## 3 Results and Discussion
### Seasonal Variability  
Figure 2 shows the time series of the daily and monthly median NO\({}_{2}\) VCDs (upper panel) and the corresponding surface concentrations (bottom panel) retrieved by the MAX-DOAS for the period of study. The data are aggregated separately for the urban (SE and S) and rural (W and SW) azimuths (blue and green lines with markers, respectively). The gray dots represent the MAX-DOAS daily median values, while the red line with markers indicates the monthly median values, derived from the in situ instrument. The in situ NO\({}_{2}\) time series is plotted against the MAX-DOAS surface NO\({}_{2}\) concentrations only, in order to compare meaningful quantities. The error bars of the monthly median values indicate the monthly variability as the interquartile range (IQR), i.e., the difference between the 75 th and 25 th percentiles. In June 2020 and April 2022, the MAX-DOAS was configured to record spectra only in the SW direction and thus measurements pointing towards urban areas are not available during these months. Here, and in the following sections, the MAX-DOAS and in situ surface concentrations are presented in different axes (due to their different units) to demonstrate the co-variability  
Figure 2: The time series of NO\({}_{2}\) vertical columns (**upper**) and surface concentrations (**bottom**) retrieved by the MAX-DOAS in Thessaloniki, Greece (gray dots represent the daily medians, while the blue and green lines with markers represent the monthly medians for the urban and rural azimuth viewing directions, respectively). The monthly median NO\({}_{2}\) surface concentrations measured by the in situ instrument are shown in red (right ordinate). The error bars of the monthly medians represent the variability as the interquartile range. of the two datasets since a quantitative comparison between the two systems is beyond the scope of this study. It also follows that since these two time series are not temporally synchronized, even though both medians represent the daylight measurements, the monthly median NO\({}_{2}\) concentrations have not necessarily been derived by measurements acquired at the same time intervals during each day. In this work, we mainly aim to conduct a qualitative comparison and investigate whether similar temporal NO\({}_{2}\) variation patterns are captured by the two instruments, indicating that they can equally contribute to air quality monitoring. In a previous study covering a shorter time period [39], a direct 1:1 comparison between MAX-DOAS and in situ NO\({}_{2}\) hourly mean concentrations showed that the MAX-DOAS was low-biased by \(\sim\)55-60%. This was attributed to the fact that the MAX-DOAS probes air masses along the line of sight of a few kilometers, while the air quality station measurements are conducted in situ. A similar comparison of the daily and monthly mean surface concentrations for the whole time period can be found in Appendix A. The NO\({}_{2}\) VCDs and surface concentrations show a similar seasonal cycle, i.e., lower values during summer and higher values during winter. Similar results using MAX-DOAS measurements in urban environments have been reported for other locations (e.g., [18; 44; 45; 46]). In urban environments, NO\({}_{2}\) is mainly distributed in the lowermost troposphere, usually within the first kilometer, since its emission sources are mainly located close to the ground (e.g., road transport emissions). Thus, the NO\({}_{2}\) VCDs are mostly affected by the concentration levels in the lowermost layers of the troposphere and hence the vertical columns and surface concentrations are expected to reveal similar patterns. The seasonal NO\({}_{2}\) variability captured for the urban and rural azimuths is similar for the whole period, with average differences (rural-urban) of about \(-\)5% for the VCDs and \(-\)18% for the surface concentrations.