New MECA report calls for implementation of particle number (PN) limits by North American regulators
19 July 2013
|Particle number vs. particle mass for various LDV engine technologies. Source: MECA, Ford Motor Company. Click to enlarge.|
North American environmental regulators should consider taking action on particle number (PN) limits as a complement to their already world-best PM mass standards, according to a new report released by the Manufacturers of Emission Controls Association (MECA).
The report, “Ultrafine Particulate Matter and the Benefits of Reducing Particle Numbers in the United States”, first summarizes the current understanding of the potential adverse health impacts of ultrafine particles (UFPs, particles that are finer than 0.1 microns in diameter); then outlines various control strategies and technologies that can be used to meet current and upcoming EPA standards; documents the successful use of diesel particulate filters (DPFs) to meet and exceed US and European emission standards; and then makes the argument for PN limits.
The report was prepared for MECA by a team from environmental consulting firm Gladstein, Neandross & Associates.
Regulators in the United States, California, and Europe have made major policy advances to reduce the human health impacts from car, truck, bus, non-road diesel engines, and other transportation-related pollution over the past decade.
The new US Tier 3 regulations, proposed earlier this year (earlier post), will further advance the regulatory regimen by lowering the average sulfur content in gasoline from today’s 30 parts-per-million (ppm) to 10 ppm and introduce new tailpipe emission standards for all new cars, light trucks, and sport-utility vehicles, starting in the 2017 model year.
Tier 3 mostly harmonizes with the California Air Resources Board’s (ARB’s) LEV III emission standards for light-duty vehicles. LEV III includes the most stringent PM mass limit in the world—1 mg/mile, starting in 2025; EPA has a 3 mg/mile PM limit starting in 2017.
All of this great progress has occurred against the backdrop of an increasing understanding of the strong evidence linking particulate matter emissions from vehicles with a wide range of adverse health impacts, including increased asthma emergencies, cancer, heart and lung diseases, and premature death. As we look ahead, we see some clouds forming on the horizon that deserve attention.—“Ultrafine Particulate Matter White Paper”
Two of those clouds are UFPs and PNs, the report suggests. The report notes the growing concern in the public health community about the contribution of UFPs to the overall health impacts of PM. While, because of their small size, UFPs are not a major factor in measurements of overall PM mass, they constitute the largest contributor to overall particle numbers. This is an especially important issue in urban areas and near busy highways and other major roads, the report says.
While the body of epidemiological and toxicological studies on UFPs is not as robust as the body of literature on the health impacts of overall PM mass, we see emerging trends in the research that suggest evidence of potential health impacts. In light of these trends, Europe has adopted first-ever limits on particle number (PN) as a way to ensure that diesel particulate filters (DPFs) are used and UFPs are reduced. In addition, Europe will soon begin implementation of a PN limit for gasoline-fueled cars that are equipped with direct injection, which will accelerate the introduction of PM reduction technologies such as gasoline particulate filters (GPFs), high pressure spray guided injectors and other combustion control technologies for PM in the European car market. This is a topic that deserves additional research and attention in the US.
Adding a PN limit in the light-duty sector would create additional, bonus emissions benefits of an additional $35.1 - $80.0 billion beyond the benefits of the proposed Tier 3 emissions standards over the life of these vehicles (including another roughly 900 premature deaths and 56,000 lost work days annually).—“Ultrafine Particulate Matter White Paper”
|PN emissions from turbocharged and normally aspirated engines. Source: MECA, Hyundai. Click to enlarge.|
The composition and quantity of UFPs vary, in part, based on the type of engine and the manner in which the engine is operated. Naturally aspirated port-fueled gasoline engines have produced the least PM per mile or per brake horsepower hour, both by particle mass and particle number. However, newer technologies such as gasoline direct injection (GDI) engines emit significantly higher levels of PM emissions than DPF-equipped diesel engines in terms of particulate number.
The report noted that PN emissions from GDI engines are intermediate—in both mass and particle number—to the emissions from gasoline PFI and older diesel engines that are not equipped with DPFs. Gasoline engines using turbocharging also show increased PM number emissions. In contrast, newer diesel engines have lowered UFP tailpipe emissions through the use of diesel particulate filters, achieving PM mass and particle number emissions similar to normally aspirated, port-fueled gasoline engines and compressed natural gas (CNG) engines.
At the end of the report, MECA makes several recommendations for EPA and ARB to consider to help achieve the maximum environmental and health benefits from their current and upcoming on-road and off-road emission standards:
EPA and ARB should add a PN limit to their regulatory structures for mobile sources. Adding a PN limit to EPA’s Tier 3, Highway Diesel, and Non-road Diesel emissions standards would help ensure that diesel and gasoline particulate filters are used to reduce both the mass of PM and the number of UFPs and other particles, the report suggests.
For the same reasons, the California Air Resources Board (CARB) should consider adding a PN limit to its LEV-III program in its upcoming midterm review, as well as to its highway and non-road diesel regulatory programs.
Both EPA and ARB should consider a new set of heavy-duty diesel engine PM standards that would be equivalent in stringency to CARB’s 1 mg/mile standard for light-duty vehicles. Emissions testing has shown that, when equipped with a DPF, 1 mg/bhp-hr is a technologically feasible emissions threshold for new heavy-duty diesel engines, according to the review.
This level is seen in certification testing of diesel engines with DPFs, which regularly yields engines that exceed the current PM standard by more than 90%. It is time, the report suggested, for EPA and ARB to consider a new round of PM standards that would lock the existing over-compliance in place.
EPA should increase its in-use compliance monitoring of non-road diesel engines that are certified without DPFs. There is ample evidence showing that engine-based strategies are prone to higher in-use emissions than DPF-equipped engines, due to cold starts, extra idling time, poor maintenance, and other factors. Given the complex nature of the non-road diesel engine sector—involving dozens of engine families and a wide array of duty cycles—in-use field testing is especially important. EPA should allocate extra compliance and enforcement resources to following up with in-use emissions testing of any Tier 4 engines that are certified without DPFs.
EPA and ARB should coordinate activities to develop a methodology for measuring UFP and particle numbers. Concerns have been raised by both agencies about whether the European Particulate Measurement Programme (PMP) is suitable for setting a PN limit in the US context. At the same time, the agencies have raised concerns about the ability of their existing framework to measure PM mass emissions at very low levels (e.g., <3 mg/mile).
The report suggests that the two agencies work together to develop a single methodology that could be used to support a PN limit or other UFP standard in the US. Because of the relative ease in measuring particle number versus particle mass at the 1 mg/mile level, the agencies should agree on a scientifically sound conversion factor between particle number and particle mass. This may provide a more robust measurement technique than a mass measurement, and may be used in conjunction with mass-based epidemiological data to estimate health impacts of ultrafine PM.
Both Federal and state governments should play a greater role in accelerating the retirement or retrofitting of older, dirtier diesel engines and the introduction of cleaner diesel replacements.
Environmental agencies around the world should tighten evaporative emission limits as a way to control secondary organic aerosols. The California and US LEV III/Tier 3 evaporative emissions programs provide the most comprehensive approach to minimizing evaporative and refueling emissions from gasoline vehicles, a significant source of secondary organic aerosol-based particulates. Other major world air quality agencies in major automobile markets should adopt US style evaporative and refueling emission requirements, the report suggested.
Founded in 1976, MECA is a national association of companies that manufacture a variety of emission control technologies for cars, trucks, buses, and off-road vehicles and equipment, as well as stationary internal combustion engines.
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