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Measuring automotive exhaust particles down to 10 nanometres

Periodic Reporting for period 2 - DownToTen (Measuring automotive exhaust particles down to 10 nanometres)

Reporting period: 2018-04-01 to 2019-12-31

Over the past decade, the European Union (EU) has taken several steps to reduce the human health impacts of particulate matter (PM) from transport, complementing the control of the mass of PM with limits on the numbers of non-volatile (or solid) particles (PN). A minimum threshold diameter at 23 nm particle diameter has been set in order to include the smallest soot particles and exclude volatile nucleation mode ones.

Concerns have been raised, however, that this lower size might not be appropriate for some vehicle technologies because high concentrations of solid sub-23 nm particles have been found. In particular, for port-fuel injection vehicles, mopeds, motorcycles, diesel particle filter regenerations, and CNG vehicles, evidence had shown that there is a considerable percentage of particles below 23 nm.

The DTT consortium was established to develop further knowledge on particle emissions from light duty vehicles and a robust and sound method for the measurement of sub -23 nm particles. The main aims of the project were (Figure 1):
• To increase the understanding of the nature and the characteristics of sub-23 nm particle emissions.
• To provide a robust sampling and measurement methodology for laboratory and RDE measurements.
• To assess the effectiveness of technical measures for reducing particle emissions.
• To provide input on emissions factors for particle number emissions from current and future technologies to enhance air quality modelling tools.
"Following an evolutionary process, the DTT consortium has developed both laboratory and portable versions of a sampling system going through a gradual procedure, basing on fundamental investigations around the lab system (Figure 2 and 3) and then solving practical problems in converting the system for portable use (Figure 4 and 5). This was then deployed in several laboratories to assess the effects of different vehicle technologies, fuels, regulatory cycles, temperatures, operating conditions and aftertreatment systems. From the overall results (Figures 6, 7 and 8), a number of important messages emerged:

General
• A novel sampling and measurement system was developed, for which the DTT consortium has demonstrated capability to measure both solid and total particle numbers. It offers a reasonable penetration efficiency for primary particles and a reasonably stable dilution ratio. When a catalytic stripper (CS) is in-line, the system was found to be artefact free.
• Most vehicle technologies were found to be compliant with the 6x1011 #/km limit for both >23 nm and >10 nm particles. For moderate power drive cycles, emissions of vehicles with particle filters are always below the limit value. Improvement in current filter applications will be required to bring PN10 emissions below the limit with a suitable engineering safety margin in severe. This is also expected to bring PN<10 below the limit value. The highest emissions were observed from PFI passenger cars, motorcycles and mopeds, currently not subject to PN legislation. Technology improvements (including particle filters) are required to meet the current limit for PN10. Average PN emissions from gasoline hybrids in ICE mode under severing conditions may also require a GPF even for PN23.

Specific cases with significant number of particles below 23nm
• PN emissions from the cold start are higher from the average cycle in general. PN<23 from this phase can dominate the overall emissions from spark ignition technologies.
• Diesel PN emissions during active DPF regenerations can be dominated by PN<10 for short periods.
• GDI vehicles may emit <23 nm soot particles under very high load transients; these particles seem to mostly fall above the PN10 threshold rather than below 10 nm.
• CNG vehicle emissions were found to be significantly elevated below 10 nm. A simple retrofit of a GPF has been seen to reduce both PN10 and PN<10 to well below the limit value. A prototype CNG operating on in-cylinder injection exhibited PN<23/PN23 ratios that were below 2.5 times.
• In RDE measurements, the measured PN10 and PN23 emissions are influenced by the soot loading both in diesel and gasoline filter applications. Higher PN emissions are detected after regeneration. PN10 and PN23 emission levels over RDE are usually above the WLTP levels.
• Total Particle Number (TPN) emissions also seem lower than levels measured for corresponding combustion technologies one decade ago. This suggests that emission control technologies introduced for solid particle control also led to reductions in total particle number. Specific GDI operation events, such as high accelerations and steady state high speeds, can increase emissions of such particles, to levels more than an order of magnitude higher than SPN emissions."
• A Portable Emissions accuracy Test bench (PET) was developed. The PET can used to compare PEMS with lab analyzer results while the PEMS is being shaken.
• An aerosol gas exchange system (AGES) for nanoparticle sampling at elevated temperatures was developed and modeled (Figure 9). With an application targeted design, particle dilution can be avoided, which can lead to improved results in many fields of aerosol measurement.
• A new mobile aerosol characterization system has been developed, suitable for chemical analysis of particle-associated volatile substances. The instrument is a combination of a novel infrared-radiation-based evaporation system (HELIOS) with a new efficient atmospheric ionization source (SICRIT) connected to an ion-trap mass spectrometer (Figure 10).
• A methodology to measure total and secondary particles has been further tested and developed, suitable for various studies related to exhaust particle measurements. A new short-residence time oxidation flow reactor (TSAR - TAU Secondary Aerosol Reactor) was used for secondary aerosol formation measurements.
• A Multicomponent Modal Aerosol Model (MMAM) coupled with the software STAR-CCM+ was developed to model particle formation and evolution in the DTT and in the CVS systems. The tool developed is helpful for the assessment of the differences in particle properties/concentrations in different diluting configurations and for studying the relative rates of different aerosol processes such as nucleation, condensation and coagulation.
• A commercial AVL PN PEMS system utilizing an Electrical Particle Counter (EPC) and a dilution concept similar to the DTT sampling system was successfully modified for 10 nm measurements. This demonstration EPC PEMS exhibited similar particle losses with the DTT PEPS, and a detection efficiency similar to that of commercial 10 nm CPCs.

The direct impact of DTT is the expected contribution that the project and the project partners can make towards the development of robust and effective EU emissions control regulation with focus on particle emissions. DTT results are being used to scientifically underpin the post-Euro 6 emission standard development at the EU level. Furthermore, DTT methodology as well as knowledge gained can be used by the automotive industry to develop calibration services and better emission control devices.