The importance of surface adsorption on the washout of semivolatile organic compounds by rain

The washout of semivolatile organic compounds (SOCs) by rain is an important removal mechanism from the atmosphere and an important loading mechanism to terrestrial and aquatic systems. In this paper, the rain washout equation is modified to include gas-phase SOC adsorption to the surface of raindrops. The relative influence of gas adsorption is a function of the air–water interface adsorption constant, Kia, Henryʹs law CONSTANT, H, the fraction of the SOCs associated with particles, φ, and the diameter of the raindrop, dR. Theoretical gas-phase washout ratios were calculated assuming a rainfall intensity of 10 mm/h and spherical raindrops for polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs). Results for PAHs indicate that gas-phase washout including surface adsorption exceed those calculated considering only Henryʹs law dissolution for low volatility PAHs. This shift in gas-phase partitioning being dominated by dissolution to the bulk water of the rain drop to being dominated by adsorption to the raindrop surface occurs at sub-cooled liquid saturated vapor pressures around 10−5 Pa, corresponding to PAHs with greater than or equal to five aromatic rings. Similar calculations for PCBs indicate similar gas-phase washout ratios (10 to 104), and a similar shift from dissolution to adsorption in the range of sub-cooled liquid saturated vapor pressures of 10−2.7 to 10−1.4 Pa. Calculations for polychlorinated dibenzo-p-dioxins (PCDDs) indicate that adsorption dominates all gas-phase washout of PCDDs. Kinetic calculations indicate that even the least volatile SOCs reach equilibrium with the rain within hundreds of meters of the bottom of a cloud, depending on rainfall rate. Given the theoretical domination of gas-phase adsorption to the surface of raindrops, traditional rain samplers may underestimate actual washout ratios and ultimately loadings to terrestrial and aquatic surfaces because of the loss of water surface area upon collection.