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Background  Portable direct-reading instruments by light-scattering method are increasingly used in airborne fine particulate matter (PM 2.5 ) monitoring. However, there are limited calibration studies on such instruments by applying the gravimetric method as reference method in field tests.    Methods  An 8-month sampling was performed and 96 pairs of PM 2.5  data by both the gravimetric method and the simultaneous light-scattering real-time monitoring (QT-50) were obtained from July, 2015 to February, 2016 in Shanghai. Temperature and relative humidity (RH) were recorded. Mann-Whitney U nonparametric test and Spearman correlation were used to investigate the differences between the two measurements. Multiple linear regression (MLR) model was applied to set up the calibration model for the light-scattering device.    Results  The average PM 2.5  concentration (median) was 48.1μg/m 3  (min-max 10.4–95.8μg/m 3 ) by the gravimetric method and 58.1μg/m 3  (19.2–315.9μg/m 3 ) by the light-scattering method, respectively. By time trend analyses, they were significantly correlated with each other (Spearman correlation coefficient 0.889, P<0.01). By MLR, the calibration model for the light-scattering instrument was Y(calibrated) = 57.45 + 0.47 × X(the QT – 50 measurements) – 0.53 × RH – 0.41 × Temp with both RH and temperature adjusted. The 10-fold cross-validation R 2  and the root mean squared error of the calibration model were 0.79 and 11.43 μg/m 3 , respectively.    Conclusion  Light-scattering measurements of PM 2.5  by QT-50 instrument overestimated the concentration levels and were affected by temperature and RH. The calibration model for QT-50 instrument was firstly set up against the gravimetric method with temperature and RH adjusted.

Validation of a light-scattering PM2.5 sensor monitor based on the long-term gravimetric measurements in field tests