Influence of chemical size distribution on optical properties for ambient submicron particles during severe haze events

Despite of extensive efforts on investigation into characteristics of severe haze pollution in megacities of China, the accurate relationships among the aerosol composition, mass-size distribution and optical properties during pollution episodes remain poorly understood. Here, we conducted in situ measurements of the mass size distribution of submicron aerosol (PM1) species by using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS), particle light scattering by a Cavity Attenuated Phase Shift ALBedo monitor (CAPS-ALB) and a Photoacoustic Extinctionmeter (PAX) during the winter of 2017 in Shanghai, China. The average PM1 concentration was 85.9 ± 14.7 μg/m3 during the haze episodes, of which was ∼7 times higher than that of clean period (12.1 ± 3.1 μg/m3). Organic aerosol (OA) and inorganic species (SO42− + NO3− + NH4+) contributed 39.9% and 51.2% of the total mass of PM1 during the haze episodes, respectively. OA exhibited a single or bimodal distribution during the haze episodes with the peak concentration of 51.8 μg/m3. There were no obvious differences between ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4) during the haze episodes, which exhibited single peak distributions at the sizes of 650-700 nm and ∼700 nm, respectively. The peak positions of OA, NH4NO3 and (NH4)2SO4 in clean period were in the range of 450-500 nm, 550-600 nm and 450-500 nm with the peak concentrations of 5.5 μg/m3, 3.1 μg/m3 and 3.8 μg/m3, respectively. The increased scattering coefficients in the haze episodes were positively correlated with higher secondary inorganic aerosols and organic aerosol (OA). The high scattering coefficient contribution fraction peak diameter of NH4NO3 and (NH4)2SO4 were in the range of 600-800 nm and 600-750 nm with the peak scattering coefficient of 352.6 Mm−1 and 165.7 Mm−1, respectively. The size distribution of scattering for OA showed bimodal modes during all episodes. OA and NH4NO3 were the largest contributors to scattering coefficients of PM1 during the haze episodes, accounting for 45.5% and 37.8%, respectively. The contribution of (NH4)2SO4 to the scattering (24%) exceeded that of NH4NO3 during clean period. Our results elucidated that substantial changes in the aerosols optical properties due to the changes in chemical compositions and size distribution during the haze events.