Influence of Relative Humidity and Composition on PM2.5 Phases in Northeast Asia

In Northeast Asia, the elevated levels of fine particulate matter (PM2.5) are an environmental concern, yet their physicochemical properties have been poorly characterized. Herein, we determined the phase states of PM2.5 in 92 filter samples collected from four different cities─Beijing, Seoul, Seosan, and Ulaanbaatar─during 2020–2022, within a temperature range of ∼290–293 K. We noted a distinct trend in the boundary relative humidity (RH) of liquid and semisolid phases within these PM2.5 samples. As the inorganic fraction increased, the RH of the liquid phase decreased, whereas that of the semisolid phase increased. This behavior was strongly influenced by the chemical composition of PM2.5. By incorporating ambient RH data from each city, we estimated the prevalent PM2.5 phase states within the planetary boundary layer of Northeast Asia. Our findings revealed that the dominant phase states of PM2.5 in these urban areas were liquid and semisolid. Additionally, we showed a critical threshold based on the aerosol liquid water content (ALWC) in PM2.5: a primarily liquid phase for ALWC/PM2.5 ratios of ≥∼0.5 and a predominantly semisolid phase for ALWC/PM2.5 ratios of <∼0.5. These insights could contribute to a better understanding of the mechanisms underlying aerosol pollution in Northeast Asia.