Impact of the particle mixing state on the hygroscopicity of internally mixed sodium chloride–ammonium sulfate single droplets: a theoretical and experimental study

Abstract

Sodium chloride (NaCl) is the main constituent of sea-salt aerosols. During atmospheric transport, sea-salt aerosols can interact with gases and other particles including secondary aerosols containing ammonium sulfate ((NH₄)₂SO₄). This paper reports on the deliquescence relative humidity (DRH) of internally mixed sodium chloride–ammonium sulfate (NaCl/(NH₄)₂SO₄) coarse particles by means of an acoustic levitation system fitted with a confocal Raman microscope (CRM). The chemical composition and physical state of individual levitated particles of different initial NaCl mole fractions were monitored during the deliquescence cycle by CRM. Experimental results were compared to the data predicted by the thermodynamic model E-AIM (Extended-Aerosol Inorganics Model). We demonstrated that NH₄Cl, Na₂SO₄ and NH₄NaSO₄·2H₂O are formed in recrystallized particles and coexist with NaCl and (NH₄)₂SO₄. All these products are randomly distributed within the particles. Deliquescence curves described two or three-stage phase transitions depending on the initial composition of the droplet. Significant discrepancies between the model and the laboratory experiments were observed for NaCl mole fractions varying between 0.40 and 0.77 due to a divergence between the predicted and the truly present products in the particles’ solid fraction during the humidification cycle.