Seasonal changes and temperature-dependent accumulation of polycyclic aromatic hydrocarbons in high-altitude soils

Polycyclic Aromatic Hydrocarbons (PAHs) are a group of priority pollutants whose occurrence in the environment is mainly of anthropogenic origin. In this paper, the effect of the slope exposure on the PAH contamination and the seasonal change of the PAH concentrations in soils were discussed on the base of several soil samples taken on three dates in 2007 (early May, end of July and beginning of November) from the south and north aspect at 1900 m a.s.l. on the central Italian Alps. The first characteristic was the higher PAH contamination in soils from the north in comparison to those from the south in all seasons. The North-South Enrichment Factors were calculated as the ratio between the north and the south concentration, giving a range from 1.4 to 1.9 for lighter PAHs (from 2 to 4 rings). These values were in agreement with theoretical calculations based on temperature-specific octanol-air partition coefficients (predicted North-South Enrichment Factors from 1.6 and 2.0). For heavier PAHs (from 5 to 6 rings), lower differences were observed according to the gas/particle distribution of these compounds. A second finding was the consistent differences in normalised concentrations during the three periods of the year. The majority of compounds showed a significant decreasing trend from the beginning of May to the end of July, according to the year cycle of the physical processes (deposition vs. volatilisation) and the biological processes (uptake and/or biotransformation). Only few compounds showed different trends presumably due to seasonal-specific local emission sources.