## 4 Results
### \(O_{3}\) Sensors  
For O\({}_{3}\) (O\({}_{3}\)), the noncorrected sensor values were plotted against the reference values to identify the degree of correlation. Figures 11-13 show both time series and scatter plots between the measurements from low-cost and reference sensors. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **NO\({}_{2}\)** \\ **Sensors** \\ \end{tabular} } & \multicolumn{2}{c}{**Before LASSO Regression**} & \multicolumn{2}{c}{**After LASSO Regression**} \\ \cline{2-5}  & **Linear Coefficient** & \(\mathbf{R^{2}}\) & **Linear Coefficient** & \(\mathbf{R^{2}}\) \\ \hline N1 & 0.8827 & 0.23 & 1.0396 & 0.27 \\ N2 & 0.8598 & 0.22 & 0.9321 & 0.26 \\ N3 & 0.9256 & \(-\)0.028 & 1.081 & 0.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Linear coefficient and correlation degree (\(\mathbf{R^{2}}\)), before and after LASSO regression of NO\({}_{2}\) low-cost sensors’ measurements with respect to reference measurements. Figure 10: Node 3, NO\({}_{2}\) corrected measurements with respect to reference measurements. (**a**) Time series of N3 (NO\({}_{2}\)) corrected and reference measurements; (**b**) Scatterplot of N3 (NO\({}_{2}\)) corrected and reference measurements. Figure 9: Node 2, NO\({}_{2}\) corrected measurements with respect to reference measurements. (**a**) Time series of N2 (NO\({}_{2}\)) corrected and reference measurements; (**b**) Scatterplot of N2 (NO\({}_{2}\)) corrected and reference measurements. Figure 11: Node 1, O\({}_{3}\) noncorrected measurements with respect to reference measurements. (**a**) Time series of N1 (O\({}_{3}\)) noncorrected and reference measurements; (**b**) Scatterplot of N1 (O\({}_{3}\)) noncorrected and reference measurements. Figure 12: Node 2, O\({}_{3}\) noncorrected measurements with respect to reference measurements. (**a**) Time series of N2 (O\({}_{3}\)) noncorrected and reference measurements; (**b**) Scatterplot of N2 (O\({}_{3}\)) noncorrected and reference measurements. Figure 13: Node 3, O\({}_{3}\) noncorrected measurements with respect to reference measurements. (**a**) Time series of N3 (O\({}_{3}\)) noncorrected and reference measurements; (**b**) Scatterplot of N3 (O\({}_{3}\)) noncorrected and reference measurements. To determine the optimal value of the \(\lambda\) coefficient, cross-validation of the LASSO fit deviation, calculated for each sensor, between the low-cost sensor measurements and the reference O\({}_{3}\) measurements was performed. Figures 14a, 15a and 16a show the O\({}_{3}\) measurements' cross-validated deviance diagram of each low-cost sensor (N1, N2, N3) and reference, respectively. Figures 14b, 15b and 16b show the scatter plot between the O\({}_{3}\) measurements of each low-cost sensor (N1, N2, N3) and reference. The scatter plot also shows the trend line for \(\lambda\) = 0 and for \(\lambda\) = 1.4. Figure 14: O\({}_{3}\) N1, cross-validated deviance diagram and scatter plot with different values of \(\lambda\). (**a**) The cross-validated deviance diagram of O\({}_{3}\) measurements between low-cost N1 and reference sensors; (**b**) Scatter plot of O\({}_{3}\) measurements between low-cost N1 and reference for \(\lambda\) = 0 and \(\lambda\) = 1.4. Figure 15: O\({}_{3}\) N2, cross-validated deviance diagram and scatter plot with different values of \(\lambda\). (**a**) The cross-validated deviance diagram of O\({}_{3}\) measurements between low-cost N2 and reference sensors; (**b**) Scatter plot of O\({}_{3}\) measurements between low-cost N2 and reference for \(\lambda\) = 0 and \(\lambda\) = 1.4. Figure 16: O\({}_{3}\) N3, cross-validated deviance diagram and scatter plot with different values of \(\lambda\). (**a**) The cross-validated deviance diagram of O\({}_{3}\) measurements between low-cost N3 and reference sensors; (**b**) Scatter plot of O\({}_{3}\) measurements between low-cost N3 and reference for \(\lambda=0\) and \(\lambda=1.4\). Table 3 shows the values of the \(\lambda\) and B coefficients of the LASSO regression for each low-cost O\({}_{3}\) sensor.