## 2 Data and Methodology
### S5p/tropomi  
The TROPOMI satellite instrument is a nadir-viewing imaging spectrometer that is mounted on the Sentinel-5 Precursor (S5P) platform. S5P was launched on 13 October  
Figure 1: The locations of the MAX-DOAS instrument (white star) and the in situ NO\({}_{2}\) measurement site (purple triangle). The four different azimuth viewing directions of the MAX-DOAS observations are represented by the solid red lines. The base map is taken from © Google Maps, [[https://www.google.com/maps](https://www.google.com/maps)]([https://www.google.com/maps](https://www.google.com/maps)), accessed on 20 January 2023.  
2017 and flies in a sun-synchronous orbit with a local overpass time at approximately 13:30 local time. TROPOMI covers a large wavelength range that extends from the UV to the shortwave infrared and provides daily global coverage. The main objective of the Copernicus Sentinel-5P mission is to perform atmospheric measurements with high spatial resolution around the globe. The initial, already high, spatial resolution of TROPOMI was 3.5 \(\times\) 7 km\({}^{2}\) and from 6 August 2019 it was further improved to 3.5 \(\times\) 5.5 km\({}^{2}\). The NO\({}_{2}\) data are continuously validated by the Mission Performance Center Validation Data Analysis Facility (MPC VDAF, [[http://mpc-vdaf.tropomi.eu/](http://mpc-vdaf.tropomi.eu/)]([http://mpc-vdaf.tropomi.eu/](http://mpc-vdaf.tropomi.eu/)), accessed on 20 January 2023). In this study, we used the v2.2 5 SP/TROPOMI NO\({}_{2}\) tropospheric vertical column density dataset that is publicly available via the ESA Sentinel-5P Product Algorithm Laboratory (SSP-PAL) containing modified Copernicus Sentinel data processed by S[&]T ([[https://data-portal.s5p-pal.com/products/no2.html](https://data-portal.s5p-pal.com/products/no2.html)]([https://data-portal.s5p-pal.com/products/no2.html](https://data-portal.s5p-pal.com/products/no2.html)), accessed on 20 January 2023). The overpass data for Thessaloniki cover the period from June 2020 to November 2022. Since clouds have a very strong impact on the column retrievals of all species (including NO\({}_{2}\)), we filtered the TROPOMI data with a quality assurance value (qa_value) > 0.99, a criterion that filters out cloudy pixels for a cloud fraction > 20%. This strict filtering choice was made in order to ensure that all measurements under cloudy conditions were discarded from the analysis, even though the recommendation from the TROPOMI Algorithm Theoretical Basis Document (ATBD) of the tropospheric NO\({}_{2}\) data products ([[http://www.tropomi.eu/data-products/nitrogen-dioxide](http://www.tropomi.eu/data-products/nitrogen-dioxide)]([http://www.tropomi.eu/data-products/nitrogen-dioxide](http://www.tropomi.eu/data-products/nitrogen-dioxide)), accessed on 20 January 2023) is to filter the data with a qa_value > 0.75. We opted for the strict filtering since cloud presence is relatively less frequent in Thessaloniki and thus during the period of study the satellite sampling and the number of collocated TROPOMI and MAX-DOAS measurements was large enough to provide statistically robust comparison and validation results. The NO\({}_{2}\) concentration levels in Thessaloniki are mainly due to local NO\({}_{\rm x}\) emission sources, especially within the city center [40; 41; 42; 43], where the MAX-DOAS system is installed. The relatively larger satellite footprint compared to the MAX-DOAS geometry would mainly result in a smoother perception of the NO\({}_{2}\) concentration. Hence, the TROPOMI tropospheric NO\({}_{2}\) column data are spatially averaged within 5 km from the overpass location.