Hygroscopicity of internally mixed particles glycine/NaNO₃ studied by FTIR-ATR technique

Internal mixed aerosol particles in atmosphere often take the drastic physical property changes owing to interactions between compositions. Hygroscopicity and phase transition behavior of mixed particles composed of glycine and NaNO₃ with different organic to inorganic molar ratios (OIRs) have been investigated by using Fourier transform infrared attenuated total reflection spectroscopy (ATR-FTIR). It is found that pure glycine particles effloresce when the relative humidity (RH) is ∼64.5% and do not deliquesce on humidifying. The efflorescence and deliquescence point for pure NaNO₃ particles are ∼54% RH and ∼91.9% RH, respectively. For the internal mixed glycine/NaNO₃ particles, there exhibit various hygroscopicities and solid-aqueous phase transitions dependent upon the OIR. When the OIRs are 2:1 and 1:1, NaNO₃ particles cannot crystallize. When the OIRs are 1:2, 1:4 and 1:8, crystalline ratios are ~31%, ~45% and 72%, NaNO₃ effloresce at ∼44.7% RH, ~35.2% RH and ~35% RH, respectively. For all studied mixture particles, glycine can crystallize. For OIRs of 2:1, 1:1, 1:2, 1:4 and 1:8, the crystalline fractions of glycine are ~78%, 89%, 93%, 94% and 95%, respectively, corresponding to the ERH range of 74% − 42%, 73% − 35.9%, 65.5% − 19.2%, 35.2% − 8.1% and 35% − 3.8%. It shows that both NaNO₃ and glycine crystals are favored by more enriched NaNO₃. On hydration, the DRHs of NaNO₃ for mixtures with OIR of 1:2, 1:4 and 1:8 are ~87.4%, ~87% and 85.4%, respectively. Glycine in mixtures deliquesces at ~99% (incomplete deliquescence), ~98.3%, ~97.3%, ~93.3%, ~85.4% RH, respectively, with decreasing OIR. The results suggest the positive effect of NaNO₃ on the deliquescence of glycine and NaNO₃.