Investigating the Phase State of PM2.5 in Seoul and Beijing using Optical Microscopy and Poke-and-Flow Technique

MIJUNG SONG, Rani Jeong, Daeun Kim, Yanting Qiu, Xiangxinyue Meng, Zhijun Wu, Andreas Zuend, Yoonkyeong Ha, Changhyuk Kim, Haeri Kim, Sanjit Gaikwad, Kyoung-Soon Jang, Ji Yi Lee, Joonyoung Ahn, Jeonbuk National University

     Abstract Number: 49
     Working Group: Aerosol Physics

Abstract
The phase state of aerosol particles plays a crucial role in understanding their properties, but information on real-world aerosols remains limited. This study aims to explore the phase state of fine mode aerosols (PM2.5) in two megacities, Seoul and Beijing, during December 2020 – January 2021. Daily PM2.5 filter samples were collected and analyzed using optical microscopy combined with the poke-and-flow technique. The phase states of the PM2.5 bulk were determined as a function of relative humidity (RH) and compared to ambient RH ranges in both cities. Results indicate that PM2.5 in Seoul exhibited a liquid to semisolid phase state, whereas in Beijing, it was primarily semisolid to solid. The liquid phase was dominant on polluted days in Seoul, while a semisolid state prevailed on clean days. These findings can be attributed to the aerosol liquid water content, which was higher in Seoul due to differences in aerosol chemical compositions. Furthermore, the phase states of PM2.5 in both cities were found to be interconnected with particle size distribution. This study contributes to a deeper understanding of aerosol fundamental physical properties and their connection to PM2.5 in urban polluted atmospheres.