ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-4105-2008 Depletion of gaseous polycyclic aromatic hydrocarbons by a forest canopy Choi S.-D. 1 Staebler R. M. 2 Li H. 1 3 Su Y. 1 4 Gevao B. 1 5 Harner T. 2 Wania F. 1 Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario, Canada Science and Technology Branch, Environment Canada, Toronto, Ontario, M3H 5T4, Canada now at: Worsfold Water Quality Centre, Trent University, Peterborough, Ontario, Canada now at: Science and Technology Branch, Environment Canada, Toronto, Ontario, Canada now at: Department of Environmental Science, Environment and Urban Development Division, Kuwait Institute for Scientific Research, Kuwait 31 07 2008 8 14 4105 4113 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/8/4105/2008/acp-8-4105-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/4105/2008/acp-8-4105-2008.pdf

Rapid uptake of gaseous polycyclic aromatic hydrocarbons (PAHs) by a forest canopy was observed at Borden in Southern Ontario, Canada during bud break in early spring 2003. High volume air samples were taken on 12 individual days at three different heights (44.4, 29.1, and 16.7 m) on a scaffolding tower and on the forest floor below the canopy (1.5 m). Concentrations of PAHs were positively correlated to ambient temperature, resulting from relatively warm and polluted air masses passing over the Eastern United States and Toronto prior to arriving at the sampling site. An analysis of vertical profiles and gas/particle partitioning of the PAHs showed that gaseous PAHs established a concentration gradient with height, whereas levels of particulate PAHs were relatively uniform, implying that only the uptake of gaseous PAHs by the forest canopy was sufficiently rapid to be observed. Specifically, the gaseous concentrations of intermediate PAHs, such as phenanthrene, anthracene, and pyrene, during budburst and leaf emergence were reduced within and above the canopy. When a gradient was observed, the percentage of PAHs on particles increased at the elevations experiencing a decrease in gas phase concentrations. The uptake of intermediate PAHs by the canopy also led to significant differences in gaseous PAH composition with height. These results are the most direct evidence yet of the filter effect of forest canopies for gaseous PAHs in early spring. PAH deposition fluxes and dry gaseous deposition velocities to the forest canopy were estimated from the concentration gradients.

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