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Characterization and modeling of Polycyclic Aromatic Compound uptake into spruce tree wood. Cassandra Rauert, Ajitha Kananathalingam, and Tom Harner Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01297 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017
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Environmental Science & Technology
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Characterization and modeling of Polycyclic Aromatic Compound uptake into spruce tree wood.
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Cassandra Rauerta, Ajitha Kananathalingama,b, Tom Harnera*
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a
Air Quality Processes Research Section, Environment and Climate Change Canada, 4905 Dufferin St. Toronto, Ontario M3H 5T4, Canada
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b
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* Corresponding author:
[email protected]; tel. +1 416 739 4837
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University of Toronto at Scarborough, 1265 Military Trail Toronto, Ontario M1C 1A4, Canada
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Abstract This study highlights the potential of uptake into tree inner wood via direct-transfer through bark, as one contributing mechanism to describe atmospheric uptake of polycyclic aromatic compounds (PACs) into trees. The uptake of PACs into blue spruce tree wood was measured, with wood-air partition coefficients (KWOOD_AIR) determined for 5 PACs. A correlation between the octanol-air partition coefficient (KOA) and KWOOD_AIR for these 5 chemicals was determined and the KWOOD_AIR for 43 PACs were derived. A ratio of solubility (activity) difference between tree wood and octanol was also determined for these chemicals from this correlation. Finally, the derived KWOOD_AIR values were further applied to calculate an air volume sampled by the inner wood layer (cambium) of a tree during a one year growth (sampling) period. PACs with a log KWOOD_AIR > 6 remained in the linear sampling phase over one year of sampling. The results further highlight the important sink that forest provide for atmospheric organic chemicals which should be considered for emissions monitoring and impact assessments from destructive events such as forest fires or clear felling of forests.
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Key Words: Air-to-tree wood partitioning, polycyclic aromatic compounds, uptake model
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Introduction
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The potential for tree inner wood to provide historic air concentration profiles of persistent organic pollutants (POPs) was first explored by Meredith and Hites in 19871. Since then a few studies have successfully provided historical trends for a range of POPs, from analysis of tree cores.2-6 Non-polar organic chemicals in the atmosphere that are in contact with a tree are taken up into the inner wood (cambium) layer, Figure 1. There is thought to be negligible radial diffusion into the tree3 and translocation in the xylem (cells that transport water from the roots to the stem/foliage of a tree) or phloem (tissue that translocates nutrients e.g. sugars around the tree) is minimal.7 This results in the chemical staying in that specific wood layer, or tree ring, and remaining largely intact in the case of persistent chemicals. The next year a new layer of cambium grows, capturing chemicals the tree is then
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exposed to, in that particular years’ growth. Thus by analysing the separate layers (or tree rings), we may be able to recreate a historic profile trend of atmospheric levels of organic chemicals. However, there is still an inadequate understanding as to the mechanisms in play for the take up of organic chemicals into this cambium layer.
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Trees can take up contaminants in the environment by a variety of mechanisms including absorbing contaminants from soil/water via the root system with translocation around the tree; deposition of air borne contaminants onto foliage with subsequent translocation around the tree in the phloem; or direct deposition through the bark into the inner wood. For this reason vegetation has been used extensively in phytoremediation of contaminated land, in particular removing heavy metals from soil and water.8 For hydrophobic semi-volatile organic compounds (SVOCs), such as the polycyclic aromatic compounds (PACs), uptake from soil via water into the tree roots is limited7,9 and the primary mechanism for uptake has been suggested to be via atmospheric deposition. 4,9 The exception are the smaller 2 ring PACs (such as Naphthalene and methyl-Naphthalene) which are water soluble and uptake via the roots may be a contributing mechanism to total tree burden.4 A range of PACs have been reported in the foliage of many different tree species10 and previous studies have theorised atmospheric uptake of PACs into the cambium may be primarily via gaseous and particle disposition onto tree foliage with subsequent vertical translocation around the tree via the phloem.4 The recent study by Yang et al.11 investigated this migration into the inner tissue of camphor leaves. PACs in foliar dust and gas-phase chemicals were shown to migrate via the cuticular wax into the inner layer with gas-phase transfer more important in the summer months. However the transfer of PACs around the tree via the phloem is still uncertain and the study by Wang et al. did not detect evidence of PAC translocation in the phloem of accumulated PACs from leaves. 7
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A secondary mechanism of atmospheric uptake is direct mass transfer through the bark, involving a combination of diffusion through the bark layer and/or infiltration through the bark layer and direct deposition to the cambium. The air to bark partitioning of persistent organic pollutants (POPs) has previously been modelled by Zhao et al.12 but there is a knowledge gap as to how the contaminants would then taken up into the cambium layer. Gas phase chemicals have been suggested to reach the cambium via lenticels (a group of cells forming a pore) in the bark, providing channels for gas phase chemicals7 to diffuse directly into the cambium and perhaps trapping fine particles.1 This would be largely influenced by the bark properties of the particular tree species. Zhao et al. reported the pore size of a range of trees with conifers having some of the larger pore sizes (1.07 and 1.18 μm for hoop and mason pine respectively).12 Therefore, particles
