Size Analysis of Automobile Soot Particles Using Field-Flow

Soot samples are prepared for FFF analysis using a three-step procedure, where a layer of soot particles is focused between the layers of n-hexane and...
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Environ. Sci. Technol. 2001, 35, 1005-1012

Size Analysis of Automobile Soot Particles Using Field-Flow Fractionation WON-SUK KIM,† SUN HUI KIM,† D A I W O O N L E E , * ,† S E U N G H O L E E , ‡ CHEOL SOO LIM,§ AND JUNG HO RYU§ Department of Chemistry, Yonsei University, Seoul 120-749, Korea, Department of Chemistry, Hannam University, Taejon 306-791, Korea, and Motor Vehicle Emission Research Laboratory, National Institute of Environmental Research, Seoul 122-706, Korea

Soot particles emitted from various automobile engines are analyzed for size distributions using field-flow fractionation (FFF). Soot samples are prepared for FFF analysis using a three-step procedure, where a layer of soot particles is focused between the layers of n-hexane and water, followed by dispersing of particles in water containing 0.05% Triton X-100. The mean diameters determined by FFF show similar trends with those obtained from dynamic light scattering (DLS) and scanning electron microscopy (SEM). Data from FFF are also compared with those from an on-line scanning mobility particle sizer (SMPS). SMPS size distributions extend further to larger size than those of FFF distributions, which indicates the three-step sample preparation procedure effectively disaggregates the agglomerated particles. Although the amount of particulate matter (PM) emitted from a heavy-duty diesel engine is much higher than that from a light-duty diesel engine, the size distributions of soot particles show no significant difference between heavy- and light-duty diesel engines. The engine-operating mode (engine speed and load rate) does not seem to affect significantly the size distribution of soot particles. It was found that the PM from a turbocharged diesel engine contains a higher percentage of particles smaller than 100 nm than an engine with a naturally aspirated (NA) air-inhalation system. As for gasoline engines, the PM collected after the catalytic converter has a narrower size distribution than those collected before and has a higher percentage of particles smaller than 100 nm.

Introduction Automobile exhaust is a complex mixture of many components including toxic gases and particulate matter (PM). PM in automobile exhaust is mostly fine carbonaceous particles (“soot”) that are coated with a mixture of various toxic chemicals. The PM causes visibility-reduction, acid rain, and climate perturbations, and so on (1, 2). The PM can also cause serious health problems by penetrating and delivering coated chemicals into human respiratory systems (3, 4). It * Corresponding author phone: +82-2-2123-2635; fax: +82-2364-7050; e-mail: [email protected]. † Yonsei University. ‡ Hannam University. § National Institute of Environmental Research. 10.1021/es001329n CCC: $20.00 Published on Web 02/07/2001

 2001 American Chemical Society

has been reported that the submicron airborne particles, not the organic chemicals adsorbed onto the particles, are responsible for the tumor response, owing to the particles overloading the lung clearance system (5-7). Smaller particles are considered to be more harmful as they remain in the air for a longer period of time (larger ones settle to the ground more quickly) and are more likely to get inhaled into lungs. Smaller particles penetrate into deeper portions of the lungs, and the cleaning can take a long period of time (months or even years). It has been also reported that most toxic elements are concentrated on the smallest respirable particles (8, 9). Development of more efficient engines (such as a highpressure injection system or turbo-charging system) and the implementation of various after-treatment devices (such as a particulate trap or catalytic converter) have resulted in the gradual reduction in PM emission over the last few decades (10). However the use of a more efficient engine may result in an increase in the emission of smaller particles (11, 12). The United States Health Effects Institute (USHEI) reported that particles smaller than 100 nm in diameter are strongly suspected to deposit on the alveolus and that modern highpressure direct injection engines emit small particles at least 1 order of magnitude higher in number concentration than those from older engines (13). Increasing concern about the health risk of combustiongenerated particles has provoked more stringent emission regulations. Current regulations are mainly concerned on the total PM amount, not the size of the particles. As described earlier, to fully assess the environmental impact of soot particles, it is important to analyze the particle size as well as the toxic elements associated with the particles. Current emission regulations mainly concern the total PM amount (not the particle size distribution). Thus, earlier studies have been focused primarily on diesel engines, especially on heavy-duty diesel engines (buses and trucks) as they emit more PM than gasoline engines. According to a report from the Korean Environmental Ministry (KEM), 98% of PM and 82% of NOx are released from diesel vehicles despite their comprising only 22% of the total vehicle (14). Gasoline engines emit less PM than diesel engines (4), and currently no after-treatment devices are required for gasoline engines except for the catalytic converter, whose function is to remove toxic gases such as NOx. As the harmfulness of soot particles increases as the particle size decreases, there have been an increasing number of studies on particle formation and PM emission of gasoline and light-duty diesel engines as well as heavy-duty diesel engines (15-17). A number of methods have been tested for the size analysis of soot particles in automobile exhaust. The on-line method includes EAA (electrical aerosol analyzer) (18, 19), TOF-MS (time-of-flight mass spectroscopy) (20), and SMPS (scanning mobility particle sizer) (21, 22). The off-line method includes a cascade impactor (23) and an electric low-pressure impactor (19). The off-line methods are less convenient but allow a pretreatment of samples and collection of fractions in different size ranges for further analysis. Field-flow fractionation (FFF) is a separation technique that is useful for analysis of particulates (24, 25) and macromolecules (26). It has been shown that FFF is potentially useful for off-line size analysis of soot particles (27). Unique features and merits of FFF as a sizing tool have been described in detail elsewhere (24, 25, 27). One of the merits of FFF is that it actually separates particles according to their size. Thus, the FFF elution profile (“fractogram”) is a direct representation of size distribution and can be directly VOL. 35, NO. 6, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Specifications of Engines Tested in This Study engine fuel administration system injection pressure (bar) air-to-fuel ratio compression ration cylinder number max. torque/rpm (kg‚m/rpm) displacement (cc) manufacturer

DE12T

DE12

Musso

mechanical fuel injection pump 820 24-25:1 17.1:1 6 110/1300 11 051 DAEWOO

mechanical fuel injection pump 820 20-21:1 17.1:1 6 81.5/1400 11 051 DAEWOO

mechanical fuel injection pump 135 20-21:1 22:1 5 25.5/2400 2874 DAEWOO

converted to the size distribution. When desired, narrow fractions of particles can be collected and be subjected to further analysis for chemical composition, shape, aggregate formation, and so on. FFF has been combined on- or off-line with such techniques as microscopy, inductively coupled plasma-mass spectrometry (ICP/MS) (28), energy-dispersive X-ray analyzer (EDX) (29), and light scattering analysis (30, 31). The purpose of this study is to analyze and compare the total PM amount and size distributions of soot particles emitted from automobile engines running at various operating modes using FFF. Both sedimentation FFF (SdFFF) and flow FFF (FlFFF) are employed. Data from FFF are compared with those from other sizing techniques such as dynamic light scattering (DLS), electron microscopy (EM), and SMPS.

Experimental Section Engines and Fuels. Table 1 shows the specifications of engines tested in this study. Five engines are tested, including one gasoline and four diesel engines. DE12 and DE12T are heavy-duty (urban bus) diesel engines equipped with a naturally aspirated air-inhalation system (NA) and with a turbo-charged system (TC), respectively. The Musso and Astra are light-duty (Jeep-style) diesel engines. The gasoline engine is a DOHC (double overhead cam) system having a displacement volume of 2 L. The gasoline fuel used in this study has the specific gravity (15/4 °C) 0.72, octane number 92.5, sulfur content 31.1 ppm, total aromatics 19.2% by volume, and vapor pressure 76 kPa at 37.8 °C. Diesel fuel has the specific gravity (15/4 °C) 0.832, octane number 55, sulfur content 40.0 ppm, and 10% distillation residue 0.08 wt %. Soot Collection. For the collection of soot particles, the engine soot is diluted and cooled below 50 °C, as the United States Environmental Protection Agency (USEPA) regulates. Cooling is important as most combustion-generated exhaust materials are in a gaseous state at high temperatures and are difficult to collect with a filter. The soot-collecting apparatus is equipped with a 630 kW ac electric engine dynamometer (APA DYNO, AVL Co, Austria), engine controller ,and a minidilution tunnel (Figure 1). For off-line measurements, a filter is placed at the exit of the dilution tunnel to collect soot particles. The dilution tunnel is constructed of electropolished stainless steel tubing with 30.4 cm in diameter. The engine exhaust is introduced along the tunnel axis near an orifice plate that ensures a rapid mixing with the dilution air. The dilution air is purified by passing through an activated carbon bed and a high efficiency particulate-free air filter and is cooled to achieve an ambient temperature of 15-30 °C upon mixing with the hot exhaust. The dilution ratio needs to be constant as the particle size-distribution, and the composition of adsorbed organic materials could vary with the dilution ratio (12, 14, 32, 33). In this study the dilution ratio is kept constant at 10 times. Particles are sampled isokinetically at the rate of 0.66 L/s through a 1.1-cm diameter tube that makes a 20° bend with the tunnel flow axis. The filters are equilibrated with the room condition for at least 1006

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Astra

gasoline multipoint injection

1700 OPEL

3 14.7:1 9.6:1 4 18.8 /4600 1998 DAEWOO

FIGURE 1. Schematic diagram of dilution system for SMPS analysis. 24 h before both the pre- and post-collection weighing in a temperature- and humidity-controlled room at 20 ( 2 °C and 48 ( 5% in relative humidity. A few milligrams of particles are collected for each engine-operating mode. Sample Preparation. A previously developed “focusing method” (27) was employed to prepare the soot samples for all off-line size analysis including FFF, dynamic light scattering, and scanning electron microscopy. This method is composed of three steps: the recovery of soot particles from a filter, the removal of soluble components from the soot particles, and then the dispersion of soot particles in an appropriate solvent system. In the first step, the collection filter is bath-sonicated for about 20 min in 10 mL of ethanol. This step is repeated 2 or 3 times to achieve a complete recovery of soot particles from the filter. The second step is to remove chemicals associated with the soot particles; it is necessary as the soluble compounds may promote the aggregation of soot particles (34). In the second step, the mixture of ethanol and soot particles obtained in the first step is mixed with n-hexane and water containing 0.05% of Triton X-100 (polyoxyethylene p-tert-octyl phenol) in a separatory funnel. The ratio, ethanol:n-hexane:water, is 1:1: 0.5 in volume. Leaving the funnel for 10 min after 20 min of shaking resulted in the soot particles being focused at the phase boundary between the organic (n-hexane) and the aqueous (ethanol and water) phase. This extraction step is repeated three times. After the removal of ethanol and n-hexane, the particle-containing aqueous phase is heated at 60 °C for an hour for a complete removal of the remaining n-hexane and ethanol. Automobile soot particles are formed in two groups: one in the nuclei mode and the other in the accumulation mode (12). When emitted from an engine, the exhaust containing soot particles is cooled, and the particles can aggregate to grow into larger particles via nucleation, adsorption, condensation, and so on. The particle aggregation is affected by several factors including temperature, dilution ratio, and the residence time in dilution tunnel; these make an accurate size determination of soot particles difficult to arrive at. Therefore, for an accurate (and consistent) size determination of soot particles, these variations need to be avoided. When

TABLE 2. Soot Samples Collected for This Study

c

sample

rpm/load rate (rpm/%)

HDS 1 HDS 2 HDS 3 HDS 4 HDS 5 HDS 6 HDS 7 HDS 8 HDS 9

Heavy-Duty Diesel Engine Soot D-13 1320/50 1320/100 2200/50 2200/100 1320/50 1320/100 2200/50 2200/100

DE12T DE12T DE12T DE12T DE12T DE12 DE12 DE12 DE12

LDS 1 LDS 2 LDS 3 LDS 4

Light-Duty Diesel Engine Soot FTP-75 (phase-1)a FTP-75 (phase-2)b FTP -75 (phase-3)c ECE (with EUDC)

Musso Musso Musso Astra 1.7

G1 G2

Gasoline Engine Soot FTP75 + ECE cycle (with EUDC) FTP75 + ECE cycle (with EUDC)

2L DOHC 2L DOHC

a Transient phase after cold start. Transient phase after hot start.

b

engine type

Stabilized phase after cold start.

engine soot is emitted into air, it is diluted about 500-1000 times, which is much higher than it is in the dilution tunnel. As the dilution ratio in the dilution tunnel (10-fold dilution in this study) is lower than that in the air, the saturation ratio is higher in the dilution tunnel. Thus the particles passing through the dilution tunnel can grow more easily than those emitted into the air and may result in an increase in the mean particle size. The aggregation of soot particle is known to be reversible (35). The final step is to disaggregate those aggregated particles before FFF measurements and to obtain a homogeneous dispersion of soot particles. The 2-3 mL of water containing 0.05% of Triton X-100 and 0.02% of NaN3 is added to the aqueous phase obtained in the second step. Then the mixture is sonicated for 10 min in a probe-type sonic dismembrator (50W, Fisher Scientific Co., Pittsburgh, PA) for the dispersion of the particles. The sonication was applied either continuously for 10 min in an ice water-bath or at 1 min-interval without an ice water bath. The soot dispersion was vortexed using a Vortex mixer (Fisher Scientific Co., Pittsburgh, PA) before the injection. Soot Samples. A total of 15 soot samples are collected and analyzed in this study (Table 2), including five samples from the DE12T engine (HDS 1-5) and four from the DE12 (HDS 6-9). The HDS 1 is collected from the DE12T engine running in a D-13 mode, which is an exhaust gas-restriction mode for regulation of heavy-duty diesel engines. It is consisted of 13 steps of various combinations of engine speed (rpm) and load rate. HDS 2-9 are collected from either the DE12T or DE12 engine running at a constant combination of rpm/load rate. All heavy-duty diesel soot particles are collected using a quartz membrane filter except HDS 1, for which a glass fiber membrane filter is used. LDS 1-3 are samples collected during the 1-3 phase of the Musso engine (a light-duty diesel engine) running in the FTP-75 test cycle. The FTP-75 test cycle is an exhaust gas-restriction mode for regulation of light-duty diesel engines. Phases 1 and 2 of the FTP-75 test cycle represent the transient and stabilized period after the cold start of the engine, and phase 3 is the transient period after the hot start. LDS 4 is the sample collected from the Astra engine running at ECE cycle (with EUDC), which is a testing mode for light-duty diesel vehicles used mainly in Europe. All light-duty diesel soot particles are collected using a glass fiber membrane filter. G1 and G2 are samples collected from a gasoline engine running at a combined mode of FTP-75 and ECE cycle (with EUDC) before (“engine-out”) and after (“tailpipe-out”) the catalytic converter, respectively

FIGURE 2. Gasoline soot samples (G1 and G2). (Figure 2). All gasoline soot particles are collected using a steal-net filter. Field-Flow Fractionation (FFF). Two FFF subtechniques, sedimentation FFF and flow FFF, are employed in this study. The sedimentation FFF (SdFFF) system is the model S101 Colloid/Particle Fractionator purchased from FFFractionation, LLC (Salt Lake City, UT). The channel length, breadth, and thickness are 89.1, 2.0, and 0.0254 cm, respectively. The rotor radius is 15.1 cm. The channel volume, measured as the elution volume of acetone, is 4.49 mL. A powerprogramming (36) was used for all SdFFF experiments with the initial field of 1890 rpm, the final field of 75 rpm, and the predecay time of 5 min. The stop-flow time is 15 min, and the flow rate is 1.04 mL/min. The flow FFF (FlFFF) system is a homemade system and is similar to the Universal Fractionator Model F-1000 (FFFractionation, LLC). The length and breadth of the channel are 28.3 and 2.0 cm, respectively. The effective channel thickness was determined from the channel volume measured by the rapid breakthrough method (37). The channel volume was measured to be 1.40 mL, from which the effective channel thickness was calculated to be 0.0248 cm. The channel membrane is the Model YM-10 (Amicon, Beverly, MA), regenerated cellulose having the cutoff pore-size of MW 10 000. For the FlFFF analysis of soot particles, the channel and the cross-flow rates are 4.63 and 0.60 mL/min, respectively, and the stop-flow time is 90 s. For both SdFFF and FlFFF experiments, the elution of particles was monitored by a M720 UV/VIS detector (YoungLin Scientific Co., Anyang, Korea) with the wavelength fixed at 254 nm. A FFF elution profile (detector signal vs elution time) is converted to the corresponding size distribution by assuming the detector signal is proportional to the mass concentration, dmi/dVi, where mi and Vi are mass and volume of the ith slice of the FFF elution profile. The size distribution is obtained by plotting the relative mass, dmi/ddi vs diameter, di, where dmi/ddi ) dmi/dVi δVi/δdi ∝ detector signal δVi/ δdi. The equivalent spherical particle diameter di corresponding to a given elution volume Vi can be determined using FFF equations. The detector signal was processed using the software obtained from FFFractionation, LLC. It is noted that there will be some perturbation from the direct proportionality between detector signal and the concentration due to the dependence of the signal on the particle size as the light attenuation is mostly due to scattering rather than absorption. For all FFF experiments, samples were injected using a Rheodyne 7125 loop injector (Rheodyne, Inc., Cotati, CA), and the injection volume was 10-20 µL. The carrier liquid is doubly distilled and deionized water containing 0.05% (w/ v) of Triton X-100 and 0.02% (w/v) of NaN3. Triton X-100 was added as a dispersing agent and NaN3 as a source of salt as well as a bactericide. The pH and the ionic strength of the carrier liquid were 8.0 and 0.0041 M, respectively (34). The pH of the carrier was adjusted by adding the appropriate amount of 0.1 N NaOH. For the FlFFF experiments, the concentration of Triton X-100 is lowered to 0.01% to prevent membrane distortion. Dynamic Light Scattering (DLS) and Electron Microscopy (EM). DLS is a NICOMP 370 Particle Sizer (NICOMP Particle Sizing System Inc., Santa Barbara, CA) equipped with a He-Ne laser as the light source. The DLS experimental conditions are temperature 25 °C, viscosity 0.009 centipoise, refractive index 1.33, and the collection angle 90 degrees. VOL. 35, NO. 6, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. PM Emission for Heavy-Duty Diesel Engines sample

engine speed (rpm)/load rate (%)

PM emission (g/h)

HDS 2 HDS 3 HDS 4 HDS 5

DE12T Engine 1320/50 1320/100 2200/50 2200/100

12.66 32.80 17.90 53.29

HDS 6 HDS 7 HDS 8 HDS 9

DE12 Engine 1320/50 1320/100 2200/50 2200/100

28.90 70.52 34.02 77.18

The accuracy of the system was tested using a 50 ( 2.2 nm polystyrene latex bead (Duke Scientific Co., Palo Alto, CA). A JEOL model JSEM-5410LV (Tokyo, Japan), a highresolution scanning electron microscope (SEM), was used for electron microscopy of soot particles. Polycarbonate disk membrane (10 mm in diameter and 0.1 µm in pore size) was used to filter the soot particles for SEM analysis. The same “focusing method” explained above was used to prepare soot samples for size analysis using DLS and SEM. Scanning Mobility Particles Sizer (SMPS). SMPS employed in this study is a Model 3936 manufactured by TSI Inc. (MN). It consists of a scanning mobility analyzer (SMA), a condensation particle counter (CPC), and a computerized control and data acquisition system. The instrument enables measurements of particle number distribution in the size range from 0.005 to 0.9 µm using the electrical mobility detection technique. Particles coming from the dilution tunnel pass first through a radioactive source bipolar ion neutralizer. This brings the level of the particle charge distribution to a minimum Boltzmann’s distribution. The aerosol then enters the mobility section close to its inner surface. Clean sheath air flows close to the central rod. When a voltage scan is applied to the rod, charged particles move in the radial direction inward or outward, depending on their polarity. Particles with the right polarity and electrical mobility exit through holes at the bottom of the central rod. The entire system is automated, and data analysis is performed by an IBM-compatible computer system with customized software.

Results and Discussion PM Emission. The PM emission (the amount of PM emitted in 1 h, g/h) was measured for heavy-duty diesel engines (Table 3). For both the DE12T and DE12 engines, the PM emission increases with an increasing load rate at a constant engine speed (rpm). At 1320 rpm, an increase in the load rate from 50 to 100% resulted in an increase in the PM emission by 159% for the DE12T and 144% for the DE12 engine. At 2200 rpm, the same increase in the load rate resulted in an increase in the PM emission by 198% for the DE12T and 127% for the DE12 engine. At both 1320 and 2200 rpm, the percent increase in the PM emission due to the same amount of increase in the load rate is higher for the DE12T than for the DE12 engine. The PM emission also increases with an increasing engine speed at a constant load rate. The percent increase in the PM emission with the engine speed is not as significant as that with the load rate. At a 50% load rate, an increase in the engine speed from 1320 to 2200 rpm resulted in an increase in the PM amount by 41% for the DE12T and 17.7% for the DE12 engine. At a 100% load rate, the same increase in the engine speed resulted in an increase in the PM amount by 63% for the DE12T and 9.4% for the DE12 engine. Again, the percent increase in the PM emission due to the same amount of increase in the engine speed is higher for the DE12T than for the DE12 engine. 1008

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FIGURE 3. Size distributions of heavy-duty diesel soot particles determined by FlFFF (A) and SdFFF (B). FlFFF: The channel and the cross-flow rates are 4.63 and 0.60, mL/min respectively, and the stop flow time is 90 s. SdFFF: The initial and the final field are 1890 and 75 rpm, respectively, the predecay time (t1) is 5 min, the stop flow time is 15 min, and the flow rate is 1.04 mL/min. Comparison of SMPS and FFF. The mean diameters (Table 4) and size distributions (Figures 3 and 4) are determined using SMPS, FlFFF, and SdFFF for four heavyduty diesel soot samples, HDS 2-5. The FlFFF measurements are repeated 4 times for each sample, and the standard deviation was less than 5%. The SdFFF measurements are repeated 2 times for each sample, and the standard deviation

TABLE 4. Mean Diameters Measured by FlFFF, SdFFF, and SMPS for DE12T Heavy-Duty Diesel Soot Particles

sample

rpm/load rate (rpm/%)

PM emission (g/h)

HDS 2 HDS 3 HDS 4 HDS 5

1320/50 1320/100 2200/50 2200/100

12.66 32.80 17.90 53.29

TABLE 5. Mean Diameters of Soot Samples Measured by FlFFF, SdFFF, and DLS

mean diameter (nm) measured by FlFFF SdFFF SMPS 83 81 98 95

104 105 121 118

84 130 72 90

mean diameter (nm) measured by sample

FlFFF

SdFFF

DLS

HDS 1 HDS 2 HDS 3 HDS 4 HDS 5 HDS 6 HDS 7 HDS 8 HDS 9 LDS 1 LDS 2 LDS 3 LDS 4 G1 G2

88 83 81 98 95 125 121 111 96 88 88 102 88 194 84

103 104 105 121 118 129 145 103 105 96 101 114 107 153 102

95 83 90 112 113 94 108 88 86 103 125 253 115

FIGURE 4. Size distributions of the same samples as those in Figure 3 determined by SMPS. was less than 2%. The SMPS measurements are repeated 5 times for each sample. The SdFFF data are consistently higher than the FlFFF data for the same sample probably due to the differences between the two FFF subtechniques in the separation principle and density-dependency of the SdFFF, and so on. FlFFF measures hydrodynamic size based on a diffusion of particles, while SdFFF measures the size based on the effective mass of the particle (27). The size measured by SdFFF is thus the diameter of a spherical particle having the same effective mass as the sample particle. Also, SdFFF measurement requires the density be known, while FlFFF measurement does not require. It was assumed all soot particles have the same density of 1.3 for all the SdFFF calculations. Despite the differences, both the FlFFF and SdFFF data show similar trends (Table 4) and similar size distributions for the same sample (Figure 3). At the same engine speed, an increase in the load rate does not influence the mean size significantly. However, an increase in the engine speed (from 1320 to 2200 rpm) at the same load rate resulted in 17% and 14% increase in mean size obtained by the FlFFF and SdFFF, respectively. These FFF data contradict the SMPS data, which shows a significant increase in mean size (by 55% at 1320 rpm and by 25% at 2200 rpm) with the load rate at a constant engine speed. An increase in the engine speed (from 1320 to 2200 rpm) resulted in a reduction in mean size by 14 and 30% at 50 and 100% of the load rate, respectively. Also the SMPS size distributions (Figure 4) extend further to a larger size than those of the FFF distributions (Figure 3). One of the factors that may play a role in these discrepancies between SMPS and FFF data is the sample preparation procedure employed for FFF measurements. For FFF measurements, soot samples are pretreated by the three-step procedure (see the Experimental Section) during which the aggregated particles are found to be disaggregated. FFF elution profiles of untreated samples showed severe tailings, due to the presence of large aggregated

FIGURE 5. Mean diameters of engine soot samples determined by three different sizing techniques. particles. SEM pictures also confirm the presence of large aggregated particles. Unlike FFF, SMPS is an on-line technique that directly measures the size of the soot particles without the disaggregation process (38), and it may explain the size increase with the load rate and the extension to larger size in SMPS data. Still, it does not explain the reason mean size decreases with an engine speed-increase. Size Analysis by FFF and DLS. Mean diameters are determined using FlFFF, SdFFF, and DLS for all soot samples (Table 5 and Figure 6). In the DLS measurements, diameters are determined by assuming that size distribution is monodisperse (unimodal operation). Overall, the DLS data agree better with the FlFFF data than with the SdFFF data, probably due to the similarity between FlFFF and DLS in principle. Both FlFFF and DLS measure diffusion-based hydrodynamic diameter, while SdFFF measures diameter based on effective VOL. 35, NO. 6, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Size distributions of gasoline soot samples determined by FlFFF and SdFFF. Experimental conditions are the same as in Figure 3. mass. No significant differences (or specific trends) are found between the mean diameters of heavy- and light-duty diesel

soot particles although the heavy-duty engines emit more PM than light-duty engines. According to the FFF data, the LDS 3 (transient phase after hot start) sample has the highest mean diameter among the light-duty diesel soot samples. The size distribution of G1 (“engine-out”, particles that did not pass through the catalytic converter) is broader and extends further toward a larger size than that of G2 (“tailpipeout”, those passed through the converter) (Figure 6), resulting in much higher mean diameter. Passing through the converter, sulfur monoxides or sulfur dioxides are oxidized to sulfur trioxides, which combine with water to produce sulfuric acids. As the sulfuric acid is saturated, they act as nucleation cores for fine particles. As the engine exhaust passes through a catalytic converter, the amount of soluble organic fraction (SOF) in the exhaust is reduced by 10-30% (10), and thus the chance of aggregates being formed is lowered, resulting in a narrower size distribution. Size Analysis by SEM. SEM pictures are taken for fractions collected from SdFFF runs of HDS 4 (Figure 7A) and HDS 5 (Figure 7B). Fractions are collected for a 4 min duration at the peak maximum (20.5-24.5 min for HDS 4 and 18.5-22.5 min for HDS 5). The height and the width of approximately 100 particles are measured from the pictures and are plotted in histograms for each sample. As seen in the pictures, some particles are grouped together; this causes measuring the size to be difficult. Only the particles that are not grouped together are measured. Both the width- and height-average sizes are smaller than the SdFFF size for HDS 4, while they are larger for HDS 5. SEM is a powerful sizing technique and can give an accurate size for particulate samples having narrow distribution and well-defined shapes. SEM may not be as useful for the samples having broad distributions and irregular shapes such as the soot particles.

FIGURE 7. SEM pictures taken for fractions collected from SdFFF runs of HDS-4 (A) and HDS-5 (B) near the maximum of the peaks. 1010

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FIGURE 8. Cumulative size distributions of soot particles emitted from DE12T engine (A), from DE12 engine (B), from a light-duty diesel engine (C), and from a gasoline engine (D).

TABLE 6. Fraction of Particles Smaller than 100 nm in Various Soot Samples sample

PM emission (g/h)

number fraction e 100 nm (%)

PM emission e 100 nm (g/h)

HDS 2 HDS 3 HDS 4 HDS 5

12.66 32.80 17.90 53.29

HDS 6 HDS 7 HDS 8 HDS 9

28.90 70.52 34.02 77.18

75.5 76.8 53.4 62.8 av ) 67.1 43.7 34.4 47.8 61.3 av ) 46.8 70.2 68.0 54.7 av ) 64.3 20.6 71.2

9.56 25.2 9.56 33.5 av ) 19.5 12.6 24.3 16.3 47.3 av ) 25.1

LDS 1 LDS 2 LDS 3 G1 G2

Cumulative Size Distribution. Cumulative size distributions are determined from FlFFF size distributions for all soot samples (Figure 8), from which the fraction of particles smaller than 100 nm is determined for each sample (Table 6). This information is of interest as the harmfulness increases as the particle size decreases as mentioned earlier. For the soot samples from light-duty diesel engine and gasoline engines, the collected PM amount was too low to measure PM emission. It is interesting to note that the soot from a DE12T engine has a higher percentage (67%) of particles smaller than 100 nm than does that from a DE12 engine (47%), resulting in the emission of particles smaller than 100 nm from a DE12T to be lower by only about 20% than those from a DE12. The lower PM emission of a turbo-charging system reduces the chance of aggregation and increases the fraction of smaller particles. Among the soot samples from light-duty diesel engines, the LDS 1 and 2 have a higher percentage of smaller particles

than does the LDS 3, and the average of all three samples is 64.3% which is as high as that from the turbo-charging heavyduty diesel engine, DE12T. As for gasoline soot, 71% of the G2 particles that passed through the catalytic converter are smaller than 100 nm, while the G1 that did not pass through the converter has particles only 21% smaller. Generally, the emission of PM from a gasoline engine is of less concern, as the PM emission there is much lower than that from a heavy-duty diesel engine. Still, it is interesting to note that the use of the catalytic converter that is to remove toxic gases such as NOx or SOx results in a much higher percentage of particles to be smaller than 100 nm in the soot. In summary, the diameters of automobile (diesel and gasoline) soot particles range from a few nanometers up to about 300 nm, with a mean diameter of around 100 nm, except for the G1 sample, for which the mean size was much larger than it was in the others. Among existing off-line sizing techniques, SEM has a unique advantage, providing actual images of particles. However the use of SEM is time-consuming and inconvenient for size analysis of particles having broad distributions or irregular shapes such as soot particles as it requires each particle to be measured individually. Being fast and easy to operate, PCS is widely used for measuring particle size. Usually PCS provides accurate size for particles having relatively narrow size distributions. However, it is difficult to obtain accurate size for samples having broad or multimodal size distributions. Thus PCS may not be as useful as FFF for accurate determination of size distribution of samples having broad distributions such as soot particles. As mentioned previously, FFF elution profiles of untreated soot particles showed severe tailings, yielding the size data that are not reproducible. The purpose of the pretreatment (extraction and redispersion) of the samples is to obtain reproducible data from FFF by disaggregating the large aggregated particles. It is not known how complete the disaggregation is. It is clear however that the pretreatment VOL. 35, NO. 6, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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does disaggregate the large aggregated particles (after the pretreatment, FFF elution profiles improve and SEM pictures show the large aggregated particles to have disappeared) and yields reproducible size data.

Acknowledgments The authors wish to acknowledge the financial support of the Korea Research Foundation made in the program year of 1998 (BSRI-98-3425). The authors specially thank Prof. Kyoo-Won Lee of the Advanced Environmental Monitoring Research Center, Kwangju Institute of Science and Technology for SMPS measurements.

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Received for review June 5, 2000. Revised manuscript received October 20, 2000. Accepted December 11, 2000. ES001329N