24 Jun 2024
 | 24 Jun 2024
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

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

Hui Yang, Fengfeng Dong, Li Xia, Qishen Huang, Shufeng Pang, and Yunhong Zhang

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.

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Hui Yang, Fengfeng Dong, Li Xia, Qishen Huang, Shufeng Pang, and Yunhong Zhang

Status: open (until 05 Aug 2024)

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Hui Yang, Fengfeng Dong, Li Xia, Qishen Huang, Shufeng Pang, and Yunhong Zhang
Hui Yang, Fengfeng Dong, Li Xia, Qishen Huang, Shufeng Pang, and Yunhong Zhang


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Short summary
Atmospheric secondary aerosols often contain a mix of organic and inorganic components, which can undergo complex reactions, leading to significant uncertainty in their phase state. Using molecular spectroscopic methods, we demonstrated that the aqueous replacement reaction, unique to these mixed aerosols and promoted by the presence of ammonium, significantly alters their phase behavior. This effect complicates the prediction of aerosol phase states and the corresponding atmospheric processes.