## 4 Comparison with 55p/tropomi  
In this section, the tropospheric NO\({}_{2}\) vertical columns retrieved by TROPOMI for Thessaloniki (considering only cloud-free satellite pixels) are compared and validated using collocated ground-based MAX-DOAS measurements. Since TROPOMI data are limited to only one overpass per day, short-term (e.g., diurnal) NO\({}_{2}\) variations cannot be investigated with satellite data. TROPOMI passes over Thessaloniki between approximately 10:30 and 12:30 UTC (Figure 3), when the NO\({}_{2}\) concentrations are usually low for the reasons discussed in Section 3.2. However, NO\({}_{2}\) is a highly variable pollutant and, thus, in order to compare meaningful results, the collocated ground-based and satellite observations should have a short time difference. In this study, the collocations are calculated by averaging MAX-DOAS measurements within \(\pm\)15 min of the satellite overpass time. Figure 8 shows the time series of the collocated tropospheric NO\({}_{2}\) vertical column measurements in box-and-whisker plots over the whole period of study (30 months), from June 2020 to November 2022. The seasonal variations of NO\({}_{2}\) are well captured by both instruments and the reported seasonal patterns are similar over the period of study. The higher tropospheric NO\({}_{2}\) columns during winter are consistent with the results presented in Section 3.1. Furthermore, higher variability of NO\({}_{2}\) VCDs is found during the cold period for both ground-based and satellite observations, especially for those of the MAX-DOAS. This is expected since NO\({}_{x}\) emissions are stronger during wintertime, which, combined with the lower photolysis rate of NO\({}_{2}\) due to limited sunlight and the more variable weather patterns, results in higher spatio-temporal variability. The highest monthly variations, represented by the interquartile range, are found during December 2020, February 2021 and December 2021 (ranging between 5.72-13.17 \(\times\) 10\({}^{15}\) molecules/cm\({}^{2}\) for the MAX-DOAS and 4.90-6.59 \(\times\) 10\({}^{15}\) molecules/cm\({}^{2}\) \begin{table}
\begin{tabular}{c c c c c} \hline  & **SE (142\({}^{\circ}\))** & **S (185\({}^{\circ}\))** & **SW (220\({}^{\circ}\))** & **W (255\({}^{\circ}\))** \\ \hline  & cold/warm & cold/warm & cold/warm & cold/warm \\ \hline Morning & NO\({}_{2}\) vertical columns & \(-3.2/-0.1\%\) & \(-8.8/-3.9\%\) & \(4.0/-2.0\%\) & \(8.0/6.0\%\) \\  & NO\({}_{2}\) surface concentrations & \(0.2/\mathbf{19.4\%}\) & \(3.6/-3.3\%\) & \(-1.2/-\mathbf{14.9\%}\) & \(-2.6/-1.27\%\) \\ \hline Noon & NO\({}_{2}\) vertical columns & \(\mathbf{11.9/24.7\%}\) & \(-7.1/-7.7\%\) & \(-0.2/-12.9\%\) & \(-4.5/-4.1\%\) \\  & NO\({}_{2}\) surface concentrations & \(\mathbf{14.5/15.8\%}\) & \(-6.6/-0.6\%\) & \(-6.4/-\mathbf{17.4\%}\) & \(-1.5/2.1\%\) \\ \hline \end{tabular}
\end{table} 
Table 1: Percentage difference of the median NO\({}_{2}\) VCDs and surface concentrations for each azimuth from the average concentration of all azimuths for the morning and noon hours and for the cold and warm periods. The highest differences are marked in bold. for TROPOMI). These are consistent with Figure 2, indicating that pollution events were well captured by both ground-based and satellite instruments. Figure 9 presents the comparison of TROPOMI tropospheric NO\({}_{2}\) vertical columns against MAX-DOAS measurements in scatter plots for the four seasons of the whole period of study. Overall, TROPOMI systematically reports lower NO\({}_{2}\) columns than the MAX-DOAS. The underestimation of satellite-derived tropospheric NO\({}_{2}\) compared to MAX-DOAS observations is also found in recent publications (e.g., [54; 55; 56]) and it is more evident in highly polluted and in extremely highly polluted environments [22]. The MAX-DOAS probes air masses along the line of sight of the telescope, pointing at different elevation angles, from near the horizon to the zenith.