## 1 Introduction
Additionally, data derived from MAX-DOAS instruments are widely used for comparison and validation of multiple satellite products, including tropospheric NO\({}_{2}\) (e.g., [20; 21; 22]). Even though ground-based MAX-DOAS is a well-established remote sensing technique for the determination of trace gas concentration levels in the troposphere and the application of such instruments to air quality monitoring is widespread, the sparsity of such systems on a global scale inhibits large-scale monitoring of NO\({}_{2}\). Satellite instruments do not suffer from this limitation; they provide daily global coverage and, thus, their importance to air quality, climate change and understanding of the dynamics in the boundary layer is widely recognized. Satellite observations contribute to the continuous monitoring of NO\({}_{2}\) on both regional and global scales, estimation of NO\({}_{x}\) emission sources and provide valuable information for the understanding of NO\({}_{2}\) transport phenomena. Over the past decades, NO\({}_{2}\) has been continuously monitored from space by multiple satellite instruments, such as the Global Ozone Monitoring Experiment (GOME) on ERS-2 [23], Scanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) on ENVISAT [24], Ozone Monitoring Instrument (OMI) on EOS-Aura [25], Global Ozone Monitoring Experiment-2 (GOME-2) on MetOp A, B and C platforms [26] and, most recently, since October 2017 from the TRPOPsheric Monitoring Instrument (TRPOMI) on board Sentinel-5 Precursor (S5P) [27]. The S5P/TROPOMI NO\({}_{2}\) observations have already demonstrated their monitoring capabilities in various air quality-related fields, such as estimating NO\({}_{x}\) emissions at the urban scale [28; 29] and evaluating the effect of the recent COVID-19 pandemic on NO\({}_{2}\) levels [30; 31], while an extensive validation against global ground-based measurements has been presented in [22]. The main disadvantages of such satellite instruments are their low temporal sampling with one or two overpasses per day at any given location, which does not allow the observation of diurnal cycles, as well as their coarse spatial resolution. With respect to the spatial sampling, the pixel size of the space borne instruments has been greatly improved, initially starting from several hundreds of kilometers (40 \(\times\) 320 km\({}^{2}\) for GOME) to a few kilometers (13 \(\times\) 24 km\({}^{2}\) for OMI) and currently to 3.5 \(\times\) 5.5 km\({}^{2}\) for TROPOMI. This improvement, joined by the continuous efforts in improving the NO\({}_{2}\) data quality by updating the retrieval algorithms of 5P/TROPOMI, highly increases the usability of this dataset in numerous different scientific and research fields. The main aims of this study are to assess the ability of the MAX-DOAS measurements to monitor the air quality in Thessaloniki in terms of tropospheric NO\({}_{2}\) by comparison with in situ data, and to validate the corresponding SSP/TROPOMI product, accounting also for the spatial features detected by the MAX-DOAS. By establishing the linkages among the three different sources of information on NO\({}_{2}\) in the area, we will set the scene for a future synergistic use of these datasets to achieve a more comprehensive characterization of NO\({}_{2}\) in Thessaloniki. This article is structured as follows: In Section 2, a description is given for the instruments and data that are used in this study. In Section 3, the results of the NO\({}_{2}\) spatio-temporal variability using MAX-DOAS observations are presented along with comparisons with in situ measurements. In Section 4, the comparison and validation of SSP/TROPOMI observations using ground-based MAX-DOAS data from Thessaloniki are discussed, and in Section 5 the main conclusions of this study are summarized.