## Appendix B Prevailing Meteorological Conditions over Thessaloniki  
This section describes briefly the dominant wind patterns over Thessaloniki for the period of study (June 2020 to November 2022). Wind speed and direction (averaged over 10 min intervals) are continuously monitored by a meteorological station that is installed on the rooftop (\(\sim\)25 m above ground) of the Department of Meteorology and Climatology building, which is located inside the University campus at a horizontal distance of about 300 m from the location of the MAX-DOAS instrument. The wind data are publicly available via the corresponding website ([[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).  
Figure 10 shows the wind rose diagrams over Thessaloniki for summer and winter. In summer, the prevailing wind directions are S-SW and are usually characterized by low wind speeds (on average \(<\) 3 m/s), while northern winds are less frequent. The presence of southern winds is a result of the coastal orientation of the city (Figure 1) and the temperature difference between the land and the sea (which is more pronounced during the warm period due stronger solar radiation) that allow the formation of sea breeze circulation [57; 58; 59]. The wind pattern is different in winter with the dominant winds blowing from N-NW directions being generally stronger (reaching speeds of over 8 m/s) than in summer. According to [57], the wind patterns in the cold period are caused by the combination of low-pressure systems over the Mediterranean Sea and anticyclones over central Europe.  
The variability of the NO\({}_{2}\) concentration levels is not only related to the spatial distribution of its emission sources, but it is also associated with certain meteorological parameters, such as wind direction and speed, since the latter significantly contribute to the vertical and horizontal dispersion of air pollutants. The effect of wind speed on the tropospheric vertical columns and surface concentrations of NO\({}_{2}\) in Thessaloniki for the two dominant wind directions, N-NW and S-SW, is depicted in Figure A3. For northern wind regimes, the NO\({}_{2}\) VCDs and surface concentrations have similar variation patterns, i.e., higher NO\({}_{2}\) amounts are reported for low wind speeds, followed by a clear gradual reduction with increasing wind speed. This is expected, since such wind conditions contribute more efficiently to the dispersion of NO\({}_{2}\), which is transported from the urban area towards the sea. For southern winds, a reduction in the NO\({}_{2}\) levels is also evident for low to moderate wind speeds (up to 4 m/s); however, a non-negligible enhancement is found for higher wind speeds (>4 m/s), especially for the surface concentrations. This finding is consistent with similar results in [58] and [40]. According to these studies, given the topography of the greater area of Thessaloniki, such conditions favor the accumulation of air pollutants. Furthermore, these wind-direction-related patterns of NO\({}_{2}\) could also be associated with larger-scale NO\({}_{2}\) transport events from southern Greece; however, additional investigation is required for the determination of NO\({}_{2}\) emission sources and the assessment of NO\({}_{2}\) transport phenomena, which is beyond the scope of this study.