## 2 Low-Cost Monitoring Stations and Data Collection  
Three low-cost gas monitoring stations [17] were developed in order to collect data. Initially, the three stations were installed next to official measuring points in order to perform initial calibration according to the manufacturer's instructions, as will be described in this section. This procedure was run during February 2021. Sequentially, the monitoring stations were allowed to run on the field in parallel to the official stations and the collected data during the period 14 April-13 May were studied in the frame of this work. Details of the above described procedure follow below in Sections 2 and 3. The low-cost stations are identified by a unique identifier (IDs 1, 2, 3). The study, design, and implementation of the gas-monitoring stations was carried out at the Electronic Devices and Materials Laboratory (EDML) of the department of Electrical and Electronics Engineering at the University of West Attica in Greece. Each monitoring station (Figure 1) includes a microprocessor, gas pollutant sensors, peripheral auxiliary units, and a data transmission device (Wi-Fi, GPRS). The Nucleo STM-f091 RC, STMicroelectronics, Schiphol, Netherlands, microprocessor, which offers a high processing speed and low power consumption, was used as the CPU, while electrochemical sensors, for NO\({}_{2}\) (NO2-B43F) and O\({}_{3}\) (OX-B431) manufactured by Alphasense [38], were used as gas pollutant sensors. In addition, an optical technology sensor was used for microparticle measurements: Plantower (PMS5003) [39]; while a barometric sensor (BME280) was used for barometric data acquisition. Finally, the auxiliary peripheral units are a GPS for installation location and data timing, a mini-UPS in case of occasional power loss, and a Wi-Fi or GPRS networking device for data transmission. Data are stored on an SD card in case of network transmission loss and sent when the connection is restored. To ensure the reliability and reproducibility of the results, three identical low-cost air quality monitoring stations were installed in a densely populated neighborhood in the city center of Athens, at a height of 6-8 m above ground level, surrounded by main roads with heavy vehicular traffic. The measurements from the low-cost sensors were compared with the corresponding NO\({}_{2}\) and O\({}_{3}\) reference values obtained from the Ministry of Environment  
Figure 1: Low-cost gas and particle matter monitoring station. and Energy of Greece (PERPA) [40]. The reference scientific air quality monitoring station has been installed at the Ministry of Environment and Energy of Greece facilities in the center of Athens. Since both the locations of the low-cost air quality monitoring stations and the reference station are within the same area, they have the same air quality conditions. In particular, the installed air pollutant measurement devices are for O\({}_{3}\) the HORIBA APOA-360 automatic analyzer with ultraviolet absorption measurement method, and for NO\({}_{2}\) the HORIBA APNA-360 automatic analyzer with a chemiluminescence measurement method. The pollutants are measured continuously throughout the 24 h period. The response time of the automatic analyzers is one minute, while the hourly average pollution values are calculated hourly. The operation of low-cost metering stations is carried out in accordance with the following sequence. Starting the station in operation, the timing signals, the analog ports, the digital input/output ports, and the communication buses with external devices (SPI, I2C, UART buses) are initialized on the CPU. During initialization, parameters such as node ID, measurement data, and GPS data are also determined. Then, the data structure packet is created for transmission, and finally the network connection (Wi-Fi or GPRS) is established. In the operation loop, measurements are taken every 10 s. In a 5 min period, the average of the measurements of each sensor together with the data from the GPS are structured into a transmission packet and sent to the server.