AIR QUALITY ASSESSMENT AND METAL SOURCE APPORTIONMENT IN TEHRAN, IRAN: TREE BARK BIOMONITORING

Metal-rich particulate matter (PM) emitted into the atmosphere from industrial sites and vehicles contributes to degradation of air quality in urban environments, and can lead to adverse health effects. Tehran suffers from substantial air pollution due to heavy road traffic and industrial activity, including power plants, a cement factory and an oil refinery all located within city limits, as well as a constant influx of natural dust from Syria, Iraq, and Arabian countries. Topographic barriers prevent ventilation and dispersion of air pollutants.

In order to assess the degree and sources of atmospheric metal pollution in Tehran, we employed bio-monitoring using pine tree bark, an ideal natural trap for atmospheric particulate matter. Samples were collected in transects from potential point sources towards the city center, including a background sample from a rural area NW of the city. Major and trace element abundances and Pb isotope ratios were measured in 29 tree bark samples by ICP-OES/MS and TIMS, respectively.

Mo, Zn, Pb, Cu, Cd, As, Mn, and Ni are moderately to significantly enriched in Tehran tree bark compared to the background sample, with Enrichment Factors (EF) for these metals ranging from 2 to 10. Calculated Geo-accumulation indices (I_geo) ranging from 2 to 5 are further indicative of moderate to strong pollution. Pb concentrations in all samples are very high, ranging from 22 to 656 ppm in tree bark ash.

Pb isotope ratios exhibit large ranges and define a linear array in all Pb-Pb isotope plots, consistent with mixing between a relatively radiogenic natural background source and a distinctly unradiogenic anthropogenic source most pronounced near the airport. Other potential contaminant sources have Pb isotope ratios intermediate between these endmembers, thus their contributions cannot be discerned by Pb isotopes alone. However, plots of Pb isotope ratios versus element/Al ratios (e.g. Pb/Al, Mo/Al) record the presence of additional contaminant sources including the cement factory and road traffic. Samples from the city center are compositionally intermediate to these sources, suggesting that natural dust as well as anthropogenic contaminants from the airport (aviation fuel?), cement factory and road traffic all contribute significantly to the metal load of atmospheric particulate matter in Tehran.