## 5 Summary and Conclusions
The seasonal NO\({}_{2}\) variations were well captured by both instruments, with increased VCDs during the cold period due to enhanced emissions and a reduction during the warm period as a result of stronger photochemical destruction of NO\({}_{2}\). Even though a generally good correlation was found between MAX-DOAS and TROPOMI (R = 0.72 and R = 0.89 for the daily and monthly means, respectively), TROPOMI underestimates the tropospheric NO\({}_{2}\) VCDs by approximately 55% in winter, 21% in spring and 40% during autumn and summer. The consolidated median and mean absolute differences (i.e., without seasonal discrimination) are \(-\)1.19 and \(-\)1.93 (\(\times 10^{15}\) molecules/cm\({}^{2}\)), respectively, and they correspond to a mean fractional bias of approximately \(-\)37%, which is in agreement with the latest S5P MPC VDAF 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). This clearly negative bias is mainly caused by the much larger satellite footprint, resulting in a smoother perception of the NO\({}_{2}\) concentrations, while the MAX-DOAS measurements are affected more by local emissions. Conceptualization, A.B. and D.K. ; Investigation, D.K. ; Methodology, D.K., M.-E.K., A.B. and D.B. ; Resources, D.K., M.-E.K. and P.T. ; Software, D.K. and M.-E.K. ; Formal analysis, D.K. and M.-E.K. ; Data curation, D.K., M.-E.K. and P.T. ; Validation, D.K. and M.-E.K. ; Supervision, A.B. and D.B. ; Writing--original draft, D.K. ; Writing--review and editing, D.K., M.-E.K., A.B., D.B. and P.T. All authors have read and agreed to the published version of the manuscript. Not applicable. The MAX-DOAS data used in this study are available from the corresponding author upon request. The air quality monitoring station data are available from the Municipality of Thessaloniki, Department of Environment, upon request. The meteorological data are publicly available from [[https://mete03.geo.auth.gr/stations/orofi_Data.HTML](https://mete03.geo.auth.gr/stations/orofi_Data.HTML)]([https://mete03.geo.auth.gr/stations/orofi_Data.HTML](https://mete03.geo.auth.gr/stations/orofi_Data.HTML)), accessed on 20 January 2023. The S5P/TROPOMI data are publicly available from the Sentinel-5P Product Algorithm (S5P-PAL; 55P-PAL Data portal) and contain modified Copernicus Sentinel data processed by S[&]T.  
We acknowledge support of this work by the project \"PANhellenic infrastructure for Atmospheric Composition and climatE change\" (MIS 5021516), which is implemented under the Action \"Reinforcement of the Research and Innovation Infrastructure\", funded by the Operational Programme \"Competitiveness, Entrepreneurship and Innovation\" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund). Results presented in this work have been produced using the Aristotle University of Thessaloniki (AUTh) High Performance Computing Infrastructure and Resources. M.-E.K. would like to acknowledge the support provided by the AUTh IT Center throughout the progress of this research work. We thank [PERSON] ([EMAIL_ADDRESS]), [PERSON] ([EMAIL_ADDRESS]) and [PERSON] ([EMAIL_ADDRESS]) for the free use of the QDOAS software, [PERSON] the free use of the VLIDORT radiative transfer code package and [PERSON] ([EMAIL_ADDRESS]) for providing the MMF algorithm. The authors declare no conflict of interest.