Water Adsorption vs Phase Transition of Aerosols Monitored by a Quartz Crystal Microbalance

A quartz crystal microbalance (QCM) with a high sensitivity of 0.1 ng was applied to monitor the oscillation frequency variation (Δf) of standard single species, two-component systems with typical ambient aerosol compositions, and ambient aerosol filter samples as a function of relative humidity (RH) and determine their deliquescence RH (DRH) and phase transition. Δf is associated with the adsorption or desorption process of water molecules for solid samples and physical properties of the sample film during solid-to-aqueous phase transition (deliquescence). During the pre-deliquescence stage, the water adsorption process led to the increased mass with decreasing Δf, especially for the hydrates such as MgCl₂ and Ca(NO₃)₂, which have more than 20% and 40% increased mass, respectively. The water adsorption process might cause a mass deviation of ambient particulate matter measurement using similar instrument principles. During the deliquescence stage, the observed rapid increasing Δf with RH was caused by a significant change in the physical properties (such as density and viscosity) of the sample film. The determined DRH for a given single-component system is consistent with the results estimated from the thermodynamic models. For a complex system, the QCM can determine the DRH^1st well as the eutonic point and track the possible variation of the physical properties of inorganic or with organic acid mixture systems. During the post-deliquescence stage, the gradual increasing trend of Δf with RH for Ca(NO₃)₂ and an external mixture of NaCl-Ca(NO₃)₂ was mainly contributed by a stronger RH dependent of physical properties for Ca(NO₃)₂(aq). Overall, this study provides the possible physical properties variation of common aerosol composition as a function of RH, which was consistent with the results calculated from the thermodynamic models. The stronger water adsorption for MgCl₂ and Ca(NO₃)₂ with solid-like viscosity at RH < DRH might lead to different chemical reactivities in the atmospheric chemistry in addition to the radiative forcing of aerosols caused by the hysteresis.