## 3 Results and Discussion
### Diurnal Cycles  
In the boundary layer, NO\({}_{2}\) is a short-lived product, with typical lifetime of a few hours, depending on the season [47], and, as a result, the NO\({}_{2}\) amounts are expected to be highly variable during the day. The diurnal variability of the NO\({}_{2}\) surface concentrations retrieved by the MAX-DOAS and the in situ instrument for all seasons is presented in Figure 3, in order to evaluate the performance of the instruments under different pollution levels and to investigate their diurnal cycles in more detail. The seasonal hourly mean NO\({}_{2}\) concentrations were calculated when at least 100 measurements were available. In this section (and also in Section 3.3), all azimuth viewing directions of the MAX-DOAS (Figure 1) were included in the calculation of the means.  
The in situ instrument reports NO\({}_{2}\) concentrations throughout the day (and also during the night), while the MAX-DOAS can only operate when solar scattered radiation is present. Thus, the secondary NO\({}_{2}\) peak that is evident in the in situ data at \(\sim\)16:00-17:00 UTC during autumn and winter and at -19:00-20:00 UTC during spring and summer cannot be revealed by the MAX-DOAS. However, both instruments show very similar diurnal cycles for all seasons. The Pearson's correlation coefficients (R) for the two datasets are displayed in the plots and they range from 0.90 during winter to 0.98 during spring, indicating that the two systems can equally contribute to air quality monitoring.  
The early morning NO\({}_{2}\) peak (\(\sim\)1.9-2.6 \(\times\) 10\({}^{11}\) molecules/cm\({}^{3}\) in winter, spring and autumn) in the MAX-DOAS data that is found during the rush hours (i.e., between \(\sim\)04:00 and 06:00 UTC) is due to the increased morning traffic emissions and to the accumulated NO\({}_{2}\) in the boundary layer during the night. This morning peak is reduced by 25-30% during summer (reaching \(\sim\)1.61 \(\times\) 10\({}^{11}\) molecules/cm\({}^{3}\)). Afterwards, the NO\({}_{2}\) concentrations rapidly drop as a result of enhanced consumption processes (photochemical destruction). The reduction is stronger and faster in summer and spring due to increased solar radiation. The NO\({}_{2}\) surface concentrations reach minimum values for all seasons between \(\sim\)11:00 and 13:00 UTC (approximately 0.25 and 1.25 \(\times\) 10\({}^{11}\) molecules/cm\({}^{3}\) in summer and winter, respectively), when the sun is high, leading to an increased photolysis rate in addition to the reduced traffic-related emissions. In the afternoon and evening, the photolysis rate weakens and NO\({}_{2}\) begins to accumulate, leading to the secondary peak as mentioned already. This afternoon increase is only marginally captured by the MAX-DOAS during winter and summer due to the lack of sunlight and, hence, observations. The high NO\({}_{2}\) concentrations that were reported in the morning by both instruments and at night by the in situ instrument only are also attributed to the shallow boundary layer due to the absence of solar radiation and the lower air temperatures. Especially during winter, when the boundary layer height in Thessaloniki reaches minimum values [48], the vertical displacement of \(\sim\)100 m of the two sites is no longer negligible. In this case, even though a very good correlation is found (R = 0.9), the MAX-DOAS reports higher morning surface  
Figure 3: The diurnal variability of NO\({}_{2}\) surface concentrations revealed by the MAX-DOAS (black) and the in situ (red) measurements for all seasons. The gray-shaded rectangles indicate the range of SSP/TROPOMI overpass times for Thessaloniki.  
NO\({}_{2}\) concentrations than in the other seasons, with a higher reduction rate between 06:00 to 09:00 UTC compared to the in situ instrument.