPOMI NO2 VCDs at Polluted Urban stations and a valley station, while overestimations are found at high-altitude stations. Considerable random uncertainties in TROPOMI HCHO VCDs lead to low correlations (r2 = 0.08-0.11) and high random errors (0.27-0.33 DU) across environments. Averaging collocated data points prior to intercomparison effectively reduces random biases, whereas increasing the spatial collocation range introduces biases due to spatial averaging effects. Tropospheric HCHO-to-NO2 ratios (FNRtrop) retrieved from Pandora observations indicate that Polluted Urban (0.82 +/- 0.08) and Rural/Background (1.64 +/- 0.07) stations are generally under VOC-limited and NOx-limited O3 production regimes, respectively, while summertime increases in FNRtrop put Polluted Urban stations in a transitional range, yielding higher O3 production efficiencies. TROPOMI-derived FNRtrop shows good agreement with Pandora in Polluted Urban stations (Delta FNRmedian = 0.18), whereas random error increases in rural areas with lower tropospheric NO2.