The Impact of Aqueous Phase Replacement Reaction on the Phase State of Internally Mixed Organic/ammonium Aerosols

Abstract. Aerosol phase state is crucial for air quality, climate, and human health. Atmospheric secondary aerosols are often internally mixed with organic and inorganic components, particularly dicarboxylic acids, ammonium, sulfate, nitrate, and chloride. These complex compositions enable aqueous reaction between organic and inorganic species, significantly complicating aerosol phase behaviour during aging and making phase predictions challenging. We investigated carboxylate/ammonium salt mixtures using in-situ infrared spectroscopy. The di- and tri- carboxylates included sodium pyruvate (SP), sodium tartrate (ST), and sodium citrate (SC), while the ammonium salts included NH₄NO₃, NH₄Cl, and (NH₄)₂SO₄. Our results demonstrated that aqueous replacement reactions between carboxylates and ammonium salts was promoted by the formation and depletion of NH₃ as relative humidity (RH) changed. Solid NaNO₃, SP, and Na₂SO₄ formed in SP/ammonium aerosol at 35.7 %~12.7 %, 64 % and 65.5 %~60.1 % RH, respectively. In contrast, reactions between ST or SC and (NH₄)₂SO₄ was incomplete due to the gel structure of SC or ST at low RH. Upon hydration, the deliquescence RH of Na₂SO₄ in SP/(NH₄)₂SO₄ (88.8 %–95.2 %) and NaNO₃ in SP/NH₄NO₃ (76.5–81.9 %) are higher than those of pure inorganic aerosols. Unexpectedly, aqueous Na₂SO₄ crystallized upon humidification in ST/(NH₄)₂SO₄ particles at 43.6 % RH and then deliquesced with increasing RH. This is attributed to decreased viscosity and increased ion mobility, which overcome the kinetic inhibition of ion movement, leading to nucleation and growth of Na₂SO₄ crystal. Our findings highlight the intricate interplay between chemical components within organic/inorganic aerosol, the impact of replacement reactions on aerosol aging and phase state, and subsequently on atmospheric processes.