Phase state of PM2.5 in Seoul and Beijing and its effect on particle size distribution

<p>A phase state of ambient aerosols is a key parameter to understand physicochemical properties of aerosols such as growth rate, size distribution, and mass concentration that affect climate and human health. However, only limited information of real aerosol phase states has been obtained. Herein, to explore phase states of fine particulate matter (PM_2.5) in megacities, we collected daily PM_2.5 filter samples during December 2020 &#8211; January 2021 in Seoul and Beijing. Optical microscopy and a poke-and-flow technique were used to observe the morphologies and phase states of the PM_2.5. Results showed that the average phase states of the PM_2.5 were significantly different in the two megacities. The PM_2.5 existed as a liquid state to a semisolid in Seoul while it existed as a semisolid to a solid in Beijing. These differences were governed by the chemical composition of PM_2.5 and aerosol liquid water content. Furthermore, the phase state of PM_2.5 in two megacities was correlated with particle size distribution. Our findings can provide an understanding of the phase states of real-world aerosols and how the phase state is linked to chemical compositions, ALWC, and size distributions of PM_2.5.</p>