EFFECTS OF TRACE METALS ON PARTICULATE MATTER 


FORMATION IN A DIESEL ENGINE: METAL CONTENTS FROM 


FERROCENE AND LUBE OIL 


D. G. LEE1)*, 

1)School of Mechanical Engineering, RIMT, Pusan National University, Busan 609-735, Korea
 

*Corresponding author. e-mail: donglee@pusan.ac.kr; 

mrz@umd.edu 

A. MILLER2),

2)NIOSH/Spokane Research Lab, WA 99207, USA
 

 K. H. PARK3), 

3)Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology, 


Gwangju 500-712, Korea
 

and M. R. ZACHARIAH4)* 

4)Department of Mechanical Engineering and Department of Chemistry and Biochemistry, 


University of Maryland, MD 20742, USA
 

ABSTRACT−Diesel particulate matter (DPM) often contains small amounts of metal as a minor component but this 

metal may contribute to adverse health effects. Knowledge of the mechanism for particle formation as well as the size 

preference of the trace metals is critical to understanding the potential for health concerns. To achieve this, the size and 

the composition of each particle should be optimally measured at the same time. Single particle mass spectrometer 

(SPMS) would be the best tool for this objective. In this paper, we therefore will introduce new findings about the 

mechanism and distribution of the trace metals in DPM, derived from a study where an SPMS was used to analyze freshly 

emitted DPM. 

1. INTRODUCTION 

Diesel Engines emit a tri-modal size distribution of Diesel 

particulate matter (DPM) at concentrations as high as 109 

particles/cm3 (Kittelson, 1998). Ultrafine particles i.e. the 

nuclei- and accumulation-mode particles are known to be 

the most harmful because of their deep penetration and 

easy deposition into the human lung. Also coarse-mode 

DPM often contains alkali and transition metal species 

such as Ca, Al, Na, Ba, Zn, Fe, Se, Mg and so on (Gross 

et al., 2000). It has been thought that the Diesel-generated 

metals originate from lube oil via the reverse blow-by 

process, mechanical wear of break pads, and combustion 

of metal-containing fuel additives such as ferrocene or 

cerium. But, there have been no reports to give answers 

to the following questions: how much metal is in a 

particle (single particle analysis of metals), whether or 

not metal contents are uniformly distributed in size (size 

preference of metals), and how and where they form 

(mehanism of metal formation). These fundamental 

questions motivate this research. 

Recent interest in adverse health effects of DPM 

initiated research relevant to in-situ measurement of the 

size of DPM at various engine modes. For example, 

Kittelson and Zachariah’s group have reported that metal 

content in DPM plays a catalytic role in carbon oxidation 

inside the engine or even in a flame, using tandem 

differential mobility analyzer (TDMA) (Kim et al., 

2005a; Jung et al., 2005; Higgins et al., 2003, Kim et al., 

2005b). 

Hence, the technology for sizing nanoparticles seems 

to be well established, while in-situ quantitative single 

nanoparticle mass and size analysis doesn’t do that to 

date. An ideal tool if it exists should be capable of 

simultaneously measuring the size and chemical composition of single nanoparticles. Aerosol time-of-flight mass 

spectroscopy (AMS) might be the closest approximation 

to such a tool. The Prather group indeed reported the 

simultaneous measurement of size and composition of 

particles (Noble and Prather, 2000). 

But, as AMS employs light scattering for sizing particles 

as well as triggering the ionizing laser, the measurable 

size of particles are often limited to those larger than 

200nm. This hinders the use of that machine to analyze 

the nuclei- and accumulation-mode particles that are 
more important in terms of the potential for adverse 

health effects. Also the power of the conventional laser 

installed in the AMS may appear to be not enough for 

complete ionization of a particle, because the resulting 

mass spectra consist of only molecular positive and negative 

ions. The different polarity of such ions facilitates the 

rapid recombination and charge transfer between ions 

with larger ionization potential (IP) and those with less IP 

(Mahadevan et al., 2002). This yields some bias in the 

measured composition of the particle. For this reason, the 

majority of research has focused on reporting the qualitative rather than quantification assignment of molecular 

species composing an environmental particle. 

We recently showed that the use of a much stronger 

laser pulse seemed very likely to solve these problems 

(Lee et al., 2005). Also we demonstrated that the SPMS 

with the capability of simultaneous measurement of size 

and composition of particles could be used to elucidate 

various nanoscale phenomena, for example, finding solid 

state reaction kinetics inside a nanoparticle (Mahadevan 

et al., 2002), and size-resolved surface reaction kinetics 

(Park et al., 2005). In this study, we applied the SPMS to 

explore the composition of DPM, namely the origin of 

metals and size preferences of metals originating from 

metallic fuel additives and lube oil. In addition, we 

extract statistical data from the single particle analyses 

and eventually, address the mechanism of formation of 

metals. 

2. EXPERIMENTAL PROCEDURE 

Our SPMS consists of an aerodynamic inlet, a source 

region for particle-to-ion conversion with a free firing 

high powered Nd:YAG laser, a linear time-of-flight (TOF) 

tube and a multichannel plate detector (MCP), as shown 

in Figure 1.

Figure 1. Schematic of single particle mass spectrometer. 

 The aerodynamic lens inlet is employed to 

separate particles from the carrier gas and collimate them 

such that they can be injected through differentially 

pumped chambers to the ionization region, albeit with 

some traveling losses en route. As configured, positive 

ions formed from a particle by multi-photon laser ionization are accelerated along the ~1 m long linear TOF tube 

and detected with the MCP. More details regarding this 

SPMS design are described elsewhere (Mahadevan et al., 

2002; Lee et al., 2005; Park et al., 2005). 

The source of diesel particulate for this work was an 

Onan-Cummins “QuietDiesel” genset powered by a three 

cylinder, 1.5 liter Isuzu engine. This unit is capable of 

providing 10 kW of continuous AC power at a fuel flow 

rate of about 4 kg/hr. In order to maintain a steady 60 Hz 

of AC current from the generator, the fuel flow to the 

engine is controlled by an electronic governor actuator 

that maintains a constant engine speed of 1800 rpm. The 

load on the engine is provided by loading the generator 

outlet with a resistive load bank made by the Simplex 

Company. 

For most of our experiments, the engine was run at two 

conditions i.e. no-load and 6 kW load unless otherwise 

noted. The corresponding fuel flow rates were 1.0 kg/hr 

and 2.25 kg/hr. It should be noted that the size 

distribution of particles generated by this relatively small 

engine varies somewhat from that of typical heavy-duty 

engines. Under normal operating conditions it produces 

no discernable nuclei mode and the accumulation mode 

decreases with engine load rather than increases as it does 

for most larger engines (Kittelson, 1998). 

Figure 2. Sampling and dilution of DPM using exhaust 

probe and ejector dilutor. 

Sampling of exhaust gases and DPM was accomplished using a probe of 3 mm ID inserted into the 10 cm ID 

exhaust tube. The samples were diluted using an ejectortype dilutor. Cold-flow calibration using a bubble flow 

meter at the dilutor inlet and a rotameter at the outlet, 

yielded a dilution ratio of approximately 10:1 for this 

setup. For measurements of particle size distribution with 
the SMPS, the flow was further diluted as shown in 

Figure 2, using a second ejector dilutor. To investigate the 

effects of metals on DPM formation, we first added 

ferrocene to the fuel as an iron source, the amount of 

which was varied from 20 ppm to 60 ppm. Secondly 1 % 

lube oil (SAE 15W-40) was directly doped to the fuel 

tank to see the effect of enhancing the level of metallic 

lube oil additives that might result through the reverse 

blow-by process. The amount of doped metals is too 

large to simulate normal lube oil consumption, but is 

used as a means of adding metals to particles to elucidate 

the catalytic effect. 

3. EFFECTS OF METALS ON DPM SIZE 

As described in the previous section, we tested two metal 

sources such as ferrocene and lube oil. First we would 

like to show the effect of the ferrocene on DPM size. 

Figure 3a shows that when the fuel is doped with 60 ppm 

iron, the size distribution of particles contains a distinct 

nuclei mode that increases with engine load. This increase 

in self-nucleated metal particles reflects higher metal 

throughput, since the increase in load results in higher 

fuel flow rates and thus higher metal throughput. As the 

doping rate is reduced to 20 ppm (Figure 3(b)), the iron 

content is lowered and the nuclei mode eventually 

disappears at 0 kW engine load as illustrated in the figure. 

Figure 3. Size distributions of DPM at various engine 

loads which is doped with (a) 60 ppm Fe and (b) 20 ppm 

Fe. 

For both doping levels (Figures 3(a) and 3(b)), the size 

distribution is strongly bimodal in the range measured by 

the SMPS and the dilution-corrected total number 

concentrations vary from about 5×107

 #/cc to about 5×108 

#/cc (higher numbers being reflective of the increasing 

number of nuclei mode particles at high engine load). 

The accumulation mode particles are shown to decrease 

as load (and thus both engine temperature and metal 

content) goes up, especially during the initial increase 

from 0 kW to 4 kW load. 

When 1% lube oil is doped to the fuel, the nuclei mode 

of DPM begins to appear even at the relatively low 

engine load of 1 kW (see Figure 4). 

Figure 4. Size distributions of DPM at various engine 

loads which is doped with 1% lube oil. 

More clearly, the 

figure verifies the gradual decrease in the number of the 

accumulation mode particles with the engine load. The 

overall trend of changes in the size distribution is very 

similar to that in Figure 3. Despite this, we note that there 

is a remarkable difference between the cases of ferrocene 

and lube oil. For 1% doping of lube oil, more than 1000 

ppm of the total metals are doped to the diesel fuel, which 

is much higher than in the ferrocene case (see table 1). 

Much higher levels of metals may enhance carbon 

oxidation in the engine, leading to reduction in number of 

preexisting carbon particles (Jung et al., 2003). In turn, 

gaseous metals do not find sufficient sites for condensation. It is probable that the metals instead tend to selfnucleate. The self-nucleation process should be pronounced at higher doses of metals, i.e. in the lube oil case, 

which may explain the more pronounced nuclei mode. In 

this case, increased number of nuclei particles may lead 

to vigorous coagulation between themselves and with 

accumulation mode particles. The same trend was observed 

by Kyto et al. (2002). 

4. COMPOSITIONAL ANALYSIS OF DPM 

4.1. Classes of DPM according to Mass Spectra 

The nuclei mode of DPM is generally considered to 

consist of two particle types; nonvolatile inorganic ash 

that nucleates at high temperatures in the engine 

(Kittelson et al., 2002), and condensed volatile materials 
such as sulfates and unburned fuel or oil vapors which 

self nucleate later during the dilution/cooling of the 

exhaust aerosol (Sakurai et al., 2003). The accumulation 

mode on the other hand, which would be the third type of 

particle, is said to consist mainly of “spherules” i.e. 

primary particles of elemental carbon, coagulated into 

agglomerates, with other materials adsorbed onto the 

surface. Particle mass spectra are sorted into two classes 

according to the ratio of hydrogen (H) to carbon (C) as a 

criterion, i.e. organic carbon (OC when H/C > 1) and 

elemental carbon (EC when H/C < 1). The third class is 

pure metal where metal to carbon ratio is larger than 10. 

These are metal nanoparticles (presumably self-nucleated 

in the engine). Figure 5 shows distinct differences in 

morphology and size for the three classes of particles 

generated during combustion of iron-doped fuel. 

Figure 5. TEM images representing the three classes of 

DPM doped with Fe at 6 kW load and 60 ppm doping. 

The dark spot of the figure, say (a), corresponds to Fe 

rich particles, i.e. the third class, which was confirmed by 

energy dispersive X-ray spectroscopy (EDS). The nonagglomerate light gray particles of (b) are organic carbon, 

while large agglomerates of (c) contain elemental carbon 

and iron as well, which is attributed to coagulation between them. Typical mass spectra also show consistently 

the variation in composition of the three classes (not 

shown here). 

The classification process was also carried out for the 

case of combustion of lube-oil doped fuel. According to 

Table 1, the oil contains various metals, so one may 

predict that TEM images and EDS of the DPM should 

show a composite of the four metal elements such as S, 

Ca, Zn, and P. 

Table 1. Elemental components and their properties of 

lube oil of SAE 15W-40 (John Deere TY6391). 

Elements Weight 

(%) 

IP 

(kJ/mol) 

Psat 

torr o

C 

Ca 0.29 590 ~10−8 282 

Zn 0.14 960 8×10−4 280 

P 0.13 419 1 261 

Mg 0.04 738 ~10−5 282 

S 0.33 1000 1 189 

C 1087 ~10−8 1657 

Fe 762 ~10−8 892 

Also the relative abundance of each 

element in such composites may be expected to decrease 

in that order. But, surprisingly, the particle mass spectra 

show that the metals in the form of isolated metal-rich 

spherules primarily consist of Ca rather than other metal 

elements. Actually, this is not that big of a surprise 

because S, the most abundant species, are likely to exist 

in vapor phase due to their high saturation vapor pressure 

(Table 1). 

Figure 6 shows that the morphology of the DPM with 

dose of lube oil is pretty similar to that of the Fe-doped 

DPM. EDS measurements for DPM denoted by circle in 

the figure clearly show that the dark areas in DPM 

contain a lot of Ca and the nuclei mode particles are selfnucleated hydrocarbon and metal rich particles. 

Figure 6. TEM images representing the three classes of 

DPM doped with 1% lube oil at 6 kW engine load. 

This figure also shows, perhaps better than Figure 5, 

that the light gray particles, i.e. organic carbon particles 

are more abundant than metal-rich nuclei. 

4.2. Species Size Distribution 

More than 500 mass spectra for each condition were 

collected and analyzed by our own computer program 

that is able to search and analyze peaks in a mass 

spectrum, and thereby estimate contents of the elements 
composing a parent particle. Such given total area of 

peaks versus time of flight in a mass spectrum is 

converted to volume equivalent size of the corresponding 

particle by an adequate volume-peak area correlation 

(Lee et al., 2005). The whole analyses for 500 mass 

spectra data is finished within half an hour. We can 

extract any statistical relations from a collection of the 

analysis results. This is a unique and striking benefit of 

single particle analysis and makes the SPMS so useful in 

nanotechnology and any research fields dealing with 

nanoparticles as well. 

4.2.1. Fe-doped DPM 

Before looking at detailed particle stoichiometry, we 

present the SPMS results for the three classes of DPM 

(explained in the previous section), in terms of the 

frequency distribution of each class as a function of 

particle size. Figure 7(a) shows that at low engine load 

and high ferrocene doping rate, the “elemental carbon” 

particles are widely distributed, with a peak at around 70 

nm, and constitute the bulk of the particulate material, 

which coincides with the distribution of agglomerate 

particles i.e. the accumulation mode particles shown in 

Figure 3(a). Figure 7(a) also shows that the “organic 

carbon” particles are distributed in the lower size range 

(< 50 nm) i.e. the lower-side tail of the accumulation 

mode. 

Figure 7. Size distributions of three species of DPM 

doped with 60 ppm Fe at (a) 0 kW; (b) 6 kW engine load. 

Under the conditions for Figure 7(a) we see 

virtually no “pure iron” particles. This is probably due to 

higher carbon generation at low load, preferentially 

facilitating Fe adsorption onto carbon particles rather 

than nucleation of Fe. The result is that at the lower load, 

the nuclei-mode iron particles as measured by mobility 

diameter (Figure 3a), are fewer in number and almost all 

less than 30 nm in diameter, and in that range the massspectrometer has poor particle transmission efficiency 

(Mahadevan et al., 2002). 

At higher engine load, the increase in Fe throughput 

combined with inherent soot reduction yields a higher Fe/ 

C ratio, so the pure iron particles have increased in number 

and grown in size to a point where the aerodynamic lens 

is able to transmit them more efficiently to the ionization 

laser. Figure 7(b) shows that at 6 kW engine load, about 9 

% of the detected particles (by number) are pure iron 

nanoparticles. Note that in this figure the size distribution 

of the elemental carbon particles has not changed significantly, which is consistent with the SMPS measurement 

in Figure 3(a). 

4.2.2. 1% Lube oil-doped DPM 

When various metals in the form of lube oil are added to 

the fuel, the species distribution is somewhat different 

from the previous. Three interesting things are found in 

this case as follows: 1) no distinct self-nucleation of 

metals even at higher total dose of metals, 2) dominance 

of Ca rich particles even in accumulation mode, i.e. they 

are not just in the nuclei mode, 3) apparent underestimation of nuclei mode particles as compared to in the 

iron doping case. All points are essentially inter-related 

through the thermo-physical properties (vapor pressure) 

of metal and the mechanism of metal formation. Though 

more Ca is added than Fe, the higher vapor pressure of 

Ca enables it to exist in vapor phase for a longer time 

period during the expansion process. The vapor thus has 

time to condense into the preexisting DPM rather than 

self nucleate as in the case for Fe. This is a possible route 

for Ca forming. 

But, here one may raise an issue related to the amount 

of Ca exceeding the other species (contained in the oil) 

and even carbon. Elemental carbon seems very likely to 

be much more reduced due to the high dose and the 

catalytic effect of Ca (compare Figures 3 and 4). The 

function of Ca for enhancing carbon oxidation was demonstrated with thermo-gravimetric analysis by Miyamoto et 

al. (1998). Also, since much of the sulfur leaves the tailpipe as vapor-phase SO2 due to its high vapor pressure, 

only a few percent of sulfur is known to convert to 

particulate sulfate. Thus, the recondensed Ca may be 

primary species detected by SPMS. 

Unfortunately, we are not in a position to give an 

answer as to whether Ca is self-nucleated or not, because 

it is also possible that the Ca particles in the accumulation 

mode may have resulted from violent coagulation between self-nucleated Ca particles. There remains another 

question, why the big nuclei-mode peak in Figure 4 is not 

shown in Figure 8, which is not consistent with Fe-doped 

DPM. We would note that the size measured by SMPS is 
electrical mobility size (~aerodynamic size), while the 

size measured by SPMS is volume equivalent size. In 

other words, even if the Fe rich and Ca rich particles have 

the same aerodynamic size and pass though the aerodynamic lens with similar transmission efficiency, the 

heavier Fe rich particles can have smaller volume. 

Therefore, in contrast to the Ca, the detectable volumeequivalent size range of Fe particles with the SPMS spans 

to smaller size. This may be the answer to the question 

posed in the third point (apparent underestimation of 

nuclei mode). 

Figure 8. Size distributions of three species of DPM 

doped with 1 % lube oil at (a) 0 kW; (b) 2 kW; (c) 6 kW 

engine loads. 

4.3. Stoichiometry of Species in Metal-doped DPM 

To confirm the above-mentioned mechanism of metal 

formation, we draw a plot of metal-to-carbon ratios 

against carbon core size for a large number of particles. 

Figure 9 demonstrates that the ratio is strongly correlated 

with the preexisting carbon core size and proportional to 

D −0.94

m,c . 

Figure 9. Metal fraction in 20 ppm Fe-doped DPM as a 

function of carbon core size at 0 kW engine load. 

Note that the diameter Dm,c representing mobility 

size of the carbon core is estimated only from the carbon 

contents in the mass spectra with a mass spectra-to-size 

converting formula (Lee et al., 2005). Also, note that in 

the free molecular regime, the recondensation rate of 

volatile species is proportional to the surface area of 

preexisting particles, while in the continuum regime it is 

proportional to the size. In turn, the ratio should have size 

dependence of D −2

m,c for the free molecular regime or 

D −1

m,c for the continuum regime. The proximity of the 

dependence to D −1 

m,c reflects that the iron vapors condense onto the carbon cores during the rapid expansion 

process inside engine, where the pressure is still high. On 

the other hand, as engine load or the amount of dose 

increases, nucleation and subsequent coagulation between 

nuclei and carbon agglomerates make the data scattered 

from the correlation seen in Figure 9. This becomes more 

pronounced at all conditions of lube-oil doping case, 

which is explained mainly by the higher dose of metals. 

5. CONCLUSIONS 

We demonstrate for the first time that single particle mass 

spectroscopy can be successfully used to characterize 

individual diesel exhaust particles in terms of size and 

composition. From the data sets, we extract several 
interesting relations which are meaningful in statistical 

view. When small amounts of metal are introduced to the 

engine as fuel additives, the metal vapor from the 

disintegration of metals in the engine condenses onto the 

pre-existing carbon spherules somewhere during the 

expansion process inside the engine. As more metals are 

added, the governing mechanism is shifted from condensation to nucleation and subsequent coagulation between 

nuclei and pre-formed agglomerates. The relative dominance between nucleation and coagulation is determined 

by not only the dose of metals but thermo-physical 

properties of metal vapor. 

ACKNOWLEDGEMENT−This work is part of the project 

“Development of Partial Zero Emission Technology for Future 

Vehicle” and is also supported by the Core Environmental 

Technology Development Project for Next Generation (Project 

No. 102-041-029). We are grateful for these financial supports. 

REFERENCES 

Gross, D. S., Galli, M. E., Silve, P. J., Wood, S. H., Liu, 

D.-Y. and Prather, K. A. (2000). Single particle characterization of automobile and diesel truck emissions in 

the Caldecott tunnel. Aerosol Sci. Technol., 32, 152−

163. 

Higgins, K. J., Jung, H., Kittelson, D. B., Roberts, J. T. 

and Zachariah, M. R. (2003). Kinetics of diesel nanoparticle oxidation. Environ. Sci. Technol., 37, 1949−

1954. 

Jung, H., Kittelson, D. B. and Zachariah M. R. (2005). 

The influence of a cerium additive on ultrafine diesel 

particle emissions and kinetics of oxidation. Comb. 

Flame., 142, 276−288. 

Jung, H., Kittelson, D. B. and Zachariah, M. R. (2003). 

The influence of engine lubricating oil on diesel 

nanoparticle emissions and kinetics of oxidation. SAE 

Paper No. 2003-01-3179. 

Kim, S. H., Fletcher, R. A. and Zachariah, M. R. (2005a). 

Understanding the difference in oxidative properties 

between flame and diesel soot nanoparticles: the role 

of metals. Environ. Sci. Technol., 39, 4021−4027. 

Kim, H., Lee, S., Kim, J., Cho, G., Sung, N. and Jeong, Y. 

(2005b). Measurement of size distribution of diesel 

particles: effects of instruments, dilution methods, and 

measuring positions. Int. J. Automotive Technology 6, 

2, 119−124. 

Kittelson, D. (1998). Engines and nanoparticles: a 

review. J. Aerosol Sci., 29, 575−588. 

Kittelson, D., Johnson, D. and Watts, W. (2002). Diesel 

Aerosol Sampling Methodology, CRC E-43 Final 

Report available at http://www.crcao.com, Georgia, 

USA. 

Kyto, M., Aakko, P., Nylund, N.-O. and Niemi, A. 

(2002). Effect of lubricant on particulate emissions of 

heavy duty diesel engines. SAE Paper No. 2002-01­

2770. 

Lee, D., Park, K. and Zachariah, M. R. (2005). Determination of size distribution of polydisperse nanoparticles 

with single particle mass spectrometry: the role of ion 

kinetic energy. Aerosol Sci. Technol., 39, 162−169. 

Mahadevan, R., Lee, D., Sakurai, H. and Zachariah, M. 

R. (2002). Measurement of condensed-phase reaction 

kinetics in the aerosol phase using single particle mass 

spectrometry. J. Phys. Chem. A, 106, 11083−11092. 

Miyamoto, N., Zhixin, H. and Hideyuki, O. (1998). 

Catalytic effects of metallic fuel additives on oxidation 

characteristics of trapped diesel soot. SAE Paper No. 

881224. 

Noble, C. A. and Prather, K. A. (2000). Real-time single 

particle mass spectrometry: a historical review of a 

quarter century of chemical analysis of aerosols. Mass 

Spectrum. Rev., 19, 248−274. 

Park, K., Lee, D., Rai, A., Mckherjee, D. and Zachariah, 

M. R. (2005). Size-resolved kinetic measurements of 

aluminum nanoparticle oxidation with single particle 

mass spectrometry. J. Phys. Chem. B, 109, 7290−7299. 

Sakurai, H., Tobias, H. J., Park, K., Zarling, D., Docherty, 

K. S., Kittelson, D. B., McMurry, P. H. and Ziemann, 

P. J. (2003). On-line measurements of diesel nanoparticle composition and volatility. Atmospheric Environment, 37, 1199−1210.