Temperature and Driving Cycle Significantly Affect Carbonaceous Gas and Particle Matter Emissions from Diesel Trucks

Abstract

The present study examines the effects of fuel [an ultralow sulfur diesel (ULSD) versus a 20% v/v soy-based biodiesel-80% v/v petroleum blend (B20)], temperature, load, vehicle, driving cycle, and active regeneration technology on gas- and particle-phase carbon emissions from light and medium heavy-duty diesel vehicles (L/MHDDV). The study is performed using chassis dynamometer facilities that support low-temperature operation (-6.7 °C versus 21.7 °C) and heavy loads up to 12 000 kg. Organic and elemental carbon (OC-EC) composition of aerosol particles is determined using a thermal-optical technique. Gas- and particle-phase semivolatile organic compound (SVOC) emissions collected using traditional filter and polyurethane foam sampling media are analyzed using advanced gas chromatograpy/mass spectrometry methods. Study-wide OC and EC emissions are 0.735 and 0.733 mg/km, on average. The emissions factors for diesel vehicles vary widely, and use of a catalyzed diesel particle filter (CDPF) device generally mutes the carbon particle emissions in the exhaust, which contains ~90% w/w gas-phase matter. Interestingly, replacing ULSD with B20 did not significantly influence SVOC emissions, for which sums range from 0.030 to 9.4 mg/km for the L/MHDDVs. However, both low temperature and vehicle cold-starts significantly increase SVOCs in the exhaust. Real-time particle measurements indicate vehicle regeneration technology did influence emissions, although regeneration effects went unresolved using bulk chemistry techniques. A multistudy comparison of the toxic particle-phase polycyclic aromatic hydrocarbons (PAHs; molecular weight (MW) ≥ 252 amu) in diesel exhaust indicates emission factors that span up to 8 orders of magnitude over the past several decades. This study observes conditions under which PAH compounds with MW ≥ 252 amu appear in diesel particles downstream of the CDPF and can even reach low-end concentrations reported earlier for much larger HDDVs with poorly controlled exhaust streams. This rare observation suggests that analysis of PAHs in particles emitted from modern L/MHDDVs may be more complex than recognized previously.

Figure 1.

Carbon-based pollutant emission factors for diesel vehicles (V1, V2, and V3) averaged over all test conditions. SVOCs characterize test sums of GC-MS identified species in the Q_f and PUF sampling arrays. OC and EC values were determined using Q_f only. Outliers (red circle), means (bold line), and median are shown. Box ends define the 25th and 75th percentiles; whiskers define the 10th and 90th percentiles. Note that the “∑SVOC” class sums include only GC-MS-identified compounds measured with Q_f-PUF array, underestimating the total SVOC emission contribution. Additionally, THC includes CH₄ and may include a fraction of the OC and SVOCs.

Figure 2.

Means and ranges of SVOC class-based emissions sums by driving cycle, vehicle, and temperature. Lower (−6.7 °C) and higher (21.7 °C) temperature tests are indicated by triangle symbol direction. Lower seasonal temperatures equate to higher emissions. Ranges are indicated using error bars. To assist in across-vehicle comparisons, the gray reference lines are set to 538 μg/km, which is the mean SVOC class-based emissions sum.

Figure 3.

Multistudy comparison of particle-phase PAH emission factors from diesel trucks. Averages are used or calculated, and vehicles are sorted by size (engine displacement) if possible. Additionally, if required, emission factors using fuel weight were converted using the fuel density and economy described in the main text. The Supporting Information contains further details about each study, compound abbreviations, and references.