## 4 Comparison with 55p/tropomi
The measurements are sensitive to local emissions located on this line of sight and at distances up to a few kilometers. The NO\({}_{2}\) columns that are reported by satellite instruments are essentially averaged over the sub-satellite pixel area (3.5 \(\times\) 5.5 km\({}^{2}\) for SSP/TROPOMI), which contains different emissions sources, resulting, generally, in the underestimation of the NO\({}_{2}\) columns. The highest correlation between the two instruments is found in winter (R = 0.75), while significantly lower correlations are found during spring (R = 0.6), autumn (R = 0.6) and summer (R = 0.44). For all seasons, a negative bias is found for TROPOMI with median differences of \(-\)2.91 \(\pm\) 3.77 \(\times\) 10\({}^{15}\) molecules/cm\({}^{2}\) in winter, \(-\)0.57 \(\pm\) 2.13 \(\times\) 10\({}^{15}\) molecules/cm\({}^{2}\) in spring, \(-\)1.04 \(\pm\) 1.55 \(\times\) 10\({}^{15}\) molecules/cm\({}^{2}\) in summer and \(-\)1.42 \(\pm\) 2.97 \(\times\) 10\({}^{15}\) molecules/cm\({}^{2}\) in autumn. These biases correspond to mean fractional biases of about \(-\)55% during winter, \(-\)21% in spring and \(-\)40% in summer and autumn. Figure 8: Time series of monthly mean tropospheric NO\({}_{2}\) vertical columns in Thessaloniki from collocated ground-based MAX-DOAS and SSP/TROPOMI observations for the period June 2020–November 2022. Figure 10 shows the consolidated comparison (i.e., without seasonal discrimination) of TROPOMI and MAX-DOAS tropospheric NO\({}_{2}\) columns in a scatter plot (Figure 10a) over the whole period of study. The blue markers indicate the daily collocated observations, while the red dots represent the monthly mean vertical columns. Figure 10b shows the frequency distribution of the absolute daily differences between TROPOMI and MAX-DOAS. The black and red dashed lines indicate the median and mean difference, respectively, while the solid magenta lines represent the 25 th and 75 th percentiles of the distribution. The correlation coefficient of the daily collocated data is 0.72, with slope and offset values of 0.37 and 1.24, respectively. For the monthly means, the statistics are significantly improved with R = 0.89 and slope and offset values of 0.49 and 0.71, respectively. A clear negative bias is found with median and mean differences of \(-1.19\) and \(-1.93\) (\(\times 10^{15}\) molecules/cm\({}^{2}\)), respectively (which corresponds to a mean fractional bias of approximately \(-37\%\)), and an interquartile range of \(2.89\times 10^{15}\) molecules/cm\({}^{2}\). These results are consistent with the findings in the latest issue of the quarterly Routine Operations Consolidated Validation Report ([[http://mpc-vdaf.tropomi.eu/](http://mpc-vdaf.tropomi.eu/)]([http://mpc-vdaf.tropomi.eu/](http://mpc-vdaf.tropomi.eu/)), accessed on 20 January 2023), where for the SSP/TROPOMI tropospheric NO\({}_{2}\) product, a median bias of \(-1.8\times 10^{15}\) molecules/cm\({}^{2}\) (about \(-35\%\)) was found over the full satellite mission using NDACC-affiliated MAX-DOAS data from 30 measurement sites. Figure 9: Scatter plots of the daily collocated tropospheric NO\({}_{2}\) VCDs that are reported by the MAX-DOAS and SSP/TROPOMI for all seasons. The gray shaded areas represent the 95% confidence interval of the linear regression fit (solid black line).