## 2 Materials and Methods
### Hardware Design for DYU Air Box  
As shown in Figure 2a, a TIGO ESP32 Wi-Fi module was adopted in the proposed wireless DYU Air Box to aggregate the BME680 temperature sensor (Bosch Sensortec, Kusterdingen, Germany), humidity and IAQ sensor (Tongdy Sensing Technology Corporation, Beijing, China), BH1750 light sensor (Rohm, Kyoto, Japan), SCD30 CO\({}_{2}\) gas sensor (Sensirion, Stifafa, Switzerland), and PMS7003T PM sensor (Panteng Technology, Nanchang, China). The BH1750, BME680, SCD30, and PMS7003T sensing devices were firstly integrated in the TIGO ESP32 shield as shown in Figure 2b which further illustrates a 4-pin 2.54 mm dip header that is assigned to connect the PMS7003T sensor with a UART interface, and three 4-pin 2.54 mm dip headers are arranged for the BH1750, BME680, and SCD30 sensors with the I\({}^{2}\)C interface standard. There is a built-in 1.14-inch ISP display with a resolution of 135 \(\times\) 240 in the TIGO ESP32 Wi-Fi module, which was connected to the standard SPI interface. The PMS7003T PM sensor with the standard Tx/Rx interface and 5 V\({}_{\rm DC}\)/GND connections was connected to GPIO 27/26 and 5 V\({}_{\rm DC}\)/GND of the TIGO ESP32 module. Furthermore, the BH1750, BME680, and SCD30 sensors with the I\({}^{2}\)C interface are connected in parallel with the GPIO 22/21 (SCL/SDA) and the 3.3 V\({}_{\rm DC}\)/GND of the TIGO ESP32 module, respectively. The layout of the DYU Air Box is in the top layer of a PCB board. The shield layout design for the proposed wireless multi-sensor module was carried out using the open-source EasyEDA 6.5.5 software [20], and both top and bottom layers of the resulting PCB layout for the proposed interdisciplinary and heterogeneous sensing module are shown in Figure 2c. As compared to the commercially available PIM485 with Enviro+ for Raspberry Pi, the customized wireless multi-sensor module has the same standard series communication interfaces including all I\({}^{2}\)C, UART, and SPI interfaces. Figure 1: Schematic of wireless multi-sensor EMIoT system on Da-Yeh University campus. The TIGO ESP32 Wi-Fi module is one of the most compact devices in the EspressIF-ESP32 family with a 240 MHz Xtensa 32-bitLX6 microprocessor-based IoT microcontroller unit (MCU) featuring a low-cost, low-power system on a chip (SoC) microcontroller with 4M-byte flash memory and analog, digital, UART, I\({}^{2}\)C, and SPI interfaces [21]. Figure 3a shows a digital 4-in-1 BME680 sensor [22] facilitating temperature, humidity, pressure, and gas measurement, in which a single MEMS die with temperature and humidity sensors acquires the ambient temperature and RH parameters. The MEMS die of the BME680 gas sensor is a metal-oxide-based sensor which detects and measures air quality for concentration level of ethanol and breath volatile organic compound (b-VOC) mixtures by absorption and subsequent oxidation/reduction on its sensitive layer; furthermore, the Bosch 2.4.0.0 Software Environmental Cluster (BSEC) algorithm provides an AQI level by taking pressure, temperature, and humidity parameters into consideration. Sensing signal multiplexer, amplification, and I\({}^{2}\)C/SPI series data communication are implemented by the following application-specific integrated circuit (ASIC). The reading of the temperature sensor is firstly oversampled twice and then goes through a built-in infinite impulse response (IIR) filter which can effectively reduce the bandwidth of the temperature signal and increase the resolution of the digital reading of temperature up to 20 bits. Considering the humidity reading does not fluctuate rapidly, the acquired data of the humidity sensor are directly sampled and converted to a digital reading with 16-bit resolution. It should be noted that the BME680 gas sensor has detection ability for all VOCs/contaminants in the surroundings such as the outgassing of paint, furniture, or garbage. In addition,  
Figure 2: Schematics of wireless DYU Air Box with heterogeneous and multi-sensor module: (**a**) Wiring connection; (**b**) sensor shield; (**c**) PCB layout.