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 pathway, ice and polar stratospheric cloud formation. The traditional understanding assumes that phase transitions are thermodynamically equilibrium, yet this is not the case at the molecular level. Current understanding can not account for these phenomena, since the interaction with water vapor induces modifications in both the composition and local chemical microenvironment of aerosols. Our findings demonstrate that these inconsistencies can be reconciled through elucidation of the microscopic dynamic processes governing phase transformation for aerosol. We propose a novel method which is accurate in determining the phase transition point and identification of micro-dynamics phase transition processes for ammonium sulfate aerosols by using 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 also explore that there are 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) particles. Furthermore, hydrogen bonds between liquid water and sulfate dissociate, reducing liquid sulfate concentration. Sulfate and ammonium ions in the bulk phase gradually approach each other, further expelling residual water. 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 and transformation mechanisms of atmospheric aerosols.