The Carbon Dioxide-Equivalent Benefits of Reducing Black Carbon Emissions from U.S. Class 8 Trucks Using Diesel Particulate Filters: A Preliminary Analysis
[1,183 KB] Published: July 2009, Revised: September 2009

Debates over climate change have shifted from scientific evidence of anthropogenic contribution to the greenhouse effect to the development of strategies to mitigate the impacts of global warming. One strategy that has yet to be fully explored is the reduction of black carbon particles. Diesel engines represent a significant, controllable source of black carbon. Targeting these emissions offers a supplemental and parallel strategy to pursue carbon dioxide (CO₂) reductions with the advantage of a much faster temperature response and the additional benefit of health risk reductions. As part of an integrated multi-prong strategy, reducing black carbon can provide net cooling benefits in the near term while carbon dioxide reduction strategies and technologies are developed and implemented.

This paper represents Clean Air Task Force's (CATF's) attempt to quantify the CO₂- equivalent climate benefits of removing black carbon from the diesel exhaust emissions of class 8 trucks using diesel particulate filters (DPFs). The DPF is a proven, off-the-shelf technology that can reduce black carbon emissions by 90 percent or more. Large U.S. Class 8 trucks (defined as exceeding 33,000 lbs.), for example, "combination" tractor-trailer trucks, waste haulers, large buses, constitute a significant contributor to U.S. diesel sector pollution from which black carbon emissions can be controlled fleet wide with this readily available technology. However, the methodology in this paper could be applied to smaller trucks and other diesel engines as well.

Climate scientists have proposed two metrics to calculate CO₂ equivalent (CO₂e) potencies of black carbon: global warming potential (GWP) and global temperature potential (GTP). In this paper, CATF summarizes GWP and GTP estimates from the published literature and uses them to calculate carbon dioxide equivalent benefits from DPFs installed on class 8 trucks. The weight of evidence described demonstrates that black carbon reductions from DPFs are climate beneficial and warrant support as a policy priority.

CATF examined four questions that have a bearing on whether installation of DPFs on class 8 trucks would provide significant climate benefits: I.) What is the CO₂e reduction from a diesel truck equipped with a DPF? II.) What is the break-even fuel penalty, i.e., assuming DPFs cause a measurable fuel penalty, the point below which use of a DPF to reduce black carbon-related warming is beneficial? III.) How many years would black carbon reduction- related climate benefits from the installation of a DPF (measured in CO₂e) exceed the increased CO₂e from an assumed fuel penalty of 2 percent? and IV.) Using the above methods, what would the benefits be of a U.S. Class 8 class 8 truck rebuild rule in the United States?

Based on a calculated range of CO₂e values in this paper, we find Bond and Sun's (2005) twentyyear global warming potential (GWP₂₀) of 2,200 provides a reasonable "best estimate" for use in calculating CO₂e benefits. In addition, we report the results based on their one hundred-year global warming potential (GWP₁₀₀) of 680, which IPCC recognizes as a standard for such comparisons. CATF believes that the use of the longer-term GWP₁₀₀ may understate the near-term climate benefits of black carbon reductions and that these black carbon reduction benefits are better represented by the GWP₂₀.

A review of the literature finds that fuel penalties associated with retrofit DPF applications range from zero as a best estimate to a few percent. The most comprehensive, controlled field study of 20 retrofit tractor-trailer trucks that each ran 150,000 miles a year/vehicle suggests there may be no measurable fuel penalty associated with the DPF itself. This conclusion is also supported by an analysis of four years worth of fueling records, covering 1.28 million fleet miles, for 10 MTA New York City transit buses that were retrofit with a DPF. Nonetheless, given the uncertainty across studies and to be conservative in this analysis, CATF assumed a 2 percent fuel penalty.

The following is a summary of CATF's findings:

1. Installation of a DPF on a typical pre-2007 U.S. class 8 truck yields a GWP₂₀ benefit of 2000 gCO₂e/gal assuming a fuel penalty of 2 percent. This would achieve the equivalent climate benefits as eliminating the pollution from six passenger cars. In addition, retrofitting six class 8 trucks would be the equivalent of eliminating the combined CO₂ and black carbon pollution from one such pre-2007 truck. (Using GWP₁₀₀ CATF finds a net benefit of about 500 gCO₂e/gal, one quarter of the GWP₂₀ benefit.)
2. A CO₂e benefit will result as long as the increase in fuel use (fuel penalty) from the installation of the retrofit is less than 22 percent—much higher than the highest documented DPF fuel penalties. Two comprehensive in-use studies tracking fuel consumption suggest that there may, in fact, be no measurable fuel penalty from the application of DPFs to class 8 trucks. (Even using GWP₁₀₀, the breakeven fuel penalty is 7 percent, significantly higher than fuel penalty estimates.)
3. Installation of a DPF on a class 8 truck will result in climate benefits for approximately half a century.
4. Retrofitting nearly one-million class 8 trucks in the U.S. with DPFs between 2012 and 2030 would provide the total equivalent carbon dioxide reduction of 96 million metric tons (GWP₂₀) –equivalent to eliminating the annual emissions of 21 million cars or 1.8 million class 8 diesel trucks.

The weight of this evidence suggests that reductions in diesel black carbon emissions could play an important role in short-term global warming mitigation.

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