Ultrafine Particulate Matter and the Benefits of Reducing Particle Numbers in the U.S.

Over the past decade, regulators in the United States, California, and Europe have taken major steps to reduce the human health impacts from car, truck, bus, nonroad diesel engines, and other transportation-related pollution. In the United States, the Environmental Protection Agency (EPA) has implemented a series of rules that have dramatically reduced sulfur levels in gasoline and diesel fuel, opening the door to a new generation of catalysts, filters, and other emission control technologies and strategies that are making the black smoke of an old diesel bus or truck a thing of the past, and has led to the cleanest cars, trucks, and buses in the world.

Taken together, the Heavy-Duty Engine rule for trucks and buses (adopted in 2001), the Nonroad Diesel Engine rule for agricultural, construction, and other nonroad diesel engines (adopted in 2004), and the Locomotive and Marine Diesel Engine rule (adopted in 2008) will eliminate an estimated 21,400 premature deaths annually, and create more than $152 billion in net health benefits annually in 2030, according to EPA.

EPA has also taken major strides to make our cars, light trucks, and sport-utility vehicles even cleaner and more fuel-efficient than ever before. In March of this year, EPA proposed a new Tier 3 program of fuel and emission standards for these light-duty vehicles, which will lower 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, staring in the 2017 model year.

The Tier 3 proposal comes on the heels of groundbreaking steps to reduce fuel consumption and greenhouse gases from these vehicles. In 2011, EPA and the National Highway Traffic Safety Administration (NHTSA) adopted new fuel economy and greenhouse gas emissions standards for light-duty vehicles that will ultimately lead to vehicles that average 54.5 miles per gallon (equivalent to 163 grams of carbon dioxide per mile) in the 2025 model year.

The Tier 3 proposal also follows the California Air Resources Board’s (ARB’s) latest regulatory program to reduce emissions from vehicles in its state. In January 2012, ARB adopted a suite of clean cars standards, including its 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’s Tier 3 proposal, for the most part, harmonizes with LEV III up to 2025 with a 3 mg/mile PM limit starting in 2017. In this paper, we explain why EPA should consider further harmonization with ARB, by adopting the same 1 mg/mile standard.

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.

First, there is a growing concern in the public health community about the contribution of the so-called ultrafine particulates (UFPs, i.e., particles that are finer than 0.1 microns in diameter) to the overall health impacts of PM. Given their small size, UFPs are not a major factor in measurements of overall PM mass, but 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.

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 United States.

Second, it is clear that using DPFs creates emission reductions beyond what is required by the emission standards—a bonus that translates directly into additional, quantifiable health benefits enjoyed by all Americans. Indeed, the DPFs that engine manufacturers and others are using to meet existing heavy-duty and nonroad diesel emissions standards in the United States result in additional emission reductions that far exceed the applicable PM standards for highway and nonroad diesel engines—by an average of roughly 90 percent and more than 80 percent, respectively.

The environmental and health benefits of these additional emissions reductions are substantial. Over the life of today’s vehicle and engine fleets, these reductions will yield an estimated $19.1 – $43.5 billion of additional environmental and health benefits from the highway diesel sector, as well as another $5.6 – $12.9 billion in environmental and health benefits from the nonroad diesel sector. These benefits include the elimination of 349-780 premature deaths and almost 50,000 lost work days annually from the highway diesel sector and another 86-196 premature deaths, and roughly 12,238 lost work days annually from the nonroad diesel sector.

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).

Some manufacturers are starting to consider new strategies to meet EPA and CARB nonroad emission standards that do not include DPFs. Already, several engines have been certified to meet EPA’s Tier 4 interim standards without DPFs. EPA certification data shows clearly that, while these engines meet the basic standards, almost all of the additional bonus emission reductions are lost with this approach.

In addition, approaches that rely on engine-based strategies rather than DPFs are more likely to lead to increased emissions in actual use. These increased emissions are likely to result from a number of factors, including off-cycle operating conditions, poor maintenance, and excess idling. Thus, while this approach may comply with the certification requirements of EPA’s standards, it would represent a lost opportunity from the perspective of clean air and human health by leaving significant emissions and health benefits on the table if this approach becomes widespread.

Third, as the EPA/NHTSA greenhouse and fuel economy standards and the likely Tier 3 proposal are implemented, we expect to see an acceleration of the shift towards using direct injection (GDI) and turbocharging in gasoline-fueled light-duty vehicles. Already, more than half of the light-duty vehicles sold in the United States have a GDI option—and the number of GDI-equipped models is rapidly increasing. There is ample evidence that engines equipped with GDI emit UFPs and PM that is comparable to the emissions of diesel engines that do not use DPFs. As EPA finalizes its Tier 3 proposal, it will be important that the agency consider the PM and UFP impacts of a shift from port fuel injection (PFI) to GDI and turbocharging and the opportunities to capture additional PM reductions afforded by emerging PM reduction technologies, such as gasoline particulate filter (GPF) technology.

In this report, we aim to assist EPA and CARB as they consider these clouds on the horizon.

First, we summarize the current understanding of the potential adverse health impacts of UFPs. Second, we outline the various control strategies and technologies that can be used to meet current and upcoming EPA standards. Third, we document the success story of using DPFs to meet and exceed U.S. and European emission standards (including Europe’s particle number limit). Perhaps most significantly, we propose a correlation between PN and PM that can be used in conjunction with PM based health data to estimate the health benefits and surmise that a PN measurement may offer a more robust unit for determining compliance at very low PM levels. Last, we quantify the emissions and health benefits of the additional emission reductions that are enjoyed when DPFs or GPFs are used. We then outline the risk that will be created if and when engine companies abandon their highly successful DPF approach or choose to not add GPFs to their future GDI-equipped light-duty vehicles.

We close with recommendations to help the EPA and CARB achieve the maximum environmental and health benefits of their current and upcoming standards.