Controlling Phase Evolution of Inorganic Salt Systems with Polyhydroxy Substances in Atmospheric Aerosols

Abstract. Polyhydroxy compounds, as key aerosol constituents, exhibit unique hygroscopicity and modulate inorganic salt hygroscopicity due to variable hydrogen bonding. However, when intermolecular H‑bonds and ionic bonds coexist, their relative contributions to hygroscopicity and phase transitions remain unclear. This work studied the hygroscopicity of aerosols from tartaric acid (a proxy for polyhydroxy diacids), its acid and neutral salts, and their mixtures with inorganics. Cyclic humidity-induced morphological transformations show that at low RH, particles are glassy, crystalline, and gel phases from tartaric acid to sodium bitartrate to sodium tartrate, owing to progressively enhanced electrostatic forces. Unexpected phase behaviors emerge when these polyhydroxy compounds are mixed with sodium sulfate at a 1:1 molar ratio. For tartaric acid/Na₂SO₄ system, Na₂SO₄ performs “effloresce upon hydration” at 36.3% RH. For sodium bitartrate/Na₂SO₄ particles upon dehydration, solid Na₂SO₄ precipitates first, followed by SBT efflorescence. Upon following hydration, the crystallization is enhanced at 52% RH. In the sodium tartrate/Na₂SO₄ mixture, solid Na₂SO₄ and tartrate‑Na‑SO₄^2- complexes simultaneously form at 57.4% RH during humidification. Shifts in the formation–dissociation equilibrium of these complexes induce “efflorescence-deliquescence-efflorescence-deliquescence on hydration”. Further experiments on sodium bitartrate–inorganic salt mixed aerosols show that ammonium salts lose ammonia, triggering intercomponent reactions and compositional changes that yield solid tartaric acid and sodium salts at low humidity. These findings indicate that organic–inorganic intermolecular interactions regulate particle hygroscopicity and phase state, affecting aerosol chemical reactions and cloud condensation nuclei activity.