Rebounding hygroscopic inorganic aerosol particles: Liquids, gels, and hydrates

Particle rebound was studied for ten atmospherically relevant inorganics. Experiments were conducted with submicron particles in aerosol form to a relative humidity (RH) of <5% followed by progressive exposure to RH up to 95% for 2 s. At low RH, particles of MgCl2, NaCl, NH4Cl, KCl, (NH4)2SO4, and Na2SO4 crystallized. As RH increased, these particles completed the transition from rebounding to adhering close to their deliquescence RH (DRH). The onset of decreased rebound, however, was below the DRH. Rebound curves for particles of MgCl2, NH4NO3, MgSO4, and NaNO3 had different features than explained by water adsorption and deliquescence. Particles of MgCl2 had rebound curves characterized by two domains, corresponding to its two hydrates. At low RH, particles of MgSO4 and NaNO3 did not crystallize but rebound occurred, suggesting a glassy or high-viscosity though noncrystalline state. Gel formation for MgSO4 can increase viscosity, affecting rebound behavior. Particles of NH4NO3 adhered even to <5% RH, suggesting a low-viscosity state even to low RH. Particles of NH4HSO4 were investigated as a special case by exposure to 5 ppm ammonia at 10% and 90% RH. At low RH, these particles still had sufficient molecular diffusivity to maintain active heterogeneous chemistry, although with some kinetic limitations. The different behaviors between nitrates and sulfates suggest different roles of heterogeneous chemistry in regions affected by NOx compared to SO2 emissions. The results of this study could have implications for the use of different wet and dry seed particles in chamber experiments.
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