Size-resolved hygroscopic behavior of atmospheric aerosols during heavy aerosol pollution episodes in Beijing in December 2016

The size-resolved hygroscopic properties of submicron particles were observed by a Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) at an urban site in Beijing in December 2016. HTDMA was operated at the relative humidity (RH) of 30-90% with the particle size range of 30-200 nm. The probability distribution of the hygroscopic growth factor (HGF-PDF) during the measurement usually showed a bimodal pattern, namely, hydrophobic mode (HGF < 1.2) and hydrophilic mode (HGF >= 1.2) with RH >= 70%. The hydrophilic mode particles were usually more dominant for the aged particles. The HGF values were size-dependent and varied depending on particle chemical composition, and the volume-weighted mean HGF could better characterize the relationship between hygroscopicity and chemical composition for the bulk aerosols. The particle mixing state could be also known from the width of HGF-PDF and the particles were mainly externally mixed in this study because a major road with heavy traffic was nearby. For the observed new particle formation events, the HGF values of 30 nm diameter particles at 90% RH showed a decreasing trend, indicating the participation of the organics with weak hygroscopic growth behaviors. During the cumulative stage of heavy aerosol pollution episode, the small particles became more externally mixed and less hygroscopic when influenced by the primary emissions. Meanwhile, the aged particles (e.g., 200 nm) changed to a quasi-internal mixing state affected by the secondary inorganic formation process, which further enhanced the ability of water uptake. Furthermore, the formation of nitrate and ammonium played a more important role in the particle hygroscopicity than sulfate in this work. The size-resolved hygroscopic study revealed that the particles of different sizes underwent different chemical and physical processes in the atmosphere, which was important for understanding the formation and evolution of the severe aerosol pollution in North China Plain.