The interplay between aqueous replacement reaction and the phase state of internally mixed organic/ammonium aerosols

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 behavior during aging and making phase predictions challenging. We investigated carboxylate-ammonium salt mixtures using attenuated total reflection Fourier-transformed infrared spectroscopy (ATR-FTIR). The mono-, di-, and tricarboxylates 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 were promoted by the formation and depletion of NH₃ as relative humidity (RH) changed. For SP/ammonium aerosols, NaNO₃ and Na₂SO₄ crystallized from 35.7 % to 12.7 % and from 65.7 % to 60.1 % RH, respectively, which is lower than the values for pure inorganics (62.5 ± 9 %-32 % RH for NaNO₃ and 82 ± 7 %-68 ± 5 % RH for Na₂SO₄). Upon hydration, the crystalline Na₂SO₄ and NaNO₃ deliquesced at 88.8 %-95.2 % and 76.5 ± 2 %-81.9 %, which is higher than the values of pure Na₂SO₄ (74 ± 4 %-98 % RH) and NaNO₃ (65 %-77.1 ± 3 % RH). In contrast, reaction between ST or SC and (NH₄)₂SO₄ was incomplete due to the gel structure at low RH. 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 increased ion mobility in viscous particles, leading to nucleation and growth of Na₂SO₄ crystals. Our findings highlight the intricate interplay between chemical components within organic/inorganic aerosol and the impact of replacement reactions on aerosol aging, phase state, and subsequently atmospheric processes.