Retrieval of Total NO₂ Columns Using Direct-Sun Differential Optical Absorption Spectroscopy Measurements in Thessaloniki

The monitoring of trace gases in the troposphere has been routinely performed at the Laboratory of Atmospheric Physics, Thessaloniki, Greece, for a decade now, by multiple Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments. Even though measurements of trace gas concentrations in the troposphere are of great interest in terms of air quality, the MAX-DOAS technique is only sensitive to absorbers in the lowest few kilometers of the atmosphere. In this work, we present a methodology for the retrieval of total NO2 columns in the atmosphere by applying the DOAS technique on direct sun spectra (DS-DOAS) measured with a new research-grade system. The advantages and limitations of the total NO2 retrieval methodology, based on DS-DOAS, are discussed. The accuracy and the quality of the retrieved columns were assessed by comparison with a collocated Pandora system that also measures total NO2 column amounts using a similar technique with independent calibration.