Name: Measuring Sub-23 Nanometer Real Driving Particle Number Emissions Using the Portable DownToTen Sampling System (Video) | JoVE Journal
Uploaded: 2020-05-22T15:00:00.000+00:00
Duration: 8 min 59 s
Description: Learn to measure sub-23 nm particle emissions in real driving conditions using the portable DownToTen sampling system. This method enables assessment of nano-sized automotive emissions for environmental research.

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5 minutes

This work is conducted in the framework of the H2020 project DownToTen. This project has received funding from the European Union&#8217;s Horizon 2020 research and innovation programme under grant agreement Nr. 724085.

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<li>Mathissen, M., Scheer, V., Vogt, R., Benter, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Investigation+on+the+potential+generation+of+ultrafine+particles+from+the+tire-road+interface%5BTitle%5D)+AND+%22Atmospheric+Environment%22%5BJournal%5D)">Investigation on the potential generation of ultrafine particles from the tire-road interface.</a> Atmospheric Environment. 45 (34), 6172-6179 (2011).</li>
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</ol>

The authors have nothing to disclose.

This work presents the DTT sampling system and its application as a portable emission measurement system. The system was designed and constructed within the EU Horizon 2020 project DTT to enable particle number emission measurements below the current legislative particle size limit of 23 nm. The system&#8217;s versatility enables the assessment of the regulated solid particle number emissions as well as total particle emissions and studies on secondary aerosols. To interpret measurement results accurately, a calibration procedure is necessary with the DTT system. This is to evaluate the relative particle penetration for different particle sizes, to be able to calculate a correction factor that accounts for the particle losses. It is critical to provide sufficient warm up time for the sampling system itself and the rest of the experimental setup to reach thermal equilibrium and achieve accurate calibration measurement results.
The application of the DTT system for the measurement of solid particle number emissions with a lower particle size cutoff of 23 nm (current regulation) and 10 nm (experimental) is described. To be able to assess particle number emissions of a vehicle it is necessary to determine the particle number concentration and the exhaust mass flow rate. The DTT system covers the particle number concentration measurement. The exhaust mass flow is measured using an exhaust flow meter (EFM). It is critical to install the EFM according to the manufacturer&#8217;s instructions. Erroneous measurements of the exhaust flow rate directly affect the deduced emission rates. When processing the measured data, it is important to perform an accurate time alignment of the particle concentration data and the exhaust flow data. This is necessary because the emission rate is the exhaust flow rate multiplied by the particle number concentration. If the two signals are not aligned correctly, the emissions over the whole drive can significantly deviate from the real emissions.
The DTT system is not a commercial device but a versatile research tool. It is used to investigate unregulated vehicle emissions as opposed to performing certification measurements validating compliance with current regulations. The high versatility comes at the cost of increased energy and dilution air consumption. When using the system for mobile measurements, the weight added to the vehicle due to the battery (30 kg) and gas bottle (20 kg) to cover the energy and air consumption of the system must be kept in mind. The total weight added to the car when measuring the PN emissions with the DTT system is approximately 80 kg, which is comparable to another person being transported in the vehicle. The added weight can lead to slightly increased emissions, especially if the drive includes a great deal of acceleration and/or hills.
The DTT system can be used to investigate the unregulated &#60;23 nm particle number exhaust emissions. Both solid and total particle number emissions can be measured. Furthermore, it can be a useful tool to study the complex field of secondary aerosol formation. Another possible application of the system is the measurement of automotive brake wear particles. A significant fraction of the particles emitted during braking events can be smaller than 30 nm34. With a d50 of approximately 11 nm, the DTT system is suitable for studying these emissions. Although it is known that non-exhaust emissions contribute almost equally to traffic-related PM10 emissions35, non-exhaust particle emissions are still unregulated. This is due to the complex and seldom reproducible process of particle generation, making it very difficult to set regulatory actions. Furthermore, the chemical composition and the related toxicity of organic brake wear particles is still widely unknown35.
The DTT system is a useful tool to improve our understanding of both exhaust and non-exhaust traffic-related particle emissions.

<table><tbody><tr><td>2x Condensation Particle Counter 4 nm</td><td>TSI</td><td>3775</td><td>Particle counter with a cut point of 4 nm</td></tr><tr><td>5x Mass Flow Controllers (MFC)</td><td>V&amp;ouml;gtlin</td><td></td><td>Mass flow controllers for controlling the miniCast gas flows</td></tr><tr><td>AVL M.O.V.E. EFM Exhaust Flow Meter</td><td>AVL</td><td></td><td>Device for the measurement of the exhaust flow rate of vehicles</td></tr><tr><td>Catalytic Stripper</td><td>Custom made</td><td></td><td>Device for the removal of volatile compounds in an aerosol by oxidation</td></tr><tr><td>Compressed Air</td><td></td><td></td><td>Oxidation and dilution air supply for miniCast</td></tr><tr><td>Condensation Particle Counter 10 nm</td><td>AVL</td><td></td><td>Particle counter with a cut point of 10 nm</td></tr><tr><td>Condensation Particle Counter 23 nm</td><td>TSI</td><td>3790A</td><td>Particle counter with a cut point of 23 nm</td></tr><tr><td>Differential Mobility Analyzer</td><td>TSI</td><td>3085</td><td>Part of the electrostatic classifier where the particle are separeted by mobility.</td></tr><tr><td>Dilution Bridge</td><td>Custom made</td><td></td><td>Needle valve in parallel to HEPA filters. Used to adjust particle concentrations for calibration purposes</td></tr><tr><td>DownToTen Sampling System</td><td>Custom made</td><td></td><td>Custom made sampling system for the assessment of automotive sub-23 nm particle emissions</td></tr><tr><td>Electrostatic Classifier</td><td>TSI</td><td>3082</td><td>Device for the classifaction of arosol particles by electrical mobility diameter</td></tr><tr><td>Hand held Mass Flow Meter (MFM)</td><td>V&amp;ouml;gtlin</td><td></td><td>Device for measuring the inlet flow of measurement instruments</td></tr><tr><td>miniCast Soot Generator</td><td>Jing Ltd</td><td></td><td>Combastion aerosol standard, soot generator</td></tr><tr><td>Mobile Battery LPS 1500</td><td>Clayton Power</td><td></td><td>Battery for power supply of the DTT measurement system</td></tr><tr><td>Nitrogen Gas Bottle</td><td></td><td></td><td>Nitrogen for Mixing gas and quench gas supply of miniCast</td></tr><tr><td>Propane Gas Bottle</td><td></td><td></td><td>Fuel for miniCast</td></tr><tr><td>Soft X-Ray Neutralizer</td><td>TSI</td><td>3088</td><td>Device for the establishmentof the equillibrium charge distribution of aerosol particles</td></tr><tr><td>Synthetic Air Bottle 10 L</td><td></td><td></td><td>Gas Bottle for the dilution air supply</td></tr></tbody></table>

measuring sub-23 nanometer real driving particle number emissions using the portable downtoten sampling system

Measuring Sub-23 Nanometer Real Driving Particle Number Emissions Using the Portable DownToTen Sampling System

The current particle size threshold of the European Particle Number (PN) emission standards is 23 nm. This threshold could change because future combustion engine vehicle technology may emit large amounts of sub-23 nm particles. The Horizon 2020 funded project DownToTen (DTT) developed a sampling and measurement method to characterize particle emissions in this currently unregulated size range. A PN measurement system was developed based on an extensive review of the literature and laboratory experiments testing a variety of PN measurement and sampling approaches. The measurement system developed is characterized by high particle penetration and versatility, which enables the assessment of primary particles, delayed primary particles, and secondary aerosols, starting from a few nanometers in diameter. This paper provides instruction on how to install and operate this Portable Emission Measurement System (PEMS) for Real Drive Emissions (RDE) measurements and assess particle number emissions below the current legislative limit of 23 nm.

Calibration Data (Particle Penetration):
Figure 11 shows an exemplary plot of the relative particle penetration of the DTT system as a function of the particle mobility diameter. The corresponding data have been measured and evaluated as described in instruction section 1. The plot shows that the deviations between two measurement points at the same mobility diameter were less than 5%. Deviations larger than 10% indicate instabilities in the experimental setup. In this case, the calibration had to be repeated with increased warm up stabilization times. Both the warm up time (typically 30 min) and the stabilization time (typically 30 s) increased by a factor of 1.5.
The particles passing through the DTT system were lost due to diffusion and thermophoresis. Thermophoretic losses were caused by a temperature gradient drawing particles towards the walls of the sampling system. This is a particle size independent effect29; in contrast, diffusion is highly particle size dependent. A concentration gradient caused a net particle flux towards the walls where particles were lost. The diffusivity rising with lower particle size made this the dominant loss mechanism for particles ≤10 nm. The lines in Figure 11 indicating thermophoretic, diffusional, and total losses demonstrate the respective particle size dependencies. For the diffusional losses, this function was used to illustrate the approximate particle size dependency:
<img alt="Exponential decay formula P=exp(-a/D(dp)), equations, probability analysis." src="/files/ftp_upload/61287/61287equ11.jpg" data-altupdatedat="1788250015000" data-alt="Equation 11">
The penetration P depends on a fit parameter a and the diffusion coefficient D:
<img alt="Diffusion coefficient equation D=kT*Cc/(3πηdp), related to dynamic light scattering studies." src="/files/ftp_upload/61287/61287equ12.jpg" data-altupdatedat="1788250015000" data-alt="Equation 12">
The diffusion coefficient depends on the Boltzmann constant k, the absolute temperature T, the viscosity η, the particle diameter dp, and the Cunningham slip correction factor Cc, which is a function of the mean free path and the particle diameter29.
The data illustrated in Figure 11 resulted in the following mean particle penetration efficiency Pmean:
<img alt="Mean pressure calculation equation, P_mean formula, mathematical equation, academic use." src="/files/ftp_upload/61287/61287equ13.jpg" data-altupdatedat="1788250015000" data-alt="Equation 13">
The particle size where the penetration efficiency amounts to 50% is referred to as d50. The d50 describes the penetration cutoff characteristic of a system. For the DTT system the d50 was 11 nm. The d50 is shown in Figure 11.
<img alt="Penetration vs d_p graph; thermophoresis, diffusion analysis; scientific measurement results." class="xfigimg" src="/files/ftp_upload/61287/61287fig11.jpg" data-altupdatedat="1788250015000" data-alt="Figure 11"> 
Figure 11: Particle penetration as a function of particle mobility diameter. 
Points marked in blue are measurement results. The dashed lines in orange and green indicate the losses associated with thermophoresis and diffusion, respectively. The red line represents the total losses as the sum of diffusional and thermophoretic losses. The dotdashed purple line shows the average particle penetration Pmean as calculated in the calibration measurement instruction section 1. <a href="https://www.jove.com/files/ftp_upload/61287/61287fig11large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Solid Particle Number:
Figure 12 shows the particle number emission rate over time for the first ten minutes of an RDE measurement drive. The data from the DTT PEMS using a 10 nm and a 23 nm CPC are shown together with data from a commercially available 23 nm cut point system. The particle emission rates were calculated from the respective particle concentrations multiplied by the exhaust flow rate as described above in the data analysis instruction section 4. The reference instrument (AVL MOVE) relied on a diffusion charger for the particle number concentration measurement. Despite the different sensor principles, the data measured with the DTT PEMS were overall in very good agreement with the data measured by the commercially available PEMS. Sharp downwards pointing spikes in all three signals occurred because the particle measurement devices can report zero particle concentrations temporarily and zeros cannot be displayed in logarithmic plots. The particle emissions measured with the 10 nm CPC were very close to the emissions measured with the 23 nm CPC for the majority of the time period shown in Figure 12. However, right at the beginning between 10 s and 25 s there was an occurrence of significant &lt;23 nm particle emission. The DTT 10 nm signal was significantly higher than the 23 nm signal of the DTT system and the AVL MOVE. In this case, &gt;50% of the total number of particles emitted were between 10 nm and 23 nm. Cold start dynamic processes in non-thermal equilibrium can cause particle size distributions to differ from emissions from a hot vehicle30. The discussion of these complex processes is beyond the scope of this work. Further information on this topic can be found in the literature31,32,33.
<img alt="Particle event analysis; graph of DTT nm; vehicle velocity; sub-23 nm particle event data." class="xfigimg" src="/files/ftp_upload/61287/61287fig12.jpg" data-altupdatedat="1788250015000" data-alt="Figure 12"> 
Figure 12: The upper part of the figure shows the particle number emission rate over time for the first 10 mins of an RDE measurement drive. 
Data measured with the DTT PEMS using 10 nm and 23 nm CPC and a commercially available 23 nm cut point system (AVL MOVE) are used as a reference. The lower part of the figure shows the velocity of the vehicle. <a href="https://www.jove.com/files/ftp_upload/61287/61287fig12large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Presented here is the DownToTen (DTT) portable emission measurement system to assess real driving automotive emissions of sub-23 nm particles.

The current particle size threshold of the European Particle Number (PN) emission standards is 23 nm. This threshold could change because future ...

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Research

JoVE Journal

Engineering

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Learn to measure sub-23 nm particle emissions in real driving conditions using the portable DownToTen sampling system. This method enables assessment of nano-sized automotive emissions for environmental research.

Measuring Sub-23 Nanometer Real Driving Particle Number Emissions Using the Portable DownToTen Sampling System

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