Particle-bound polycyclic aromatic hydrocarbons in Malta : levels and sources

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are organic compounds that are usually found in the environment associated with combustion sources. Their presence in the environment is of concern as many of these compounds, the best studied benzo(ɑ)pyrene, are known to be carcinogenic to humans. In this study the concentrations of P AHs in airborne particulate matter and in street dust was analysed. For the airborne particulate matter, 10 localities have been chosen to represent different air pollution situations in Malta and from which two 60 hour samples were taken one for PM10 and another for PM2.5 (these fractions include particles having aerodynamic diameters smaller than 10µm and 2.5µm respectively). For the street dust, 5 samples were collected, 4 from the kerbside of very busy roads and another from a low traffic road. Particulate matter samples were analysed for their particulate matter content using a gravimetric procedure. P AH analysis was performed using gas chromatography with flame ionisation detection (GC-FID) with an extraction and clean up procedure based on Soxhlet extraction and column chromatography. The particulate matter concentrations found, 17.9-71.2µg Nm-3 for the PM10 and 8.3- 62. 7µg Nni3 for the PM2.5, show large variations which were not readily explained by meteorological conditions. PAHs were found to be ubiquitous in Malta with a concentration of l.46-23.26ng Nm-3 which on average lies on the lower side when compared to other countries and is considered to involve a low health risk, but which becomes significantly high in localities with heavy traffic loads and which are situated in the most industrialized parts of Malta. The most abundant PAHs in ambient air were found to be phenanthrene and benzo(g,h,i)perylene. Using diagnostic PAH ratios and principal component analysis (PCA), vehicular traffic was identified as the major source of these pollutants with catalyst equipped petrol vehicles and diesel vehicles being singled out as particularly important sources. An interesting unexpected positive correlation has been found between the P AH concentrations in ambient air and the atmospheric temperature. This correlation conflicts with opposite results from other studies and requires further investigation. Street dust samples suggest that concentrations of P AHs are very high at street level, 2.580-ll.821µg g-1 in dust, and that particle re-suspension may pose a significant health risk at the kerbside of the busiest arterial roads. A positive correlation was found between the carbon and the P AH content of dust, while, a slight negative correlation was found with the fineness of the dust sample. Both of these trends confirm a combustion source for PAHs.