## 4 Discussion
In addition, methodology scaling was followed to investigate the reliability of the corrected measurements through the nonlinear regression approach, which encompasses both time scaling and the seasonality scale. The evaluation of the time scaling of the experiment involved applying the nonlinear regression approach for a period of one and a half months following the main experiment (i.e., from 14 May 2021 to 31 June 2021) to all low-cost sensors. Indicative results are presented in the figures below. For simplicity and given that all nodes show high correlation with each other. Figure 17 shows the time-series and scatter plots of non-corrected NO\({}_{2}\) measurements from node N1 and reference measurements, during the time period from 14 May 2021 to 31 June 2021. Correspondingly, Figure 18 shows the time-series and scatter plots of NO\({}_{2}\) corrected measurements from N1 and reference measurements, for the time period of 14 May 2021 to 31 June 2021. In fully compliance and correspondence, Figure 19 shows the time-series and scatter plots of O\({}_{3}\) non-corrected measurements of node N1 and reference measurements, during the time period from 14 May 2021 to 31 June 2021. Respectively, Figure 20 shows the time-series and scatter plots of O\({}_{3}\) corrected measurements of node N1 and reference measurements, during time period from 14 May 2021 to 31 June 2021. In order to determine whether seasonality affects the results, the nonlinear regression approach was applied during the winter month of December 2021 (i.e., from 1 December 2021 to 31 December 2021), to all low-cost sensors, where indicative results are shown in the figures below. Figure 21 shows the time-series and scatter plots of NO\({}_{2}\) non-corrected measurements of node N1 and reference measurements, for the time period of 1 December 2021 to 31 December 2021. Figure 22 shows the corresponding time-series and scatter plots of NO\({}_{2}\) corrected measurements for node N1 and reference measurements, for the time period from 1 December 2021 to 31 December 2021. Figure 17: Time-series and scatter plots of NO\({}_{2}\) non-corrected measurements of node N1 and reference measurements, for the time period of 14 May 2021 to 31 June 2021. (**a**) Time series of N1 (NO\({}_{2}\)) non-corrected and reference measurements, (**b**) Scatterplot of N1 (NO\({}_{2}\)) non-corrected and reference measurements. Figure 19: Time-series and scatter plots of O\({}_{3}\) non-corrected measurements of node N1 and reference measurements, for the time period of 14 May 2021 to 31 June 2021. (**a**) Time series of N1 (O\({}_{3}\)) non-corrected and reference measurements, (**b**) Scatterplot of N1 (O\({}_{3}\)) non-corrected and reference measurements. Figure 20: Time-series and scatter plots of O\({}_{3}\) corrected measurements of node N1 and reference measurements, for the time period from 14 May 2021 to 31 June 2021. (**a**) Time series of N1 (O\({}_{3}\)) corrected and reference measurements, (**b**) Scatterplot of N1 (O\({}_{3}\)) corrected and reference measurements. Figure 23 shows the time-series and scatter plots of O\({}_{3}\) non-corrected measurements of node N1 and reference measurements for the time period of 1 December 2021 to 31 December 2021. Figure 24 shows the time-series and scatter plots of O\({}_{3}\) corrected measurements of node N1 and reference measurements for the time period from 1 December 2021 to 31 December 2021. For reasons of completeness, Table 9 summarizes the above presented results and specifically the R\({}^{2}\) and the \(a\), \(b\) coefficients (of linear equation \(y=ax+b\)) behavior before and after the application of the nonlinear regression model for all Nodes, N1, N2 and N3, in order to make clear the similar behavior of all sensing devices. Figure 21: Time-series and scatter plots of NO\({}_{2}\) non-corrected measurements of node N1 and reference measurements, for the time period of 1 December 2021 to 31 December 2021.