Columnar aerosol optical and radiative properties according to season and air mass transport pattern over East Asia

Evaluation of Two Low-Cost Optical Particle Counters for the Measurement of Ambient Aerosol Scattering Coefficient and Ångström Exponent

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.

Vertically-resolved profiles of mass concentrations and particle backscatter coefficients of Asian dust plumes derived from lidar observations of silicon dioxide

This study presents a method to retrieve vertically-resolved profiles of dust mass concentrations by analyzing Raman lidar signals of silicon dioxide (quartz) at 546nm. The observed particle plumes consisted of mixtures of East Asian dust with anthropogenic pollution. Our method for the first time allows for extracting the contribution of the aerosol component "pure dust" contained in the aerosol type "polluted dust". We also propose a method that uses OPAC (Optical Properties of Aerosols and Clouds) and the mass concentrations profiles of dust in order to derive profiles of backscatter coefficients of pure dust in mixed dust/pollution plumes. The mass concentration of silicon dioxide (quartz) in the atmosphere can be estimated from the backscatter coefficient of quartz. The mass concentration of dust is estimated by the weight percentage (38-77%) of mineral quartz in Asian dust. The retrieved dust mass concentrations are classified into water soluble, nucleation, accumulation, mineral-transported and coarse mode according to OPAC. The mass mixing ratio of 0.018, 0.033, 0.747, 0.130 and 0.072, respectively, is used. Dust extinction coefficients at 550nm were calculated by using OPAC and prescribed number concentrations for each of the 5 components. Dust backscatter coefficients were calculated from the dust extinction coefficients on the basis of a lidar ratio of 45±3sr at 532nm. We present results of quartz-Raman measurements carried out on the campus of the Gwangju Institute of Science and Technology (35.10°N, 126.53°E) on 15, 16, and 21 March 2010.