Crystal Nucleation and Crystal Growth and Mass Transfer in Internally Mixed Sucrose/NaNO₃ Particles

Secondary organic aerosols (SOA) can exist in a glassy or semisolid state under low relative humidity (RH) conditions, in which the particles show nonequilibrium kinetic characteristics with changing ambient RH. Here, we selected internally mixed sucrose/NaNO₃ droplets with organic to inorganic molar ratios (OIRs) of 1:8, 1:4, 1:2, and 1:1 as a proxy for multicomponent ambient aerosols to study crystal nucleation and growth processes and water transport under a highly viscous state with the combination of an RH-controlling system and a vacuum Fourier transform infrared (FTIR) spectrometer. The initial efflorescence RH (ERH) of NaNO₃ decreased from ∼45% for pure NaNO₃ droplets to ∼38.6 and ∼37.9% for the 1:8 and 1:4 sucrose/NaNO₃ droplets, respectively, while no crystallization of NaNO₃ occurred for the 1:2 and 1:1 droplets in the whole RH range. Thus, the addition of sucrose delayed the ERH and even completely inhibited nucleation of NaNO₃ in the mixed droplets. In addition, the crystal growth of NaNO₃ was suppressed in the 1:4 and 1:8 droplets most likely due to the slow diffusion of Na⁺ and NO3^- ions at low RH. Water uptake/release of sucrose/NaNO₃ particles quickly arrived at equilibrium at high RH, while the hygroscopic process was kinetically controlled under low RH. The half-time ratio between the liquid water content and the RH was used to describe the mass transfer behavior. For the 1:1 droplets, no mass limitation was observed with the ratio approaching to 1 when the RH was higher than 53%. The ratio increased 1 order of magnitude under an ultraviscous state with RH ranging from 53 to 15% and increased a further 1 order of magnitude at RH < 15% under a glassy state.