Kinetically Determined Hygroscopicity and Efflorescence of Sucrose–Ammonium Sulfate Aerosol Droplets under Lower Relative Humidity

Organic aerosols will likely form in semisolid, glassy, and high viscous state in the atmosphere, which show nonequilibrium kinetic characteristics at low relative humidity (RH) conditions. In this study, we applied optical tweezers to investigate the water transport in a sucrose/(NH₄)₂SO₄ droplet with high organic to inorganic mole ratio (OIR). The characteristic time ratio between the droplet radius and the RH was used to describe the water mass transfer difference dependent on RH. For OIR greater than 1:1 in sucrose/(NH₄)₂SO₄ droplets, the characteristic time ratio at low RH (<∼30% RH) was two orders magnitude greater than that at high RH (>∼60%). We also coupled vacuum FTIR spectrometer and a high-speed photography to study the efflorescence process in sucrose/(NH₄)₂SO₄ droplets with low OIR. The crystalline fraction of (NH₄)₂SO₄ was used to understand efflorescence behavior when the RH was linearly decreasing with a velocity of 1.2% RH min^-1. Because of suppression of (NH₄)₂SO₄ nucleation by addition of sucrose, the efflorescence relative humidity (ERH) of (NH₄)₂SO₄ decrease from the range of ∼48.2% to ∼36.1% for pure (NH₄)₂SO₄ droplets to from ∼44.7% to ∼25.4%, from ∼43.2% to ∼21.2%, and from ∼41.7% to ∼21.1% for the mixed droplets with OIR of 1:4, 1:3, and 1:2, respectively. No crystallization was observed when the OIR is higher than 1:1. Suppression of (NH₄)₂SO₄ crystal growth was also observed under high viscous sucrose/(NH₄)₂SO₄ droplets at lower RH.