Abstract
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 under-represented geographically; however, a rapid growth of air-pollution monitoring, using low-cost particle sensors, may extend observation networks. This paper presents the results of co-located measurements of aerosol optical properties, such as the aerosol scattering coefficient and the scattering AE, using low-cost sensors and using a scientific-grade polar Aurora 4000 nephelometer. A high Pearson correlation coefficient (0.94–0.96) between the low-cost 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 non-linear for the Alphasense OPC-N2 device. After regression analyses, both low-cost 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 OPC-N2, 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 low-cost sensor with Aurora 4000 are given with the RMSE of 0.23–0.24, which corresponds to 16–19%. In addition, a three-year (2016–2019) observation by SEN0177 indicates that this sensor can be used to determine an annual cycle as well as a short-term variability.
Highlights
Atmospheric aerosol is of global concern due to its detrimental health effects as well as climate effects [1,2]
The one-hour data from both low-cost sensors (SEN0177, OPC-N2) as well as the Aurora 4000 were investigated and the results of data analysis are summarized in Figure 3, Tables 1 and 2
We found that the Pearson correlation coefficient (r) for PM1, PM2.5, and PM10 mass concentration obtained from both low-cost sensors with the aerosol scattering coefficient is high and varies from 0.91 to 0.96
Summary
Atmospheric aerosol is of global concern due to its detrimental health effects as well as climate effects [1,2]. Various studies have reported that the relationship between columnar and surface aerosol microphysical properties is not a straightforward problem [4], which can be explained by complex (multi layers) vertical variability of aerosol chemical composition and size distribution. The relationship between aerosol optical depth (AOD) and the particulate matter (PM) mass concentration at the ground-level should be determined regionally, to account for its specific conditions [5]. The empirical relation between AOD and PM mass concentration measurements for different parts of the world shows a wide range of correlation coefficients (e.g., [7,8,9,10]), indicating an important role of the variations within local meteorological conditions, the occurrence of multiple aerosol layers, and variations within aerosol chemical composition [5,9]
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