Conference report: 25^th ETH Conference on Combustion Generated Nanoparticles

29 June 2022

The 25^th ETH Conference on Combustion Generated Nanoparticles was held—for the third time—as an online only event on June 21-23, 2022. The conference program included 53 presentations and 35 posters covering particle fundamentals, particles from aircraft, emission measurement, emission control, occupational and indoor exposure, health effects of air pollution, and future legislation. The conference closed with a Focus Event—‘New legislation to guide the world’, dedicated to trends in future air quality and emission regulations. The conference included an exhibitor session with the participation of suppliers of products related to emission measurement.

Emission Control

Vir2sense [C. Barro] discussed the development of a virtual sensor for soot emissions using alternative fuels. Ignition delay, heat release rate (HRR) and NOx emission models were developed and calibrated based on experiments on a 4 cylinder heavy-duty research engine. The tested fuels included diesel, hydrotreated vegetable oil (HVO), Fisher-Tropsch gas-to-liquid (GTL) fuel, and polyoximethylene dimethylether (OME3-6)—tested neat and/or in blends with diesel. The model provided an accurate depiction of the measured HRR for the various fuel. The NOx emission model showed very good predictive capability for all fuels, indicating that any change in NOx emissions between the fuels are mostly due to combustion phasing which was captured by the HRR model.

A new material, an SiC sponge, was investigated by Nagoya University [K. Yamamoto] as a potential gasoline particulate filter (GPF) substrate. The SiC sponge had a pore size of 150-460 µm and a porosity range of 82-86%. The researchers simulated the gas flow and soot deposition in the GPF using the lattice Boltzmann method (LBM). 3D pore structure of the substrate was obtained using the X-ray CT technique. Soot deposition regions during depth and surface filtration were visualized, and the effects of filtration velocity and soot size on pressure drop were discussed.

Researchers from Germany’s Paderborn University [J. Beimdiek] proposed a new type of ammonia-SCR process that involves continuous synthesis of catalytically active iron oxide nanoparticles using a spray-flame burner. The SCR reactions took place mostly in-flight, at the surface of the freshly synthesized iron oxide particles. Downstream, the catalyst particles—together with any fly ash present in the flue gas—were collected in a surface filter. The catalyst showed a good low temperature activity—a 50% NOx conversion was achieved at 175°C.

A team led by the Helmholtz Zentrum München [S. Jeong] studied particle mass and number emissions from a research marine diesel engine using a wet sulfur scrubber. Emissions were measured with three fuels: marine gas oil (MGO, S < 0.1%), high-aromatic heavy fuel oil (HFO, S = 1.3%) and high-sulfur HFO (S = 2.4%). Particle emissions with HFOs were characterized by remarkably higher particle number and mass concentrations as well as left-skewed size distributions compared to MGO. After the scrubber, the size distribution of HFOs was shifted to larger particles, while the MGO size distribution was almost unchanged. The particle number concentration of all fuel types was reduced by the scrubber up to 40% due to coagulation. Further reductions of PM and PN emissions were achieved by combining the scrubber with a wet-electrostatic precipitator (WESP).

Emission Measurement

Spain’s CIEMAT [T. Al-Wasif] investigated the influence of altitude conditions beyond the RDE boundary conditions (altitude gain limited to 1200 m over 100 km) on nanoparticle emissions from a gasoline Euro 6 vehicle. The measurements were carried out using a Skoda Yeti 1.2 TSI car, which was equipped with the EEPS particle sizing instrument. Results outside of the altitude boundaries showed 97% and 47% more total particles in urban and rural areas, respectively, compared to measurements within the RDE limits. In addition, tests carried out beyond the boundary conditions showed 17% more NOx emissions compared to the experiments within the RDE limits. The RDE elevation gain boundary is likely to be removed at the Euro 7 stage.

An experimental study with a gasoline direct injection (GDI) engine was conducted by the University of Minnesota [D. Kittelson]. The researchers measured solid particle number, size, and mass concentration; effective density; and mass-mobility exponent. The engine (1995 cc, 125 kW BMW N43B20) was fueled with E10, E30, and E50 and operated in three combustion modes: stoichiometric (2000 rpm, 7 bar BMEP); lean homogeneous (2000 rpm, 7 bar BMEP); and lean stratified (2000 rpm, 4 bar BMEP). Using the measured effective density functions and particle size distributions, solid particle mass was calculated using the integrated particle size distribution (IPSD) method and compared to black carbon mass measured with a Micro Soot Sensor (MSS). It was found that increasing the ethanol content to E30 and E50 decreased particle number and mass concentrations under lean homogeneous conditions.

Carleton University [J. Scott] reported on the development of a flame spray pyrolysis (FSP) method as a preliminary screening tool for particle emissions with alternative jet fuels, in lieu of full scale, high cost jet engine testing. The method was able to show the difference in particle size distribution with Jet A1 and ATJ fuels, and provided geometric mean mobility diameter (GMD) results repeatable within 15%. Further work is planned to stabilize the active dilution ratio, to minimize the variability in size and number concentrations.

A separate session was devoted to nanoparticle metrology. Naneos [M. Fierz] presented a new instrument to monitor aerosol volatility. The device consists of a catalytic stripper and the “Advanced Partector” diffusion charging sensor as particle detector, which measures lung-deposited surface area (LDSA), average particle diameter and particle number. The catalytic stripper can be bypassed by switching a valve every 10 seconds. Compared to a traditional particle number counter, this instrument delivers additional information on particle size and volatility in a system that can be operated without intervention over long periods of time (months).

Other talks focused on the generation of standard aerosols, including standard silver particle generator [Catalytic Instruments], generation of aircraft-like soot [ETH Zürich], and reproducible generation of secondary organic aerosols [FHNW].

Brake and Tire Emissions

Non-exhaust emissions (NEE) including brake and tire wear particles have been attracting a growing attention. As exhaust particle emissions have been controlled through the use of cleaner engines with particulate filters, brake and tire wear may be soon the dominant source of PM emissions from traffic, Figure 1. An additional concern is that these particles have high metal content, which could result in higher toxicity. Importantly, brake and tire emissions are also emitted by electric vehicles. There have been calls for NEE regulation, but their measurement remains elusive.

The German Aerospace Center (DLR) [L. Bondorf] developed a tire and brake emission test setup, Figure 2, that can be used both on a chassis dynamometer and on the road. There are two separate enclosures to collect tire and brake particles.

Experiments were conducted with a BMW i3 test vehicle. Emissions of brake particles increased with temperature and with vehicle speed. The particle size peaked at 220 nm, with smaller particles emitted during heavy braking. EV regenerative braking could reduce brake particle number emissions by up to 50%. Tire emissions were composed of very small particles ≤10 nm. PM/PN emission levels strongly depended on the drive cycle. High concentrations of background particles presented a challenge during road measurements.

Real-world brake and tire wear PM emissions in cities in Southern California were studied by the University of California, Riverside [H. Jung]. Average concentrations of near-road PM_2.5 and PM₁₀ were 10-15 and ~30 µg/m³, respectively. For PM_2.5, the contribution of brake & tire particles was either comparable or greater than the exhaust fractions (diesel + gasoline), depending on the area. For PM₁₀, the non-exhaust contributions were 2-3 times higher than the exhaust contributions. One of the challenges in the study was related to subtracting background for PM concentrations.

The source apportionment methods used in air quality studies to determine the contribution of brake & tire particles to the overall particle inventory are often based on the analysis of metals and other chemical elements that are typically found in brake components and in tires. Tofwerk AG [M. Tanner] developed a nitrogen plasma mass spectrometer that can be used for such studies. The instrument, tested in ambient air measurements near a rail line, could provide multi-element signals for more than 20 elements (Ba, Mg, Al, Zn, Ti, Mn, Sr, Zr, and others) with a 10 ms resolution.

Emission Health Effects

Several health effects presentations discussed the toxicity of aircraft emissions. Almost all aviation fuels are extracted from middle distillates, and jet engine particles are similar, while somewhat smaller, to diesel particles. Researchers from the Danish NFA [U. Vogel] exposed mice to aircraft particles collected at two airports. The exposure induced acute phase response, inflammation, and genotoxicity similar to standard diesel exhaust particles and carbon black nanoparticles, suggesting similar physicochemical properties and toxicity of jet engine particles and diesel exhaust particles.

Helmholtz Zentrum Munich [M. Delaval] conducted an in vitro cell study to evaluate the toxicity of nonvolatile particulate matter from an aircraft turbofan engine using different fuels. Nonvolatile PM from conventional fuel at low thrust levels was found to be the most hazardous, causing cell inflammation, cytotoxicity, and oxidative stress. The use of a biofuel (32% of hydroprocessed HEFA blend) reduced PM emissions, and the particles appeared less hazardous—they caused inflammation, but no cytotoxicity or oxidative stress.

University of Southern California [A. Wu] investigated the relationship between long-term airport-related ultrafine (UFP) exposures and risk of malignant brain cancers. The source of the data was the California Multiethnic Cohort (MEC) study that involved some 215,000 men and women, mostly residents of Los Angeles County). The analysis found that the risk of malignant brain cancer increased by 14% per interquartile range increase in UFP after adjusting for individual and neighborhood factors and PM_2.5 exposures.

A number of health effects presentations were also given on exposures to emissions from other sources such as wood stoves and forest fires.

Focus Event: ‘New legislation to guide the world’

The ‘Focus Event’ on the last day of the conference was opened by Andreas Mayer [VERT], who made the argument that emission legislation should be initiated, supported and controlled from the bottom-up, in a grassroots, democratic process—as opposed to being a gift granted by government. An example of such grassroots initiative is the VERT Association, which has been influencing the emission legislation in Switzerland since 1995, and following the Swiss success story in other countries worldwide. The ETH Nanoparticle Conference has been founded 25 years ago for exactly this purpose, to become an instrument of science and engineering, and has definitely been able to fulfill these expectations—in agreement with the superseding principles of human right for clean air and implementing the best available technology. One of the recent initiatives spearheaded by VERT and covered at the ETH Conference has been the introduction of PN-PTI procedures to detect defective or tampered particulate filters—now officially adopted in four European countries [P. Buerkenhoudt, CITA].

In September 2021, the World Health Organization (WHO) published new Air Quality Guidelines (AQG)—a document that will have impact on future air quality regulations and emission standards in many countries [N. Künzli, Swiss TPH]. For most pollutants, the new AQG 2021 introduces lower limit values than the prior AQG 2005. For instance, the exposure limit for PM_2.5 has been lowered from 10 µg/m³ to 5 µg/m³, as new data shows health effects still exist at lower exposures.

The world air quality trends, however, can differ significantly in different parts of the world. Emission technologies are not free—only wealthy and resourceful countries can afford wide scale implementation of clean air technologies. Air quality in the Western world is generally improving and in many countries—such as in Switzerland [Ch. Hüglin, Empa]—nearly complies with the WHO guidelines. Due to the inequality gap, the opposite trend is seen in many developing countries, where air quality continues to deteriorate and air becomes even more unhealthy, moving further away from the WHO guidelines.

* * *

The 26^th ETH Conference on Combustion-Generated Nanoparticles is scheduled to take place in Zürich, Switzerland on 20-22 June 2023.

Conference website: npc22.scg.ch