Enhanced Nitrate Fraction Allows Urban Aerosols to be in Liquid State at Lower Relative Humidity

• DOI: 10.5281/zenodo.8188863
• Publication Date: 2023-07-27
• Authors: Yuechen Liu; Zhijun Wu; Yanting Qiu; Ping Tian; Quan Liu; Ying Chen; Mijung Song; Min Hu
• Keywords: Fraction (chemistry)

Nitrate has become the primary inorganics in urban aerosols, but its effects on particle phase state, which is crucial in multiphase chemistry, are still largely unknown. Herein, particle rebound measurements were conducted to investigate the relationship between the liquid–phase–transition threshold relative humidity (RHthreshold) and the mass fraction of inorganics in dry particles (Finorg). Results revealed a negative correlation between RHthreshold and Finorg, with more nitrate leading to lower RHthreshold. Even with a relative humidity (RH) < 20%, particles with ∼50% nitrate mass fraction remained in semisolid state. Using Beijing as an example, a decrease was observed in RHthreshold from 64% in 2015 to below 53% nowadays during moderate-pollution periods (PM2.5 = 35–75 mg/m3) due to an enhanced nitrate fraction. This allowed urban aerosols to exist in liquid state at lower RH. Consequently, bulk diffusion in nitrate-dominated particles is negligible, making them key seeds for secondary aerosol formation through aqueous-phase reactions.