Nitrogen dioxide spatiotemporal variations in the complex urban environment of Athens, Greece

In this study, five years of total, tropospheric and near-surface nitrogen dioxide, ΝΟ2, observations from a Pandora spectrometer system routinely operating in the city center of Athens, Greece, are presented and compared with space-borne observations from the S5P/TROPOMI sensor and in-situ air quality monitoring station measurements from the Greek National Air Pollution Monitoring Network. Each of the three different systems exhibits its own monitoring capabilities and restrictions, based on the spatial representativeness and temporal resolution of its measurements. However, they all reveal a clear weekly pattern with low NO2 concentrations on Sundays while a diurnal cycle with higher levels of NO2 in the morning is observed from the ground due to the high NOx emissions from heavy traffic in the urban environment. A seasonal pattern is further demonstrated by both space- and ground-based remote sensing instruments with enhanced NO2 loadings in wintertime and decreased levels in summertime. Even though, as expected, S5P/TROPOMI appears to underestimate the tropospheric NO2 load, the correlation between the ground-based and space-borne data is relatively high, with corresponding correlation coefficients ranging between 0.73 in spring and 0.80 in winter. High correlations are also found between the relatively new near-surface NO2 product of Pandora and the in situ observations, ranging from 0.91 to 0.99. However, an underestimation is observed by the Pandora instrument compared to those in situ monitoring stations which are highly affected by the increased NOx emissions in the city center. For the Pandora near-surface NO2 at 0° (north) azimuthal viewing angle, the best agreement is found with the urban background in situ stations, with a negative bias at 0.99 ± 1.60 × 1011 molecules cm−3, whereas the Pandora retrievals at 39° (north-east) are closer to the urban in situ NO2 levels, with a bias of −1.10 ± 2.02 × 1011 molecules cm−3 on average.