Hydrogel network formation triggers atypical hygroscopic behavior in atmospheric aerosols

The phase state of atmospheric aerosol particles, dictated by composition, intermolecular interaction, size, and pH, profoundly impacts climate, human health, and air quality. Herein, the phase behavior of internally mixed sodium bitartrate (SBT) and ammonium sulfate (AS) aerosols with equal molar ratio was studied using microscopic imaging, confocal Raman and infrared spectroscopy. We observed atypical phase transition during hygrosopic cycles. On dehydration, micro-solids formed at relative humidity (RH) of 80.1 % without further efflorescence, namely limited efflorescence (LE). On hydration, droplets effloresced at 39.8 % RH and deliquesced at 68.8 % RH, referred as efflorescence on hydration (EH). Raman spectra show that Na₂SO₄ solids form during LE, while both Na₂SO₄ and (NH₄)₂SO₄ solids form during EH. The phase transition of the SBT/AS (1:1) droplets is size-dependent, where smaller droplets are prone to "EH" while larger droplets are prone to typical efflorescence. Combining AIOMFAC-VISC viscosity predictions with scanning electron microscopy (SEM) images, we attribute the "LE" and "EH" phenomena to the formation of an ion-organic hydrogel network structure within droplets. pH and organic/inorganic mixing ratios can collaboratively affect the stability of hydrogel network by changing the organic/inorganic mixing ratios. Our study shows that gel network formation can cause atypical phase transition in atmospheric aerosols.