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 (Δ<i>f</i>) 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. Δ<i>f</i> 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 Δ<i>f</i>, 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 Δ<i>f</i> 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 Δ<i>f</i> 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.