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

Polycyclic Aromatic Hydrocarbons (PAHs) are a major group of pollutants whose occurrence in the environment is mainly of anthropogenic origin. In this paper, we examine the effect of topographical slope exposure on PAH contamination and seasonal change in PAH concentrations in soils. We collected soil samples on three dates in 2007 (early May, end of July and beginning of November) from south- and north-facing aspects at 1900 m a.s.l. in the central Italian Alps. We found greater PAH contamination in soils from a north-facing slope than in those from a south-facing slope at all seasons. We calculated North-South Enrichment Factors as the ratio between the concentrations measured in soils from northern and southern aspects. These ratios ranged from 1.4 to 1.9 for lighter PAHs (from 2 to 4 rings). These values are consistent with theoretical calculations based on temperature-specific octanol-air partition coefficients (predicted North-South Enrichment Factors range from 1.6 to 2.0). For heavier PAHs (from 5 to 6 rings), smaller differences were observed between soils from northern and southern aspects, due to the gas/particle distribution of these compounds. We also found consistent differences in normalised PAH concentrations across the three sampling periods. The majority of compounds showed a significant decreasing trend from the beginning of May to the end of July, due to the annual cycles of physical processes (deposition vs. volatilisation) and biological processes (uptake and/or biotransformation). Only a few compounds showed different trends, presumably due to season-specific local emission sources.