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Diesel Particulate Matter [subscriber access]
DieselNet Technology Guide
Diesel Particulate Matter
W. Addy Majewski
Abstract: Diesel particulate matter (DPM) is the most complex of
diesel emissions. Diesel particulates, as defined by most emission
standards, are sampled from diluted and cooled exhaust gases. This
definition includes both solids, as well as liquid material which
condenses during the dilution process. The basic fractions of DPM are
elemental carbon, heavy hydrocarbons derived from the fuel and
lubricating oil, and hydrated sulfuric acid derived from the fuel
sulfur. DPM contains a large portion of the polynuclear aromatic
hydrocarbons (PAH) found in diesel exhaust. Diesel particulates
include small nuclei mode particles of diameters below 0.04 mm and
their agglomerates of diameters up to 1 mm.
What Are Diesel Particulates
Composition of DPM
Solid Fraction
Soluble Organic Fraction
Sulfate Particulates
Particle Size Distribution
What Are Diesel Particulates
Particulate matter-perhaps the most characteristic of diesel
emissions-is responsible for the black smoke traditionally associated
with diesel powered vehicles. The diesel particulate matter emission
is usually abbreviated as PM or DPM, the latter acronym being more
common in occupational health applications. Diesel particulates form
a very complex aerosol system. Despite considerable amount of basic
research, neither the formation of PM in the engine cylinder, nor its
physical and chemical properties or human health effects are fully
understood. Nevertheless, on the basis of what is already known, PM
is perceived as one of the major harmful emissions produced by diesel
engines. Diesel particulates are subject to diesel emission
regulations worldwide and, along with NOx, have become the focus in
diesel emission control technology.
Contrary to gaseous diesel emissions, PM is not a well defined
chemical species. The definition of particulate matter is in fact
determined by its sampling method, the detailed specification of
which is an important part of all diesel emission regulations. PM
sampling involves drawing an exhaust gas sample from the vehicle's
exhaust system, diluting it with air, and filtering through sampling
filters. The mass of particulate emissions is determined based on the
weight of PM collected on the sampling filter. It is quite obvious
that any changes in the procedure, for example using a different type
of sampling filter or different dilution parameters, may produce
different results. Standardization of sampling methods is of utmost
importance if results from different laboratories are to be
comparable. Such standards have been developed for the measurement of
PM mass in the area of public health regulations (i.e., emission
standards for diesel engines and vehicles) worldwide. Whenever
reference to "particulate matter" is made throughout these papers, it
is quietly assumed that the sampling was performed from diluted and
cooled exhaust, in accordance with these standards (unless explicitly
stated otherwise). However, it should be realized that other sampling
procedures and corresponding definitions of PM are perfectly
possible. Ongoing research in Europe is aimed at developing
standardized measuring methods based on particle number emissions,
rather than mass, for the inclusion in future emission standards in
addition to the PM mass metric [Andersson 2002]. So far no common
standard has been reached in the area of diesel occupational health
regulations, where a number of different measuring methods-and
corresponding DPM definitions-exist in parallel.
Diesel particulate matter, as specified by the US EPA procedures and
most other standards and regulations worldwide, is sampled by
filtering diluted diesel exhaust at temperatures not higher than
125¡F (52¡C). This cooling effect is typically achieved with
laboratory dilution ratios in the range 3:1 - 20:1. Devices which are
used in the laboratory to produce the mixture of air with diesel
exhaust gas are known as dilution tunnels. The intention of this
procedure is to simulate conditions at which diesel particulates are
released from vehicles into the atmosphere. The substance which is
sampled and regulated is supposed to correspond to diesel soot which
is suspended in the ambient air. Fiberglas filters which are used for
laboratory PM sampling capture solid particles, as well as liquid
droplets, or mist, which condense from exhaust gases during the
dilution process. In effect, the definition of PM extends to "any
matter"-all solid and liquid material (condensate)-present in the
diluted and cooled diesel exhaust. It should be emphasized that the
above definition of diesel particulates is to a large degree
arbitrary. Since the atmospheric dilution ratios of PM (about
500-1000) are much higher than those used in laboratory dilution
tunnels, the simulation of the atmospheric dilution is far from
perfect [Kittelson 1998a].
Diesel particulates are composed of elemental carbon particles which
agglomerate and adsorb other species to form structures of complex
physical and chemical properties. Diesel particulates have a bimodal
size distribution. They are a mixture of nuclei mode and accumulation
mode particles, schematically shown in Figure 1. Nuclei mode
particles are very small-according to most authors, their diameters
are between approximately 0.007 and 0.04 mm (micron). More recent
studies redefine the nuclei mode particle size range to be even
smaller, from 0.003 to 0.03 mm [Kittelson 2002], thus making them
comparable to certain large molecules. Nuclei mode particles are
often referred to as nanoparticles, although these two terms are not
the same. Nanoparticles are usually defined as particles below 50 nm
in diameter (0.05 mm). This is an arbitrary definition, not related
to the physical properties of diesel exhaust; nanoparticles include
practically all nuclei mode particles, but may also contain a certain
fraction of the accumulation mode.
The nature of nuclei mode particles is still studied in research
laboratories. It is believed that nuclei mode particles are primarily
volatile and consist mainly of hydrocarbon and hydrated sulfuric acid
condensates which are formed from gaseous precursors as temperature
decreases in the exhaust system and after mixing with cold air, be it
in the laboratory dilution tunnel or in the ambient air. These
volatile particles are very unstable; their concentrations strongly
depend on dilution conditions such as dilution ratio and residence
time. A small amount of nuclei mode particles may consist of solid
material, such as carbon or metallic ash from lube oil additives
[Tobias 2001][Kittelson 1998a]. Nuclei mode particles constitute the
majority of particle number-on the order of 90%-but only a few
percent of the PM mass.
Figure 1. Schematic of Diesel Particulate Matter
Accumulation mode particulates are formed by agglomeration of primary
carbon particles and other solid materials, accompanied by adsorption
of gases and condensation of vapors. They are composed mainly of
solid carbon mixed with condensed heavy hydrocarbons (Figure 1), but
may also include sulfur compounds, metallic ash, cylinder wear
metals, etc. Diameters of the accumulation mode particles are between
approximately 0.04 and 1 mm with a maximum concentration between 0.1
and 0.2 mm [Brown 2000][Kittelson 1998a]. Most PM mass emission (but
only a small proportion of the total particle number) is composed of
agglomerated particles.
Composition of Diesel Particulates
Based on analysis performed by a combination of physical and chemical
methods, PM is traditionally divided into three main fractions, which
can be further sub-categorized, as follows:
1. Solid fraction (SOL)
* elemental carbon
* ash
2. Soluble organic fraction (SOF)
* organic material derived from engine lubricating oil
* organic material derived from fuel
3. Sulfate particulates (SO4)
* sulfuric acid
* water
According to that classification, the total particulate matter (TPM)
can be defined as:
(1)TPM = SOL + SOF + SO4
Particles leaving the engine are composed primarily of solid phase,
carbon material (SOL). Both individual (nuclei mode) and agglomerated
carbon particles are formed in the combustion chamber. In the exhaust
system, depending on the temperature, the particles undergo limited
oxidation and further agglomeration. Some particles are deposited on
the exhaust pipe walls due to thermophoretic forces (i.e., mass
transfer driven by temperature gradient). Other PM precursors
including hydrocarbons, sulfur oxides, and water are present in the
hot diesel exhaust as gases or vapors.
Another source of solid material in diesel exhaust are metal ash
compounds derived from lubricating oil additives, as well as from
engine wear. Nucleation of volatile ash constituents is believed to
take place during expansion stroke in the engine cylinder. The ash
nuclei can then agglomerate to form accumulation mode particles. The
relative proportion of ash generally increases in new engines, due to
less carbon particulates and lower total PM mass.
Physical and chemical properties of PM change once the exhaust gases
enter the dilution tunnel, are mixed with air, and cooled to below
52¡C. Heavy hydrocarbons, which are derived from lubricating oil and
unburned fuel, condense or adsorb onto the surface of carbon
particles forming the organic portion of PM (SOF). If the amount of
carbon particles that can act as a "sponge" is insufficient,
hydrocarbons will nucleate forming increased numbers of volatile
(liquid) nuclei mode particles. In the dilution tunnel, the total
hydrocarbon material from the combustion chamber becomes finally
proportioned between particulate (SOF) and gas phase hydrocarbons (at
least in theory; in practice a portion of diesel hydrocarbon material
may be measured and accounted for twice: in the particulate phase and
in the gaseous phase).
Sulfuric acid in diesel exhaust is derived from the fuel sulfur.
Sulfur leaves the combustion chamber primarily in the form of sulfur
dioxide (>95%) and a small proportion of sulfur trioxide (typically
2-5%; this proportion may be significantly increased by oxidation
catalysts). In the presence of water, SO3 reacts to produce sulfuric
acid:
(2)SO3 + H2O ® H2SO4
Sulfate particulates are formed in the dilution tunnel through a
heteronucleation process from the molecules of H2SO4 and water.
During PM measurements, sulfate particulates are deposited on the
filters together with the carbonaceous material. It was once believed
that sulfuric acid is attached to or associated with carbon
particles. Later research found that sulfate particles may be also
separate from carbon particles [Walters 1988]. It is now envisioned
that sulfate particulates, while existing in the accumulation mode
mixed with carbon and organic SOF material, are also an important
source of volatile (H2SO4 - H2O) nuclei mode particles [Kittelson
2002]. Depending on the availability of metal based compounds,
sulfuric acid may also form solid (non-volatile) sulfate salts.
The composition of PM varies greatly depending on the engine
technology, test conditions, and, in the case of sulfate
particulates, the sulfur content in the fuel. An example PM
composition from a post-1994 US heavy-duty diesel engine is
illustrated in Figure 2 [Kittelson 1998a].
Figure 2. Composition of Diesel Particulate Matter
HD diesel engine, US FTP transient cycle
Diesel particulates also include (usually as a part of the SOF)
certain other classes of heavy hydrocarbon or hydrocarbon-derived
material of special interest, for example the polynuclear aromatic
hydrocarbons (PAH) or dioxins. Concentrations of these substances are
orders of magnitude lower in comparison to the primary PM components,
making them "invisible" in Figure 2. Due to extremely high biological
activity in their pure state, the presence of some of these compounds
in diesel exhaust attracted a lot of attention. PAHs and dioxins will
be discussed in more detail below.
Solid Fraction
Carbon
The solid fraction of diesel particulates is composed primarily of
elemental carbon, sometimes also referred to as the "inorganic
carbon". This carbon, not chemically bound with other elements, is
the finely dispersed "carbon black" or "soot" substance responsible
for black smoke emissions. The carbonaceous PM fraction results from
the heterogeneous combustion process in diesel engines, where solid
particle precursors are formed in both diffusion and premixed flame.
Hexagonal arrays of carbon atoms in soot particles form platelet-like
structures. The platelets are arranged in layers, typically 2 to 5,
to form leaf-shaped graphite crystallites. The crystallites are
randomly packed with their planes generally parallel to the surface
of the primary (i.e., nuclei mode) particle. These three structures
are shown schematically in Figure 3 [Broome 1971].
Figure 3. Structure of Primary Carbon Particle
The primary particles agglomerate in the cylinder, while traveling
through the exhaust system, and after discharge into the atmosphere.
The structure of an agglomerated diesel particle is shown in the
transmission electron microscope (TEM) picture, Figure 4 [Dreher
2002]. A primary particle in an agglomerate can be seen in picture
(a). The particle if formed of numerous graphite crystallites, few
nanometers in size, ordered to yield an onion-like structure. Both
the size and the arrangement of the crystallites are in reasonably
good agreement with the representation in Figure 3. The agglomerated
particle in Figure 4 (b) exhibits a grape-like structure formed of
hundreds of nuclei mode particles.
Figure 4. TEM Pictures of Agglomerated Carbon Particle
Courtesy of NMI
As apparent from Figure 4, while nuclei mode particulates are nearly
spherical, sphere is a poor approximation for the shape of the
accumulation mode particle. Agglomerated particulates often form
chain-like elongated structures of no well defined "diameter". This
presents a certain difficulty in PM sizing methods, since particle
sizes have to be expressed using various types of equivalent particle
diameters which differ depending on the measurement principle.
Ash
Another important component of the solid fraction of PM is metallic
ash. In new engines producing less carbon particulates, the relative
importance of non-carbon solid particulate emissions increases.
Particulate matter from US post-1994 HD diesel engines contain 10%
and more of ash [Abdul-Khalek 1998]. This proportion may increase
even further in future engines.
Ash emissions receive a lot of attention from the developers of
diesel particulate filter materials, which must be formulated to
resist corrosion by ash compounds. In general, diesel exhaust ash
consists of a mixture of the following components [Merkel 2001]:
* Sulfates, phosphates, or oxides of calcium (Ca), zinc (Zn),
magnesium (Mg), and other metals that are formed in the engine's
combustion chamber from burning of additives in the engine
lubricating oil. These chemicals are present in lube oil as
detergents, dispersants, acid neutralizers, anti-oxidants, corrosion
inhibitors, anti-wear and extreme pressure additives, etc. Lube oil
ash emissions can be modeled based on the known oil ash content
(typically around 1.5%) and oil consumption rate (typically 0.1-0.2%
of the diesel fuel consumption).
* Metal oxide impurities resulting from the engine wear, which are
carried into the combustion chamber by the lube oil. These include
oxides of iron (Fe), copper (Cu), chromium (Cr), and aluminum (Al).
* Iron oxides resulting from corrosion of the engine exhaust manifold
and other exhaust system components. Depending upon the metallurgy of
the exhaust system materials, these particles may also include
chromium, nickel, and aluminum.
Furthermore, if metallic fuel additives are used to facilitate the
regeneration of particulate filters, exhaust gases will contain the
corresponding metal oxide ash. Metals used for that purpose include
cerium, iron, and strontium. Metallic additives could become a source
of high numbers of solid nuclei mode particle emissions [Kittelson
2002].
Example composition of ash from a heavy-duty diesel vehicle is shown
as "Ash A" in Table 1 [Merkel 2001]. The ash was collected in a
particulate filter over a 100,000 km long experiment. Ash analysis
revealed the presence of (Zn,Mg)3(PO4)2 and CaSO4, as agglomerates of
primary particles of 100-500 nm in size. The BET surface area of the
ash was 10 m2/g.
Table 1
Composition of Diesel Particulate Ash
Oxide Ash A Ash B Ash C
Weight percent of oxide in ash
CaO 29.6 20.7 10.4
ZnO 9.9 6.9 3.5
MgO 5.5 3.9 1.9
SO3 38.8 27.2 13.6
P2O5 15.8 11.1 5.5
Fe2O3 0.41 30.3 0.14
CeO2 0.0 0.0 65.0
Ashes "B" and "C" were prepared by doping ash A with iron and cerium,
respectively, to simulate the use of additives for filter
regeneration. The amount of additive was modeled based on the assumed
concentrations of 10 ppm Fe and 40 ppm Ce in the fuel.
Soluble Organic Fraction
Composition
Hydrocarbons adsorbed on the surface of carbon particles and/or
present in the form of fine droplets form the soluble organic
fraction (SOF) of diesel particulates. The adjective "soluble" takes
its origin in the analytical technique which utilizes extraction with
solvents to isolate the organic fraction of particulates. Sometimes
this fraction is also referred to as VOF-volatile organic fraction.
VOF is usually measured by vacuum evaporation and its interpretation
is very close to that of SOF. In some laboratories, however, the
evaporated fraction may also include water-soluble material. The VOC
should be then adjusted for sulfates, which are determined by
chemical analysis. If no such adjustment is made, the interpretation
of VOC should rather be the sum of SOF and sulfate particulates, SO4.
It should be emphasized again that the SOF fraction becomes liquid
only after cooling to below 52¡C in the laboratory dilution tunnel.
At the temperatures in diesel exhaust, most of the SOF compounds
exist as vapors, especially at higher engine loads when temperatures
are high. One needs to keep this realization in mind to understand
the changes that occur to diesel particulates in such aftertreatment
devices as oxidation catalysts or particulate filters.
The proportion of SOF in the total PM may vary significantly between
engines. Particulates with low SOF content are called "dry"
particulates. PM of high SOF content is called "wet" particulate. In
wet particulates, the organic fraction may constitute over 50% of the
total PM, indicating a multi-layer hydrocarbon adsorption on the
surface of the particles. In dry particulates, the SOF content may be
as low as 10% and less. In a given engine, the SOF is strongly
dependent on the operating conditions (i.e., test modes and duty
cycles). Typically, SOF content is highest at light engine loads when
exhaust temperatures are low. This is illustrated in Figure 5, which
presents total PM emissions (represented by the size of the circles)
and their SOF content, as measured at a number of steady state
conditions from a 2.8 liter DI diesel engine [Horiuchi 1990]. All
measurements at exhaust temperatures up to 200¡C (measured at the
inlet to a catalyst) showed SOF content in excess of 50% of total PM.
At high engine loads of above 70% and temperatures of 400¡C and more,
the SOF dropped to less than 5%.
Figure 5. PM Composition at Different Engine Operating Conditions
2.8 liter, DI, turbocharged diesel engine; 0.38 wt.% S in fuel; PM
emission/SOF content (g/bhp-hr/%): A 2.48/69%; B 0.66/55%; C
1.12/65%; D 1.60/58%; E 0.85/21%; F 0.52/46%; G 0.90/10%; H 0.54/25%;
I 0.71/33%; J 1.41/2.7%; K 0.51/8.7%; L 0.45/11%; M 0.45/4.0%
This temperature trend was also confirmed by cold-start emission
tests, which were reported to produce approximately 25% higher SOF
levels than hot-start emissions [Wachter 1990]. Two-stroke engines
also tend to have higher SOF emissions than their four-stroke
counterparts. The fluctuations in SOF with engine technology and with
engine test cycle may be extremely important in developing PM control
strategies, since solids behave differently from the SOF in both
diesel oxidation catalysts and in particulate filters.
The SOF is typically composed of lube oil derived hydrocarbons, with
a small contribution from the higher boiling end diesel fuel
hydrocarbons. Diesel fuel is composed of hydrocarbons having boiling
range equal to that of aliphatic hydrocarbons with 12 to 20 atoms of
carbon in their molecules. Diesel lube oil is a continuum of many
compounds with boiling points comparable to normal aliphatic
hydrocarbons in the range between C18 and C36. Figure 6 shows an
example chromatogram of diesel particulate SOF compared with
chromatograms of diesel lube oil and diesel fuel. The particulates
were collected from a diesel engine running at low speed and high
load [Voss 1995]. The chromatogram of SOF sample closely resembles
that of diesel lube oil hydrocarbons.
Figure 6. Analysis of SOF from Diesel Particulates
Lube oil hydrocarbons are also believed to be the major contributor
to nuclei mode SOF. Experiments were reported, where the volatility
of nuclei mode particles was found to be similar to that of C24 - C32
normal alkanes, implying the lube oil origin [Kittelson 2002].
The proportion of lube oil derived SOF also shows considerable
variability. A study with pre-Euro 1 light-duty vehicles found that
the lube oil derived SOF material contributed anywhere between 19 and
88% of the total PM emission [Cartellieri 1984].
Polynuclear Aromatic Hydrocarbons
The SOF fraction contains most of the polynuclear (or polycyclic)
aromatic hydrocarbons (PAH) and nitro-PAHs emitted with diesel
exhaust gases. PAHs are aromatic hydrocarbons with two or more (up to
five or six) benzene rings joined in various, more or less clustered
forms. They attracted special attention because of their mutagenic
and, in some cases, carcinogenic character. There was a proliferation
of scientific publications on PAHs in the late 1980's. Judging by the
number of publications, there is less interest in the PAHs today.
However, these compounds are still studied by air quality agencies,
notably by the US EPA under its "air toxics" regulations.
Figure 7. Example PAH Compounds
PAHs include tens of compounds, some of them having very complex
structures (Table 2). They may also contain cyclopentane rings and
heterogeneous rings with atoms of nitrogen (PANH) or sulfur (PASH).
Table 2
PAHs Identified in Diesel Particulate Extracts [Mills 1983]
Compound M* Compound M*
Acenaphthylene 152 Pyrene 202
Dibenzofuran 168 Ethylmethylphenanthrene 220
Fluorene 166 Methylfluoranthene 216
Methylfluorene 180 Ethylmethylphenanthrene 220
Methyldibenzofuran 182 Benzo(a)fluorene 216
Dibenzothiophene 184 Methylpyrene 216
Phenanthrene 178 Naphtobenzothiophene 234
Anthracene 178 Ethylpyrene 230
Benzo(h)quinoline 179 Benzo(g,h,i)fluoranthene 228
Acridine 179 Benz(a)anthracene 228
Dimethylfluorene 194 Chrysene 228
Methyldibenzothiophene 198 Methylnaphtobenzothiophene 248
Dimethylfluorene 190 Methylchrysene 242
Methyldibenzothiophene 198 Binaphtyl 254
Methylphenanthrene 192 Benzofluoranthene 252
4h-Cyclopenta(d,e,f)phenanthrene 190 Benzo(e)pyrene 252
Ethyldibenzothiofene 212 Benzo(a)pyrene 252
Ethylphenanthrene 206 Dibenz(a,h)anthracene 278
Fluoranthene 202 Benzo(b)chrysene 278
Benzacenaphthylene 202 Benzo(g,h,i)perylene 276
Benz(d,e,f)dibenzothiofene 208 Coronene 380
* M - Molecular Mass
The US EPA introduces the term Polycyclic Organic Matter (POM),
defined as a class of air toxic compounds with more than one benzene
ring and a boiling point of 100¡C and higher. The POM class is almost
identical with the PAH compounds. A group of seven polynuclear
aromatic hydrocarbons (benz(a)anthracene, benzo(b)fluoranthene,
benzo(k)fluoranthene, benzo(a)pyrene, chrysene,
7,12-dimethylbenz(a)anthracene, and indeno(1,2,3-cd)pyrene), all
identified as probable human carcinogens, are often used by the EPA
as surrogates for the entire group of POM compounds.
PAHs in the exhaust gas are split between gas and particulate phase.
A fraction of the light, 2-ring compounds is present in the gas. The
most harmful compounds of four or more rings can be found almost
exclusively in the organic fraction (SOF) of diesel particulate
matter.
PAHs are present in diesel fuels, with their concentrations typically
varying between 1.5 and 2.5%. Some of them (0.2 - 1% of the total
fuel PAH) survive combustion and can be found in the exhaust gas
[Williams 1986]. While it is also believed that some of the heaviest
PAH compounds can be generated by pyrosynthesis in the engine
cylinder, fuel appears to be the dominant source of PAH in diesel
exhaust [Froelund 1997][Schramm 1994]. Another phenomenon occurring
in the engine is the formation of nitro-derivatives of PAHs due to
high NOx concentrations.
Emissions of PAHs typically constitute a fraction of a percent of the
total PM emission, with many studies reporting at about 0.5% of total
PM [Rogge 1993]. Concentrations of particular PAH compounds vary from
0 to about 250 mg/mile, with total PAH emission reaching anywhere
from 1 to more than 10 mg/mile, depending on engine technology. In
volumetric units, most single PAH species are measured between 0 and
100 mg/m3, with some species reaching above 150 mg/m3 [Williams 1986].
Dioxins
Dioxins is the generic term for a special group of chlorinated
polynuclear hydrocarbon compounds, which are characterized by
extremely high toxicity, suspected carcinogenicity, and resistance to
biological breakdown. Even though the term "dioxin" refers chemically
to a simple heterocyclic ring with two atoms of oxygen and with no
chlorine atoms, its meaning in environmental sciences extends over
three families of chlorinated compounds: the chlorinated
dibenzo-p-dioxins (CDDs), chlorinated dibenzofurans (CDFs) and
certain polychlorinated biphenyls (PCBs). Sometimes the term "dioxin"
is also used to refer to the well-studied and one of the most toxic
dioxins, the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD - Figure 8).
Dioxins are created primarily during incineration of
chlorine-containing wastes, manufacturing of some chlorine-containing
chemicals (PVC, herbicides, ...), chlorine-bleaching of wood pulp and
paper, and in some natural processes.
Figure 8. Example Dioxin Compounds: TCDD and TCDF
Studies detecting CDD/CDFs in used motor oils [Ballschmiter 1986],
followed by discoveries of elevated levels of CDD/CDFs in European
traffic tunnels raised concerns that motor vehicles, including diesel
engines, may be contributors to nationwide ambient dioxin levels
[Jones 1993]. In its draft dioxin health assessment document [EPA
2000], the US EPA reviewed the existing literature and provided
estimates for dioxin emission factors from internal combustion
engines, as listed in Table 3 (in pico-grams per km and pico-grams
per liter of fuel).
Table 3
Dioxin Emission Factors (US EPA 2000)
Fuel / Vehicle TEQ, pg/km TEQ, pg/L FCà, km/L
Diesel trucks* 172 946a 5.5
Unleaded gasoline (w/catalyst) 1.5 14.9 10
Leaded gasoline 45 450 10
* - highway trucks only, no data existed for non-road engines
à - fuel consumption
a - based on assumed fuel consumption of 5.5 km/L (fuel consumption
not reported in the analyzed studies)
Various dioxin compounds may have greatly different toxicities.
Emissions of dioxin mixtures, such as those in Table 3, are typically
expressed using toxicity equivalency factors (TEQ), in which the
concentration of each compound is corrected based on its toxic
effect, relative to the toxicity of TCDD and TCDF. In other words,
the TEQ data represents a calculated concentration (or mass) of
TCDD/TCDF that would have a toxic effect equivalent to that of the
real mixture.
Available literature data shows considerable spread of emission
factors. Data reviewed by the EPA varied anywhere from "non
detectable" to over 600 pg TEQ/km, resulting in a low confidence
rating assigned to the above factor of 172 pg TEQ/km. A more recent
study by EPA authors investigated dioxin emissions from six diesel
trucks, to measure emission factors from 8 to 164 pg TEQ/km, with an
average of 47 pg TEQ/km [Gullett 2002].
According to the EPA inventories, diesel engines are only a minor
source of dioxin emission in the USA, being attributed 1.1% of the
total atmospheric release (33.5 g TEQ per year in 1995, compared to a
total from all sources of 2,888 g TEQ) [EPA 2000].
It must be emphasized that certain catalytic combustion additives may
increase emissions of dioxins by orders of magnitude, as it was
reported with copper compounds [Heeb 1998]. Therefore, fuel
additives/fuel borne catalysts must always be evaluated for their
dioxin formation activity.
Sulfate Particulates
Sulfate particulates are composed primarily of hydrated sulfuric acid
and, as such, are mostly liquid. Formation of sulfate particulates
requires an interaction between molecules of H2SO4 and H2O. The
process is theoretically modeled as heteromolecular nucleation
[Baumgard 1996]. According to this theory, both sulfuric acid and
water vapor can be undersaturated and still produce particles. The
nucleation starts with small molecular clusters, which then grow into
more stable nuclei particles. Most stable sulfate particulates are
formed when the molar ratio of H2O to H2SO4 equals 8:3 (i.e., 2.67:1
or 0.49:1 by weight). Nuclei sulfate particles are composed of
approximately 8000 molecules of H2O and 3000 molecules of H2SO4. It
is believed that sulfate particulates are separate from carbon
particles and are present in the exhaust gas primarily as nuclei mode
particles.
Formation of particles in dilution tunnel depends on H2SO4 and H2O
vapor pressures which are functions of:
* fuel sulfur level
* conversion of fuel sulfur to SO3
* air/fuel ratio
* dilution tunnel temperature
* dilution tunnel relative humidity.
Dilution tunnel sampling variability can be, thus, minimized by
maintaining a constant relative humidity and using a constant sulfur
level fuel. It should be noted that PM sampling procedures which are
currently in place do not have any special requirements regarding the
relative humidity in the dilution tunnel, while, according to the
heteronucleation theory, the rate of sulfate particulates nucleation
strongly depends on that parameter.
In addition to sulfuric acid, sulfate particulates may also include
sulfate salts. The most common salt is calcium sulfate (CaSO4), which
can be formed in reactions between H2SO4 and calcium compounds
originating from lube oil additives. Various sulfates may be also
produced in reactions between sulfuric acid and exhaust system
components. Sulfate particulates of high salt content would bound
less water, as most sulfate salts are less hydrophilic than sulfuric
acid.
As discussed earlier, the TPM emission is determined by weighing the
total mass of material collected on the sampling filter. As a
consequence, the sulfuric acid, sulfate salts, and combined water are
all counted as part of TPM. The exact amount of water which is
combined with sulfate particulates changes as sulfuric acid on the
filter reaches equilibrium with moisture in the surrounding
atmosphere. Sampling filters are typically pre-conditioned at
controlled conditions until their mass stabilizes. Assuming that
sulfate particulates are pure sulfuric acid, the amount of combined
water may be determined from Figure 9 depending on the relative
humidity during pre-conditioning of the sampling filter [SAE 1995].
If filters are pre-conditioned at typical conditions of 50% relative
humidity and 25¡C, each gram of sulfuric acid is associated with 1.32
gram of water. This corresponds to an average hydration ratio of
7.19, i.e., the chemical composition of sulfate particulates would be
H2SO4 ´ 7.19 H2O (the molecular mass of H2SO4 and H2O is 98.08 and
18.016, respectively).
Figure 9. Mass of Bound H2O Per Gram of H2SO4
Samples collected for the determination of PM emissions can be
chemically analyzed for sulfates. Care should be taken while
interpreting the results. Different methods may be used for the
analysis and reporting of results for sulfates-an unregulated diesel
emission. Many laboratories would report the result as mass of the
SO4 group (molecular mass of 96.06 g/mole). Certain corrections are
necessary to estimate the corresponding overall contribution of
sulfates to the TPM emission. To be exact, the SO4 result should be
first multiplied by the molecular mass ratio of 98.08/96.06 to
convert to H2SO4. If calcium sulfate or other salts were present in
the sample, the contribution of the cations remains unknown, unless a
separate chemical analysis is performed. However, one can easily
adjust the H2SO4 figure for combined H2O by adding the water portion
which, as determined from Figure 9, often contributes more to the
total PM mass than sulfuric acid alone.
Particle Size Distribution
Size distribution is discussed in detail in Diesel Exhaust Particle Size.
An increased interest in diesel particle size distribution has been
sparked by medical research which indicates that adverse health
effects from exposure to particulates may be increasing with
decreasing particle size, even if the particles are composed of
toxicologically inert materials. In the USA, these concerns have been
reflected by the introduction in 1997 of new ambient air quality
standards for particulates smaller than 2.5 mm in addition to the
existing standards for particulates below 10 mm (PM10).
Size distributions of diesel exhaust particulates exhibit bimodal
character with two concentration peaks which correspond to the nuclei
and accumulation mode particles. The nuclei mode is usually fairly
insignificant (a few percent) if mass distributions are presented. In
particle number representation, however, the nuclei mode often
accounts for over 90% of the total particulate count. Since the late
1990's, a number of emission laboratories have undertaken PM size
distribution investigations. Quite opposite to the total particulate
mass quantification, the particulate size distribution measurements
are far from being standardized. Due to differences in particulate
sampling and the variety of measuring methods which are used, test
results from different laboratories have not been consistent. An
important objective of the ongoing research is to develop standard
measurement procedures, which would allow for reliable quantification
of particle number emissions and size distributions [Andersson
2002][Kittelson 2002].
An increased interest in the size of diesel particulates was
triggered by a study funded by the Health Effects Institute (HEI)
which investigated two heavy-duty diesel engines to find increased
numbers of nanoparticles in the engine of newer generation [Bagley
1996]. That observation, although based on a single engine
measurement, implicated that PM emissions from new technology engines
might be of more health concern, despite their lower PM mass. Later
studies have generally not confirmed these findings, suggesting that
they were characteristic to the particular engine tested by the HEI.
A very comprehensive study sponsored by the Coordinating Research
Council (CRC E-43) concluded that newer, low PM mass engines produce
generally fewer particle numbers that their older predecessors
[Kittelson 2002]. More light has been also shed on the nature of
nuclei mode particulates, indicating that health concerns in relation
to nanoparticle emissions from diesel engines, although real, may
have been exaggerated. For instance, it is now believed that nuclei
mode particles from diesel engines are composed primarily of liquid
condensates, as opposed to solid material, making them less likely to
cause harm to human lungs. Furthermore, the smallest size particles
are very short-lived in the atmosphere. A 90 % reduction (and nearly
100% in the range below 0.1 mm) of total particle number
concentrations occurs within a few minutes or a 100-1000 m distance
[Capaldo 2001]. Thus, diesel and other mobile particle sources
influence particle number concentrations in the ambient air within a
limited area mainly near roadways.
Discussion of particulate number emissions and size distributions is
not possible without more background on particle formation,
characterization, and measurement, all of the above areas involving
many unresolved issues. These topics are discusses in more detail in
the Diesel Exhaust Particle Size paper.
References
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