Direct sun total NO2 column measurements at Thessaloniki, Greece with two DOAS systems and comparisons with S5P/TROPOMI

Multiple Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments have been operating in the Laboratory of Atmospheric Physics (LAP), Thessaloniki, Greece over a decade to monitor trace gases in the troposphere. In this work, we present methodology, applied for the first time in LAP for retrieving total NO₂ columns in the atmosphere by applying the Direct Sun DOAS technique (DS-DOAS) on solar direct radiance spectra acquired with a research grade MAX-DOAS system, Delta. For the retrieval of total NO₂ two spectral fitting methods were tested using cross sections of NO₂ at two different temperatures for the tropospheric and the stratospheric component. The method that is based on concurrent spectral DOAS fittings with NO₂ cross sections at two temperatures (294 K and 220 K) (2T method) produces results of better quality as compared to the method using NO₂ cross section at one 254.5 K (1T method), with latter underestimating the total NO₂ vertical column density by up to 15%. This was verified by the good agreement (correlation coefficient R = 0.973) against the total NO₂ columns measured by an independently calibrated collocated Pandora system. Finally, the total NO₂ columns derived from the Delta instrument were compared with collocated observations by S5P/TROPOMI, showing an underestimation of the latter by up to 40% for days with high NO₂ concentrations, while the agreement is better under low NO₂ conditions. The overall agreement between the satellite and Delta system observations is characterized by a correlation coefficient of 0.82 and a mean bias of −2.63 ± 2.75 Pmolec/cm².