Crystal Nucleation and Crystal Growth and Mass Transfer in Internally Mixed Sucrose/NaNO 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/NaNO 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 NaNO decreased from ∼45% for pure NaNO droplets to ∼38.6 and ∼37.9% for the 1:8 and 1:4 sucrose/NaNO droplets, respectively, while no crystallization of NaNO 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 NaNO in the mixed droplets. In addition, the crystal growth of NaNO was suppressed in the 1:4 and 1:8 droplets most likely due to the slow diffusion of Na and NO ions at low RH. Water uptake/release of sucrose/NaNO 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.