Investigation of solid particle number measurement: Existence and nature of sub-23 nm particles under PMP methodology
Highlights
► Sub-23 nm particles were found downstream the PMP volatile particle remover (VPR) in our previous study. ► An AVL particle counter and a catalytic stripper were used to investigate the nature of these particles. ► Laboratory tests showed that some solid particles were formed from volatile precursors in the APC. ► Particle number level of the 3–10 nm particles downstream the APC was higher than that in the dilution tunnel. ► The majority of sub-23 nm particles are artifact particles formed by renucleation of semivolatiles.
Introduction
As regulation of diesel Particulate Matter (PM) mass gets more stringent, the current gravimetric method for the legal determination of emissions will have difficulty accurately quantifying PM mass emissions. Although the United States (U.S.) Environmental Protection Agency (EPA) issued an improved protocol for the gravimetric method (EPA, 2008), accuracy will continue to be an issue at the very low emission levels of new diesel vehicles equipped with aftertreatment systems. For the Euro IV heavy-duty engine limit of 0.02 g/kWh, for example, the variabilities of the repeatability and reproducibility of the current gravimetric method are more than 20% and 50%, respectively (Burtscher, 2005).
Progress in regulating diesel particle emissions by non-gravimetric means has been made in Europe. The United Nations Economic Commission for Europe-Group of Experts on Pollution and Energy (UNECE-GRPE) initiated the Particle Measurement Program (PMP) working group to develop new particle measurement techniques to supplement or replace the current gravimetric method. The PMP protocol specifies measuring solid particles larger than 23 nm. Solid particles are operationally defined by the PMP as particles that can survive after passing through an evaporation tube (ET) that has a wall temperature of 300–400 °C (UNECE, 2010). A solid particle number concentration limit of 6×1011 particles/km has been included in the Euro 5/6 standards for light-duty diesel vehicles (UNECE, 2008). The Euro VI standard for heavy-duty diesel vehicles includes a solid particle number concentration limit as well, with the proposed limits of 8×1011 particles/kWh for stationary cycles and 6×1011 particles/kWh for transient cycles (Johnson, 2010).
The PMP only measures solid particles larger than 23 nm to avoid issues with poor repeatability caused by volatile particles present in the nucleation mode of diesel exhaust (Martini et al., 2009). Exclusion of sub-23 nm particles may have some potential issues, since not all sub-23 nm or nucleation mode size range particles are volatile. Some studies have found solid particles in the nucleation mode from heavy-duty diesel vehicles operating at idle or low loads (Filippo and Maricq, 2008, Kittelson et al., 2006). Even at high load operating conditions, solid particles in the nucleation mode have been observed for heavy-duty diesel vehicles (Lähde et al., 2009, Lähde et al., 2010, Rönkkö et al., 2007). These references (Filippo and Maricq, 2008, Kittelson et al., 2006, Lähde et al., 2009, Lähde et al., 2010, Rönkkö et al., 2007) also define non-volatile or solid particles as particles that can survive after passing through a thermodenuder (Burtscher et al., 2001), but with a slightly larger temperature operating range of 270–400 °C compared with the PMP. By excluding these sub-23 nm solid particles, the full impact of solid particles is not characterized by the PMP standard (Martini et al., 2009). Regulating particle number emissions for other sectors (aviation, off-road) is under discussion (Giechaskiel et al., 2010a). If the current PMP protocol were applied to other sectors, further caution should be taken in excluding sub-23 nm solid particles. For example, solid nucleation mode particles have been found for a gasoline vehicle, when some anti-knock metal additives were used (Gidney et al., 2010). Lead anti-knock additives are also still used in gasoline for general aviation. Czerwinski et al. (2006) even found solid particles below 23 nm emitted from 2-stroke mopeds.
It is also reported that the PMP can remove almost all volatile components of diesel vehicle emissions, and that no nucleation can occur downstream of the PMP (Giechaskiel & Drossinos, 2010). However, during previous California Air Resource Board (CARB)/University of California Riverside (UCR) studies of the PMP, a significant number of appeared-to-be solid sub-23 nm particles were found downstream of the PMP volatile particle remover under conditions that were thought to be unlikely to form sub-23 nm solid particles (Herner et al., 2007, Johnson et al., 2009). In the exploratory work for applying the current PMP protocol to heavy-duty diesel engines, Giechaskiel et al. (2009) also found apparently non-volatile sub-23 nm particles downstream of the PMP system. Thus, it is important to investigate whether these sub-23 nm particles observed downstream of the PMP system are solid or volatile, and if they are solid, whether they come from the exhaust or are artifacts of the measurement system.
An alternative system commonly used by researchers to remove volatile particles is a catalytic stripper (CS) (Abdul-Khalek and Kittelson, 1995, Kittelson et al., 2005, Park et al., 2003, Swanson and Kittelson, 2010a, Swanson and Kittelson, 2010b). In contrast to the PMP system, the CS uses a different approach to remove volatile particles. It removes all volatile hydrocarbon components and sulfur components by catalytic reactions at an elevated temperature. Therefore, renucleation will not occur downstream the CS. A study comparing the volatile removal efficiency of a CS with a thermodenuder, which is another type of volatile particle remover, showed that the CS had a higher efficiency for removing volatile particles (Swanson & Kittelson, 2010a). However, no studies have been conducted to compare the PMP system with a CS in terms of volatile particle removing efficiency.
This study presents laboratory and vehicle experiments of diesel particle penetration/formation using a PMP system and a CS. This study investigated and compared the effectiveness of the European PMP system and CS in removing volatile aerosols (1) using volatile aerosols generated in the laboratory; and (2) using diluted exhaust from a heavy-duty diesel vehicle operating over various cycles on a chassis dynamometer. This study also advances our understanding of the nature of sub-23 nm particles downstream of the PMP system, which were identified in a previous work (Johnson et al., 2009).
Section snippets
PMP system, AVL particle counter advanced
The PMP system used in the current study was an AVL particle counter advanced (APC), a commercial solid particle measuring system developed by AVL List GmbH. It fulfills the most recent requirements of the PMP protocol (UNECE, 2010). A brief description of the APC is provided here. More detailed schematics and descriptions of the APC can be found in reference (Giechaskiel et al., 2010b) and the manual of the APC. The sample enters the system with a typical flow rate of 5 Lpm (liters per minute)
Laboratory test
The laboratory experiments are an important part of this study, because they allow the formation process of sub-23 nm particle to be investigated using model aerosols under well-controlled conditions. This provides an important link to the vehicle exhaust testing with the chassis dynamometer in this study and the on-road testing in our previous study (Johnson et al., 2009), both of which showed that a significant fraction of the particles downstream of the PMP were in the sub-23 nm size range.
The
Conclusions
A European PMP compliant particle measurement system, the APC, and an alternative system for removing volatile particles, the CS, were evaluated and compared using laboratory-generated model volatile particles and diluted exhaust of a DPF-equipped, heavy-duty diesel vehicle operated on a heavy-duty chassis dynamometer. The goal of this study was to investigate and characterize particles found downstream of the PMP system, with an emphasis on sub-23 nm particles. The well-controlled laboratory
Acknowledgments
The authors acknowledge California Air Resources Board (CARB) for funding (08–302) and lending instruments for this study. H.S.J. would like to thank Drs. Alberto Ayala and Jorn Herner for encouragement. The authors gratefully acknowledge AVL LIST GmbH Inc. for providing us an AVL particle counter and technical support. Drs. Barouch Giechaskiel, Richard Frazee, Linke Manfred, Siegfried Roeck, and William Silvis from AVL are particularly appreciated. We appreciate the help of Mr. Donald Pacocha,
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