## 1 Introduction  
Atmospheric pollution and air quality are important environmental and research issues worldwide. On a global scale, nitrogen dioxide (NO\({}_{2}\)) is considered a major tropospheric gaseous pollutant in most urban and industrial environments, strongly affecting air quality and human health [1]. NO\({}_{2}\) plays a key role in the photochemistry of the troposphere [2], acting as a precursor in the formation of tropospheric ozone (O\({}_{3}\)) and, hence, it indirectly affects the radiative forcing of the atmosphere [3]. In the boundary layer, NO\({}_{2}\) is a short-lived pollutant that is mainly produced by the oxidation of nitrogen monoxide (NO), and it is emitted in the troposphere as a result of both natural and anthropogenic processes. The primary natural sources are soil emissions and lightning, while the main anthropogenic sources include biomass burning and fossil fuel combustion from domestic heating sources and motor vehicles [4; 5]. Given the influence of NO\({}_{2}\) on atmospheric pollution and air quality, the importance for its accurate detection, the determination of its emission sources and the investigation of its spatio-temporal variability is self-evident. Thus, continuous monitoring of NO\({}_{2}\) concentrations has been widely performed over the past few decades using both ground-based measurements and satellite observations. Over Thessaloniki, Greece, air quality monitoring and trace gas measurements are primarily performed by ground-based in situ instruments, with some of them reporting to the European Environmental Agency [6]. These continuous in situ measurements provide valuable information on various atmospheric pollutants (including NO\({}_{2}\)) at fixed sampling stations, but their limited number does not allow a representative spatial coverage over the area of Thessaloniki. Furthermore, most of the stations have their sampling inlets installed close to the ground (a few meters above the surface) and are consequently mostly affected by local traffic emissions. Thus, these measurements are usually not representative of the average pollution levels in the local boundary layer and, moreover, they cannot provide information for the pollutants' vertical distribution. Ground-based Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a remote sensing measurement technique for the detection of aerosols and trace gases in the troposphere that represents a significant enhancement of the well-established zenith scattered sunlight DOAS instruments, which are mainly sensitive to stratospheric absorbers [7]. MAX-DOAS systems have been further developed during the past few decades and are widely used for the retrieval of tropospheric trace gas concentrations with characteristic narrowband absorption structures in the ultraviolet (UV) and visible ranges [8; 9; 10; 11; 12]. MAX-DOAS is based on the analysis of scattered radiation spectra measured at multiple elevation viewing directions using the differential optical absorption spectroscopy (DOAS) technique [13]. MAX-DOAS measurements are highly sensitive to various absorbers, such as NO\({}_{2}\), HCHO, H\({}_{2}\)O, BrO, CHOCHO and SO\({}_{2}\) in the lowest few kilometers of the atmosphere. Information about the vertical distribution of a trace gas can be obtained from a single elevation scan by applying proper inverse modeling approaches that combine the measurements with Radiative Transfer Model (RTM) simulations (e.g., [14; 15; 16; 17]). The capability of retrieving trace gas vertical profiles, vertical column densities and surface concentrations with a generally high accuracy and temporal resolution makes the MAX-DOAS systems essential for air quality monitoring, supplementing point measurements of in situ instruments and satellite observations. Ground-based MAX-DOAS systems have been used in numerous recent publications for the assessment of air pollution, especially in urban environments, and for understanding of the physical and chemical processes in the boundary layer (e.g., [18; 19]).