Polycyclic aromatic hydrocarbons (PAHs) constitute a class of organic contaminants, that can derive from the incomplete combustion of wood and fossil fuels and vehicular traffic. Once emitted they can be found in air as gas or particulate phase. Plants can remove PAHs from the air through the so-called Forest Filter Effect (FFE) accumulating these compounds in leaves and transferring them to soil. Moreover, phyllosphere microorganisms can contribute to the biodegradation of PAHs. However, no studies are available so far about the presence and source of PAH metabolites in plant leaves.

In this work, new aspects of FFE were investigated through several laboratory experiments. More specifically, the study was designed to evaluate for the first time the contribution of light and microbial communities of Quercus ilex leaves to the degradation of deuterated PAHs. Such degradation was investigated by observing the production of deuterated hydroxy-PAHs. This was performed using an UHPLC-HRMS (LC-Orbitrap). In addition, a dynamic multimedia fate model (SoilPlusVeg) was used to predict the fate of PAH and their metabolites in a terrestrial ecosystem.

Hydroxy-PAHs were produced both in light and dark conditions, while their appearance in sterile controls was negligible, highlighting the degradation ability of native phyllosphere microbial communities.

Laboratory data and model predictions showed that FFE can be enhanced by photo- and biodegradation, because these reactions could help to maintain a high air to leaf PAH gradient, since these compounds could be continuously removed. Additionally, the metabolites produced, being more polar and less hydrophobic, are likely washed off during rainfall, and transported to the terrestrial environment, restoring the PAH gradient.

Further and more realistic studies are necessary to better quantify the contribution of phyllosphere biodegradation and photodegradation on PAH removal from air and their precise role in FFE. PAH metabolite ecotoxicity needs also to be evaluated.