Richard E. Chase

Size Distributions of Motor Vehicle Exhaust PM: A Comparison Between ELPI and SMPS Measurements

Particle size measurements using the electrical low pressure impactor (ELPI) and scanning mobility particle sizer (SMPS) are compared from the perspective of characterizing the particulate matter in motor vehicle exhaust. Both steady state vehicle operation and transient drive cycles are considered, and both gasoline and diesel fueled vehicle emissions are compared. Although the ELPI and SMPS measure different physical properties, respectively, the aerodynamic diameter and mobility diameter, the steady state particle size distributions are in close agreement, except for the 37 nm impactor stage of the ELPI which may overestimate particle number by up to a factor of two relative to the SMPS. This has little effect on the volume, or mass, weighted distribution. These, too, are generally in good agreement, though discrepancies appear at large particle size due to multiple charging effects in the SMPS and to electrometer offsets and the small particle loss correction in the ELPI. Selecting particles based on their electrical mobility with the SMPS, and then measuring their aerodynamic diameter with the ELPI, reveals that diesel particulate matter with well-specified mobility diameter exhibits a wide range in aerodynamic diameter and, therefore, also in effective density. Over transient drive cycles, the ELPI provides second by second particle distributions, whereas the SMPS must be run in a fixed particle size mode and size distributions constructed from repeated tests. The ELPI registers higher instantaneous PM emission rates during transients than the SMPS due to the faster time responses of the ELPI. The time integrated ELPI and SMPS size distributions, however, remain in good agreement. The relative merits of the two instruments for steady state and transient tests are discussed.

Vehicle exhaust particle size distributions: A comparison of tailpipe and dilution tunnel measurements

This paper explores the extent to which standard dilution tunnel measurements of motor vehicle exhaust particulate matter modify particle number and size. Steady state size distributions made directly at the tailpipe, using an ejector pump, are compared to dilution tunnel measurements for three configurations of transfer hose used to transport exhaust from the vehicle tailpipe to the dilution tunnel. For gasoline vehicles run at a steady 50 70 mph, ejector pump and dilution tunnel measurements give consistent results of particle size and number when using an uninsulated stainless steel transfer hose. Both methods show particles in the 10 100 nm range at tailpipe concentrations of the order of 104 particles/cm3. When an insulated hose, or one containing a silicone rubber coupler, is used to test small 4 cylinder gasoline vehicles, a very intense nanoparticle / ultrafine mode at < ~30 nm develops in the dilution tunnel particle size distribution as the vehicle speed is increased to 60 and 70 mph. This nanoparticle mode coincides with a rise of the transfer line temperature to about 180 250 °C. It is much less evident for the full size gasoline sedan, which has cooler exhaust. Both tailpipe and dilution tunnel measurements of diesel vehicle exhaust reveal an accumulation mode peak of ~108 particles/cm3, centered at 80 -100 nm. In this case, even with the uninsulated transfer hose an intense ultrafine peak appears in the dilution tunnel size distributions. This mode is attributed to desorption and/or pyrolysis of organic material, either hydrocarbon deposits on the walls of the steel transfer hose or the silicone rubber, by hot exhaust gases, and their subsequent nucleation in the dilution tunnel. This substantially limits the ability to make accurate particle number and size measurements using dilution tunnel systems. INTRODUCTION Prompted by potential health concerns, the past few years have witnessed a growing interest in particulate matter (PM) measurements, both ambient and from a wide variety of emissions sources. These measurements are conventionally performed by recording PM mass. The ambient standards are written in terms of mass concentrations, and emission regulations are based on mass rates. However, in order to understand better the nature of the mobile source contribution to ambient PM, many research groups are currently extending their investigations to include measurements of the numbers and sizes of particles in motor vehicle exhaust. Because the standard procedure for tailpipe PM measurements utilizes a dilution tunnel to cool the exhaust and to prevent water condensation, concerns have emerged that the test cell measurements of motor vehicle PM do not reflect the “real world” emissions. The root of the concern is that vehicle exhaust, a hot, complex, mixture of gaseous emissions and particles, is transformed differently when diluted in a tunnel as compared to the “real world”. Particles are not immutable; they readily undergo transformations, such as coagulation, condensation, and adsorption, and new particles can be created by nucleation of gaseous particle precursors in the diluted and cooled exhaust. Under “real world” conditions, motor vehicle exhaust is diluted rapidly, in less than one second, and by a large amount, a dilution ratio of greater than 100, into air of variable temperature and humidity. In the test cell, the exhaust is conducted to the dilution tunnel, typically by a 10 cm diameter by 5 m long tube, where it is then diluted by a factor of between 5 and 50, depending on test conditions, using dry air at room temperature. This discrepancy between dilution times, extents, temperature, and humidity can potentially lead to significant differences in the nature of the particulate emissions.

Measuring Particulate Mass Emissions with the Electrical Low Pressure Impactor

The electrical low pressure impactor (ELPI) is a useful tool for recording transient particle size distributions, such as in motor vehicle emissions. But for sub-micron aerosols, the straightforward mass weighting and integration of these size distributions overestimates the particulate matter (PM) mass by a factor of two or more. The present article examines the sources of this discrepancy and develops an analysis that allows quantitative PM mass measurement with an accuracy of about 20%. This procedure is applied to measure motor vehicle PM emissions, and the results are compared with filter-based gravimetric determinations. Good agreement is achieved for diesel and direct injection gasoline vehicles. For particulate trap equipped diesel vehicles and conventional gasoline vehicles, the PM mass recorded by the ELPI is often substantially lower than the filter based mass owing to the gaseous adsorption artifact of the latter. Accurate work at very low emissions levels, less than ∼ 1 mg/mi, requires further study of how reliably the ELPI can provide semivolatile nanoparticle mass as well as an improved understanding of filter-based vehicle exhaust measurement.

A Comparison of Tailpipe, Dilution Tunnel, and Wind Tunnel Data in Measuring Motor Vehicle PM

ABSTRACT Comparison between particle size distributions recorded directly at the tailpipes of both diesel and gasoline vehicles and measurements made using a conventional dilution tunnel reveals two problems incurred when using the latter method for studying particle number emissions. One is the potential for particulate matter (PM) artifacts originating from hydrocarbon material stored in the transfer hose connecting the tailpipe to the dilution tunnel, and the other is the particle coagulation (as well as condensation and chemical changes) that occurs during the transport. Both are potentially generic to current PM emissions measurement practices. The artifacts typically occur as a nanoparticle mode (10–30 nm) that is 2–4 orders of magnitude larger than what is present in the vehicle exhaust and can easily be mistaken for a similar mode that can arise from the nucleation of hydrocarbon or SO4 2-components in the exhaust under appropriate dilution rates. Wind tunnel measurements are in good agreement with those made directly from the tailpipe and substantiate the potential for artifacts. They reveal PM levels for the recent model port fuel injection (PFI) gasoline vehicles tested that are small compared with the ambient background particle level during steady-state driving. The PM emissions recorded for drive cycles such as the Federal Test Procedure (FTP) and US06 occur primarily during acceleration, as has been previously noted. Light-duty diesel vehicle emissions normally exhibit a single lognormal mode centered between 55 and 80 nm, although a nonartifact nanoparticle mode in some cases appears at a 70-mph cruise up a grade.

Reducing PM Measurement Variability by Controlling Static Charge

PM (Particulate Matter) emitted by vehicles and engines is most often measured quantitatively by collecting diluted exhaust samples on filters that are weighed pre-and post-test. Static charge that builds on filters from handling can dramatically influence the measurement results, especially at low PM levels such as those produced when testing typical gasoline-powered vehicles or diesel-powered vehicles employing DPF (Diesel Particulate Filter) technology. It was found that proper grounding of equipment, furniture, and floor was insufficient to mitigate the effects of static electricity when using the traditional method of weighing from a glass Petri dish in the presence of an ionizing bar. A stainless steel EDP (Electrostatic Discharge Platform), using commercially available ionizing bars, was developed and proven to successfully reduce filter measurement variability when weighing PTFE membrane filters on a 0.1 microgram balance. This paper reports hardware design, measurements, and comparative results quantifying the variability reduction achieved with the use of the Electrostatic Discharge Platform in a weigh room under tightly controlled conditions.

A Constant-Volume Rapid Exhaust Dilution System for Motor Vehicle Particulate Matter Number and Mass Measurements

Abstract An improved version of the constant volume sampling (CVS) methodology that overcomes a number of obstacles that exist with the current CVS dilution tunnel system used in most diesel and gasoline vehicle emissions test facilities is presented. The key feature of the new sampling system is the introduction of dilution air immediately at the vehicle tailpipe. In the present implementation, this is done concentrically through a cylindrical air filter. Elimination of the transfer hose conventionally used to connect the tailpipe to the dilution tunnel significantly reduces the hydrocarbon and particulate matter (PM) storage release artifacts that can lead to wildly incorrect particle number counts and to erroneous filter-collected PM mass. It provides accurate representations of particle size distributions for diesel vehicles by avoiding the particle coagulation that occurs in the transfer hose. Furthermore, it removes the variable delay time that otherwise exists between the time that emissions exit the tailpipe and when they are detected in the dilution tunnel. The performance of the improved CVS system is examined with respect to diesel, gasoline, and compressed natural gas vehicles.