Surprising Effects of Hydrochloric Acid on the Water Evaporation Coefficient Observed by Raman Thermometry

The kinetics and energetics of cloud droplet and aerosol formation in the atmosphere are strongly influenced by the evaporation and condensation rates of water, yet the magnitude and mechanism of evaporation remains incompletely characterized. Of particular import (and controversy) is the nature of interfacial water pH and its potential effects on evaporation rate and environmental reactivity. We have used Raman thermometry measurements of freely evaporating microdroplets to determine evaporation coefficients (gamma) for two different hydrochloric acid solutions, both of which result in a significant deviation from gamma(water). At 95% confidence, we find the evaporation coefficient for 1.0 M HCl to be 0.24 +/- 0.04, a, similar to 60% decrease relative to pure water, and for 0.1 M HCl to be 0.91 +/- 0.08, a, similar to 45% increase relative to pure water. These results p suggest a large perturbation in the surface structure induced by either hydronium ions adsorbing to the water surface or by the presence of a Cl-center dot center dot center dot H3O+ ion-pair moiety in the interfacial region.