Author Contributions
Conceptualization, I.S. and I.C.; methodology, I.S., O.T., D.K. and I.C.; software, I.C.; validation, I.S., O.T., D.K. and I.C.; formal analysis, D.K. and I.C.; investigation, I.C.; resources, D.K. and I.C.; data curation, O.T. and I.C.; writing—original draft preparation, I.C. and D.K.; writing—review and editing, O.T., D.K. and I.S.; visualization, I.C.; supervision, D.K. and I.S.; project administration, I.S. All authors have read and agreed to the published version of the manuscript.
Figure 1.
Depiction of the developed air quality monitoring system.
Figure 1.
Depiction of the developed air quality monitoring system.
Figure 2.
Low-cost air quality sensor node.
Figure 2.
Low-cost air quality sensor node.
Figure 3.
The map of the experimental setup (reference and low-cost sensor nodes) at the center of Athens, Greece.
Figure 3.
The map of the experimental setup (reference and low-cost sensor nodes) at the center of Athens, Greece.
Figure 4.
Flow chart of operation of low-cost air quality measuring device.
Figure 4.
Flow chart of operation of low-cost air quality measuring device.
Figure 5.
Node 1, NO2, corrected and reference measurements: (a) Node 1, time series of corrected and reference NO2values; (b) Node 1, correlation of corrected and reference NO2values.
Figure 5.
Node 1, NO2, corrected and reference measurements: (a) Node 1, time series of corrected and reference NO2values; (b) Node 1, correlation of corrected and reference NO2values.
Figure 6.
Node 1, NO2:predicted and reference measurements: (a) Node 1, time- series of predicted and reference NO2values; (b) Node 1, correlation of predicted and reference NO2 values.
Figure 6.
Node 1, NO2:predicted and reference measurements: (a) Node 1, time- series of predicted and reference NO2values; (b) Node 1, correlation of predicted and reference NO2 values.
Figure 7.
Node 1, O3, corrected and reference measurements: (a) Node 1, time series of corrected and reference O3 values; (b) Node 1, correlation of corrected and reference O3 values.
Figure 7.
Node 1, O3, corrected and reference measurements: (a) Node 1, time series of corrected and reference O3 values; (b) Node 1, correlation of corrected and reference O3 values.
Figure 8.
Node 1, O3, predicted and reference measurements: (a) Node 1, time series of predicted and reference O3 values; (b) Node 1, correlation of predicted and reference O3 values.
Figure 8.
Node 1, O3, predicted and reference measurements: (a) Node 1, time series of predicted and reference O3 values; (b) Node 1, correlation of predicted and reference O3 values.
Figure 9.
Node 2, corrected and reference NO2measurements: (a) Node 2, time series of corrected and reference NO2values; (b) Node 2, correlation of corrected and reference NO2 values.
Figure 9.
Node 2, corrected and reference NO2measurements: (a) Node 2, time series of corrected and reference NO2values; (b) Node 2, correlation of corrected and reference NO2 values.
Figure 10.
Node 2, predicted and reference NO2 measurements: (a) Node 2, time series of predicted and reference NO2 values; (b) Node 2, correlation of predicted and reference NO2 values.
Figure 10.
Node 2, predicted and reference NO2 measurements: (a) Node 2, time series of predicted and reference NO2 values; (b) Node 2, correlation of predicted and reference NO2 values.
Figure 11.
Node 2, corrected and reference O3 measurements: (a) Node 2, time series of corrected and reference O3 values; (b) Node 2, correlation of corrected and reference O3 values.
Figure 11.
Node 2, corrected and reference O3 measurements: (a) Node 2, time series of corrected and reference O3 values; (b) Node 2, correlation of corrected and reference O3 values.
Figure 12.
Node 2, predicted and reference O3 measurements: (a) Node 2, time series of predicted and reference O3 values; (b) Node 2, correlation of predicted and reference O3 values.
Figure 12.
Node 2, predicted and reference O3 measurements: (a) Node 2, time series of predicted and reference O3 values; (b) Node 2, correlation of predicted and reference O3 values.
Figure 13.
Node 3, corrected and reference NO2 measurements: (a) Node 3, time series of corrected and reference NO2 values; (b) Node 3, correlation of corrected and reference NO2 values.
Figure 13.
Node 3, corrected and reference NO2 measurements: (a) Node 3, time series of corrected and reference NO2 values; (b) Node 3, correlation of corrected and reference NO2 values.
Figure 14.
Node 3, predicted and reference NO2 measurements: (a) Node 3, time series of predicted and reference NO2 values; (b) Node 3, correlation of predicted and reference NO2 values.
Figure 14.
Node 3, predicted and reference NO2 measurements: (a) Node 3, time series of predicted and reference NO2 values; (b) Node 3, correlation of predicted and reference NO2 values.
Figure 15.
Node 3, corrected and reference O3 measurements: (a) Node 3, time series of corrected and reference O3 values; (b) Node 3, correlation of corrected and reference O3 values.
Figure 15.
Node 3, corrected and reference O3 measurements: (a) Node 3, time series of corrected and reference O3 values; (b) Node 3, correlation of corrected and reference O3 values.
Figure 16.
Node 3, predicted and reference O3 measurements: (a) Node3, time series of predicted and reference O3 values; (b) Node 3, correlation of predicted and reference O3 values.
Figure 16.
Node 3, predicted and reference O3 measurements: (a) Node3, time series of predicted and reference O3 values; (b) Node 3, correlation of predicted and reference O3 values.
Figure 17.
Variation in Kalman filter coefficients A, B, C of the low-cost sensors; (a) variation in Kalman filter coefficients for NO2; (b) variation in Kalman filter coefficients for O3.
Figure 17.
Variation in Kalman filter coefficients A, B, C of the low-cost sensors; (a) variation in Kalman filter coefficients for NO2; (b) variation in Kalman filter coefficients for O3.
Table 1.
Fit estimation data and the mean square error (MSE) of the predicted coefficients (A, B, C).
Table 1.
Fit estimation data and the mean square error (MSE) of the predicted coefficients (A, B, C).
| N1 NO2 | N2 NO2 | N3 NO2 | N1 O3 | N2 O3 | N3 O3 |
---|
Fit to estimation data (%) | 45.47 | 46.00 | 44.00 | 62.14 | 62.11 | 62.29 |
MSE | 79.51 | 77.98 | 83.88 | 141.3 | 141.5 | 140.2 |
Table 2.
Kalman coefficients (A, B, C, D) of each gas sensor.
Table 2.
Kalman coefficients (A, B, C, D) of each gas sensor.
| N1 NO2 | N2 NO2 | N3 NO2 | N1 O3 | N2 O3 | N3 O3 |
---|
A | 0.677951 | 0.6812565 | 0.735013 | 0.861465 | 0.842696 | 0.83215 |
B | −0.00058 | −0.001061 | 0.000461 | −0.00024 | 0.000275 | −0.00026 |
C | −516.399 | −522.8114 | 566.6811 | −503.494 | 473.485 | −521.718 |
Table 3.
MAD of corrected reference values and predicted reference values of each low-cost sensor.
Table 3.
MAD of corrected reference values and predicted reference values of each low-cost sensor.
| N1 NO2 | N2 NO2 | N3 NO2 | N1 O3 | N2 O3 | N3 O3 |
---|
Corrected-Ref | 14.8 | 15.5 | 13.3 | 20.0 | 24.0 | 21.8 |
Predicted-Ref | 12.8 | 13.2 | 12.9 | 21.1 | 22.7 | 23.0 |
Table 4.
MSE of corrected–reference values and predicted–reference values of each low-cost sensor.
Table 4.
MSE of corrected–reference values and predicted–reference values of each low-cost sensor.
| N1 NO2 | N2 NO2 | N3 NO2 | N1 O3 | N2 O3 | N3 O3 |
---|
Corrected-Ref | 0.69 | 0.74 | 0.62 | 1.36 | 1.68 | 2.25 |
Predicted-Ref | 0.55 | 0.66 | 0.55 | 2.26 | 2.42 | 2.33 |
Table 5.
MAPE of corrected–reference values and predicted–reference values of each low-cost sensor.
Table 5.
MAPE of corrected–reference values and predicted–reference values of each low-cost sensor.
| N1 NO2 | N2 NO2 | N3 NO2 | N1 O3 | N2 O3 | N3 O3 |
---|
Corrected-Ref | 0.21 | 0.23 | 0.20 | 0.27 | 0.24 | 0.33 |
Predicted-Ref | 0.19 | 0.22 | 0.19 | 0.46 | 0.45 | 0.46 |
Table 6.
RMSE of corrected–reference values and predicted–reference values of each low-cost sensor.
Table 6.
RMSE of corrected–reference values and predicted–reference values of each low-cost sensor.
| N1 NO2 | N2 NO2 | N3 NO2 | N1 O3 | N2 O3 | N3 O3 |
---|
Corrected-Ref | 1.58 | 1.60 | 1.60 | 0.11 | 0.07 | 0.15 |
Predicted-Ref | 1.67 | 1.79 | 1.80 | 1.40 | 1.38 | 1.39 |
Table 7.
Comparison of the performance of air quality measurement optimization methods.
Table 7.
Comparison of the performance of air quality measurement optimization methods.
Gas | Optimization Method | R2 | Improvement Rate (%) |
---|
| Cross et al. [50] | 0.39 | 325 |
O3 | Zimmerman et al. [51] | 0.92 | 159 |
Our work | 0.82 | 137 |
| Cross et al. [50] | 0.69 | 575 |
ΝO2 | Zimmerman et al. [51] | 0.75 | 277 |
Our work | 0.80 | 200 |