Analysis of NO2 and O3 Total Columns from DOAS Zenith-Sky Measurements in South Italy

The Gas Absorption Spectrometer Correlating Optical Difference—New Generation 4 (GASCOD/NG4) is a multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument which measures diffuse solar spectra at the Environmental-Climate Observatory (ECO) of the Italian research institute CNR-ISAC, near Lecce. The high-resolution spectra measured in zenith-sky configuration were used to retrieve the NO2 and O3 vertical column densities (VCDs) from March 2017 to November 2019. These good-quality data, proven by the comparison with the Ozone Monitoring Instrument (OMI) and TROPOspheric Monitoring Instrument (TROPOMI) satellite measurements, were used to characterize the ECO site by exploiting the sinergy with in situ NO2 and O3 concentrations and meteorological data. Although stratospheric processes seem to be the main forces behind the NO2 and O3 VCDs seasonal trends, diurnal variabilities revealed the presence of a tropospheric signal in the NO2 VCDs, which had significant lower values during Sundays. Comparison with wind data acquired at the ECO observatory, at 20 m above the ground, revealed how NO2 VCDs are influenced by both tropospheric local production and transport from the nearby city of Lecce. On the other hand, no significant tropospheric signal was contained in the O3 VCDs.