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 phenanthrene vapor greater than that predicted by bulk (air-water)-phase equilibrium was observed for diameters less than 200 microm, 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, alpha = (1.4 +/- 0.4) x 10(-2) was observed for the highly surface-active (hydrophobic) phenanthrene, whereas a small alpha = (9.7 +/- 1.8) x 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.

Heterogeneous Oxidation by Ozone of Naphthalene Adsorbed at the Air-Water Interface of Micron-Size Water Droplets

“…39,40 FA is a known surrogate that exhibits properties similar to the surface-active OC in fog droplets. 39 Previous work 10 (Table 2). Thus, organic-coated surfaces of water droplets can show much higher O 3 reactivity with adsorbed PAH molecules.…”

“…A piezoceramic crystal (American Piezoceramics Inc.) placed above the orifice was mechanically vibrated to generate the desired droplet size depending on the frequency of voltage modulation. 9,10 …”

“…These experiments showed that the partitioning of PAHs between the gas and a water droplet was larger for small droplets. 9,10 This was attributed to increased partitioning of compounds to the air-water interface through adsorption. As a consequence, the equilibrium at the airwater interface required a modified partition constant given by:…”

“…22,23 Similarly, PAHs also show small mass accommodation coefficients. 9,10 However, the presence of a PAH at the interface may increase the PAH-O 3 interaction at the airwater interface of a fog droplet. 21 In the natural atmosphere, the primary oxidants are the hydroxyl radical, O 3 , and the nitrate radical.…”

“…9,10 As fresh droplets traversed the reactor, the PAH vapors were incorporated into the droplets. The PAH concentration increased with droplet/gas contact time and decreased droplet radius.…”