Conference report: US EPA Workshop on Ultrafine Particles
27 February 2015
A Workshop on Ultrafine Particles organized by the US Environmental Protection Agency (EPA) was held on February 11-13, 2015 at the EPA facility in Research Triangle Park, NC, USA. The meeting was attended by about 140 participants, with additional remote attendance via a webinar. The program covered various aspects of ultrafine particle (UFP) science, from basic research and health effects, through instruments and measurement methods to control technologies, mitigation measures and policy approaches.
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Ultrafine particles are most commonly defined as particles of diameters below 100 nm—a definition that matches that of engineered nanoparticles. UFP science is an area of growing relevance to the National Ambient Air Quality Standards (NAAQS) for particulate matter as well as vehicle emission standards—particularly for diesel and gasoline direct injection (GDI) vehicles. Among the 25 speakers at the workshop, 8 were invited European experts, which allowed for an interesting comparison of the European and US perspective. While the US NAAQS standards for PM10/PM2.5 are more protective than European ambient air standards, the EU has the most stringent engine and vehicle emission standards for particulates—by incorporating particle number (PN) emission limits, the EU standards ensure that wall-flow diesel particulate filters (DPF) be used on all affected onroad and, under the Stage V proposal, nonroad engines in the future. While particulate filters were also forced by the 2007 US EPA mass-based PM standards for onroad diesel engines, the EPA Tier 4 nonroad standards—also designed with the DPF technology in mind—have been met without the use of filters on many nonroad engine families and are not protective against UFP emissions from nonroad machinery.
Sources of UFP Emissions. Traffic is a major source of UFP emissions, especially in areas near major roadways. Diesel engines were historically a major contributor among the traffic sources. This may be changing in the future, as new diesel engines are increasingly fitted with particulate filters. An emerging new source of UFP emissions are GDI engines. GDI emissions are especially high during certain driving conditions. For instance, it was shown that during engine start-up GDI vehicles can emit 8,000 times more particles than DPF-equipped diesels [I. Khalek, SwRI]. Engine start-up emissions are especially concerning because of the engine start/stop (ESS) systems that are increasingly common in light-duty vehicles.
Diesel particulate filters, while effectively reducing solid particle number emissions, facilitate nucleation of sulfate particulates downstream of the filter at high temperatures and high numbers of liquid particles. A study in California found that total particle number concentrations were four times higher in a 2010 Volvo engine during highway cruise than in a 1998 Cummins engine [J. Dinh, CARB]. Due to this phenomenon, ambient UFP measurements in California seem to show increasing trends since the introduction of DPF equipped engines. However, toxicity assays show decreased levels of toxicity in spite of the higher numbers of (volatile) particles, suggesting that sulfate particles may have little toxic effect.
Health Effects Evidence. There has been a growing body of evidence about the health effects of UFP emissions. Most of the evidence, however, has been provided by toxicological studies—at this time there is no epidemiological evidence suggesting that UFP exposures can account in substantial ways for the PM2.5 effects. The EPA [J. Sacks] considered UFP in their review of NAAQS for particulate matter completed in 2012. The review classified the health effects evidence for UFP as “suggestive” for cardiovascular and respiratory effects, for short term exposures. Contrary to PM2.5, no causal relationship between exposure and mortality and/or other long term effects could be determined for UFP. The EPA has not proposed a separate indicator for UFP (in addition to PM10/PM2.5) in their 2012 assessment.
A review of ambient UFP research was conducted by the Health Effects Institute (HEI) [K. Walker]. Ultrafine particles, which can be deposited deeper into the lungs and have different clearance characteristics than larger particles, were found to cause health effects in animals, including translocation of particles from the lungs to other organs and brain inflammation. However, fewer studies have been conducted with humans. In particular, existing epidemiological studies—which were predominantly focused on PM2.5 exposures, not on ultrafines—provide no evidence that would link UFP exposures with long term health effects and increased mortality.
Several talks focused on recent UFP research. Annette Peters [Univ. Munich/Helmholtz Center] discussed a five city study, focused on short term health effects and involving PM2.5 and UFP sampling in Augsburg, Dresden, Ljubljana, Prague and Chernivtsi. Commenting on UFP metrics, Dr Peters said that solid particle numbers are an important indicator that can help explain many observations on health effects. Therefore, size-specific monitoring of ambient aerosols should be more widely used, while some of proposed UFP mass metrics such as PM0.5 are not relevant to health effects.
Nino Künzli [Swiss Tropical and Public Health Institute, Basel & Swiss Federal Commission for Air Hygiene, FCAH] talked about long term effects of particle exposure—long term effects are considered to be of crucial importance and they can increase more steeply with exposure than short term effects. Based on new research, it is possible that health effects attributed to PM10 may be in fact attributable to the UFP fraction of PM10. This hypothesis is based in part on the high correlation between UFP and PM10 in the Swiss SAPALDIA cohort study. For example, in the first SAPALDIA results, the association between atherosclerosis and PM10 as well as UFP was very similar. If the role of UFP exposures is confirmed in other settings, separate ambient PN standards may be necessary to provide better protection from particle exposures. The correlation between UFP and PM2.5/PM10 is however not universal—in some urban settings ambient nucleation events are possible, leading to high numbers of (liquid) UFPs [M. Zhang, Cornell Univ.].
Instruments & Methods. Imad Khalek [SwRI] discussed regulatory methods of particle emission measurements from light- and heavy-duty vehicles. The US methods have been improved by the EPA Part 1065 regulation—the changes were necessary for the 2007 PM standard of 0.01 g/bhp-hr. While this standards forced DPF technology on heavy-duty engines, DPF engines are certified at PM emission levels about 90% below the standard. Hence, there are no regulatory safeguards that would prevent the replacement of DPFs with a less efficient control technology in the future. Considering the state-of-the-art in measurement technology, the existing mass standard could be tightened by some 80%, but a EU-style PN limit would provide much less variability of the measurement. In light-duty vehicles, the EU PN regulations for gasoline engines are equivalent to about 0.5 mg/mi, which is twice as stringent as the California 2025 standard of 1 mg/mi.
Jon Andersson [Ricardo] talked about the history of the particle number legislation in Europe and the PMP test method. The adoption of the EU PN limits was possible thanks to a strong political will to ensure that wall-flow particulate filters be used on all new diesel engines. While this objective has been achieved—with over 80 million vehicles equipped with DPFs—some elements of the PMP test method used for particle number measurements remain questionable. One of the issues is the 23 nm cut off point. In the case of DPF-equipped diesels, the 23 nm cut off is not considered a problem, because the DPF is highly effective in controlling particle numbers of all sizes, including those below 23 nm. However, as the PMP method was extended to GDI engines (possibly without GPFs), concerns exist that vehicles that meet the regulatory limit for particles above 23 nm may be still emitting high numbers of smaller size particles. The PMP program was recently reactivated to examine the possibility of lowering the cut off point to 10 nm, as well as to simplify the complex and expensive calibration procedures.
Workshop presentations and the audio/video recordings of the event are expected to be available from the workshop website within a few weeks. The organizers are also planning to produce a paper summarizing the presentations and discussions at the workshop, which would also be posted on the website.
Workshop website: www2.epa.gov/air-research/epa-workshop-ultrafine-particles