Calibration of Portable Particulate Matter-Monitoring Device using Web Query and Machine Learning.

BACKGROUND

Monitoring and control of PM are being recognized as key to address health issues attributed to PM. Availability of low-cost PM sensors made it possible to introduce a number of portable PM monitors based on light scattering to the consumer market at an affordable price. Accuracy of light scattering-based PM monitors significantly depends on the method of calibration. Static calibration curve is used as the most popular calibration method for low-cost PM sensors particularly because of ease of application. Drawback in this approach is, however, the lack of accuracy.

METHODS

This study discussed the calibration of a low-cost PM-monitoring device (PMD) to improve the accuracy and reliability for practical use. The proposed method is based on construction of the PM sensor network using Message Queuing Telemetry Transport (MQTT) protocol and web query of reference measurement data available at government-authorized PM monitoring station (GAMS) in the republic of Korea. Four machine learning (ML) algorithms such as support vector machine, k-nearest neighbors, random forest, and extreme gradient boosting were used as regression models to calibrate the PMD measurements of PM. Performance of each ML algorithm was evaluated using stratified K-fold cross-validation, and a linear regression model was used as a reference.

RESULTS

Based on the performance of ML algorithms used, regression of the output of the PMD to PM concentrations data available from the GAMS through web query was effective. The extreme gradient boosting algorithm showed the best performance with a mean coefficient of determination (R) of 0.78 and standard error of 5.0 μg/m, corresponding to 8% increase in R and 12% decrease in root mean square error in comparison with the linear regression model. Minimum 100 hours of calibration period was found required to calibrate the PMD to its full capacity. Calibration method proposed poses a limitation on the location of the PMD being in the vicinity of the GAMS. As the number of the PMD participating in the sensor network increases, however, calibrated PMDs can be used as reference devices to nearby PMDs that require calibration, forming a calibration chain through MQTT protocol.

CONCLUSIONS

Calibration of a low-cost PMD, which is based on construction of PM sensor network using MQTT protocol and web query of reference measurement data available at a GAMS, significantly improves the accuracy and reliability of a PMD, thereby making practical use of the low-cost PMD possible.