A HRTEM/EDX approach to identification of the source of dust particles on urban tree leaves

Dust on tree leaves in the urban area of Hangzhou, China, was analyzed in terms of heavy metal contents and magnetic properties. Morphological and chemical composition of the dust particles were analyzed using a high resolution transmission electron microscope equipped with an energy-dispersive X-ray analyzer (HRTEM/EDX). Results indicated that the dusts contained high concentrations of Cd (mean 2.62), Cu (63.7), Zn (535.9) and Pb (150.9 mg kg−1). Magnetic susceptibility of the dusts was in a range of (16–856) × 10−8 m3 kg−1. It was shown that the dusts close to industrial area and busy road intersection had higher heavy metal contents and magnetic susceptibility. The dusts showed a strong positive inter-correlation for the concentrations of Fe, Mn, Cr, Zn, Pb, and Cu in addition to magnetic susceptibility, which suggests that the dusts had a common source for the heavy metals and magnetic carriers. We found that the dust particles were composed mainly of Fe-rich near-spherical, plate and agglomerate particles, and Ca-rich, S-rich and silicate particles, and that iron oxide spherules (0.2–0.5 μm in diameter) and larger iron-bearing particles were the magnetic carriers. Ca in the dusts was present in the forms of CaCO3 and CaCO3/CaSO4 internal mixture. The Fe-rich, Ca-rich and S-rich particles in dusts could be directly related to nearby polluting activities, such as coal combustion, traffic emission and industrial activity. The identification of the main sources of dusts on tree leaves can help in controlling the polluting sources in urban areas. The close correlation between magnetic susceptibility and heavy metal concentration makes it possible to use the magnetic technique as a non-destructive and time-efficient tool for biomonitoring of the atmospheric dust pollutants.