## 3 Results and Discussion
### Spatial Distribution
Figure 7 shows the normalized median NO\({}_{2}\) VCDs and surface concentrations for the four azimuths (\(x\) axis, see also Figure 1). The data were normalized by dividing the medians with the average VCD or surface concentration of all azimuths for the corresponding period, while the error bars indicate the variability for each azimuth as the interquartile range. The top row of panels represents the NO\({}_{2}\) VCDs and the bottom row the NO\({}_{2}\) surface concentrations, while each column refers to a different time period of the day. In the morning, the normalized NO\({}_{2}\) VCDs and surface concentrations along the different azimuths reveal a generally homogenous spatial distribution for the cold period (Figure 7, blue dots). This is probably related to the relatively longer lifetime of NO\({}_{2}\) in the troposphere, due to weaker  
Figure 6: An example of the NO\({}_{2}\) VCDs diurnal variability for all azimuth viewing directions of the MAX-DOAS during a clear-sky summer day (30 July 2022). photochemical destruction, that allows stronger dispersion of the pollutant, leading to better horizontal mixing. Furthermore, meteorological parameters, (e.g., wind direction and speed) can also affect the horizontal dispersion of NO\({}_{2}\). Although a detailed investigation of these effects is beyond the scope of this study, the influence of wind speed and direction is briefly addressed in Appendix B. During the warm period (Figure 7, red squares), the NO\({}_{2}\) VCDs also show a similar (homogenous) horizontal distribution. However, despite the large variability, the NO\({}_{2}\) surface concentrations for the SE direction are generally higher than in the other azimuths, while the lowest concentrations are reported for the SW direction. The SW direction points towards the sea (Figure 1) and the effective horizontal path extends over the bay, where NO\({}_{2}\) is expected to be significantly lower than in the urban environment. Even though the W direction is also towards the sea, it crosses part of the western sector of the urban Thessaloniki, where industrial activities take place, and thus the reported NO\({}_{2}\) concentrations are generally higher than in the SW direction. Finally, the SE direction crosses the entire city center and an urban area of persistent traffic-related NO\({}_{\text{x}}\) emissions, resulting in locally increased NO\({}_{2}\) loads. During noon hours, when the NO\({}_{2}\) levels overall significantly drop, the median NO\({}_{2}\) VCDs and surface concentrations are higher for the SE direction in both periods. This effect is stronger during the warm period due to the enhanced NO\({}_{2}\) photolysis rates and the generally weaker winds that suppress the dispersion, leading to less homogenous horizontal distribution. Similar results have been reported in [46]. For the same reasons that are discussed for the morning hours, the median NO\({}_{2}\) concentrations for the SW direction are significantly lower than the other viewing directions, especially during the warm period. Figure 7: Distribution of the normalized MAX-DOAS measurements of NO\({}_{2}\) VCDs (**top row**) and surface concentrations (**bottom row**) for all azimuth viewing angles during morning (**left column**) and noon (**right column**) hours. Data are presented separately for the cold (blue) and warm (red) periods. The variability of NO\({}_{2}\) in all azimuths is large, due to highly variable emission sources, and thus quantitative estimates cannot be easily unveiled from Figure 7. Hence, the percentage differences of the median NO\({}_{2}\) VCDs and surface concentrations for each azimuth from the average of all directions are summarized in Table 1 for the cold and warm periods. In the morning, the median NO\({}_{2}\) surface concentrations are \(\sim\)20% above average for the SE direction and \(\sim\)15% below average for the SW direction during the warm period.