Measurements of Evaporation Kinetics of Aqueous Aerosols

The rates of water evaporation and condensation govern the growth, size distribution, and number concentration of cloud droplets, yet the details of these fundamental processes remain poorly understood. In this work, the effect of acetic acid on the evaporation rate of water is studied using a liquid microjet technique. A collimated train of micron-sized droplets containing 1M acetic acid is injected into a vacuum chamber, and their temperature is measured at various distances from the jet nozzle using Raman thermometry. The data are compared to an evaporative cooling model to determine the evaporation coefficient, γ. In previous studies, γ was determined for pure water, D2O, 3M ammonium sulfate, and 4M sodium perchlorate to be 0.62 ±0.09, 0.57 ±0.06, 0.58 ±0.05, and 0.47 ±0.02, respectively. It is hypothesized that more surface-active species have a greater effect on evaporation rates. Acetic acid has been shown to be surface-active and is therefore representative of the small, surfactant-like molecules often found in aerosols. Preliminary results show a slight reduction in the evaporation coefficient for acetic acid compared to pure water. These experimental findings are being compared with calculations from the Chandler group at UC Berkeley in order to better understand the molecular mechanism of water evaporation.