Physicochemical characterization of nitrated and oxy-polycyclic aromatic hydrocarbons in the atmospheres of two French alpine valleys
Résumé
The objective of this study was to characterize the NPAHs and OPAHs in ambient air on various sites (traffic, suburban, rural and altitude). Atmospheric concentrations of these classes of compounds are of interest because they include potential mutagens and carcinogens. Samplings were performed within the framework of the French research program POllution des Vallées Alpines (POVA) during the winter 2002-2003 and the summer 2003. Both, ambient air particulate and gas phases were collected and particle size distribution was also studied. Results show that, OPAH concentration levels were of the same order of magnitude as PAHs while NPAH concentrations were one to two orders of magnitude lower. Study of source specific ratios (2-nitrofluoranthene/1-nitropyrene) clearly showed these compounds had a primary origin at the sites close to the sources of pollution whereas production of secondary NPAHs by gas phase reactions was prevalent at the rural sites far to the direct sources of pollution. Because a weak formation of secondary NPAHs by gas reaction in winter, the relative contribution of NPAHs from primary sources increased during this season. Nevertheless, in winter under specific conditions, evidence of secondary NPAH formations was observed at suburban and traffic sites (snowfalls) and rural site (pollutants accumulation and snowfalls). Despite their lower atmospheric concentrations, carcinogenic risk (estimated using toxic equivalent factors) attributed to the NPAHs could reach 20% of the total risk. This risk becomes more critical on the sites away from pollution sources, where the formation of the secondary compounds like NPAHs, have been highlighted. The fraction of PAHs, OPAHs and NPAHs associated with the particle phase was strongly depending on their vapour pressure and the ambient conditions. During both seasons, PAHs, OPAHs and NPAHs were mainly associated (>80%) with fine particles (Dp<1.3 µm). The differences of physicochemical properties of the classes of compounds could account in their particle size distribution.