Interpretation of Pandora NO2 measurements over Beijing and application to satellite validation

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, in August 2021. 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/). The Beijing-RADI Pandora is included in the data suite which is routinely used for TROPOMI S5P validation. The use of Pandora total and tropospheric NO2 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. 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. The Pandora total and tropospheric NO2 vertical column densities (VCDs) and surface concentrations collected during the first year of operation 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., monthly averages vary between 50% and 60% in the winter and between 60% and 70% in the 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.