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

In Northeast Asia, the elevated levels of fine particulate matter (PM_2.5) are an environmental concern, yet their physicochemical properties have been poorly characterized. Herein, we determined the phase states of PM_2.5 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 PM_2.5 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 PM_2.5. By incorporating ambient RH data from each city, we estimated the prevalent PM_2.5 phase states within the planetary boundary layer of Northeast Asia. Our findings revealed that the dominant phase states of PM_2.5 in these urban areas were liquid and semisolid. Additionally, we showed a critical threshold based on the aerosol liquid water content (ALWC) in PM_2.5: a primarily liquid phase for ALWC/PM_2.5 ratios of ≥∼0.5 and a predominantly semisolid phase for ALWC/PM_2.5 ratios of <∼0.5. These insights could contribute to a better understanding of the mechanisms underlying aerosol pollution in Northeast Asia.