Experimental observation of a pH profile at an evaporating or growing vapor-liquid interface

Gas uptake, heterogeneous chemical reactions and collision-induced charge transfer are examples of processes, which occur in clouds, that are particularly sensitive to the specific chemical environment at the surface of a cloud particle. During evaporation or growth the surface region, just below a liquid-air interface, can exhibit a large length-scale buildup (or dilution) of ions caused, at least in part, by the flux of water molecules leaving (or coming to) the liquid surface. The resulting enhancement or dilution of ion concentration means incoming vapor molecules experience a surface-region-specific chemical environment that is different from the interior or bulk environment.

We have built a UV confocal microscope to measure the pH profile at a liquid interface for solutions with bulk pH ranging from 4-6. We find a characteristic length scale for a pH profile (surface enhancement during evaporation and deficit during growth) of about 220 um with the magnitude of the hydronium ion enhancement being as large as 2x the bulk concentration for solutions undergoing moderate (2.1 um/min) evaporation rates. Solutions held in equilibrium show no detectable surface hydronium ion enhancement. But for evaporating or growing solutions the amount of enhancement (or deficit in the case of growth) appears dependent both upon the evaporation or growth rate and the type of ions used to set bulk pH.

The potential effect of surface ion concentration enhancements to collision-induced thunderstorm electrification processes will be discussed.