Comparison of Phase States of PM 2.5 over Megacities, Seoul and Beijing, and Their Implications on Particle Size Distribution.

Although the particle phase state is an important property, there is scant information on it, especially, for real-world aerosols. To explore the phase state of fine mode aerosols (PM _2.5 ) in two megacities, Seoul and Beijing, we collected PM _2.5 filter samples daily from Dec 2020 to Jan 2021. Using optical microscopy combined with the poke-and-flow technique, the phase states of the bulk of PM _2.5 as a function of relative humidity (RH) were determined and compared to the ambient RH ranges in the two cities. PM _2.5 was found to be liquid to semisolid in Seoul but mostly semisolid to solid in Beijing. The liquid state was dominant on polluted days, while a semisolid state was dominant on clean days in Seoul. These findings can be explained by the aerosol liquid water content related to the chemical compositions of the aerosols at ambient RH; the water content of PM _2.5 was much higher in Seoul than in Beijing. Furthermore, the overall phase states of PM _2.5 observed in Seoul and Beijing were interrelated with the particle size distribution. The results of this study aid in a better understanding of the fundamental physical properties of aerosols and in examining how these are linked to PM _2.5 in polluted urban atmospheres.