Enhanced Nitrate Fraction: Enabling Urban Aerosol Particles to Remain in a Liquid State at Reduced Relative Humidity

Nitrate has become the primary inorganic compound in urban aerosol particles, but its effects on particle phase state, which is crucial in multiphase chemistry, remains largely unknown. Herein, particle rebound measurements were conducted to explore the relationship between the liquid–phase–transition threshold relative humidity (RH_threshold) and the inorganic compounds mass fraction in dry particles (F_inorg). Results revealed negative correlations between RH_threshold and F_inorg, with more nitrate leading to lower RH_threshold. Even with RH < 20%, particles with ∼50% nitrate mass fraction remained in non-solid state. Taking Beijing as an example, decreases were observed in RH_threshold from 64% in 2015 to below 53% nowadays during the moderate-pollution periods (PM_2.5 = 35–75 μg/m³) due to an enhanced nitrate fraction. This allowed urban aerosol particles to exist in liquid state at lower RH, and consequently, kinetic limitation by bulk diffusion in nitrate-dominated particles might be negligible, making them key seeds for secondary aerosol formation through multiphase reactions.