Observational study of aerosol hygroscopic growth on scattering coefficient in Beijing: A case study in March of 2018

A humidified nephelometer system was deployed to measure the aerosol scattering coefficients at RH < 30% and RH in the range of 40 to 85% simultaneously in megacity Beijing in March 2018. The aerosol optical properties and aerosol hygroscopicity of two sizes (PM₁₀ and PM₁) during the pollution period, dust period and a new particle formation event (NPF) were analyzed. During the pollution period, the scattering and absorption coefficients increased dramatically with the accumulation of pollutants, while scattering Ångström exponent (SAE), submicron scattering fraction (Rsp), submicron absorption fraction (Rap) decreased, as well as single scattering albedo (SSA) rose slightly, which indicated the increasing contribution of larger particle to scattering and absorption, and enhanced the scattering ability of aerosols. The average PM₁₀ mass scattering efficiency is 3.86 ± 1.19 m² g^-1 with a range of 2.05-5.74 m² g^-1 during the pollution period, and 0.40 ± 0.05 m² g^-1 during the dust period. Rsp at wavelength of 550 nm varied from 55.8% to 89.3% during the measurement period, with the average of 64.8% ± 5.2% and 73.1% ± 6.8% during the pollution period and dust period, respectively, which suggests that the aerosol scattering coefficient is mainly affected by fine particles. The average PM₁₀ and PM₁ aerosol scattering hygroscopic growth factors f(80%) are 1.75 ± 0.05 and 1.75 ± 0.04 during the pollution period, 1.14 ± 0.09 and 1.15 ± 0.06 during the dust period, 1.59 ± 0.05 and 1.60 ± 0.06 during the NPF event period, respectively. Aerosol scattering hygroscopic growth factors showed a strong correlation with the scattering Ångström exponent which suggests the hygroscopicity is much stronger for fine particles (SAE > 1.5) than the coarse particles (SAE < 1.0).