Magnetic susceptibility of dust-loaded leaves as a proxy of traffic-related heavy metal pollution in Kathmandu city, Nepal

Dust-loaded tree leaves from Kathmandu have been analyzed for magnetic susceptibility (χχ) and heavy metal (HM) contents. For 221 samples of leaves of cypress (mainly Cupressus corneyana), silky oak (Grevillea robusta) and bottlebrush (Callistemon lanceolatus  ), χχ has a range of (0.01–54)×10−8 m3 kg−1 with a median of about 10.0×10−8 m3 kg−1. Trees situated close to the busy road intersections, near the main bus station and sectors of roads with steep slope yield elevated susceptibility.Chemical analysis of 20 samples of varying susceptibility by atomic absorption spectrometry yields the following maximum HM contents: Fe (1.3 wt%), Mn (281.9 ppm), Zn (195.2 ppm), Cu (41.5 ppm), Pb (38.4 ppm), Ni (8.1 ppm), Cr (6.4 ppm), Co (4.1 ppm) and Cd (1.2 ppm). The logarithmic susceptibility on dry mass basis (χdχd) shows significant linear relationship with HM contents: Pearson's correlation coefficient r>0.8r>0.8 with Zn, Fe, Cr; r>0.7r>0.7 with Mn, Cu; r>0.6r>0.6 with Pb, Ni. Magnetic phases are of soft (magnetite/maghemite) and hard (hematite) coercivities. Microscopy of magnetic extracts reveals spherules (mostly of 2–20 μm diameter) originated from vehicle exhausts through the combustion process as well as crystalline grains of lithogenic origin.The dust accumulation in leaves took place mainly after monsoon (beginning of October 2001) till the sampling period (first half of February 2002). Despite the dependence of susceptibility and HM contents on a variety of spatial and temporal factors (amount of particulate matter (PM), efficiency of deposition/removal of PM by wind, precipitation, birds etc.), a significant correlation of susceptibility to HM implies that the former serves as an effective proxy of metallic pollution. Hence, susceptibility-based bio-monitoring technique is recommended as an economic and rapid tool for assessment of environmental pollution in urban areas like Kathmandu.