## 2 Data and Methodology  
### MAX-DOAS Observations  
Monitoring of trace gases in the troposphere is routinely performed at the Laboratory of Atmospheric Physics (40.634\({}^{\circ}\) N, 22.956\({}^{\circ}\) E, 60 m above sea level), in the Aristote University of Thessaloniki (LAP.AUTH), Greece, by multiple MAX-DOAS instruments. These systems have participated in various intercomparison and validation campaigns, such as the Cabauw Intercomparison of Nitrogen Dioxide Measuring Instruments 2 (CINDI-2, [[http://www.tropomi.eu/data-products/cindi-2/](http://www.tropomi.eu/data-products/cindi-2/)]([http://www.tropomi.eu/data-products/cindi-2/](http://www.tropomi.eu/data-products/cindi-2/)), accessed on 20 January 2023) and the TROPOomi valLdation eXperiment 2019 (TROLIX'19, [[https://ruisdael-observatory.nl/trolix19-tropomi-validation-experiment-2019/](https://ruisdael-observatory.nl/trolix19-tropomi-validation-experiment-2019/)]([https://ruisdael-observatory.nl/trolix19-tropomi-validation-experiment-2019/](https://ruisdael-observatory.nl/trolix19-tropomi-validation-experiment-2019/)), accessed on 20 January 2023), and they contribute data to global projects and networks, e.g., the Fiducial Reference Measurements for Ground-Based DOAS Air-Quality Observations (FRM4 DOAS, [https://ferm4 doas.aeronomie.be/](https://ferm4 doas.aeronomie.be/), accessed on 20 January 2023), the Quality Assurance for Essential Climate Variables (QA4 ECV, [[http://www.qa4](http://www.qa4) ecv.eu/]([http://www.qa4](http://www.qa4) ecv.eu/), accessed on 20 January 2023) and the Network for Detection of Atmospheric Composition Change (NDACC, [[https://ndacc.larc.nasa.gov/](https://ndacc.larc.nasa.gov/)]([https://ndacc.larc.nasa.gov/](https://ndacc.larc.nasa.gov/)), accessed on 20 January 2023). The measurement site is located in an urban, moderately polluted environment, at the center of the city of Thessaloniki. The MAX-DOAS instruments have been gradually upgraded over the past decade by making changes to their design, using newer and improved spectrographs and by updating the operating software. Additionally, the processing and calibration of the recorded spectra have been further developed in order to increase their accuracy, improve their stability over time and provide data of higher quality. In this study, the spatio-temporal variability of NO\({}_{2}\) over Thessaloniki is assessed by analyzing spectra recorded by a 2D MAX-DOAS system, Phaethon, which operates at LAP.AUTH since June 2020, using the DOAS technique. The entrance optics of the instrument are mounted on a two-axis tracker that allows for a full azimuthal and elevation rotation (0-360\({}^{\circ}\)). The system is configured to perform elevation scans, from the zenith to near the horizon, at different azimuth directions in the southern sector, as illustrated in Figure 1, since the other azimuths are partly or fully blocked by buildings of the city and the university campus. The elevation scans consist of measurements of scattered radiation spectra at the elevation angles of 30, 15, 12, 10, 8, 6, 5, 4, 3, 2 and 1\({}^{\circ}\), with about 1 min integration time per angle, in addition to a zenith-sky measurement (90\({}^{\circ}\)) prior to each scan that is used as a reference Fraunhofer spectrum (FRS) for the determination of the trace gas differential Slant Column Densities (dSCDs). The recorded spectra are analyzed using the QDOAS (version 3.2, September 2017) software that is developed by BIRA-IASB ([[https://uv-vis.aeronomie.be/software/QDOAS/](https://uv-vis.aeronomie.be/software/QDOAS/)]([https://uv-vis.aeronomie.be/software/QDOAS/](https://uv-vis.aeronomie.be/software/QDOAS/)), accessed on 20 January 2023). The NO\({}_{2}\) dSCDs are retrieved in the visible range, using the spectral fitting window 425-490 nm, following the recommendations from the CINDI-2 campaign ([PERSON] et al., 2018).