Influence of soot carbon on the soil-air partitioning of polycyclic aromatic hydrocarbons

Soil-air partitioning is one of the key processes controlling the regional and global cycling and storage of polycyclic aromatic hydrocarbons (PAHs). However, the specific processes dominating the partitioning of PAHs between these two environmental compartments still need to be elucidated. Stable and distinct atmospheric conditions paralleling different soil properties are found at Tenerife island (28degrees18'N, 16degrees29'W), which is located in permanent inversion layer conditions, and they provide interesting model cases for the study of air-soil partitioning. Analysis of phenanthrene, pyrene, fluoranthene, and chrysene showed concentrations 4- to 10-fold higher below than above the inversion layer. Similarly, soil total organic carbon (TOC) and black carbon (BC) were 11 and 3 times higher, respectively, below the inversion layer than above. The octanol-air partition coefficient (K-OA) derived model provides a good description of PAH soil-air partitioning coefficients (K-P) below the inversion layer but underpredicts them in the area dominated by deposition of long-range transported aerosols without inputs of organic matter from local vegetation. Inclusion of soot carbon in the soil-air partitioning model results in good agreement between predicted and measured K-P in this area but in overpredicted K-P values for the soils under the vegetation cover, which shows that the influence of soil soot carbon on PAH air-soil partitioning depends on its abundance relative to soil organic carbon. Absorption into organic matter is the dominant process in soils containing high organic carbon concentrations, whereas adsorption onto soot carbon becomes relevant in soils with low organic carbon and high soot content.