## 3 Results  
The results of nonlinear regression as a correction factor in ozone (O\({}_{3}\)) and nitrogen dioxide (NO\({}_{2}\)) concentration measurements are presented here. Given that low-cost electrochemical gas quality sensors consisting of four electrodes (working electrode, auxiliary electrode, counter electrode, and reference electrode) were used, the procedure during the initial calibration and normalization of the primary measurements is then presented [34]. The first step is to correct the working electrode value according to manufacturer by the specific properties of each sensor (electronic zero value) and the temperature coefficient. 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 target gas concentration is suggested by Alphasense's Equation (2) [48], 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 second step is to calculate the concentration (ppb) of gas pollutant. 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. The third step is to calculate a calibration function. 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 [15] are calculated during the calibration period when co-locating the low-cost monitoring station close to official instruments. The formula that yields the final corrected values by combining the two correction factors and the sensor measurements is shown in Equation (4)  
\[GAS_{corrected_{Measurement}}=\left(\frac{(GAS_{Measurement}+A)}{B}\right) \tag{4}\]  
where \(GAS_{corrected_{Measurement}}\) is the calibrated gas sensor value, \(Gas_{Measurement}\) is the corrected measurement concentration value of the gas pollutant by Equation (3), where \(A\) represents the level factor, which increases or decreases the measurement values to be corrected, and \(B\) represents the scaling factor, from which the corrected values of measurement are derived. The last step of the procedure is the conversion of ppb to \(\mu\)g/m\({}^{3}\). The initial calibration period was carried out in February 2021, in the field, by co-installing low-cost sensors next to a reference instrument. This process resulted in the determination of the \(A\) and \(B\) coefficients for each low-cost gas sensor.