Does the forest filter effect prevent semivolatile organic compounds from reaching the Arctic?

Abstract

Forests act as efficient filters for many airborne semivolatile organic compounds (SOCs). However, most simulations of an organic chemical's long-range transport in the atmosphere do not account for this filter effect. In this study, forests are introduced into an existing zonally averaged global distribution model (Globo-POP) to investigate how such a change affects a chemical's potential to undergo long range transport and accumulation in the Arctic, as quantified by the Arctic contamination potential (ACP). Simulation results indicate that the ACP of a "space" of perfectly persistent hypothetical organic chemicals, defined by log KOA and log KAW, is reduced by introducing forests in the global model. Depending on partition characteristics, this reduction can be as large as a factor of 2. Model calculations also indicate that it is mostly the boreal forests, specifically boreal deciduous forests, which play a key role in this respect. Sensitivity analyses establish the deposition velocity to boreal forests, especially for gaseous compounds, as one of the most influential parameters controlling this global forest filter effect. The extent of the effect is further sensitive to the forest density and precipitation rate in the boreal zone, and the degradation rates of the chemical. Specifically, degradation in the forest canopy may enhance the effect and further reduce an SOC's long range transport to remote regions. Simulations for three PCB congeners suggest that forests may reduce concentrations in air, ocean, and freshwater at the expense of increased concentrations in forest soils and may lead to substantially increased overall global residence times.