## 2 Low-Cost Monitoring Stations and Data Collection
If the acknowledgement of receipt of the packet is successful, a check is made to see if there are any records on the memory card that can be sent and then the operation stream is restarted. In case the acknowledgement of receipt of the packet fails, then the data are stored in the memory card and the loop starts again. The electrochemical sensors used are from Alphasense [41] and use a four-electrode technology (working electrode, auxiliary electrode, counter electrode, reference electrode); the working electrode is exposed to ambient air as the oxidation or reduction of the gas to be measured takes place. The auxiliary electrode is an electrode with the same characteristics as the working electrode, which is placed in an electrolyte and therefore does not come into contact with the target gas. Because of its isolation from external conditions, it serves as a reference for the measurements of the working electrode. The counter electrode balances the reaction of the working electrode, where if the working electrode increases one molecule due to oxidation by the target gas, then the counter electrode must decrease another molecule to generate an equivalent current. The reference electrode operates according to the potential of the working electrode so that it always operates under the correct conditions. When the sensor is exposed to the target gas, the oxidation or reduction phenomenon occurs at the working electrode, which results in the creation of a current flowing from the working electrode to the opposite electrode or vice versa, depending on the phenomenon, with respect to the target gas. The generated current corresponds to the concentration of the target gas and can be used for calibration. An individual sensor board (ISB) circuit, as reported by Alphasense, provides both working electrode and auxiliary electrode measurements in mV. The conversion of the working and auxiliary electrode voltages to the target gas concentration is suggested by Alphasense's Equation (2) [42] which includes the ambient temperature. \[WE_{c}=(WE_{u}-WE_{e})-n_{T}*(AE_{u}-AE_{e}) \tag{2}\]  
where \(WE_{c}\) represents the corrected value of the working electrode, \(WE_{u}\) represents the measured value of the working electrode, \(AE_{u}\) represents the measured value of the auxiliary electrode, \(n_{T}\) represents the ambient temperature coefficient, \(WE_{e}\) represents the electronic zero value of the working electrode, and \(AE_{e}\) represents the electronic zero value of the auxiliary electrode. The values of \(WE_{e}\) and \(AE_{e}\) are given by the manufacturer for each sensor. The \(Gas_{Measurement}\) concentration measurement is given by dividing the corrected output voltage \(WE_{c}\) by \(Sensor_{Sensitivity}\), as shown in Equation (3); the sensor sensitivity is also given for each sensor by the manufacturer. \[Gas_{Measurement}=\frac{WE_{c}}{Sensor_{Sensitivity}} \tag{3}\]  
where \(Gas_{Measurement}\) is the concentration measurement of the gas pollutant, \(WE_{c}\) is the corrected working electrode value of the target gas given by Equation (2), and \(Sensor_{Sensitivity}\) is given by the sensor manufacturer. According to the manufacturer, a function must be applied when calibrating the sensors so that the corrected values of the measurements with this function are close to the reference values. Two correction factors [17] are calculated during the calibration period when colocating the low-cost monitoring station close to official instruments.