Responses of Ammonium Sulfate Particles Coated with Glutaric Acid to Cyclic Changes in Relative Humidity: Hygroscopicity and Raman Characterization

Abstract

Atmospheric particles, which may have an organic coating, exhibit cyclical phase changes of deliquescence and crystallization in response to changes in the ambient relative humidity (RH). Here, we measured the hygroscopicity and Raman spectra of solid ammonium sulfate ((NH4)2SO4) particles initially coated with water-soluble glutaric acid in two consecutive cycles of deliquescence and crystallization utilizing an electrodynamic balance. (NH4)2SO4 particles with glutaric acid coating (49 wt % glutaric acid) had different hygroscopicity and morphology in the two cycles. Once the particles deliquesced, the dissolution of the solid (NH4)2SO4 core and the glutaric acid coating formed mixed (NH4)2SO4−glutaric acid solution droplets, which was confirmed by Raman characterization. Coating studies with either deliquescence or crystallization measure ments, or one complete cycle of these two measurements may not fully assess the effects of the organic coatings on aerosol hygroscopicity. We also present an analysis on the kinetic and chemical effects of organic coating on aerosol hygroscopicity. Glutaric acid coating does not impede the evaporation and condensation rates of water molecules compared to the rates of (NH4)2SO4 particles in the two cycles. The coating likely affects the hygroscopicity of aerosol particles through dissolution and its chemical interactions with (NH4)2SO4