Simultaneous measurements of PM1 and PM10 aerosol scattering properties and their relationships in urban Beijing: A two-year observation.

The aerosol scattering properties of submicron (PM1) and sub-10 μm particles (PM10) under dry conditions (RH <30%) were investigated in Beijing from 2018 to 2019. Using the simultaneous measurement of PM1 and PM10, the scattering properties of super micron (PM10-1) particles were also calculated. At 550 nm, the average of scattering coefficient (σsp) and asymmetry parameter (g) were 208.7 ± 204.9 Mm-1 and 0.61 ± 0.04 for PM10, 140.6 ± 130.2 Mm-1 and 0.60 ± 0.04 for PM1, and 69.8 ± 82.2 Mm-1 and 0.62 ± 0.04 for PM10-1, respectively, while the backscattering ratio (b) values were 0.13 ± 0.02 for PM10 and PM1, and 0.12 ± 0.02 for PM10-1. The mass scattering efficiencies (MSE) for PM10, PM1 and PM10-1 were 2.43 ± 2.37, 3.67 ± 0.96, and 1.73 ± 1.82 m2 g-1, respectively. In 2019, σsp decreased by approximately 18.4% for PM10, and 16.7% for PM1 compared with those in 2018, which was quite similar to the decrease of 17% and 19% for PM10 and PM2.5 mass concentrations during the same time period. The scattering Ångström exponent (SAE450/700), which was 1.88 ± 0.29 for PM1 and 1.50 ± 0.27 for PM10 indicated size distributions dominated by fine mode aerosols. This is also evidenced by the high submicron scattering ratio (Rsp) (71.1% ± 7.9%). The high SAE, Rsp, and high PM1 σsp in the study suggest that control of fine particle pollution is important to reduce overall PM pollution in urban Beijing. In addition, with an increase in σsp, b, Rsp, and SAE gradually decreased, while g and MSE increased. The clearly scattering coefficient-dependent MSE suggests that high aerosol loading and high MSE both play an important role in degraded visibility during heavy pollution periods.