Polycyclic aromatic compounds in air - importance of re-volatilisation from soils

Polycyclic aromatic hydrocarbons (PAHs) and their nitrated and oxygenated derivatives (NPAHs, OPAHs) are semivolatile air pollutants which may re-volatilise following atmospheric deposition to soil. Here, we determined polycyclic aromatic compounds' (PACs) concentrations in air, topsoil and air equilibrated with soil at a rural grassland site and a boreal forest site. The samples were analysed by GC-MS/MS and the flux was studied by fugacity calculations and a micrometeorological flux-gradient method. In soil at the boreal site, OPAH and NPAH contributed much higher fractions to PAC contamination of air and soil than at the rural site, reflecting the conversion of primary PACs during transport from sources. The same number of PACs in soil have reached equilibrium concentration with air, despite one site being in a polluted and PAC source region and the other in a much less polluted and PAC receptor region. The direction of air-soil exchange was found insensitive to the modest day-night temperature variation at both sites. Upward diffusive flux was observed for one PAH, triphenylene, at the rural grassland site, and for three 3-ring PAHs and naphthaldehyde at the boreal forest site. There, emission fluxes were quantified and ranged 0.5-12 ng m^-2 h^-1. The results indicate that low molecular weight PAH secondary emissions in a receptor region may be of same order of magnitude as domestic primary emissions.