## 3 Results and Discussion
### Day of the Week Variability  
NO\({}_{2}\) is emitted in the troposphere by anthropogenic activities (e.g., industrial and road transport emission sources) that usually follow a weekly cycle. Normally, such activities are enhanced during the working days, and they are significantly reduced at weekends, leading to strong weekday-to-weekend variations of NO\({}_{2}\) concentrations in urban environments, a phenomenon that is known as the \"weekend effect\". The weekend effect has been investigated in the past using both ground-based MAX-DOAS data (e.g., [18; 19; 42; 49; 50]) and satellite observations (e.g., [51; 52; 53]). In these studies, a significant reduction of the NO\({}_{2}\) concentration in weekends is reported, with a clear Sunday minimum of up to 50% compared to the working days. Here, we investigate the day of week variability of NO\({}_{2}\) surface concentrations in Thessaloniki using MAX-DOAS observations and in situ measurements for comparison. Figure 4 shows the average weekly cycles of NO\({}_{2}\) surface concentrations for all seasons measured by both instruments. The two datasets reveal similar weekly patterns with Pearson's correlation coefficients that range between 0.93 (in spring) and 0.99 (in autumn). The weekend effect for all seasons is investigated by comparing the mean NO\({}_{2}\) surface concentrations on the weekends (Saturday and Sunday) and the traditional resting day of Sunday alone with the mean concentration during the working days (Monday to Friday). Significantly lower NO\({}_{2}\) concentrations are found for both datasets during weekends for all seasons, i.e., the MAX-DOAS (in situ) reports a reduction of 19% (18%) in winter, 29% (31%) in spring, 30% (28%) in summer and 30% (27%) in autumn. The reduction rate is even larger on Sundays, i.e., 30% (29%) in winter, 37% (40%) in spring, 40% (39%) in summer and 43% (41%) in autumn. The two systems reveal that the weekend effect is more evident during the warm period and less distinct in winter. The stronger reduction in the warm period is associated with the shorter atmospheric lifetime of NO\({}_{2}\), due to higher photochemical destruction rates, resulting in less accumulation in the working days. Additionally, the expansion of the boundary layer to higher altitudes in summer due to stronger solar radiation allows for a greater dispersion of NO\({}_{2}\), leading to lower concentrations at the lowermost layers compared with the cold period. A reduction of the NO\({}_{2}\) vertical column with increasing  
Figure 4: The day of week variability of NO\({}_{2}\) surface concentrations revealed by the MAX-DOAS (black) and the in situ (red) measurements for all seasons. boundary layer height in Thessaloniki, especially during summer, was also found by [43] using MAX-DOAS measurements and chemical transport model simulations. Figure 5 shows the diurnal variation of tropospheric NO\({}_{2}\) VCDs (top row) and surface concentrations (bottom row) in a weekly cycle retrieved by the MAX-DOAS over Thessaloniki for each day of the week. The analysis was conducted separately for the cold period (October-February, left column) and the warm period (May-September, right column). The variability of the NO\({}_{2}\) levels in Thessaloniki from Monday to Friday is not very pronounced; however, the weekend effect is strong for both time periods. On Saturdays (dark blue line in Figure 5), the NO\({}_{2}\) amounts are generally lower than on the working days but are of comparable magnitude in the early morning and late afternoon. The strongest reduction of the NO\({}_{2}\) vertical column and surface concentrations is found on Sundays (purple line in Figure 5) throughout the day. This is mainly related to the decreased human activities on the resting day and especially to the absence of NO\({}_{x}\) emissions from heavy road transport to and from work.