Responses of Ammonium Sulfate Particles Coated with Water-Soluble Glutaric Acid to Cyclic Changes in Relative Humidity: Hygroscopicity

Atmospheric particles, which may have organic coating, exhibit cyclical change in physical state and size in response to ambient relative humidity (RH) change. We measured the hygroscopicity of solid ammonium sulfate (AS) particles initially coated with water-soluble glutaric acid in two consecutive cycles of deliquescence and crystallization utilizing an electrodynamic balance. We also give an analysis on the kinetic and chemical effects of glutaric acid coating on aerosol hygroscopicity. We found that AS particles with glutaric acid coating had different hygroscopicity upon repeated humidification and dehumidification. Once the particles deliquesced, the dissolution of the solid AS core and the glutaric acid coating formed mixed AS/Vglutaric acid solution droplets, which was confirmed by Raman characterization. Glutaric acid coating does not impede the evaporation and condensation rates of water molecules compared to the rates of AS particles. The coating likely affects the aerosol hygroscopicity through dissolution and its interactions with AS. Coating studies with either deliquescence or crystallization measurements, or one complete cycle of these two measurements may not fully assess the effects of the organic coatings on aerosol hygroscopicity.