Soils and surface waters are secondary sources of polycyclic aromatic compounds

Abstract

Polycyclic aromatic hydrocarbons (PAHs) in the atmospheric environment are almost exclusively formed in combustion processes. Oxygenated and nitrated PAHs are co-emitted with parent PAHs from fossil fuel and biomass combustion processes, and many are formed in photochemical and microbiological reactions of PAHs in air and soil. As semivolatiles resisting biodegradation in soils and surface waters to some extent, polycyclic aromatic compounds (PACs) i.e., PAHs and their derivatives, can be subject to re-volatilisation., which may turn soils and surface waters from sinks into secondary sources and enhances the long-range transport potential of PACs by multihopping (grasshopper effect). The significance of these secondary sources for PAC abundances in ambient air is unknown and is not accounted for in emission inventories. Gaps in PAH emission inventories have been indicated by field studies in various countries. We determined the concentrations of 15 parent, 10 oxygenated and 17 nitrated PAHs in air and soils at a rural and near-coastal northern European site and a central European rural background site, and in air and surface seawater at two off-shore sites in the eastern Mediterranean and along NW-SE transects in the Mediterranean. Directions of air-soil and air-sea exchanges were derived from the substances’ fugacities. At the central European site, a number of 2-4 ring PACs were found to volatilise from grassland and more from forest soils in summer, and much less in winter. Conversely, at the receptor site in northern Europe, net deposition of PACs prevails and re-volatilisation occurs only sporadically. In the Mediterranean, 3-4 ring PAHs and dibenzofuran are found to volatilise in most seasons. Existing data on air-surface exchange of PACs is notably scarce, and methodological uncertainties persist in quantifying air-soil exchange. As very little is known about the spatial and seasonal distributions of PACs soil burdens and net mass fluxes, an assessment of the significance of soils and surface waters as secondary sources of PACs in the air of source and receptor areas is not possible.