Evaluation of the first year of Pandora NO₂ measurements over Beijing and application to satellite validation

Abstract. Nitrogen dioxide (NO₂) is a highly photochemically reactive gas, has a lifetime of only a few hours, and at high concentrations it is harmful to human beings. Therefore, it is important to monitor NO₂ with high-precision, time-resolved instruments. To this end, a Pandora spectrometer has been installed on the roof of the laboratory building of the Aerospace Information Research Institute of the Chinese Academy of Sciences in the Olympic Park, Beijing, China. The concentrations of trace gases (including NO₂, HCHO, O₃) measured with Pandora are made available through the open-access Pandora data base (https://data.pandonia-global-network.org/Beijing-RADI/Pandora171s1/). In this paper, an overview is presented of the Pandora NO₂ data collected during the first year of operation, i.e., from August, 2021, to July, 2022. The data show that NO₂ concentrations were high in the winter and low in the summer, with diurnal cycle where the concentrations reach a minimum during day time. The concentrations were significantly lower during the 2022 Winter Olympics in Beijing, showing the effectiveness of the emission control measures during that period. The Pandora observations show that during northerly winds clean air is transported to Beijing with low NO₂ concentrations, whereas during southerly winds pollution from surrounding areas is transported to Beijing and NO₂ concentrations are high. The contribution of tropospheric NO₂ to the total NO₂ VCD varies significantly on daily to seasonal time scales, i.e., close to 50 % in autumn and winter, and close to 70 % in spring and autumn. The comparison of Pandora-measured surface concentrations with collocated in situ measurements using a Thermo Scientific 42i-TL Analyzer shows that the Pandora data are low and that the relationship between Pandora-derived surface concentrations and in situ measurements are different for low and high NO₂ concentrations. Explanations for these differences are offered in terms of measurement techniques and physical (transport) phenomena. The use of Pandora total and tropospheric NO₂ vertical column densities (VCDs) for validation of collocated TROPOMI data, resampled to 100×100 m², shows that although on average the TROPOMI VCDs are slightly lower, they are well within the expected error for TROPOMI of 0.5 Pmolec ⋅ cm⁻² + 0.2 to 0.5 ⋅ VCD_trop The location of the Pandora instrument within a sub-orbital TROPOMI pixel of 3.5×5.5 km² may result in an error in the TROPOMI-derived tropospheric NO₂ VCD between 0.223 and 0.282 Pmolec.cm^-2, i.e., between 1.7 % and 2 %. In addition, the data also show that the Pandora observations at the Beijing-RADI site are representative for an area with a radius of 10 km.