## 5 Summary and Conclusions  
In this study, the spatio-temporal variability of tropospheric NO\({}_{2}\) over Thessaloniki, Greece, was investigated using vertical column and surface concentration data that were retrieved by MAX-DOAS observations during the period June 2020-November 2022. The NO\({}_{2}\) seasonal, diurnal and weekly cycles were investigated and the surface concentrations that were retrieved by the MAX-DOAS were compared with in situ data from a nearby air quality station, which is mostly unaffected by local emissions and, hence, is more representative of the average NO\({}_{2}\) levels in the local boundary layer. The comparison of the two ground-based datasets was conducted in order to evaluate the performance of the MAX-DOAS instrument in the context of air quality monitoring in Thessaloniki. The tropospheric NO\({}_{2}\) VCDs and surface concentrations, retrieved by the MAX-DOAS, show a similar seasonal cycle, i.e., lower values during summer and higher values during winter. The higher columns in winter are due to enhanced anthropogenic emissions (e.g., domestic heating) and the longer lifetime of NO\({}_{2}\) in this season. Hourly mean NO\({}_{2}\) surface concentrations measured by the MAX-DOAS reveal a clear diurnal pattern with an early morning peak during the rush hours, followed by a reduction around noon as a result of stronger photolysis rates. The increased morning concentrations are due to the enhanced traffic-related NO\({}_{x}\) emissions and the accumulation of NO\({}_{2}\) in the boundary layer during the night. The morning peak during summer is significantly reduced (by approximately 25-30%) compared with the other seasons. Additionally, the NO\({}_{2}\) day of the week variability and the weekend effect were investigated. The NO\({}_{2}\) surface concentrations retrieved by the MAX-DOAS were significantly lower on the weekend (by 20-30%), especially on Sundays (by 30-40%), compared with the working days. In all cases, the NO\({}_{2}\) temporal variations that were captured by the MAX-DOAS are in agreement with the in situ measurements with Pearson's correlation coefficients ranging from 0.90 to 0.99, indicating that the two systems are equally capable of monitoring the variability of NO\({}_{2}\) over Thessaloniki. The NO\({}_{2}\) spatial variability was investigated by comparing the concentrations reported by the MAX-DOAS for four azimuth viewing directions. In order to reduce the effect of the NO\({}_{2}\) temporal variability, the analysis was conducted separately for the morning hours, when the NO\({}_{2}\) levels are high, and for noon hours, when the NO\({}_{2}\) concentrations reach minimum values. On a long timescale (i.e., during the 30 months in this study), the spatial  
Figure 10: The consolidated comparison of tropospheric NO\({}_{2}\) vertical columns (**a**) between the MAX-DOAS and 55P/TROPOMI for the daily (blue) and monthly mean (red) collocated measurements and the frequency distribution (**b**) of their absolute differences. NO\({}_{2}\) variability is masked by the strong short-term temporal variations. However, despite the large variability, higher NO\({}_{2}\) concentrations of up to 25% were found for the SE azimuth that crosses the entire city center and extends over an urban area compared with azimuths towards the sea (e.g., SW). Differences between the SE and SW directions were statistically significant at the 95% confidence level for all cases, except for the VCDs in the morning. The tropospheric NO\({}_{2}\) VCDs in Thessaloniki, retrieved by the MAX-DOAS, were compared with collocated measurements of the TROPOMI instrument on board the Sentinel-5P satellite, aiming at the validation of the satellite product against the well-established ground-based observations. The comparisons were conducted for all seasons in order to investigate any seasonal effects and to evaluate their agreement under both low and high NO\({}_{2}\) concentration levels.