Morphological and Elemental Classification of Freshly

Emitted Soot Particles and Atmospheric Ultrafine Particles

using the TEM/EDS

Laarnie Tumolva,1 Ji-Yeon Park,1 Jae-suk Kim,1

and Kihong Park1

1Research Center for Biomolecular Nanotechnology, Department of Environmental Science and

Engineering, Gwangju Institute of Science and Technology, Gwangju, Korea

Arthur L. Miller,2

2National Institute for Occupational Safety and Health, Spokane, Washington, USA

Judith C. Chow,3 John G. Watson,3

3Desert Research Institute, Reno, Nevada, USA

The Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS) were used to determine morphology

and elemental composition of a variety of freshly emitted soot particles(acetylene flame, candle flame, kerosene flame, diesel exhaust,

electric arc, plastic burning, styrofoam burning, wood burning

[white oak and pine bark], and rice straw burning), which can be

possible candidate soot in the ambient atmosphere, and ultrafine

particles sampled in urban, industrial, and coastal sites during ultrafine particle formation events (combustion and photochemical

events). By using mobility-classified non-refractory ((NH4)2SO4) 

and refractory (Polystyrene latex (PSL) and salt (NaCl)) particles, limitation of the TEM was tested. Data showed that the TEM

method can be used to examine shapes of both volatile particles

such as (NH4)2SO4 (100 nm) at low, but not high magnification

(refer to low and high beam intensity, respectively), and nonvolatile particles like NaCl (100 nm) and PSL (84 nm) at either

low or high magnification. Distinct differences in morphological

properties such as primary particle diameter, fractal dimension,

and microstructure were observed among the different types of

fresh soot particles. The atmospheric ultrafine particles were classified as agglomerates, sulfate mixtures (spherical), metallic oxides

(spherical and polygonal), C-rich refractory (not agglomerated),

C-rich non-refractory (not agglomerated), Si-rich (spherical), Narich (porous), or P-containing (non-spherical) particles. At the urban Gwangju site, a higher fraction of fresh and aged agglomerates

was observed than at other sites. The C-rich non-refractory and

sulfate mixtures were often observed in the photochemical event.

The C-rich refractory particles were abundant at the Gwangju

and Yeosu sites. The coastal Taean site had few agglomerates due

to limited anthropogenic combustion source.

This research was supported by the Korea Research Foundation 

Grant (KRF) (No. 2007-331-D00222) and the Korea Science and Engineering Foundation (KOSEF) (No. R01-2007-000-10391-0 and 2009­

007-3019). 

Address correspondence to Kihong Park, Research Center for 

Biomolecular Nanotechnology, Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology, 

Gwangju, Korea. E-mail: kpark@gist.ac.kr 

INTRODUCTION

Fine and ultrafine particles in the ambient atmosphere are of 

current interest due to their potential impact on human health, 

radiation balance, cloud formation, visibility impairment, and 

atmospheric chemistry. In particular, ultrafine particles (<100 

nm) may have especially high levels of toxicity and reactivity due to their elevated surface-area-to-volume ratio (Peters 

et al. 1997; Oberdorster  ̈ 2000). A high concentration of ultrafine particles may also lead to an increase in alveolar deposition, 

as has been reported in animal studies and in vitro cell studies 

(Oberdorster  ̈ et al. 2005). It has been shown that the morphological properties of atmospheric particles play an important role 

in their transport behavior, deposition patterns in the human 

respiratory system (Oberdorster  ̈ 2001) and interactions in the 

human lung (Stearns et al. 2001). Optical properties of irregular 

particles may significantly differ from a compact sphere (Bond 

and Bergstrom 2006). 

Atmospheric ultrafine particles are either emitted directly 

from a wide range of sources or formed by gas-to-particle conversion processes. The major types of ultrafine particles in the 

ambient atmosphere include sulfates, nitrates, and ammonium 

(Friedlander 2000) and carbonaceous particles which are comprised of both organic and elemental carbon (OC and EC), and 

the OC can dominate the bulk mass of soot (50–90%) (Katrinak et al. 1992), depending on the type of sources and processes (Szidat et al. 2007; Grieshop et al. 2009). Soot particles 

are defined here as carbonaceous particles having an irregular agglomerate structure consisting of spherical primary particles (Xiong and Friedlander 2001; Park et al. 2003; Wentzel 

et al. 2003; Hu and Koylu 2004; Jung et al. 2004; Park et al. 

2004; Kocbach et al. 2006). They originated from a variety 

of anthropogenic combustion sources and biomass burning 

(Friedlander 2000; Xiong and Friedlander 2001). The morphology of particles has been found to differ among sources and combustion conditions. For example, vehicle exhaust particles (i.e., 

typically produced from high temperature combustion process) 
are usually highly agglomerated having a high surface-areato-mass ratio (Kocbach et al. 2006). The morphologies of ultrafine particles on and near a freeway were found to be aggregate, electron-opaque sphere, electron-transparent sphere, 

irregularly shaped particles, and particles with multiple inclusions (Barone and Zhu 2008). Polymer burnings, such as plastic bags and styrofoam, usually occurring in waste incinerators, can produce soot particles with highly toxic substances 

(e.g., dioxin). Burning biomass fuels (e.g., woods, agricultural 

residues, etc.) through a variety of different combustion processes (e.g., flaming and smoldering) can emit a range of carbonaceous particles (Posfai et al. 2003). Particles from biomass 

burning can also contain toxins due to the inclusion of cellulose, hemicellulose, and lignin in the biomass fuel (McKenzie 

et al. 1995). Due to the high surface-area-to-mass ratio of the 

agglomerated soot particles, they may have high ability of reactivity, and provide condensational sink for toxic vapor compound such as dioxin. Most previous studies have focused on 

morphological properties of specific soot particles from flame 

and diesel engines (Dobbins 2007), vehicle exhaust and residential wood smoke (Kocbach et al. 2006), diesel and Palas 

(Wentzel et al. 2003), or biomass burning (Posfai et al. 2003) 

rather than comparing morphological properties among a variety of soot particles. In addition to those soot particles, spherical or non-spherical sulfates, organics, metals, and so on can 

constitute the atmospheric ultrafine particles in an urban area 

(Friedlander 2000). Although there have been a lot of studies on morphological and elemental properties of ultrafine particles (Maynard 2000), there has been limited database on 

the properties of atmospheric ultrafine particles during ultrafine particle formation events (photochemical and combustion 

event). 

In this study, limitation of the Transmission Electron Microscopy (TEM) (i.e., high vacuum condition and high electron beam intensity) was first tested by using mobility-classified 

non-refractory (e.g., ammonium sulfate ((NH4)2SO4)) and refractory (Polystyrene latex (PSL) and sodium chloride (NaCl)) 

particles. Morphology and elemental composition of a variety 

of freshly emitted carbonaceous soot particles from acetylene 

flame, candle flame, kerosene flame, diesel exhaust, electric arc, 

plastic burning, styrofoam burning, wood (white oak and pine 

bark) burning, and rice straw burning, which can be possible 

candidate soot in the ambient atmosphere, were then examined. 

Atmospheric ultrafine particles were classified into several distinct types according to their morphology and elemental composition sampled in urban, industrial, and coastal sites during 

ultrafine particle formation events (photochemical and combustion events). Additionally, the number of each type of particles 

at the different sites was determined. 

EXPERIMENTAL

Figure 1 provides a schematic outline of the current experimental setup. Atmospheric ultrafine particles were sampled at 

three different sites (Gwangju, Yeosu, and Taean in Korea). 

FIG. 1. A schematic outline of the experimental setup (DMA: Differential 

Mobility Analyzer, CPC: Condensation Particle Counter, TEM: Transmission 

Electron Microscopy, EDS: Energy Dispersive Spectroscopy). 

Detailed description on the sampling sites was given in our previous 

paper (Park et al. 2008). Gwangju is an urban site surrounded 

by multiple pollution, including agricultural, traffic, industrial, 

and residential activities. Yeosu is an industrial site, located 

in a coastal area where petrochemical and steel manufacturing 

industries are present, and Taean is a coastal and continental 

background site. The atmospheric particles were drawn through 

a PM10 inlet, dried out by a dryer (Nafion MD-110, USA), 

and introduced into either the Differential Mobility Analyzer 

(DMA) or the Scanning Mobility Particle Sizer (SMPS). The 

SMPS consisted of a DMA (TSI 3081, USA) and a condensation 

particle counter (CPC) (TSI 3022A, USA). The size distribution 

of the ambient aerosols was monitored continuously to determine the peak mode diameter of ultrafine particles (<100 nm) 

with the highest concentration. When enhanced ultrafine particle concentration was observed (i.e., ultrafine particle event), the 

particles with an ultrafine mode diameter were selected by the 

DMA and collected on a TEM grid using either a nano-aerosol 

sampler (TSI 3089, USA) or a filter sampler. 

For the laboratory-generated aerosols tests, (NH4)2SO4 

(Sigma Aldrich, USA) polystyrene latex (PSL)(84 ± 9 nm, 

Duke Scientific, USA), and NaCl (Sigma Aldrich, USA) particles were produced using a Collision atomizer (TSI 3076, USA). 

The water solution was continuously aerosolized into droplets 

through compressed air in the atomizer. Droplets containing 

any suspended particles or dissolved solids were dried out by 

a series of two diffusion driers. The liquid droplets containing suspended particles were evaporated through driers (i.e., 

water solvent was evaporated), enabling only the particles to 

be suspended in the air. Any dissolved solids in the droplets 

were crystallized during the drying process into particle phases 
in the air. A DMA was used to select particles of a specified 

mobility size. 

As summarized in Table 1, the fresh soot particles were produced from an acetylene flame, a candle flame, a kerosene flame, 

diesel exhaust, an electric arc, plastic burning, styrofoam burning, wood burning (white oak and pine bark), and rice straw 

burning. 

TABLE 1
 

Summary of all particles tested
 

Particle type Mobility size, nm Source or generation method 

Atmospheric submicron particles a20 nm, 25 nm, 30 nm, 35 nm, 40 nm, Urban Gwangju, Korea, industrial Yeosu, 

50 nm, 70 nm, 80 nm, 90 nm, 100 nm, Korea, coastal Taean, Korea 

130 nm, 160 nm 

Ammonium sulfate particles a100 nm Atomization 

PSL particles a84 nm Atomization 

Sodium chloride particles a100 nm Atomization 

Acetylene flame soot b5 nm ∼ 50 nm Flame 

Candle flame soot b10 nm ∼ 100 nm Flame 

Kerosene flame soot b20 nm ∼ 600 nm Flame 

Diesel exhaust soot b20 nm ∼ 300 nm Diesel generator 

Electric arc soot b20 nm ∼ 300 nm Palas electric arc 

Plastic burning soot b10 nm ∼ 200 nm Plastic bag 

Styrofoam burning soot b10 nm ∼ 200 nm Styrofoam 

Wood burning soot b30 nm ∼ 300 nm White oak 

Wood burning soot b30 nm ∼ 100 nm, c200 nm ∼ 10 um Pine bark 

Rice straw burning soot b30 nm ∼ 600 nm Rice straw 

amobility size selected by the DMA.
 

bparticle size range measured by the SMPS.
 

cparticle size range measured by the PSD Analyzer.
 

A 2 inch flame acetylene torch was used to produce 

acetylene flame soot particles that were sampled on a TEM 

grid at a dilution ratio of 17 and sampling time of 20 min. For 

the generations of candle and kerosene flame soot particles, a 

10 inch shroud was used as a housing to prevent outside air from 

disturbing the flame. The generated soot particles were extracted 

through a 6 mm hole of the glass sampling probe with an internal 

diameter of 10 mm and diluted with 20 liter per minute (lpm) 

of clean dry air. Particles were collected at a flow rate of 1 lpm 

on a TEM grid which was taped on a filter holder. The height 

of flame for each fuel used was noted. To generate the diesel 

soot particles, particles were sampled from a diesel generator 

(Onan-Cummins powered by a three cylinder, 1.5 liter Isuzu engine) operated at 4 kW and diluted with filtered air at a flow rate 

of 5 lpm and subsequently collected on a TEM grid using a filter 

sampler for a period of 40 min. A Palas electric arc (GfG 1000, 

USA) operated at 950 a.u. current was used to produce electric 

arc soot particles, which were subsequently diluted with air at a 

dilution ratio of 8 and collected on a TEM grid for a sampling 

duration of 40 min. A plastic bag and styrofoam burned in an 

aluminum can to produce soot. A stove was used to burn wood 

and rice straw and the soot was collected. For the wood combustion particles, white oak was used to produce particles in the 

flame phase, while pine bark was used to produce particles in 

the smoldering phase. Both were sampled on a TEM grid at a 

dilution ratio of 40 and a sampling duration of 25 min. The dried 

rice straw, a common residue found in Korea after harvesting 

rice, was burned to produce the rice straw soot in both the flaming and smoldering phases. The combustion conditions were 

identified by measuring the exhaust temperature and CO/CO2 

ratio. To determine the size distribution of the soot particles, the 

SMPS was used. 

The TEM grid used in the study was carbon film coated 200 

mesh copper grid (Electron Microscopy Sciences, USA). Samples were stored in a polystyrene Petri dish and equilibrated in 

a desiccating chamber at a constant temperature (∼25◦C), and 

relative humidity (∼20%) before imaging. Morphologies of individual particles using the TEM (JEOL JEM-2100, USA) and 

their elemental compositions were determined by Energy Dispersive Spectroscopy (EDS) (OXFORD INCAx–sight, USA). 

The TEM was operated at a 200 kV accelerating voltage and 

102 μA beam current. By measuring the spectrum of blank areas between particles, the background signal of the TEM grid 

was corrected. Digital Micrograph 3 software (Gatan Inc., USA) 

was used for the TEM image analysis. 

RESULTS AND DISCUSSION

Morphological and Elemental Classification

of Laboratory-Generated Particles

To examine particle shape changes under a low vacuum condition and high electron beam intensity in the TEM, (NH4)2SO4, 
PSL, and NaCl particles were used. Figure 2 shows the TEM 

images of 100 nm mobility-classified (NH4)2SO4 particles at 

both low (left) and high (right) magnification (i.e., low and high 

electron beam intensity, respectively). 

FIG. 2. TEM images of mobility-classified particles: (a) 100 nm ammonium sulfate ((NH4)2SO4) under low (left) and high (right) magnification, (b) 84 nm 

Polystyrene latex (PSL) under high magnification, and (c) 100 nm sodium chloride (NaCl) particles under high magnification. 

The (NH4)2SO4 particles were spherical at low magnification, but had a residue with 

a beam-damaged outer area at high magnification, indicating 

that for non-refractory particles like (NH4)2SO4, the original 

shape cannot be accurately determined at high magnification. 

On the other hand, NaCl and PSL showed no significant difference between low and high magnification. Their projected area 

equivalent sizes obtained from the TEM images were similar 

to the mobility size (100 nm) for NaCl and the specified size 

(84 nm) for PSL, also suggesting that the shape of such refractory particles remained unchanged. 

To quantify the morphological properties of soot agglomerate that consists of spherical primary particles, the number 

and size of the primary particles, the fractal dimension, and the 

agglomerate size (e.g., length, width, projected area equivalent 

diameter) can be used. In this study, the soot particles are considered as having a chain-like agglomerated shape and a dominant element of C in its background-corrected EDS spectrum. 

Table 2 summarizes morphological and elemental properties of 

the soot or carbonaceous particles tested in this study. Their corresponding TEM images are also shown in Figure 3. Distinct differences in the morphological properties among different types 

of soot particles were found. As shown in Table 2, the kerosene 

flame soot showed the highest mean primary particle diameter 

(33.9 ± 17.5 nm), while the acetylene flame soot exhibited the 

lowest (23.5 ± 7.4 nm). Precursor carbon radicals and kinetic 

factors may affect the size distribution of primary particles. The 

precursor carbon radicals undergo carbonization, forming polycyclic aromatic hydrocarbons (PAH), which is the precursor of 

soot (Pugmire et al. 2002). For example, the diesel exhaust soot 

and some of the flame soot, depending on the engine load and 

flame conditions, showed similar PAH composition and formation (Dobbins 2007), leading to a similar primary particle size. 

The kinetic factors to form precursor molecules may also affect primary particle size. The soot from burning styrofoam and 

plastic contained a significant amount of condensed species, as 

shown in Figures 3b and c, and their primary particles were 

indistinct. 

The fractal dimension was determined by using the relationship between the primary particles and the number of primary 

particles calculated from the projected areas of agglomerates 
as well as the ratio of the maximum length to primary particle 

diameter (Park et al. 2004). The mean value of the maximum 

length of the agglomerate ranged from 135 ± 42 nm (candle flame soot) to 1157 ± 659 nm (kerosene flame soot). By 

considering the overlap of primary particles to calculate their 

number (Oh and Sorensen 1997), we obtained a higher number 

of primary particles in the agglomerate, resulting in a higher 

fractal dimension. With the exception of the soots from electric arc, plastic burning, and styrofoam burning, the number of 

primary particles in all types of agglomerates showed a linear 

dependence on the ratio of the maximum length to the primary 

particle diameter, suggesting that they are all fractal-like. The 

fractal dimension of the soots from electric arc, plastic burning, 

and styrofoam burning was not determined due to their indistinct 

primary particles. 

As shown in Figure 3 and Table 2, the fractal dimension decreased as the agglomerate shape became more irregular and less 

compact. 

TABLE 2
 

Summary of morphological and elemental properties of laboratory-generated soot or carbonaceous particles
 

Primary particle diameter (nm) Maximum length (nm) 

Source 

Standard Standard 

Mean deviation Mean deviation 

Fractal Major 

dimension Microstructure elements 

Minor 

elements 

Acetylene flame 

Candle flame 

Kerosene flame 

Diesel exhaust 

Electric arc 

Plastic burning 

Styrofoam burning 

White oak burning in 

the flaming phase 

Pine bark burning in the 

smoldering phase 

Rice straw burning in 

the flaming phase 

Rice straw burning in 

the smoldering phase 

23.5 

26.3 

33.9 

24.0 

—a 

—b 

—b 

32.7 

—c 

29.2 

—c 

7.4 

6.2 

17.5 

3.5 

—a 

—b 

—b 

9.9 

—c 

5.1 

—c 

180 

135 

1157 

262 

249 

527 

347 

445 

—c 

143 

—c 

72 

42 

659 

222 

107 

320 

180 

376 

—c 

90 

—c 

2.11 

2.16 

1.69 

1.69 

—a 

—b 

—b 

1.72 

—c 

1.84 

—c 

Ordered layers 

Ordered layers 

Ordered layers 

Ordered layers 

Amorphous 

Amorphous 

Amorphous 

Ordered layers 

Amorphous 

Ordered layers 

Amorphous 

C 

C 

C 

C 

C 

C 

C 

C, K 

C, 

C, K 

C, K 

O, Fe 

O, Si 

O, Si 

O 

O, Si 

Si, O 

O 

Cl, O, S, Si 

O, S 

Cl, O, S, Si 

O, S, Si, Al 

a not determined due to indistinct primary particles.
 

bnot determined due to heavy coating.
 

cnot agglomerated (spherical).
 

The candle flame and acetylene flame soot indicated 

the highest fractal dimensions (2.16 and 2.11, respectively), 

while kerosene flame and diesel soot had the lowest value (i.e., 

1.69). In other words, the candle flame soot had the most compact structures, while the diesel exhaust and kerosene flame soot 

had the most branched structures among the different types of 

soot tested. Based on fractal dimension analysis, the soot can be 

categorized as an aggregation either formed by particle-cluster 

(fractal dimension >2), the primary particles diffuse to a cluster 

of primary particles, or cluster-cluster (fractal dimension <2), 

the clusters of primary particles attach to each other. 

Data for the particle mobility size distribution (Table 1), 

measured with the SMPS, and length (Table 2), measured with 

the TEM, showed that agglomerates produced from the laminar 

flame (i.e., acetylene and candle flames) had a more uniform 

size compared to those produced from the turbulent flame (i.e., 

kerosene), diesel engine combustion, and wood burning which 

produced both large and small agglomerates. This might have 

occurred because soot precursors coexist with agglomerates in 

the turbulent processes (Hu and Koylu 2004). A laminar flame 

is characterized by the flame and smoke flowing in a uniform 

direction. In a turbulent flame, however, the flow of flame and 

smoke is chaotic. The kerosene flame generated very large agglomerates and the heavy soot and aggregate collisions in the 

dense smoke observed in this flame may have contributed to the 

presence of superaggregates (Kim et al. 2006). 

In the smoldering phase of pine bark combustion, the ignited 

fuel charred and released smoke continuously, and particles 

were sampled directly from the smoke plume. Figure 4 shows 

TEM images of pine bark particles in the smoldering phase with 

a significant amount of spherical particles (i.e., tar ball, left figure). This is in contrast to particles from burning white oak in 

the flaming phase, as shown in Figure 3f. Mckenzie et al. (1995) 

reported that the pine bark burning had lower combustion efficiency, favoring the smoldering phase, and emitted a higher 
carbon monoxide (CO) concentration and less oxygenated organic species, compared to the other parts of ponderosa pine 

wood. We also found that these spherical particles were refractory, as they were not sensitive to the increased electron beam 

intensity (i.e., high magnification), and that they contained inorganic species (e.g., S). 

FIG. 3. TEM images of soot particles under high magnification from: (a) electric arc, (b) styrofoam burning, (c) plastic burning, (d) candle flame, (e) acetylene 

flame, (f) wood (white oak) burning in the flaming phase, (g) rice straw burning in the flaming phase, (h) diesel exhaust, and (i) kerosene flame. 

As shown in Figure 5, the acetylene flame, diesel exhaust, rice 

straw burning, and wood burning (white oak) soots all exhibited 

crystalline structures having ordered layers, while the plastic 

burning and wood burning (pine bark burning in the smoldering 

phase) soot showed an amorphous microstructure. The electric arc and styrofoam burning soot also showed an amorphous 

microstructure. The electric arc soot experienced a high cooling rate. If the cooling rate is fast, carbon clusters do not have 

enough energy and time to have an ordered crystalline structure, 

leading to a disordered structure (Liu et al. 2004). For polymers, 

the monomers formed upon heating are created by a scission of 

the chain (Panagiotou and Levendis 1994). The polystyrene, a 

major component of the styrofoam container, had a branched 

structure with aromatics, while the polyethylene, a major component of the plastic bag, had a straight chain structure. Both 

fuels were harder to burn than other fuels and their flames were 

faint with a low sooting. They also had a significant amount of 

condensable species. It is possible that these conditions led to 

the amorphous structure of their primary particles. 

The EDS data, included in Table 2, showed that C and O 

elements were found in all types of soot. Since elemental composition of particle free area was measured along with the EDS 

to account for background contribution, C, O, and Cu elements 

found in the EDS spectrum mostly come from particles. K and 
other inorganic species were detected in particles from wood 

(white oak and pine bark) burning and rice straw burning, suggesting that K could be used as a marker for fresh soot particles 

from biomass burning. The presence of K and other inorganic 

species such as Ca, Mg, and S makes the soot from vegetative 

burning more polar, hygroscopic, and probably more prone to 

aging. A bulk analysis of the soot from wood smoke showed a 

substantial fraction of inorganic salt, possibly enhancing their 

water solubility and hygroscopicity (Mavrocordatos et al. 2002). 

FIG. 4. TEM images of wood (pine bark) burning particles in the smoldering phase. 

Morphological and Elemental Classification

of Atmospheric Ultrafine Particles

The properties of mobility-classified ultrafine particles sampled at the urban Gwangju, industrial Yeosu, and coastal Taean 

sites in Korea were investigated. Because of the diversity of 

atmospheric particles, we limited our attention to the ultrafine 

mode particles when an increase of ultrafine particle number 

concentration was observed, which was classified into photochemical and combustion events by Park et al. (2008). The 

number concentration of ultrafine particles was continuously 

monitored by the SMPS. 

Our classification of atmospheric ultrafine particles is based 

on their morphology, elemental composition, and beam sensitivity. The agglomerates were classified as either “fresh agglomerates” or “aged agglomerates.” The “fresh agglomerate” contained a major element of C along with a few minor elements and 

distinct primary particles, similar to the agglomerates reported 

in Table 2 and Figure 3, while the “aged agglomerate” contained 

multi elements and a visible coating or were coagulated with 

other types of particle. Since it is difficult to identify source of 

agglomerated soot particles in the ambient air, the atmospheric 

soot particles were roughly classified into “fresh” and “aged” 

agglomerates. The classification of laboratory-generated soot 

particles, however, provides useful information on the type of 

soot particles, which may be observed in the other atmospheric 

or indoor condition. 

Figure 6a shows a fresh agglomerate with no visible coating and a major element of C in its EDS spectrum. Figure 6b 

shows an aged agglomerate which was included in the sphere, 

having multi elements (e.g., S, Si, C). This particle type is 

similar to those found by Okada et al. (2003). When the electron beam in the TEM was intensified to magnify particle, the 

spherical species decomposed, suggesting that they are volatile. 

Figure 6c illustrates an aged agglomerate coagulated with spherical particles, resulting in the existence of multi elements such 

as C, O, Si, S, K, and Fe. These aged agglomerates may have 

undergone atmospheric processing (i.e., condensation and coagulation), leading to their more compact structure with additional 

chemical species (Zhang et al. 2008). The restructuring may 

depend on the origin of the soot. For instance, the soot agglomerates from the electric arc may collapse into more compact 

agglomerates when they are exposed to high relative humidity, 

but the diesel-exhaust particles may exhibit only minimal restructuring due to the presence of a hydrophobic organic coating 

such as lubricating oil, PAHs, and unburned hydrocarbons (Xue 

et al. 2009). In addition, we classified the atmospheric ultrafine 

particles as either sulfate mixtures (spherical), metallic oxides 

(spherical and polygonal), C-rich refractory (spherical), C-rich 

non-refractory (spherical), Si-rich (spherical), Na-rich (porous), 

or P-containing (non-spherical) particles, as summarized in 

Table 3. In case of sulfate mixtures they are spherical and usually 

contained S, coexisting with other elements. The classification 

of C-rich refractory (beam-resistant) and C-rich non-refractory 

(beam-sensitive) particles was determined by their response to 

the intensified electron beam in the TEM. The C-rich refractory 
particles were not agglomerated and had a major element of C, 

and exhibited no visible morphological change under the strong 

electron beam intensity in the TEM. 

FIG. 5. Microstructure of soot or carbonaceous particles from: (a) acetylene flame, (b) rice straw burning in the flaming phase, (c) diesel exhaust, (d) white oak 

burning in the flaming phase, (e) kerosene flame, (f) pine bark burning in the smoldering phase, and (g) plastic burning. 

Figure 7a shows a TEM image of a sulfate mixture particle. When the electron beam in the TEM was intensified, a 

hollow area was frequently observed, indicating the presence 

of volatilized materials (Okada and Heintzenberg 2003). This 

type of spherical particle was particularly observed in the photochemical event (Park et al. 2008). The corresponding EDS 

spectrum showed a minor S, suggesting that sulfates evaporated 

under strong electron beam and high vacuum conditions in the 

TEM. Figures 7b and c show Cu-rich and Zn-rich metallic oxide 

(polygonal), respectively. The metallic oxide particles, often observed in urban and industrial sites, are opaque and often coated. 

The Si-rich particle is spherical and sometimes contains minor 

Al, possibly originating from fly ash in a polluted atmosphere 

(Li et al. 2003; Niemi et al. 2006). The Na-rich particle contains 

Na, S, and K. The Na-S-K-O and porous morphology might be 

resulted from burning of biomass residuals or other waste material (Niemi et al. 2006). The P-containing particle usually has 

an abundant C with a minor P and may have a biological origin. Although microstructure of atmospheric particles was not 

examined in details, all of particles shown in Figures 6, 7, and 

8 seem to be amorphous except for some fresh agglomerates. 

Regardless of above morphological and elemental classification, two types of particles, refractory and non-refractory, can 

be identified based on their stability under high beam intensity in the TEM. The non-refractory particles are volatilized in 
the TEM, leaving a residue, which is similar to the response 

of the laboratory-generated (NH4)2SO4 particles, as shown in 

Figure 2. The non-refractory particles were the sulfate mixture 

and C-rich non-refractory particles. The particles classified as 

metallic oxides, C-rich refractory, Si-rich, and Na-rich, were refractory, comparable to the laboratory-generated PSL and NaCl 

particles, shown in Figure 2. 

FIG. 6. TEM and EDS data of agglomerate particles sampled in the ambient atmosphere for: (a) fresh agglomerate, (b) aged agglomerate (coated), and (c) aged 

agglomerate (coagulated). 

Comparison of Atmospheric Ultrafine Particles Among

Different Sites

The number of each type of atmospheric ultrafine particles classified above was determined among the three different 

sites during ultrafine particle formation events. Note that particles more volatile than (NH4)2SO4 were not included in the 

TEM analysis. Results for the laboratory-generated (NH4)2SO4 
particles
 atmospheric ultrafine 

3
 

mobility-classified 

TABLE 

of properties elemental and morphological of Summary 

Particle Morphological Major Minor Mobility Number of particles Number of particles Number of type description elements elements size (nm) at Gwangju at Yeosu particles at Taean Fresh agglomerate Distinct primary C O, Si, Cu 20, 25, 30, 35, 50, 46 4 3 

particles without 70, 80, 90 heavy coating 

Aged agglomerate Coated, coagulated, or C, O, S, Cu, Ca, Si, Ti, Fe, Na, K, 25, 30, 35, 40, 50, 49 7 0 

mixed with other Mg Cl, Al 70, 80, 90, 100, 

chemical elements 160 

Sulfate mixture Spherical morphology S, C, O Ca, Mg, K, Cu, 20, 25, 30, 35, 50, 78 5 3 

and presence of Na, Al 70, 80, 160 beam-damaged portion at high magnification 

Metallic oxides Opaque spherical or C, O, Cu, Al, Si, Ti, K, Mg, Cl 20, 25, 30, 35, 40, 185 29 11 

polygonal Ca, Fe, Cr, Zn 50, 70, 80, 160 

C-rich refractory Not agglomerated and C O, Si, Cu 20, 25, 30, 35, 50, 278 60 7 

beam-resistant at high 80, 130, 160 magnification 

C-rich non-refractory Not agglomerated and C O, Cu, Si 20, 25, 30, 35, 40, 146 25 8 

beam-sensitive at 50, 70, 80, 100, high magnification 130 

Si-rich Spherical and Si, C, O, Cu Al, Ca, Mg, K, 30 , 35, 40, 50, 5 24 3

beam-resistant Na 70, 80, 160 

Na-rich Porous and rounded Na, C, K, Cl, Cu O, S 35, 40, 50, 80 5 18 0 

edges 

P-containing Elongated or C, O, P Si, Cu, Ca, Zn, 35, 70, 80, 100 21 1 1 

aggregated Fe, Cl 

Total 813 173 36 
particles (<100 nm) showed that they did not evaporate under 

low electron beam intensity and partially evaporated under high 

electron beam intensity. The C-rich non-refractory particles defined here showed a similar behavior to the (NH4)2SO4 particles. 

Table 3 compares the number of each type of ultrafine particles in 

the three sampling sites. We observed relatively higher fraction 

of agglomerates (both fresh and aged) at the urban Gwangju site 

and the appearance of fresh agglomerates was consistent with 

the combustion event at this site (Park et al. 2008). In this combustion event, caused primarily by traffic and residential heating, the peak mode size of ultrafine particles was 70–100 nm, 

while a lower mode size (10–50 nm) was observed in the 

photochemical event. In the photochemical event at the urban 

Gwangju site, C-rich non-refractory and sulfate mixtures were 

often observed. The C-rich refractory, metallic oxide particles, 

and Si-rich particles are abundant at the industrial Yeosu site. 

FIG. 7. TEM-EDS data of: (a) sulfate mixture, (b) Cu-rich metallic oxide (polygonal), and (c) Zn-rich metallic oxide (polygonal). 
The metallic oxides can be contributed by the steel industries at 

the Yeosu site. The Si-rich particles can originate from petrochemical industries where zeolites or silicates are abundantly 

used as catalysts (Mravec et al. 2005). At the background Taean 

site the ultrafine particles were not as diverse as those from 

urban or industrial site, and few agglomerates were observed 

due to the absence of anthropogenic combustion sources in 

this area. 

FIG. 8. TEM-EDS of: (a) Si-rich particle, (b) Na-rich particle, and (c) P-containing particle. 

CONCLUSIONS

This study was conducted to examine morphological and 

elemental properties of fresh soot particles produced from a 

variety of fuels and processes, and to classify atmospheric ultrafine particles sampled at three ambient sampling sites (urban 

Gwangju, industrial Yeosu, and coastal background Taean) during ultrafine particle formation event. A laboratory test showed 

a significant morphological change for non-refractory particles 
like (NH4)2SO4 (100 nm) between low and high magnification, 

while refractory NaCl (100 nm) and PSL (84 nm) showed no 

difference, suggesting that morphological properties of more 

volatile particles than (NH4)2SO4 cannot be accurately determined by the TEM method. Distinct differences in morphological properties were observed among the different types of fresh 

soot particles. The diesel soot particles had the lowest fractal 

dimension (1.69), suggesting that they are the most irregular 

among the fresh soot particles tested here. An examination of 

the microstructures of the primary particles revealed that plastic burning, styrofoam burning, electric arc, pine bark burning 

(in the smoldering phase), and rice straw burning (in the smoldering phase) soot had amorphous microstructures, while others exhibited crystalline structures having ordered layers. The 

soot produced from burning wood and rice straw showed K in 

its EDS spectrum, potential marker for biomass burning emissions. A comparison between wood combustion in the flaming 

phase (white oak) and the smoldering phase (pine bark) revealed 

that the flaming phase produced fractal-like particles, while the 

smoldering phase produced a significant number of tar ball-like 

particles (spherical). The atmospheric ultrafine particles were 

classified as being fresh agglomerate, aged agglomerate, sulfate mixture (spherical), metallic oxides (spherical and polygonal), C-rich refractory, C-rich non-refractory (spherical), Si-rich 

(spherical), Na-rich (porous), or P-containing particles (nonspherical). The fresh and aged agglomerates were found to be 

abundant at the urban Gwangju site, and C-rich non-refractory 

and sulfate mixtures were often observed in the photochemical 

event at this site. The Si-rich and C-rich refractory particles 

were abundant at the industrial Yeosu site. The coastal Taean 

site did not have as diverse particle types as compared to other 

sites and had least agglomerates due to limited anthropogenic 

combustion sources. 

REFERENCES

Barone, T. L., and Zhu, Y. (2008). The Morphology of Ultrafine Particles on 

and Near Major Freeways. Atmos. Environ. 42:6749–6758. 

Bond, T. C., and Bergstrom, R. W. (2006). Light Absorption by Carbonaceous Particles: An Investigative Review. Aerosol Sci. Technol. 40:27– 

67. 

Dobbins, R. A. (2007). Hydrocarbon Nanoparticles Formed in Flames and 

Diesel Engines. Aerosol Sci. Technol. 41:485–496. 

Friedlander, S. K. (2000). Smoke, Dust, and Haze Fundamentals of Aerosol

Dynamics, 2nd Edition. Oxford University Press, New York, p. 372. 

Grieshop, A. P., Logue, J. M., Donahue, N. M., and Robinson, A. L. (2009). Laboratory Investigation of Photochemical Oxidation of Organic Aerosol from 

Wood Fires 1: Measurement and Simulation of Organic Aerosol Evolution. 

Atmos. Chem. Phys. 9:1263–1277. 

Hu, B., and Koylu, U. (2004). Size and Morphology of Soot Particulates Sampled from a Turbulent Nonpremixed Acetylene Flame. Aerosol Sci. Technol.

38:1009–1018. 

Jung, H., Kittelson, D. B., and Zachariah, M. R. (2004). Kinetics and Visualization of Soot Oxidation using Transmission Electron Microscopy. Combust.

Flame. 136:445–456. 

Katrinak, K. A., Rez, P., and Buseck, P. R. (1992). Structural Variations in Individual Carbonaceous Particles from an Urban Aerosol. Environ. Sci. Technol.

26:1967–1976. 

Katrinak, K. A., Rez, P., Perkes, P. R., and Buseck, P. R. (1993). Fractal Geometry of Carbonaceous Aggregates from an Urban Aerosol. Environ. Sci.

Technol. 27:539–547. 

Kim, W., Sorensen, C. M., Fry, D., and Chakrabarti, A. (2006). Soot aggregates, Superaggregates, and Gel-Like Networks in Laminar Diffusion Flames. 

J. Aerosol Sci. 37:386–401. 

Kocbach, A., Li, Y., Yttri, K. E., Cassee, F. R., Schwarze, P. E., and Namork, 

E. (2006). Physicochemical Characterisation of Combustion Particles from 

Vehicle Exhaust and Residential Wood Smoke. Part. Fibre Toxicol. 3:1–10. 

Li, J., Anderson, J. R., and Buseck, P. R. (2003). TEM Study of Aerosol Particles 

from Clean and Polluted Marine Boundary Layers over the North Atlantic. J.

Geophys. Res. 108:8-1-8-14. 

Liu, Y., Xiaolong, S., Tingkai, Z., Jiewu, Z., Hirscher, M., and Philipp, F. (2004). 

Amorphous Carbon Nanotubes Produced by a Temperature Controlled DC 

Arc Discharge. Carbon. 42:1852–1855. 

Mavrocordatos, D., Kaegi, R., and Schmatloch, V. (2002). Fractal Analysis of 

Wood Combustion Aggregates by Contact Mode Atomic Force Microscopy. 

Atmos. Environ. 36:5653–5660. 

Maynard, A. D. (2000). Overview of Methods for Analysing Single Ultrafine 

Particles. Phil. Trans. R. Soc. Lond. A. 358:2593–2610. 

Mckenzie, L. M., Hao, W. M., Richards, G. N., and Ward, D. E. (1995). Measurement and Modeling of Air Toxins from Smoldering Combustion of Biomass. 

Environ. Sci. Technol. 29:2047–2054. 

Mravec, D., Hudec, J., and Janotka, I. (2005). Some Possibilities of Catalytic 

and Noncatalytic Utilization of Zeolites. Chem. Pap. 59:62–69. 

Niemi, J. V., Saarikoski, S., Tervahattu, H., Mak ̈ ela,  ̈ T., Hillamo, R., Vehkamaki,  ̈

H., Sogacheva, L., and Kulmala, M. (2006). Changes in Background Aerosol 

Composition in Finland During Polluted and Clean Periods Studied by 

TEM/EDX Individual Particle Analysis. Atmos. Chem. Phys. 6:5049–5066. 

Oberdorster  ̈ , G., Oberdorster  ̈ , E., and Oberdorster  ̈ , J. (2005). Nanotoxicology: 

An Emerging Discipline Evolving from Studies of Ultrafine Particles. Environ. Health Pers. 113:823–839. 

Oberdorster  ̈ , G. (2001). Pulmonary Effects of Inhaled Ultrafine Particles. Occup.

Environ. Health.74:1–8. 

Oberdorster  ̈ , G. (2000). Toxicology of Ultrafine Particles: In Vivo Studies. Phil.

Trans. R. Soc. A. 358:2719–2740. 

Oh, C., and Sorensen C. M. (1997). The Effect of Overlap between Monomers 

on the Determination of Fractal Cluster Morphology. J. Colloid Interface Sci.

193:17–25. 

Okada, K., and Heintzenberg, J. (2003). Size Distribution, State of Mixture, 

and Morphology of Urban Aerosol Particles at Given Electrical Mobilities. 

J. Aerosol Sci. 34:1539–1553. 

Panagiotou, T., and Levendis, Y. (1994). A Study on the Combustion Characteristics of PVC, Poly(styrene), Poly(ethylene), and Poly(propylene) Particles 

under High Heating Rates. Combust. Flame. 99:53–74. 

Park, K., Cao, F., Kittelson, D. B., and McMurry, P. H. (2003). Relationship 

Between Particle Mass and Mobility for Diesel Exhaust Particles. Environ.

Sci. Technol. 37:577–583. 

Park, K., Kittelson, D. B., and McMurry, P. H. (2004). Structural Properties 

of Diesel Exhaust Particles Measured by Transmission Electron Microscopy 

(TEM): Relationships to Particle Mass and Mobility. Aerosol Sci. Technol.

38:881–889. 

Park, K., Park, J., Kwak, J., Cho, G., and Kim, J. (2008). Seasonal and Diurnal 

Variations of Ultrafine Particle Concentration in Urban Gwangju, Korea: 

Observation of Ultrafine Particle Events. Atmos. Environ. 42:788–799. 

Peters, A., Wichmann, H. E., Tuch, T., Heinrich, J., and Heyder, J. (1997). 

Respiratory Effects are Associated with the Number of Ultrafine Particles. 

Am. J. Respir. Crit. Care Med. 155:1376–1383. 

Posfai, M., Simonics, R., Li, J., Hobbs, P. V., and Buseck, P. R. (2003). Individual 

Aerosol Particles from Biomass Burning in Southern Africa: 1. Compositions 

and Size Distributions of Carbonaceous Particles. J. Geophys. Res. 108:19­

1-19-13. 

Pugmire, R. J., Solum, M. S., Jiang Y. J., Sarofim A. F., Veranth, J., Schobert 

H. H., and Pappano, P. J. (2002). The Study of Soot Formation by Solid State 

NMR Spectroscopy . Fuel Chem. Div. Preprints. 47:733–735. 
Stearns, R. C., Paulauskis, J. D., and Godleski, J. J. (2001). Endocytocis of 

Ultrafine Particles by A549 Cells. Am. J. Respir. Cell. Mol. Biol. 24:108–115. 

Szidat, S., Prevot A. S. H., Sandradewi, J., Alfarra, M. R., Synal, H.-A., Wacker, 

L., and Baltensperger, U. (2007). Dominant Impact of Residential Wood 

Burning on Particulate Matter in Alpine Valleys during Winter. Geophys.

Res. Lett. 34: L05820. 

Wentzel, M., Gorzawski, H., Naumann, K.-H., Saathoff, H., and Weinbruch, S. 

(2003). Transmission Electron Microscopical and Aerosol Dynamical Characterization of Soot Aerosols. J. Aerosol Sci. 34:1347–1370. 

Xiong, C., and Friedlander, S. K. (2001). Morphological Properties of Atmospheric Aerosol Aggregates. Proc. Natl. Acad. Sci. 98:11851–11856. 

Xue, H., Khalizov, A. F., Wang, L., Zheng, J., and Zhang, R. (2009). Effects 

of Coating of Dicarboxylic Acids on the Mass-Mobility Relationship of Soot 

Particles. Environ. Sci. Technol. 43:2787–2792. 

Zhang, R., Khalizov, A. F., Pagels, J., Zhang, D., Xue, H., and McMurry, 

P. H. (2008). Variability in Morphology, Hygroscopicity, and Optical Properties of Soot Aerosols During Atmospheric Processing. Proc. Natl. Acad. Sci.

105:10291–10296.