Direct sun total NO2 column measurements at Thessaloniki, Greece with two DOAS systems and comparisons with S5P/TROPOMI.
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| Title: | Direct sun total NO2 column measurements at Thessaloniki, Greece with two DOAS systems and comparisons with S5P/TROPOMI. |
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| Authors: | Nikolis, Dimitrios1 (AUTHOR) dnikolis@auth.gr, Bais, Alkiviadis1 (AUTHOR), Karagkiozidis, Dimitris1 (AUTHOR), Koukouli, Maria-Elissavet1 (AUTHOR), Balis, Dimitris1 (AUTHOR) |
| Source: | Atmospheric Environment. Jul2025, Vol. 352, pN.PAG-N.PAG. 1p. |
| Subject Terms: | *Environmental sciences, Atmospheric physics, Optical spectroscopy, Light absorption, Statistical correlation |
| Abstract: | 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 NO 2 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 NO 2 two spectral fitting methods were tested using cross sections of NO 2 at two different temperatures for the tropospheric and the stratospheric component. The method that is based on concurrent spectral DOAS fittings with NO 2 cross sections at two temperatures (294 K and 220 K) (2T method) produces results of better quality as compared to the method using NO 2 cross section at one 254.5 K (1T method), with latter underestimating the total NO 2 vertical column density by up to 15%. This was verified by the good agreement (correlation coefficient R = 0.973) against the total NO 2 columns measured by an independently calibrated collocated Pandora system. Finally, the total NO 2 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 NO 2 concentrations, while the agreement is better under low NO 2 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. • Cross sections at 294 K and 220 K are used for the DOAS retrieval of total NO 2 column from direct-sun spectra. • The retrieval methodology was evaluated by comparison to a collocated Pandora system with a mean difference of −3%. • Based on two years of observations, the TROPOMI onboard the S-5P satellite underestimates the total NO 2 column by 21%. • The total NO 2 column at Thessaloniki has a distinct annual course (higher levels in winter) ranging from 3 to 35 Pmolec/cm2. [ABSTRACT FROM AUTHOR] |
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| Database: | GreenFILE |
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