Atmospheric deposition of semivolatile organic compounds to two forest canopies

Abstract

The deposition of polycyclic aromatic hydrocarbons, hexachlorobenzene and polychlorinated biphenyls, dibenzo- p-dioxins and dibenzofurans to a coniferous and a deciduous forest canopy was measured by simultaneously sampling bulk deposition below the canopies and in an adjacent clearing for one year. In addition, ambient air was sampled continuously, with separate analysis of the gaseous and particle-bound phases. The deposition of almost all compounds was higher under the forest canopies than in the clearing. The excess deposition to the forest sites was attributable to equilibrium partitioning between the atmosphere and the canopy vegetation, kinetically limited gaseous deposition, or particle-bound deposition. Which of these deposition processes dominated for a given compound was shown to depend on the octanol–air partition coefficient of the chemical and its gas/particle partitioning. Deposition velocities–to our knowledge the first for SOCs to forests–were calculated by dividing the excess deposition by the air concentrations. The gaseous deposition velocities were 0.78 cm s -1 to the coniferous canopy (annual weighted average) and 3.6 cm s -1 to the deciduous canopy (6 month summer average). These values are high compared to deposition velocities to forest canopies that have been measured for inorganic gases, reflecting the fact that lipophilic organic chemicals are taken up by the leaf/needle cuticle and not just via the stomata. The dry particle bound deposition velocities for particle diffusion and impaction were 0.05 and 0.73 cm s -1 for the coniferous and deciduous canopies, respectively. These values are considerably lower than the gaseous deposition velocities, underlining the importance of gaseous deposition for the accumulation of semivolatile organic compounds in forest ecosystems.