Uptake of Aromatic Hydrocarbon Vapors (Benzene and Phenanthrene) at the Air-Water Interface of Micron-Size Water Droplets

Abstract

Uptake of aromatic hydrocarbon vapors (benzene and phenanthrene) by typical micrometer-sized fog-water droplets was studied using a falling droplet reactor at temperatures between 296 and 316 K. Uptake of phenan-threne vapor greater than that predicted by bulk (air-water)-phase equilibrium was observed for diameters less than 200 μm, and this was attributed to surface adsorption. The experimental values of the droplet-vapor partition constant were used to obtain the overall mass transfer coefficient and the mass accommodation coefficient for both benzene and phenanthrene. Mass transfer of phenanthrene was dependent only on gas-phase diffusion and mass accommodation at the interface. However, for benzene, the mass transfer was limited by liquid-phase diffusion and mass accommodation. A large value of the mass accommodation coefficient, α = (1.4 ± 0.4) × 10^?2 was observed for the highly surface-active (hydrophobic) phenanthrene, whereas a small α = (9.7 ± 1.8) × 10^?5 was observed for the less hydrophobic benzene. Critical cluster numbers ranging from 2 for benzene to 5.7 for phenanthrene were deduced using the critical cluster nucleation theory for mass accommodation. The enthalpy of mass accommodation was more negative for phenanthrene than it was for benzene. Consequently, the temperature effect was more pronounced for phenanthrene. A linear correlation was observed for the enthalpy of accommodation with the excess enthalpy of solution. A natural organic carbon surrogate (Suwannee Fulvic acid) in the water droplet increased the uptake for phenanthrene and benzene, the effect being more marked for phenanthrene. A characteristic time constant analysis showed that uptake and droplet scavenging would compete for the fog deposition of phenanthrene, whereas deposition would be unimpeded by the uptake rate for benzene vapor. For both compounds, the characteristic atmospheric reaction times were much larger and would not impact fog deposition.