Laboratory Study of pH at the Air–Ice Interface

A good understanding of pH at the air–ice interface is required in order to better understand phenomena occurring on frozen media. In this study, we used glancing-angle laser-induced fluorescence in conjunction with the surface- and pH-sensitive fluorescent dyes harmine and acridine to investigate pH at the air–ice interface. We followed changes in the surface region pH due to the freezing of water samples containing HCl, HNO₃, or NaOH. Freezing leaves the surface pH largely unchanged with respect to the prefreezing pH, indicating that protons are not strongly excluded during freezing. Exclusion of chloride to the ice surface on freezing was inferred via the enhanced quenching of the acridine fluorescence lifetime upon freezing samples containing HCl. Changes in surface pH due to the deposition of HCl­(g) to frozen water surfaces were smaller than those seen on liquid surfaces, for the same acid loading