| The pH in aqueous solutions is important in a number of atmospheric processes, particularly reactions involving water and acid aerosols. Accurate knowledge of the pH is therefore necessary to understand such processes. Bulk pH is easily measured using pH electrodes or pH paper, and the pH at the water-air interface at equilibrium has recently been measured using surface specific techniques. However, during phase change the pH at, and near, the water-vapor interface changes. The objective of this work is to measure the pH profile not only at, but also near the surface of aqueous solutions while evaporation or condensation is occurring at the water-air interface.; A confocal-cryostat microscope was developed for depth profiling chemical compositions of aqueous solutions. The cryostat allowed for control of the temperature and chemical composition of both the liquid sample and the vapor above the sample.{09}A dual-peak fluorescent probe was used to chemically detect the pH.{09}Depth profiling of the pH in mildly acidic solutions (hydrochloric, sulfuric, and potassium phthalate) was performed to depths of more than 1 nun, with a resolution of 45 mum. The resulting pH profiles had no significant changes with depth for solutions at equilibrium, but the pH profiles showed significant decreases in surface and subsurface pH down to depths of ∼500 mum during evaporation. These profiles depend on the evaporation rate, a larger evaporation rate having a lower surface pH. These profiles were also pH dependent, with more acidic solutions having a greater increase of H3O + near the surface and deeper non-bulk behavior. Strong temperature dependence was also found, with a subsurface pH maximum found ∼100 mum below the surface for evaporating solutions near the maximum water density of 4°C. The pH profiles also showed increases in surface and subsurface pH down to depths of ∼150 mum during condensation growth. |
