Metals emitted from heavy-duty diesel vehicles equipped with advanced PM and NOX emission controls

Abstract

Emission factors for elemental metals were determined from several heavy-duty diesel vehicles (HDDV) of 1998-2007 vintage, operating with advanced PM and/or NOX emissions control retrofits on a heavy-duty chassis dynamometer, under steady state cruise, transient, and idle conditions. The emission control retrofits included diesel particulate filters (DPF): catalyzed and uncatalyzed, passive and active prototype vanadium- or zeolite-based selective catalytic reduction (SCR) systems, and a catalyzed DPF fitted on a hybrid diesel electric drive vehicle. The prototype SCR systems in combination with DPF retrofits are of particular interest because they represent the expected emissions controls for compliance with PM and NOX regulations in 2010. PM samples from a full-exhaust dilution tunnel were collected on bulk filters, and on a Personal Cascade Impactor Sampler (PCIS) for total and water-soluble elemental analysis. All the DPFs significantly reduced emissions of total trace elements (>85% and >95% for cruise and for the Urban Dynamometer Driving Schedule (UDDS), respectively). However, we observed differences in the post-retrofit metals emissions due to driving cycle effects (i.e., exhaust temperature) and type of retrofit. In general, the metals emissions over cruise conditions (which leads to higher exhaust temperatures) were substantially different from the emissions over a transient cycle or while idling. For instance, during cruise, we observed higher levels of platinum (1.1 ± 0.6-4.2 ± 3.6 ng km-1) for most of the retrofit-equipped vehicle tests compared to the baseline configuration (0.3 ± 0.1 ng km-1). The vanadium-based DPF + SCR vehicle during cruise operation exhibited emissions of vanadium (562 ± 265 ng km-1) and titanium (5841 ± 3050 ng km-1), suggesting the possible release of actual SCR wash-coat (V2O5/TiO2) from the catalyst under the higher temperatures characteristic of cruise operation. The vanadium emissions exhibited a bi-modal mass size distribution, with modes at <0.25 μm and 1.0-2.5 μm size ranges for the vanadium-based SCR system. For the DPF + SCR systems, a greater fraction of the metal emissions from the zeolite-based system is water-soluble compared to emissions from the vanadium-based system.