• sea salt;
  • deliquescence;
  • crystallization;
  • phase contrast imaging;
  • X-ray;
  • synchrotron radiation

X-ray phase contrast imaging (PCI) based on synchrotron radiation was introduced for the first time as an in situ imaging way to investigate sea salt phase change, i.e. deliquescence and crystallization in atmospheric environment. A performance on the deliquescence of pure NaCl, which is the dominant component in sea salt, demonstrated that this technique can directly observe the change of core particle and differentiate the outer water layer clearly in solid-aqueous system of ~100 µm scale. The imaging results showed that sea salt particle deliquesced on a large scale of relative humidity (RH) between 34 and 97% RH as a solid–liquid drop, while no clear deliquescence RH was observed during the process. According to the drop size growth curve, sea salt deliquescence can be divided into three steps, namely water accumulating step (34–75% RH), bulk melting step (75–86% RH) and delay dissolving step (>86% RH), which are most probably dominated by grouped components as MgCl2/CaCl2/MgSO4, NaCl and Na2SO4/KCl/K2SO4, respectively. Instead at a sole RH, the crystallization of sea salt solution occurred at a range of 46–58% RH, which well agreed with the theory proposed by Ge et al. The aqueous-solid condition provided by sea salt deliquescence and crystallization may greatly enhance the heterogeneous chemical reactions in atmosphere. Copyright © 2012 John Wiley & Sons, Ltd.