Environ. Sci. Technol. 2005, 39, 2229-2238
Comparison of Mass-Based and Non-Mass-Based Particle Measurement Systems for Ultra-Low Emissions from Automotive Sources MARTIN MOHR,* URS LEHMANN, AND JOSEF RU ¨ TTER EMPA (Swiss Federal Laboratories for Materials Testing and Research), Ueberlandstrasse 129, CH-8600 Du ¨ bendorf, Switzerland
Drastic reduction in particle emissions of diesel-powered vehicles and new findings on the health impact of particles raise the question of a more sensitive measurement procedure. In this paper, 16 different particle mass measurement systems are compared on a diesel heavyduty engine equipped with a particle filter to investigate their feasibility for particle characterization for future ultralow concentration levels. The group of instruments comprises mass-related methods (filter methods, laser-induced incandescence, photoacoustic detection, photoelectric charging, combined inertial and mobility sizing, opacity) as well as non-mass-related methods (CPC, diffusion battery, diffusion charger, ELPI, light scattering). The instruments are compared on the basis of repeatability, limit of detection, sensitivity, time resolution and correlation with the regulated gravimetric filter method, and elemental carbon fraction (EC). Several time-resolved methods show good performance and give reliable results. Opacimeters and light scattering, however, reveal shortcomings at these low concentrations. For all time-resolved advanced methods, poor correlation with the regulated filter method is observed, but most of them show good correlation with the EC fraction of the particles. This outcome demonstrates the crucial role of the sampling conditions for measurement methods that do not exclude volatile material from detection. A clear improvement in sensitivity is observed when non-mass-based instruments are applied (e.g., number or surface-related methods). The results reveal that reliable measurement methods exist for future measurement procedures. However, a change in the measurement method will lead to a discontinuity in the inventories, making it difficult to compare the particle emissions from future and past vehicle generations.
Introduction In most countries, particle emissions are restricted in terms of mass determined by the weighing of filter samples with a microbalance using a specified filter collection procedure. Forced by steadily reduced limit values, the continuous improvement in engine technology and the development of after-treatment systems have resulted in a sharp reduction in the particle emissions of present-day automotive road * Corresponding author phone: +41 1 8234190; fax: +41 1 8234041; e-mail:
[email protected]. 10.1021/es049550d CCC: $30.25 Published on Web 03/02/2005
2005 American Chemical Society
vehicles (1, 2). As a consequence, the effective limit of this measurement method is being approached, and the feasibility of measurement for further reduction of limit values is questionable. In addition, the gravimetric filter method does not distinguish between solid (soot) and condensed components (e.g., sulfates, hydrocarbons). The latter depends greatly on the sampling conditions and can be predominant for engines optimized for low particle emissions or downstream of an efficient particle trap. This situation has led to a considerable increase in uncertainty in quantification of the emission and to misinterpretation of the efficiency of after-treatment systems. This situation can hinder correct emission inventories that are the basis for decisions in air pollution policy-making. Another drawback of the filter collection method is the poor temporal resolution. For engine development, methods are needed that allow transient operation conditions with time resolution of 1 s and below to be studied. Another aspect is the possible impact of ultrafine particles on human health. Recent studies have suggested an association with symptoms in patients suffering from respiratory and cardiavascular diseases (3, 4). As a consequence, there is a need to consider an alternative or complement method to the current gravimetric approach for certification purposes. EMPA carried out a comparison study of a large number of particle measurement systems that show potential for future legislative purposes and/or research. In contrast to earlier comparison studies (5-8), the objective was to study the performance of the measurement systems for very low concentration levels. Criteria that have been used for the evaluation include repeatability, limit of detection, sensitivity, response time, and degree of correlation of time-integrated results between the instruments. The selection of investigated instruments comprised a wide variety of methods that include mass-related methods as well as non-mass-related methods. The study was carried out in the framework of a collaborative program (PMP) of several European countries for the evaluation of a new particle measurement system (9). The program is running under the auspices of the UNECE WP29/ GRPE Group.
Experimental Section Particle Measurement Instruments. A total of 16 particle measurement instruments, comprising state-of-the-art and prototype technologies, were investigated in a 3-week measurement program. The group consisted of nine massrelated methods, including three filter collection methods and six time-resolved methods, and seven non-mass-related methods including four number-related and one each of length-, surface-, and volume-related methods. All non-massbased methods were capable of time-resolved measurements. The measurements were undertaken in cooperation with representatives of the instrument manufacturers. The 16 instruments are described in the following sections, and Table 1 lists the relevant details of the measurement systems. Gravimetric Filter Method (GFM). The filter sample is drawn out from a full-flow dilution tunnel (CVS) and is diluted once more in a mini-tunnel. The method used in this program was based on European regulation (1). However, to evaluate the potential of an improved method, some specifications of the U.S. Federal Register 2007 (2) were adopted: single filter with a collection efficiency g99% (Pallflex, TX40), face velocity, a pre-classifier (50% cutpoint at 2.5 µm), quality of primary and secondary dilution air, thermal isolation of transfer lines and pre-classifier, conditioning of filters, and microbalance workstation. VOL. 39, NO. 7, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TSI Matter Eng. Dekati sensors DC2 EDB ELPI LS
EAD3070A EDB200 ELPI (10 L/min) PM-300
turbul. diff. charging, electrical detection elec. diffusion battery, electrical detection impactor principle, electrical detection laser light scattering
photoelectrical charging, electrical detection light extinction, opacimeter light extinction, opacimeter laser light extinction, opacimeter cond. particle counter, laser light scattering diffusion charging, electrical detection Matter Eng. Hartridge AVL Wizard TSI Matter Eng.
standard prototype standard standard
Y Y Y Y
raw gas line raw gas line raw gas line raw gas line
double stage ejector, heated rotating disk, heated double stage ejector, heated internal, heated
95/10 123/21 95/10 10
mass volume volume volume number active surface length number number number 123/21 6 >4 95/10 >6 Y N Y Y N
CVS tunnel CVS tunnel secondary dilution tunnel CVS tunnel raw gas line double stage ejector, heated raw gas line partial flow dilution tunnel
mass >6 secondary dilution tunnel CVS tunnel N
standard modified prototype standard prototype prototype standard filter sample, weighing with microbalance GFM
code
instrument
manufacturer
principle 9
gravimetric filter method LII LI2SA PM MEXA 1370PM PA PASS MM mass monitor DMM CM coulometric filter analysis PAS PAS2000 OPM1 DPSO-1 OPM2 Opacimeter 439 OPM3 DQL CPC CPC3022A DC1 LQ1-DC
metrics dilution factor higher/lower emission dilution system sampling location state of time-resolved development measurements
TABLE 1. Investigated Particle Measurement Systems 2230
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Laser-Induced Incandescence (LII). LII allows the monitoring of the elemental carbon (EC) concentration with a high time resolution. Using high-energy laser pulses, soot particles within the probe volume are heated to their vaporization temperature. Subsequently, the enhanced thermal radiation is detected. The maximum signal provides soot mass concentration and is practically unaffected by absorbed material and volatile particles. In addition, the specific surface area can be evaluated from the signal decay time. The instrument used for this work (LI2SA, Esytec GmbH) has a time resolution of 20 Hz (10-12). Coulometric Analysis (CM). CM is a widespread approach for the determination of carbon content in filter samples and is specified in international standards (13, 14). In a twostep process, the filter sample is heated, and the carbonaceous compounds are oxidized to CO2 by means of a catalytic converter. CO2 is absorbed in a basic chemical solution and quantified by electrochemical titration. The instrument used for this work, Coulomat 702 DR (Juwe GmbH), operates in two steps: In a first step, the filter sample is heated in a nitrogen gas flow to 773 K and subsequently in an oxygen flow to 923 K. In this way, the organic bound carbon (OC) and EC fraction are separated. Mass PM Analyzer (PM). This instrument, Mexa 1370 PM (Horiba Ltd.), operates in a similar way to the method described above. In this case NDIR detectors are applied to separately measure the mass of PM components after an off-line process of vaporization and oxidation (15). The particulate-laden filter is installed in a furnace and heated to 1253 K in a nitrogen gas flow and subsequently in an oxygen gas flow. The sulfate and SOF components are vaporized and detected as SO2 and CO2 after deoxidization and oxidation, respectively. Finally, the soot trapped at the filter is oxidized and is also detected as CO2, too. Photoelectric Aerosol Sensor (PAS). The PAS measures the electrical charge of the aerosol after illumination by UV radiation (in this case λ ) 222 nm). The particles are charged by photoelectric effect, which depends on the surface material and the size of the particles. Subsequently, the particles are collected on an electrically insulated filter, which is connected to a current amplifier. Although the PAS is very sensitive to adsorbed material on the surface (e.g., large response to PAH compounds), recent investigations also indicate a moderate response for EC mass concentration (16). This induced the manufacturer of the PAS2000 instrument (Matter Engineering AG), which was used in this study, to calibrate PAS as an EC mass sensor for fresh diesel exhaust. Photoelectric sensors have been previously used in many applications in the field of characterization of emissions and atmospheric measurements (17-23). The time resolution is of the order of 1 s. Opacimeters (OPMs). The OPMs are based on the extinction of light by particles. By definition they measure visible smoke (i.e., extinction coefficient, k value; unit, m-1) using green light. The value is usually converted to a mass concentration by calibration. The attenuation of light is caused by scattering and adsorption and is a function of concentration, size, and composition of the particles. Opacimeters have been widely used with good success for emission measurements in the past (24-27). The response times are typically well below 1 s. Today, low particle concentration necessitates advanced development to achieve a viable signal. Three different opacimeters were investigated in this study. The AVL 439 (AVL List GmbH) (OPM1) is a conventional opacimeter with optical path length of 430 mm and a single wavelength detector for green light. The DPSO-1 (Hartridge) (OPM2) is an advanced instrument with a broadband light lamp, which uses multiple sensors to detect opacity at three wavelengths in the range between ultraviolet and red over an optical path length of 400 mm. The three signals are used to extract a dominant particle size based on Mie theory for
extinction. The DQL (Wizard) (OPM3), also operates at three wavelengths (laser diodes), but in contrast to the OPM2, it operates in the range between red (637 nm) and infrared (811 nm, 1316 nm) and at a significantly extended optical path length of 10 m to improve the sensitivity of the instrument. For the opacimeters OPM1 and OPM2, the measured k values were converted to a mass concentration using the manufacturer’s calibration of 1.5E-07 mg/(cm3‚ mK). OPM3 displays the measurement values as particle volume applying the Lorentz-Mie theory. Mass Monitor (MM). The DMM (Dekati Ltd., Finland) combines different aerosol measurement principles: electrical charging and detection and inertial and electrical mobility size classification. Inertial size separation is done in a cascade low-pressure impactor (six stages), and for electrical mobility classification a grade mobility analyzer is used. Current carried by charged particles defines the amount of particles in each size range and is measured using a multichannel electrometer. The comparison of aerodynamic and mobility size measurement is used to calculate the effective density of the particles (28), and together with the determined particle size distribution, a time-resolved particle mass is calculated. A sampling system with a thermal treatment is recommended since a non-symmetrical distribution (e.g., due to the existence of a nucleation mode) increases the uncertainty of the calculation for density. More details and examples of first successful applications can be found in refs 29 and 30. Photoacoustic Sensors (PAs). The PAs measure the absorption of a modulated IR laser beam by the particles that results in a modulated heating of the particles and, subsequently, leads to modulated gas expansion. The resulting pressure wave is recorded by a microphone in an acoustic resonator optimized for the modulated frequency. For particles from engine exhaust, the dominant light absorbing component is soot, and the absorption efficiency is independent of particle size as long as it is smaller than the wavelength of light. Photoacoustic instruments have been used for particle quantification for emission sources and ambient measurements for many years (6, 22, 31-36). The photoacoustic instrument PASS (TU Munich) used in this study was a prototype version and was operated at a wavelength of 809 ( 2 nm. The temporal response was of the order of 1 s (37). Condensation Particle Counters (CPCs). CPCs are based on the detection of light scatter pulses of particles. To achieve a measurable light intensity, the particles pass a condensation tube where alcohol vapor condenses on their surface. For this project a CPC 3022A (TSI) was used. This instrument has two detection modes depending on the particle concentration. At low concentration, the single counting mode detects the individual pulses of light; at concentrations above 104 cm-3, the photodetector measures the amount of light scattered by the cloud of droplets as they pass by the measurement chamber. The lower 50% cut point is at a particle diameter of 7 nm, and the time response is a few seconds. CPCs have excellent sensitivity and are very common for particle counting for emission and ambient measurements. Further details on the design and characteristics of CPCs are presented in ref 38. Electrical Low-Pressure Impactor (ELPI). The instrument is a further development of a conventional cascade impactor. Whereas the size classification is still based on the inertial mass of the particles, the particles are quantified by their electrical charge obtained upstream in a unipolar corona charger. The current value of each stage is proportional to the number of particles collected. The ELPI (Dekati Ltd.) used in this work has a number size distribution of 12 channels in the range between 8 nm and 10 µm with a time resolution of about 1 s. However, it has been shown that the sensitivity of the size channels above 1 µm is too low for
diesel exhaust aerosol. Further details of the method are presented in ref 39. Diffusion Charger (DC). The operating principle of this method is based on diffusional charging of the particles, followed by a current measurement caused by the trapped particles in a Faraday cage. Diffusion chargers can be fairly simple and compact instruments with good time resolution of 1 s and below (40). However, the metrics determined by this method is not very easy to interpret, since the signal is proportional to a particle dimension that is between length and surface area of the particles (i.e., ∝ d1.13) experimentally determined in ref 41. As a consequence, the two instruments used in this study were calibrated on different parameters. The LQ1-DC (Matter Engineering) (DC1) was calibrated to active surface area, as opposed to the EAD (TSI) (DC2), which was calibrated to particle length. Electrical Diffusion Batteries (EDBs). EDBs combine the principles of a conventional diffusion battery and the method of the diffusion charger. After charging, the particles pass several series of electrically insulated screens to which they eventually attach by diffusion. The particle charge is captured and amplified for each series of screens and the backup filter. The prototype of Matter Engineering consisted of a total 5 size channels that are used to calculate a monomodal or bimodal log-normal size distribution function of the particle number with a time resolution of about 1 s (42). Light Scattering (LS). In contrast to nephelometers in which the signal is produced by a group of particles, the PM-300 instrument (Sensors) used in this project detects scattered light pulses of single particles when they cross the laser beam (43). In this way the particles can be counted, and their size can be determined with a high time resolution. The gain in size information as opposed to CPC comes at the cost of the counting efficiency of small particles. For this instrument the manufacterer reports a minimum detectable particle size of 300 nm, which is significantly above the typical size of diesel exhaust particles. Particle Sampling. The intention of the work was to compare whole measurement systems, consisting of a detection unit and a sampling and conditioning system, and not to look at individual detection instruments out of context. There is broad agreement that sampling and conditioning of the aerosol sample have a significant bearing on the result of the measurement. The large number of instruments with their different requirements in flow rate, temperature, and concentration range as well as their sensitivity to volatile particles and pressure fluctuations made it impossible to define a common solution for the sample conditioning that would suit all instruments. For this reason the conditioning was defined individually for each instrument in agreement with the manufacturer to achieve the best results, knowing that a comparison of absolute concentrations is hindered. The samples for instruments were taken either from the primary full-flow CVS tunnel or from the raw exhaust gas line. For sampling from the CVS tunnel, all probe openings were in the same plane at the end of the 4.5 m long tunnel. Due to the large number of sampling points and the small diameter of the exhaust pipe, two locations had to be defined for the raw gas sampling. The planes were about 0.5 m apart and at least 6 times the diameter of the exhaust pipe downstream of the last bend to prevent any interference. The probes for the filter sampling for coulometric analysis and one opacimeter (OPM2) were located in the raw gas line about 1 and 2 m downstream of the other probes, respectively. For the raw gas sampling, the transfer lines to the first dilution unit or detection unit were heated for all instruments. Table 1 presents an overview of the sampling location and dilution systems for the individual instruments. Engine Test Facilities. The engine tests were carried out on a heavy-duty diesel engine, manufactured by Volvo VOL. 39, NO. 7, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Technical Details of the Engine Volvo HD engine certification level displacement no. of cylinders rated power rated speed max torque engine technology after-treatment system
[-] [L] [-] [kW] [1/min] [N‚m] [-]
Euro 3 7 in-line, 6 190 2200 1030 turbocharger (waste gate), intercooler CRT system (oxi-cat and particle trap)
(Go¨teborg, Sweden), on the heavy-duty test bench at EMPA. This type of engine is typically installed in buses and was certified in combination with a particle filter system CRT according to the Euro 3 regulations (1). More technical information on the engine is provided in Table 2. To obtain a data set with a wider concentration range, a bypass was installed parallel to the CRT system to enable the particle concentration in the exhaust line to be increased. Using two gates, one in the bypass and the other upstream of the division, the particle emission level could be adjusted without changing the back-pressure settings of the engine. For this study, measurements at two different emission levels were carried out, which were both well below today’s emission standards. For measurements at the higher emission level, the gate of the bypass was adjusted to about 40% below the future emission standard Euro 4, corresponding to specific PM emission during the European transient driving cycle (ETC) of about 17 mg/kWh corresponding to about 2 mg/ m3. NOx analyzers were installed upstream and downstream of the particle trap to determine the NO conversion rate of the CRT/bypass system. In this way, the stability of the flow split at the bypass could be checked for each individual test cycle. For the measurements at the lower emission level, the bypass was removed resulting in specific emission of about 2 mg/kWh and 0.3 mg/m3, respectively. All engine tests were undertaken with diesel fuel, according to standard CEC-RF06-99, with less than 10 ppm sulfur. The engine was run with high-quality lubricant oil, with the SAE classification 10 W-40 and about 3900 ppm sulfur. Test Program. The engine test program comprised transient and steady-state tests cycles. The main focus was put on the European transient cycle (ETC), which is the official transient certification test cycle in Europe and is run in hotstart conditions (1). For the steady-state tests, five operating modes were selected from the European steady-state cycle (ESC) (1) for a “single-mode test cycle”. The modes were run for 15 min, one after the other without interruption. To obtain information about the time response of the particle systems, a step change test was defined, consisting of a repeated switch between two loads at constant speed, with a 3-min stabilization phase between two switches. At the beginning of each day, background measurements were carried out.
Results and Discussion Repeatability. Measurements during multiple ETC test cycles were performed to assess the repeatability of the particle measurement systems. The average and the standard deviation were calculated for seven test runs, which were carried out on three different days. Figure 1 shows the results determined over the total cycle of 1800 s for the higher emission and the lower emission configuration. The analysis of the subcycles, which consist of urban, rural, and highway parts, did not reveal any significant dependence of the repeatability on the engine load. As the concentration of the lower emission level is about a factor of 10 lower and close 2232
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FIGURE 1. Repeatability of the measurement systems. The relative standard deviation determined for seven ETC test cycles is shown for the higher and lower emission configuration, respectively. EC, elemental carbon fraction; 0, one outliner excluded. to the background concentration, it is not surprising that repeatability is better for the higher emission level for all measurement systems. The best results for the coefficient of variation (COV) were obtained for the LII and the PM. Although most of the instruments show similar or better repeatability as compared to the gravimetric filter method (GFM) for the higher emission level, a greater increase in varation is observed for most non-mass-based instruments when the exhaust flow completely passed the trap. The very high variation exhibited by the DC2 for the lower emission level could be explained by a zero-point drift. The coulometric method (CM) also showed unexpected high variability throughout the whole test program, which was probably caused by a malfunction in the instrument because blind tests also showed unusually wide scattering. Due to lack of sensitivity and high cross-sensitivity to NO2, none of the opacimeters displayed a particle related signal for the lower emission level. In the case of the OPM1 this could already apply to the high emission level. Simultaneous measurements of the gaseous components and engine parameters were carried out to distinguish between variations in the measurement systems from those caused by the engine. The results show that the engine operation was very stable and do not give any indication of a relevant contribution to the overall variation. Only for the hydrocarbon, measured by flame ionization detection (FID), high variation has to be accepted downstream of the catalytic oxidizing converter where the concentrations were very low. The following COVs were determined for the lower emission configuration: 0.1% (engine power), 0.5% (exhaust flow), 2.5% (NO), 40% (T.HC). Limit of Detection (LOD). Another important criterion for the suitability of a measurement system for future application is the limit of detection. In this study, characteristic values were determined for the specific setups with exhaust gas line, sampling systems, and particle measurement instruments. From our point of view, LODs from the complete systems are more relevant to the assessment of measurement systems than of isolated instruments. However, these values do not necessarily correspond to the specification of the instruments given by the manufacturers. The particle measurement systems were characterized during the seven ETC test cycles in the full trap configuration (i.e., no bypass was installed in the exhaust line parallel to the particle trap). We decided to calculate the LODs from these tests and not from the background tests (engine shut off), as the absolute concentrations downstream of the trap were similarly low and the repeatability of the ETC tests was significantly better than for the background tests. Moreover, for the ETC tests the conditions are closer to real engine exhaust measurements regarding temperature and flow rate. The LOD was determined by taking three times the standard deviation of the seven ETC measurements with the lower emission
FIGURE 2. Sensitivity of particle measurement systems. The ratio of concentrations measured for the higher and lower emission configuration is plotted. configuration. As can be seen from Table 5, the LODs for the systems show wide differences. These differences are not only explained by the noise but also indicate a different response in relation to exhaust and particle properties. For the mass-related systems, LII, PA, PAS, PM EC, and MM show the lowest limit of detection. This is explained by the fact that these methods are not affected by low-volatility particles, which are present predominantly downstream of a veryefficient trap (Figure 4). In the case of the MM, the volatile fraction is suppressed by the hot dilution principle. Among the filter collection methods the PM analyzer exhibits the lowest LOD, whereas the performance of coulometric method again is uncommonly high. No useful results were obtained for any of three opacimeters. The interference with NO2 prevented the evaluation of the signals for the LOD. For the number-related measurement system, CPC and ELPI show the lowest LODs with about 4000 and 5000 particles/cm3, respectively. While the concentrations may seem to be very high, it has to be borne in mind that they reflect the whole measurement system, including the dilution system. Sensitivity. Sensitivity is the criterion that demands significant improvement to meet the requirements of measurement systems for future applications. Characteristic information about the sensitivity of the individual systems was determined by the ratio between the concentrations measured for the higher and lower emission configurations. We are aware that this ratio does not correspond to the real sensitivity as given by the signal/noise ratio. Nevertheless, we consider that the calculated ratio provides a useful information on the capability of the instruments to distinguish between two low emission levels or to quantify the efficiency of particle traps. The ratios for the ETC test cycle are shown in Figure 2. The exceptionally wide difference of about 2 orders of magnitude between the instruments indicates that not only differences in detection sensitivity but also other particle properties are measured by the methods. The low sensitivity of the filter collection method can be mainly explained by adsorbed material on the filter for the post-trap sample (44). However, non-filter methods (e.g., LII and PA) show similarly low ratios, although they are not affected by condensed material. A large ratio was expected especially for these instruments, since they detect EC only, and the EC concentration is almost zero downstream of an efficient trap. A possible explanation could be deficiency in resolution of the sensors or the A/D converters for electronic signal processing. Slightly better results were obtained for the filter samples (PM EC, CM EC) with EC selective detection. However, much higher sensitivity was observed for the CPC and those instruments whose detection are based on electrical current measurement induced by a group of electrically charged particles (MM, PAS, DC, EDB, ELPI). The results indicate that this principle seems to be far more sensitive than the mass-based methods. The high sensitivity of the
FIGURE 3. Absolute particle mass concentrations measured during the ETC for the higher emission configuration.
FIGURE 4. Absolute particle mass concentrations measured during the ETC for the lower emission level configuration (i.e., complete exhaust flow through particle trap). condensation particle counter is not surprising since this is an inherent quality of single particle counting. Despite single particle counting, the laser scattering instrument (LS) showed a low sensitivity, which gives a clear indication that the instrument does not cover the full size range of diesel exhaust particles. Small particles are not detected due to the very low intensity of their scattered light (I ∝ r6), if they are not enlarged by condensation. Absolute Values. Due to the fact that the instruments apply completely different principles and measure different particle properties, discrepancies in the absolute values have to be expected. For example, an instrument that measures only elemental carbon should detect an equal or lower mass than an instrument that is calibrated for total mass. As can be seen from Figure 3, the mass concentration varies by about a factor of 1.4 for the filter-based methods (GFM, PM, CM) and a factor of 2.2 for the soot-sensitive instruments (LII, PM EC, PA, CM EC) for the higher emission level. Exceptions are PAS and the opacimeter (OPM2), which measure much higher concentrations, indicating wrong calibration and crosssensitivity to NO2, respectively. Much higher variation is observed for the lower emission level (Figure 4), where the soot fraction is almost zero downstream of the particle trap and most of the mass is contributed by nonsolid material. The filter methods (GFM, PM, CM) show good agreement within 10%. Obviously, the soot-sensitive methods (LII, PM EC, PA, CM EC, PAS) detect a significantly lower concentration, but the agreement is not satifactory. Some of the instruments are able to distinguish between different components of the particles or their detection is based on a selective component, (e.g., soot). For clarity, and with the consequence of simplification, the particle components are grouped in Figures 3 and 4. It is notable that the soot-sensitive instruments (LII and PA) measure significantly higher values than PM and CM for the EC fraction. This applies particularly to the lower emission configuration (Figure 4). VOL. 39, NO. 7, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Correlation Factors of Non-Mass-Based Systems with Gravimetric Filter Method (GFM) and EC Fraction Determined by Coulometric Analysis (CM EC), Respectively R2
CPC
ELPI
DC1
DC2
EDB
OPM3
LS
GFM CM EC
0.17 0.95
0.40 0.90
0.47 0.97
0.50 0.98
0.38 0.97
0.57 0.96
0.03 0.15
TABLE 5. Limit of Detection of the Investigated Particle Measurement Systems as Determined in This Worka
FIGURE 5. Absolute particle number concentrations measured during the ETC for the higher emission level configuration.
code
unit
GFM LII PM PM EC PA MM CM CM EC PAS
[mg/Nm3]
a
[mg/Nm3] [mg/Nm3] [mg/Nm3] [mg/Nm3] [mg/Nm3] [mg/Nm3] [mg/Nm3] [mg/Nm3]
LOD 0.189 0.037 0.061 0.004 0.029 0.004 0.357 0.139 0.002
code OPM1 OPM2 OPM3 CPC DC1 DC2 EDB ELPI LS
unit
LOD
[mg/Nm3] [mg/Nm3] [µm3/Nm3] [1/Ncm3] [µm2/Ncm3] [mm/Ncm3] [1/Ncm3] [1/Ncm3] [1/Ncm3]
0.951 4040 97 0.181 16700 5340 89600
N ) standard conditions (273 K, 1013 hPa).
point is an average of several test runs. Table 3 presents the correlation of the particle measurement systems with the gravimetric filter method and EC concentration measured by CM. Very good correlation with GFM was achieved by the other filter method PM (Mexa 1370PM) with a unity slope of 1.0, a small offset of -0.04, and a R 2 of 0.95. It is evident that the identical filter sampling with the GFM contributed to this very good result. All other instruments, including the non-mass-based methods show rather poor correlation with the GFM. Correlation coefficients were found to be less than 0.6, and the slopes for the mass-related instruments ranged between 0.08 and 0.86. All instruments show a smaller slope than the GFM, denoting a higher particle mass concentration measured by GFM. For most advanced instruments including the non-mass-related methods, the correlation with the EC fraction showed significantly better results with values for R 2 > 0.94 (ELPI: 0.90). This outcome demonstrates that the poor correlation of most instruments with GFM is caused by a mass fraction of condensed material that is not detected by other instruments. It is notable that even number-based instruments show good correlation with the elemental carbon fraction of the particle mass, if the aerosol is conditioned in such a way that condensation of low-volatility material is suppressed (Table 4). This finding also implies that the mean particle size did not change significantly for the different test cycles, representing different mean engine speed and load. Response Time. Time-resolved measurement of particle emissions is a desirable but not a compelling criterion for the evaluation of future measurement systems. Examples of second-by-second data measured by the fast-response instruments are plotted for a sequence of 3 min of the
FIGURE 6. Absolute particle number concentrations measured during the ETC for the lower emission level configuration. The comparison of the number-related instruments showed good agreement between CPC and ELPI for both emission levels and also for EDB in the case of the higher emission level (Figures 5 and 6). A peculiarity of ELPI is that the number concentration displayed by it is affected by the density of the particles due to the impactor principle. Since in our case, as in most situations, the density is not known, we used the default value of unit density for the data evaluation. Similarly to ELPI, the number concentration determined by EDB results from the different size classes. However, since diffusion is independent of the particle density, the measurement by the EDB does not require any correction for density. The light scattering instrument (LS) shows a clearly lower concentration as compared to the other instruments for the higher emission level, whichsas already mentioned abovesreflects the fact that small particles are detected less efficiently. However, no explanation was found for the high values measured for the lower emission level. Correlation between Instruments. For the correlation between the instruments, 16 measurement points were included in the analysis. These points comprised two different transient test cycles, one steady-state test cycle and five single operation modes at higher and lower emission level. Each
TABLE 3. Correlation of Particle Mass Measurement Systems with Gravimetric Filter Method (GFM) and EC Fraction Determined by Coulometric Analysis (CM EC), Respectively GFM
LII
PM
PM EC
PA
0.497 0.093 0.53
0.998 -0.041 0.96
0.274 -0.049 0.56
0.636 0.004 0.56
1.11 0.14 0.95
1.01 0.84 0.36
0.59 -0.01 0.94
1.37 0.09 0.95
MM
CM
CM EC
OPM1
PAS
OPM2
0.855 -0.156 0.54
0.565 0.449 0.35
0.431 -0.016 0.51
0.272 1.381 0.30
3.411 -0.571 0.53
0.083 0.406 0.040
0.44 1.49 0.24
7.57 -0.23 0.95
0.41 0.27 0.36
GFM slope offset R2 slope offset R2 2234
9
1.19 0.77 0.51
CM EC 1.88 -0.06 0.95
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1.40 0.41 0.78
FIGURE 7. Time-resolved data for a 3-min sequence of the ETC for the higher emission configuration. Upper curve: engine power. Lower curve: particle concentration measured by mass-related instruments.
FIGURE 8. Time-resolved data for a 3-min sequence of the ETC for the higher emission configuration. Upper curve: engine power. Lower curve: particle concentration measured by number-related instruments.
transient ETC cycle for the higher emission configuration (Figures 7-10). Most of the instruments show clear and fast
response of particle concentration to engine power. Exceptions are OPM3 and OPM1, which have a slow response or VOL. 39, NO. 7, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 9. Time-resolved data for a 3-min sequence of the ETC for the higher emission configuration. Upper curve: engine power. Lower curve: particle concentration measured by opacimeter instruments.
FIGURE 10. Time-resolved data for a 3-min sequence of the ETC for the higher emission configuration. Upper curve: engine power. Lower curve: particle concentration measured by diffusion charger instruments. measure a particle concentration almost unaffected by engine power. Overall Measurement System Evaluation. From the results presented above, it can be concluded that several time-resolved particle measurement methods are feasible to tackle future low concentration emissions. Most massbased as well as non-mass-based systems achieve a lower limit of detection and better repeatability than the GFM. However, they all show poor correlation with the regulated filter method. Only the other filter method with the identical sampling (i.e., the PM analyzer) revealed excellent correlation with GFM. This indicates the delicate role of sampling conditions in the total mass measurement. On the other hand, fast response systems that are insensitive to volatile matter yield very good correlation with the EC mass concentration. Exceptions were the OPMs and the LS, which disclosed clear shortcomings in the measurements. Although improvement by multiple wavelength and enlarged optical path length is achieved, the sensitivity is not sufficient for concentrations 2236
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below the future emission limit. For the LS, the sharp decrease in intensity of scattered light with declining particle size limits the lower size detectection and prevents correct number measurement of diesel exhaust particles. Among the mass-based methods with time resolution, LII and PA would best meet future requirements for legislative purposes. Their signal is directly proportional to soot mass concentration, demonstrated by very good correlation to EC mass concentration of R 2 ) 0.95, which is in agreement to the findings of ref 6. However, too high absolute values were measured compared to the EC concentrations determined by the filter methods in the post-trap configuration, where no EC is present. The mass monitor also showed good performance, but its principle is more complicated as the mass is calculated from different non-mass-related signals. Concerning the sensitivity, LII and PA do not provide significant improvement in sensitivity as compared to GFM. Much better results for sensitivity are observed for the nonmass-based instruments CPC, ELPI, and DCs and to a lesser
extent EDB. A positive feature of ELPI and EDB is the fact that they display a number size distribution, but both also have disadvantages. The number concentration measured by ELPI is affected by the particle density and needs individual calibration, which is a time-consuming and demanding task. A drawback of the EDB is the susceptibility of the diffusion screens to pollution, which directly affects the sizing and quantification. Therefore, the measurement principles of choice for non-mass-based measurement are CPC and DC. Both are relatively simple and show good repeatability and excellent sensitivity. In addition, the CPC excels in the dynamic range (up to 109) and has easy-to-interpret metrics. To ensure good performance in repeatability, the nucleation of volatile material in the sampling line has to be avoided. This can be achieved by hot dilution and/or an evaporation unit, known as thermodesorber (45). In conclusion, there is no doubt from the measurement technology point of view that future ultra-low emissions can be tackled in a reliable way. However, the definition of particulate matter has to be limited to solid material and is ideally quantified by number. Changing the measurement method in such a way may lead to ambiguity in concentrations measured using today’s approach relate to those with the new technique. Although this may cause a discontinuity in the inventories, the reliability and the significance of particle emission data for atmospheric modeling and policy-making will increase.
Acknowledgments This work was financially supported by the Swiss Federal Roads Authority (ASTRA) and the Swiss Agency for the Environment, Forests and Landscape (BUWAL). The filter holder and pre-classifier assembly were supplied by Rupprecht & Patashnick. We thank Robert Wolf and co-workers from the Swiss Institution for Accident Insurance (SUVA) for carrying out the coulometric analysis. We gratefully acknowledge the following people who operated the individual instruments during the campaign: Harald Beck and Christoph Haisch, Technical University Munich; Hirokazu Fukushima, Horiba Ltd.; Roland Sommer, Friedrich-AlexanderUniversita¨t Erlangen-Nu ¨ rnberg; Ville Niemela¨, Dekati Ltd.; Thomas Mosimann, Matter Engineering AG; Mike Jones, Assembly Technology & Test Ltd.; Thore Simon, Sensors Inc.; Oliver Bischof, TSI GmbH; and Edgar Laile, Wizard Zahoransky KG. We also thank our colleague Roland Graf for operating the test engine bench.
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Received for review March 24, 2004. Revised manuscript received January 20, 2005. Accepted January 20, 2005. ES049550D
