## 3 Results and Discussion
### Spatial Distribution  
Since in urban environments, especially in large cities, emission sources of various sizes and strengths are distributed in different areas, the NO\({}_{2}\) concentrations can vary rapidly in space. In situ (point measurements) or MAX-DOAS observations at one azimuth (1D systems), that provide information about the NO\({}_{2}\) content in a particular direction, are usually not representative for the NO\({}_{2}\) amounts in the surrounding areas. However, by using MAX-DOAS measurements at multiple azimuths (2D systems), valuable information for the assessment of the NO\({}_{2}\) spatial distribution can be retrieved, allowing for the investigation of possible NO\({}_{2}\) horizontal inhomogeneities and a better characterization of the NO\({}_{2}\) field around the measurement site. In this study, the spatial variability of NO\({}_{2}\) over Thessaloniki was investigated by comparing the NO\({}_{2}\) vertical columns and surface concentrations measured at different azimuth viewing directions (see Figure 1) over the period of study. These viewing geometries can be roughly distinguished to those representing urban/suburban environments, across the city center and pointing to heavy-traffic roads (azimuths of 142\({}^{\circ}\) and 185\({}^{\circ}\)) and to those pointing towards the sea (220\({}^{\circ}\) and 255\({}^{\circ}\)). On short timescales (e.g., on a daily basis), spatial differences in the NO\({}_{2}\) amounts can  
Figure 5: The mean diurnal variability of NO\({}_{2}\) vertical columns and surface concentrations derived by the MAX-DOAS observations for each day of the week for the cold and warm periods. occur. Figure 6 shows an example of the diurnal variability of the NO\({}_{2}\) VCDs during a sunny summer day for all azimuth directions, which are represented by lines of different colors. On this day, 15% of the data were flagged as invalid by the inversion algorithm (MMF) and thus they were rejected from the analysis, leading to some missing values in the time series. The morning peak and the secondary afternoon increase of NO\({}_{2}\) that were related to enhanced traffic emissions, along with the strong reduction of NO\({}_{2}\) at noon due to increased photochemical reactions (see also Section 3.2), are well captured in all viewing directions. However, the NO\({}_{2}\) columns in the SE direction were systematically and significantly larger compared to the other azimuths, especially in the morning, likely related to the inhomogeneous emissions within the city. As already discussed in Section 3.2, the concentration of NO\({}_{2}\) in the boundary layer changes rapidly with time (both in the short- and long-term), prohibiting a detailed investigation of its horizontal distribution. Even on a daily basis, a full measurement sequence for all viewing directions lasts for about 1 hour (~15 min per elevation scan for each azimuth and ~2 min per scan for dark signal measurements). This constrains the time step of measurements at each azimuth angle to at most one per hour. The NO\({}_{2}\) variability over the whole period of study (~2.5 years) was analyzed separately for the cold and warm periods, aiming at the investigation of possible differences in spatial distribution patterns. In order to reduce the effect of the NO\({}_{2}\) temporal variability, the analysis was focused on two periods of different, but less variable, NO\({}_{2}\) levels, i.e., in the morning, when NO\({}_{2}\) is high, and during noon hours, when the NO\({}_{2}\) concentrations reach minimum values. Anthropogenic activities that lead to the emission of tropospheric NO\({}_{2}\) peak at different times during the cold and warm periods (Figure 5). Hence, in our analysis, morning hours were considered as between 06:00 and 09:00 UTC for the cold period and between 04:00 and 07:00 UTC for the warm period. Similarly, the time intervals 10:00-12:00 and 11:00-14:00 UTC were used for the noon hours for the cold and warm periods, respectively.