Identification of micro-dynamics phase transition processes for ammonium sulfate aerosols by two-dimensional correlation spectroscopy

Abstract. Phase transitions of particles are importance because it could influence reactive gas uptake, multiphase chemical reactions pathways, ice and polar stratospheric cloud formation. Electron microscopy-based techniques detect phase transitions through macroscopic properties but cannot resolve efflorescence kinetics at the molecular level, whereas spectroscopic methods, while enabling molecular-level tracking of aerosol transformations and water uptake quantification, remain limited in characterizing the dynamics of efflorescence phase transition due to insufficient temporal resolution to resolve intermolecular interactions and capture complex intermediate states. In this study, the efflorescence phase transition of AS aerosol is tracked by using the fourier transform infrared (FTIR) spectroscopic technique. A novel method has been developed that not only accurately determines the phase transition region but also enables the identification of micro-dynamic phase transition processes through analysis of the molecular structural change sequence based on two-dimensional correlation spectroscopy. During efflorescence transition processes, we measured the phase transition point at 39 ± 0.8 % (RH), and its start and end points at 41 ± 0.8 % (RH) and 36 ± 0.8 % (RH), respectively. We further identified four distinct micro-dynamics steps during the efflorescence processes. Initially, there was a gradual loss of liquid water for the solution droplets. Subsequently, it formed the supersaturated ammonium sulfate (AS) aerosols. Furthermore, hydrogen bonds between liquid water and sulfate dissociate, reducing aqueous sulfate concentration. The replacement of H2O by NH4+ in the sulfate hydration shell drives a bond reorganization that strengthens sulfate-ammonium interactions at the expense of original sulfate-water bonds. The efflorescence occurs and forms crystal/solid AS. Eventually, the remaining liquid water molecules eventually detach from the AS system, completing the liquid-to-solid phase transition. This method will help improve comprehending of the transport and deposition of inhaled aerosol. Moreover, these insights will spur fundamental research into the formation, transformation and nucleation mechanisms of atmospheric aerosols.