Polycyclic aromatic hydrocarbons in highway plants and soils. Evidence for a local distillation effect

Poa trivialis grasses and soils from the side slope of a high-traffic highway were analyzed for their PAH content by gas chromatography coupled to mass spectrometry. The ratios of volatile, low-molecular weight PAHs versus high-molecular weight PAHs increase with side slope height. For instance, naphthalene/pyrene values increase from 0.3 to 3.4 in plants. Acenaphthene/fluoranthene values increases from 0.017 to 0.123 in soils. Moreover, soil PAHs can be classified into two categories according to variations of absolute concentrations with height: low-molecular weight PAHs showing an increase and high-molecular weight PAHs showing a decrease. These results demonstrate the occurrence of an atmospheric distillation effect which favour the concentration of high-molecular weight PAHs near the PAH source. Environmental implications of such a phenomenon are discussed.