Emissions of PM2.5-Bound Trace Metals from On-Road Vehicles: An Assessment of Potential Health Risk

Abstract

The present study elucidates on PM2.5 (particle aerodynamic diameter ≤ 2.5 μm) bound trace metals characterization from on-road light-duty vehicles during on-road operation and their health risk assessment for adults and children. The vehicles assessed in present work included 4-wheelers passenger cars with a different age group of Bharat Stage (BS) II, III, and IV and different fuel type [diesel, gasoline and compressed natural gas (CNG)]. To understand the particle losses, particle formation, and homogenous mixing, firstly, a new portable dilution system (PDS) was designed for diluting the exhaust with adequate aerosol formation and growth, and evaluated beforehand under controlled condition in laboratory for diesel vehicles over a wide range of dilution ratio (30:1, 60:1, and 90:1). For on-road experiments, a PDS, a heated duct, a gas analyzer, an exhaust velocity probe, a temperature, and relative humidity probe were mounted on Aerosol Emissions Measurement System (AEMS) and it was towed behind the vehicle. Total 46 experiments were performed on a mixed traffic route in Delhi city, and PM2.5 mass was collected on Teflon and quartz filter using multi-stream PM2.5 sampler. Total 17 trace metals (Al, Ag, As, Ba, Co, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Se, Sr, Ti, V, and Zn) were characterized on Teflon filters using Induced Coupled Plasma-Mass Spectrometry (ICP-MS). Out of these metals, the non-carcinogenic and carcinogenic risks for adults and children were calculated for 5 metals namely Cr, Mn, Ni, Zn, and Pb. Trace metals concentration was highest in exhaust emitted from 4W-diesel followed by 4W-gasoline, 3W-CNG, and 2W-gasoline. Trace metals such as Cr, Mn, Ni, Zn, and Pb were present in high amount (25.2 ± 8.6 μg m−3). The carcinogenic risk from Cr was considerably higher than tolerable risk (10−4), while the risk from other metals such as Ni, As, Cd, and Pb was within the range of safe (10−6) and tolerable (10−4) level. Overall, the human health risks associated with the exposure to PM2.5 emitted from gasoline and CNG vehicles were higher than that from diesel vehicle. This estimated risk in this work can help in refining the burden of disease and crafting policy to help reduce the exposures. This study is limited only for PM2.5 bound trace metals and associated health risk from on-road vehicle emissions. However, poly-aromatic hydrocarbons (PAHs), perfluorinated compounds (PFCs), and semi-volatile compounds in vehicular exhaust can also impose a severe risk to human health which needs to be assessed to evaluate combined risk.