Publication
Evaluation of Two LowCost Optical Particle Counters for the Measurement of Ambient Aerosol Scattering Coefficient and Ångström Exponent 

Markowicz K., Chiliński M. 
Publication type:

Sensors20(9), 2020, art. 2617, 10.3390/s20092617 
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The aerosol scattering coefficient and Ångström exponent (AE) are important parameters in the understanding of aerosol optical properties and aerosol direct effect. These parameters are usually measured by a nephelometer network which is underrepresented geographically; however, a rapid growth of airpollution monitoring, using lowcost particle sensors, may extend observation networks. This paper presents the results of colocated measurements of aerosol optical properties, such as the aerosol scattering coefficient and the scattering AE, using lowcost sensors and using a scientificgrade polar Aurora 4000 nephelometer. A high Pearson correlation coefficient (0.94–0.96) between the lowcost particulate matter (PM) mass concentration and the aerosol scattering coefficient was found. For the PM10 mass concentration, the aerosol scattering coefficient relation is linear for the Dfrobot SEN0177 sensor and nonlinear for the Alphasense OPCN2 device. After regression analyses, both lowcost instruments provided the aerosol scattering coefficient with a similar mean square error difference (RMSE) of about 20 Mm−1, which corresponds to about 27% of the mean aerosol scattering coefficient. The relative uncertainty is independent of the pollution level. In addition, the ratio of aerosol number concentration between different bins showed a significant statistical (95% of confidence level) correlation with the scattering AE. For the SEN0177, the ratio of the particle number in bin 1 (radius of 0.15–0.25 µm) to bin 4 (radius of 1.25–2.5 µm) was a linear function of the scattering AE, with a Pearson correlation coefficient of 0.74. In the case of OPCN2, the best correlation (r = 0.66) was found for the ratio between bin 1 (radius of 0.19–0.27 µm) and bin 2 (radius of 0.27–0.39 µm). Comparisons of an estimated scattering AE from a lowcost sensor with Aurora 4000 are given with the RMSE of 0.23–0.24, which corresponds to 16–19%. In addition, a threeyear (2016–2019) observation by SEN0177 indicates that this sensor can be used to determine an annual cycle as well as a shortterm variability.