The phase state of aerosol particles plays a crucial role in the mass transfer processes between gas and particles, which is essential for determining aerosol multiphase chemistry ^{[1, 2]}. Understanding the characteristics of aerosol phase state is important for comprehending the mechanisms behind secondary aerosol formation and improving air quality. Recent studies have shown notable changes in the chemical composition of urban aerosol particles, with an increase in the mass fraction of inorganic components, particularly nitrate emerging as the dominant component ^[3]. However, the impacts of these changes on the phase state of urban aerosol particles remain largely unknown.
Fig. 1 The schematic of the influences of enhanced nitrate fraction on the phase state of urban aerosol particles
In this study, we investigated the effects of inorganic compounds, specifically nitrate, on the phase state of urban aerosol particles. We conducted rebound behavior measurements on both laboratory-generated mixtures and regenerated ambient particles from water-extracted filters collected in the field. This allowed us to explore the relationship between the liquid-phase-transition threshold relative humidity (RH_threshold) and the mass fraction of inorganic compounds in dry particles (F_inorg). Our results revealed a negative correlation between RH_threshold and F_inorg, with more nitrate leading to lower RH_threshold. Even at relative humidity (RH) levels below 20%, particles with approximately 50% nitrate mass fraction remained in a semisolid state. Using Beijing as an example, a decrease in RH_threshold from 64% in 2015 to below 53% during moderate-pollution periods (PM_2.5 = 35 - 75 μg/m³) due to an enhanced nitrate fraction was observed. This indicates that enhanced nitrate fraction enables urban aerosol particles to exist in a liquid state at reduced RH (see Fig. 1). Consequently, based on Stokes-Einstein Equation, bulk diffusion in nitrate-dominated particles is negligible, suggesting their potential role as key seeds for secondary aerosol formation through aqueous-phase reactions.
 

Urban aerosol; Particulate nitrate; Phase state; Liquid water content; bulk diffusion