Real-world particulate, GHG, and gaseous toxic emissions from heavy-duty diesel and natural gas vehicles

This study assessed the real-world emissions from two heavy-duty natural gas vehicles certified to the optional 0.02 g/bhp-hr nitrogen oxides (NOx) standard and one diesel vehicle equipped with diesel particulate filter (DPF) to control particulate matter (PM) mass emissions and selective catalytic reduction (SCR) to control NOx emissions. Testing was performed on four different routes typical of goods movement in the greater South Coast Air Basin using a mobile emissions laboratory equipped with a constant volume sampling (CVS) system. This study emphasized the characterization of solid particle number (SPN) and total particle number (TPN) emissions, carbonyl compounds, as well as the greenhouse gas (GHG) emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Results showed that SPN and TPN emissions were several orders of magnitude higher for the natural gas vehicles compared to those of the DPF-equipped diesel vehicle. Natural gas vehicles under motoring conditions were found to have a strong nucleation mode of ultrafine particles below 23 nm. PM mass chemical speciation of metals and trace elements indicated that the source of PM mass from natural gas vehicles was lubricant oil combustion. The low molecular weight formaldehyde, acetaldehyde, and acrolein were the predominant aldehydes in the exhaust of all vehicles, with the formaldehyde emissions below the emissions limits for both diesel vehicles and natural gas vehicles. Emissions of CO2 were lower for the natural gas vehicles, as a result of the lower carbon-to-hydrogen (C/H) molar ratio of natural gas compared to diesel fuel. Similarly, the CO2 equivalent emissions for the natural gas vehicles were about 20% lower than those of the diesel vehicle. CH4 emissions were the dominant fraction of total hydrocarbons for the natural gas vehicles, while for the diesel vehicle CH4 concentrations were negligible. N2O emissions were very low for the natural gas vehicles, whereas N2O formation was favored in the SCR-equipped diesel vehicle. [Display omitted] •Natural gas vehicles showed more than an order of magnitude higher SPN and PM emissions than diesel.•About 56% of TPN emissions reside below the 23 nm size threshold under high engine load conditions.•Strong nucleation mode particles below 23 nm observed during motoring conditions.•PM formation from natural gas vehicles was related to lubricant oil combustion.•Formaldehyde, acetaldehyde, and acrolein were the dominant aldehydes in vehi