Scientists and regulators have long known that PM is a mixture of different-sized particles. Within the PM category, ultrafine particles (UFPs) make up the smallest size fraction (d ≤ 100 nm). In the United States, PM is regulated on a mass basis as part of engine, vehicle, and ambient air quality standards. A small fraction of PM is attributed to UFPs, but when quantified on a number basis, UFPs may account for more than 90% of the total number of engine PM. Both animal and human studies have provided evidence for respiratory and cardiovascular effects associated with exposure to UFPs. Currently, there are no regulatory standards that directly control UFPs in the United States. U.S. EPA concluded that more research is needed to further investigate the role of UFPs on PM-mortality associations in its 2009 Integrated Science Assessment. In recognition that UFPs may be associated with potential health impacts, U.S. EPA has encouraged conducting UFP measurements at near-road monitoring sites.

Laboratory dynamometer emission tests have evaluated the total particle number emissions (PN) from HDVs with and without aftertreatment devices. Previous on-road tests have measured the concentration of solid particles, which are defined as particles that remain in the aerosol phase after thermal treatment at around 300 ˚C with diameters between 23 nm to 2.5 μm, using the Particle Measurement Programme (PMP) method. However, limited studies have examined total particle number emissions from HDVs driving on road, which are typically formed during dilution rather than combustion. Therefore, the purpose of this study is to help improve the understanding of real-world total particle number emission patterns of various HDV technologies, which will also help to better understand the impacts of HDV on near-roadway UFP concentrations.

—Wang et al.

The team selected the vehicles in the study to represent four emission technology groups currently prevalent in California:

1. One MY 2007 (Vehicle Nº 1) diesel engine with no SCR and certified to a 2.3 g/bhp-hr NO_x family emission limit (FEL);

2. Two MY 2013 (Nº 2 and Nº 5) and a MY 2014 (Nº 4) diesel engines equipped with SCR;

3. One MY 2013 compressed natural gas (CNG) (Nº 3) 12-L engine with a three-way catalyst (TWC) certified to the 0.2 g/bhp-hr NO_x standard; and

4. One hybrid diesel vehicle (Nº 6) with a MY 2011 engine with no SCR and certified to a 0.47 g/bhp-hr NO_x FEL.

All diesel vehicles were equipped with a DOC and a DPF.

Six route types represented real-world driving conditions associated with freight operation in California, including steep hill climb; highway speed; congestion; stop-and-go operations at ports and rail yards; and urban “last-mile” delivery routes.

Route-specific PNEFs (in log scale) of the six vehicles compared to a reference study of MY 1998 diesel vehicles in the transient and cruise at 50 mph on the dyno. Credit: ACS, Wang et al. Click to enlarge.

All test vehicles pulled the West Virginia University’s Transportation Emissions Measurement System (TEMS), which was affixed to a flatbed trailer along with an on-board power generator. The TEMS housed a full-scale constant volume sampling (CVS) and a suite of gaseous and particle emission measurement systems. Two dilution systems were used for measuring total particle number concentrations in this study.

Sampling configuration. Credit: ACS, Wang et al. SI. Click to enlarge.

Broadly, they found that for vehicles with OEM DPFs and Selective Catalytic Reduction Systems, PNEFs under highway driving (i.e., 3.34 × 10¹² to 2.29 × 10¹³ particles/mile) were larger than those measured on urban and drayage routes (i.e., 5.06 × 10¹¹ to 1.31 × 10¹³ particles/mile).

They suggested that this likely occurs because a significant amount of nucleation mode volatile particles were formed when the DPF outlet temperature reached a critical value—usually over 310 °C, which was commonly achieved when vehicle speed was sustained over 45 mph. A model year 2013 diesel HDV produced approximately 10 times higher PNEFs during DPF active regeneration events than non-active regeneration.

Resources

• Tianyang Wang, David C. Quiros, Arvind Thiruvengadam, Saroj Pradhan, Shaohua Hu, Tao Huai, Eon S. Lee, and Yifang Zhu (2017) “Total Particle Number Emissions from Modern Diesel, Natural Gas, and Hybrid Heavy-Duty Vehicles During On-Road Operation” Environmental Science & Technology doi: 10.1021/acs.est.6b06464