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Conference report: 19th ETH Conference on Combustion Generated Nanoparticles

24 July 2015

The 19th ETH Conference on Combustion Generated Nanoparticles was held in Zürich, Switzerland, on June 29-July 1, 2015. The Conference program included a total of 50 presentations and 71 posters. A Focus Event: Cleaner Air for Megacities was held on the third day of the program. The conference also included an exhibition with 17 suppliers of emission measurement instruments, emission control systems and related products showcasing their technology. The meeting was attended by about 400 registrants.

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Regulatory Trends. Alternative metrics for particulate emissions and ambient particle pollution have been a recurring theme at the ETH Conference. Joop Van Ham [EFCA, The Netherlands] criticized current European policies on particle pollution as missing adequate tools to protect the public health and to provide guidance on addressing priority sources of emissions. Emission standards and regulations should focus on the ultrafine fraction in the 30-170 nm range—a crucial factor for cardiovascular and olfactory induced effects. Other potentially relevant metrics include black carbon and secondary organic aerosols (SOA). The relevant definition of ultrafine particles was further discussed by David Kittelson [University of Minnesota]. Particle surface area is an interesting metric that can capture particles in both nuclei and accumulation modes. Among the various types of surface area (BET, aerodynamic, lung deposited), the lung deposited surface area (LDSA) appears to be most relevant to particle health effects.

Jon Andersson [Ricardo] talked about the origins of the European particle number (PN) emission legislation, emphasizing that the adoption of PN emission limits was only possible thanks to the strong political will that existed at the time among the stakeholders. The PN limits that are now a part of the light-duty Euro 5 standards, heavy-duty Euro VI and the proposed nonroad Stage V regulation are orders of magnitude stronger than the corresponding mass based standards, as well as the US PM standards for light- and heavy-duty engines—the PN regulation effectively mandates high efficiency DPFs. One of the outstanding issues with the PMP particle number measurement method is the 23 nm cut-off point. While not considered a problem for DPF-equipped diesels (where the filter effectively captures smaller particles), in low carbon emitting engines—e.g., direct injection gasoline (GDI) engines—high numbers of particles may be emitted below the current 23 nm measuring range. Therefore, it may be desirable to lower the PMP cut-off point to 10 nm.

The US EPA [B. Hasset Sipple] presented a summary of their recent workshop on ultrafine particles. The EPA has started their periodic review process of the National Ambient Air Quality Standards (NAAQS) for particulate matter—a process that may eventually lead to the adoption of ultrafine particle (UFP) standards in the United States. However, one of key issues that make this process challenging is the lack of universally accepted UFP definition. A comparison and evaluation of different studies is often difficult or impossible due to the lack of a common UFP metric.

Notwithstanding the need for more relevant particle emission metrics, air quality improvements are possible by enforcing stringent mass based PM standards. In many settings and geographical areas, UFP exposures and particle numbers are correlated to particle mass. For instance, the Swiss SAPALDIA study found very good correlation between particle number exposures in Switzerland and both PM2.5 and PM10 [N. Probst-Hensch, TPH]. Therefore, strong PM mass based standards may be also effective in controlling UFP exposures.

California continues to reduce emissions and the state is on target to meet their future air quality and GHG targets [W. Robertson, California ARB]. California ARB has conducted extensive research on PN emissions and proposed to include an alternative PN compliance option in the LEV III standards, but the PN approach was withdrawn from the final regulation. In spite of the lack of PN limits, vehicles that comply with LEV III PM standards of 3 mg / 1 mg per mile are using the same control technology as EU vehicles that comply with PN standards. All heavy-duty highway engines also use particulate filters to comply with a mass-based PM standard, and to ensure NOx control levels necessary for meeting the current NOx standard, as well as the anticipated future NOx standards. Further tightening of NOx limits for heavy-duty engines by as much as 90% is expected, to meet California NOx emission reduction targets of 80% by 2023 and of 90% by 2032. Particulate emissions from off-road engines, however, remain a concern. About two thirds of off-road engine families are emission certified without a DPF, due to the more relaxed Tier 4 PM and NOx limits compared to standards for onroad engines. Depending on the actual emission levels, California ARB may consider strengthening the off-road emission regulations.

Engine Emissions. Talks on nanoparticle emissions from internal combustion engines covered emissions from diesel and GDI engines, as well as emissions with alternative fuels. West Virginia University [A. Thiruvengadam] reported on their real world PM/PN measurements from heavy trucks in California, confirming the high efficiency of diesel particulate filters in reducing particle emissions. The study also found ultrafine particles in emissions from a stoichiometric natural gas heavy-duty engine.

Will Northrop [University of Minnesota] presented the results of an investigation of particle emissions from diesel low temperature combustion (LTC) modes. Two light duty diesel engines were used in the study, one operated under the premixed charge compression ignition (PCCI, diesel fuel) mode, the other under the reactivity controlled compression ignition (RCCI, diesel + gasoline) regime. LTC combustion produced increased emissions of semi-volatile particles—PM emissions from both engines were mostly semi-volatile. Particle shrinking experiments produced results similar to the volatility of lube oil particles in the C28-C32 range. A catalyzed diesel particulate filter effectively reduced semi-volatile PM emissions, even under the low operating temperatures—presumably through adsorption of the hydrocarbon material over the filter surface.

GDI engines have been a growing source of nanoparticle emissions. A recent real world emission study found that PN emissions from six Euro 6 GDI vehicles were 10-100 times higher than those from DPF-equipped diesel cars [Axel Friedrich]. The European PN emission limit for GDI engines will likely require the use of gasoline particulate filters (GPF) on GDI vehicles. If GPFs are not widely adopted, the upcoming RDE regulations should include PN measurements from GDI cars. GDI vehicles can also be a source of PAH emissions. In a study by EMPA [M. Muñoz], three GDI vehicles tested on a chassis dynamometer produced bezo(a)pyrene and a number of other PAH compounds. The PAH emissions were generally higher under cold start conditions. In tests with an uncatalyzed GPF positioned downstream of a three-way catalyst, PAH emissions actually increased in some cases, which was attributed to PAH storage and release phenomena in the filter.

Nanoparticle emissions from a gasoline SI engine were significantly reduced and the engine efficiency was increased when methanol reforming products were added to the fuel [R. Fleischman, Technion]. In another study on fuel effects [M. Vojtisek-Lom, University of Prague], butanol blend of 25% was found to reduce particle emissions from a Ford Ecoboost Euro 6 engine, while E15 did not reduce particle emissions.

Health Effects. Maria Costantini [HEI] summarized the results of the Advanced Collaborative Emission Study (ACES)—a major research effort to characterize emissions from US EPA 2007/2010 heavy duty diesel engines. Phase 1 and 2 of the study concluded that regulated and unregulated emissions in 2007/2010 engines (with particulate filters) were substantially lower compared to emissions from 2004 engines. Phase 3 of the study examined biological effects—a lifetime exposure of rats to 2007 engine emissions found no increase in tumor formation, in contrast to long-term exposure studies at similar dilutions with traditional diesel exhaust, where tumors and pre-cancerous changes in the lungs were found.

Health hazards from PM2.5 emissions from different sources—brake wear, wood combustion (3 ovens), diesel combustion, poultry farm and tire wear—were evaluated in a mice exposure study [M. Gerlofs-Nijland, RIVM, The Netherlands]. The effects varied depending on the PM2.5 source; in some cases the lowest dose already created the most effects. Increased hazards were associated with wood combustion particles in case of inefficient combustion, as well as brake wear particles.

University of Pennsylvania [G. Reto] reported on ambient sampling and toxicological analysis of particles from biomass combustion. Many plants (e.g., grasses, cane, bamboo) contain silica, which can be emitted as cristobalite—a known carcinogen—when the biomass is burned. Another study [V. Soppa, University of Düsseldorf] found changes in lung function and blood pressure from exposure to common indoor aerosols, including particles from candle burning, bread toasting, and sausage frying.

Cleaner Air for Megacities. The Focus Event started with an introduction on the inherent problems of urban growth by Andreas Mayer [TTM], followed by a historical perspective on air pollution control in Los Angeles given by David Parrish [University of Colorado]. Los Angeles pollution levels in the 1940s reached extremely high levels, never matched by any other city in the world. In 1948, Los Angeles experienced pollution episodes with ambient 24 hr average PM10 concentrations exceeding 600 µg/m3, while annual averages above 150 µg/m3 persisted for years. These PM10 concentrations were significantly higher than current pollution levels in Beijing (2011 average of 114 µg/m3) or the recent pollution episode in Paris (a peak PM10 of 120 µg/m3 on 15 March 2015). Thanks to the coordinated action in the region, VOC emissions in Los Angeles were reduced by a factor of 50 over the period of 1980-2010. And yet, after 5 decades of effort, the air is still not as clean as needed.

The remaining talks focused on current air quality programs in Bogotá, Tehran, as well as cities in China. One of the major programs implemented in Bogotá has been the development of a Bus Rapid Transit system that relies on high capacity buses and dedicated bus lanes [A. Mayer, TTM]. A pilot DPF retrofit project for Bogotá public buses was started in 2013.

China expected rapid air quality improvements from the introduction of EU-style vehicle emission standards. Due to a combination of reasons—from congested traffic patterns to poor enforcement of regulations—the air quality in large Chinese cities has not improved as anticipated. Future emission standards, China 6, are supposed to be better aligned with the actual conditions in China, drawing not only on the European but also on US regulations [Ma Dong, VECC-MEP China]. Other actions and policies under consideration include implementation of enhanced in-use vehicle emission standards, remote emission measurements for in use vehicles, as well as urban traffic demand management and control of vehicle population.

In Tehran, a pilot DPF retrofit program has been coordinated by VERT [Volker Hensel]. A group of stakeholders including VERT, TÜV Süd and several engine manufacturers have been also working with the Iranian government to adopt emission regulations for new heavy-duty diesel engines [Mathias Karsten, TÜV Süd]. The proposed requirements include Euro III emission standards for gaseous pollutants, a Euro IV PM limit and a PN limit of 1×1012 intended to force diesel particulate filters on all diesel engines. The standards would have to be met using high sulfur diesel fuel (300 ppm S), thus limiting the available DPF technologies to fuel borne catalyst (FBC) and burner regenerated filters.

Next year will mark an anniversary of the conference—the 20th ETH Conference on Combustion Generated Nanoparticles will be held on 13-17 June 2016.

Conference website: nanoparticles.ch