Influence of Amorphous Humic Acid Solids on the Phase Transition Behavior of Nitrate Aerosols

The phase state of atmospheric aerosols plays a crucial role in regulating the atmospheric chemistry and radiative forcing. Humic acid (HA), a key organic component and highly reactive species in aerosols, may influence the phase transition behaviors of mixtures with inorganic salts, although these effects remain poorly understood. In this study, we employed microscopic imaging and infrared spectroscopy to investigate the hygroscopic growth and phase transitions of sodium nitrate (SN)/HA mixed particles across various mass ratios. During dehumidification, HA effloresced at relative humidity (RH) levels above 80%, forming an amorphous solid that inhibited water transport across the air-water interface, thereby lowering the efflorescence relative humidity (ERH) of SN. As the organic fraction increased, the ERH of SN dropped from 49.8% in the 40,000:1 mixture, approaching that of pure SN, to 36.0% in the 40:1 mixture, and then increased to 51.2% in the 1:1 mixture. In the 1:4 SN/HA mixture, the SN efflorescence was completely suppressed during dehumidification. These observations can be attributed to two competing effects: the inhibition of SN efflorescence by amorphous HA solids and the heterogeneous nucleation of SN induced by embedded organic crystals within HA solids. Additionally, higher aerosol acidity significantly reduced the ERH of SN, while the deliquescence relative humidity (DRH) in the 1:1, 4:1, and 40:1 mixtures remained almost unchanged due to the presence of HA solids. These results provide deeper insights into the phase state evolution of HA-containing atmospheric aerosols and highlight the importance of considering the amorphous solid state in the evaluation of aerosol physicochemical properties.