Introduction
Atmospheric tar balls (TBs) comprise a unique class of carbonaceous aerosol
particles emitted during biomass burning (Pósfai et al., 2003, 2004;
Adachi and Buseck, 2011). TBs are understood to be part of the family of
atmospheric brown carbon (BrC) (Hand et al., 2005; Andreae and Gelencsér,
2006), as they absorb light in the visible range of the solar spectrum yet
are distinctly different from BC in microstructure, morphology and in other
properties (as summarized by Petzold et al., 2013). Since these particles are
fairly abundant in biomass burning plumes and are able to absorb solar
radiation quite efficiently in the visible (Hand et al., 2005; Alexander et
al., 2008) and up to the near-IR range (Hoffer et al., 2017), TBs may have a
considerable effect on the Earth radiation budget (Chung et al., 2012). It
should be noted here that other authors (Chakrabarty et al., 2010; China et
al., 2013; Sedlacek et al., 2017; Sedlacek III et al., 2018) found less
absorbing “tar ball” particles with chemical properties (C / O ratio)
and optical parameters resembling those of humic-like substances (HULIS). TBs
can be unambiguously identified by electron microscopy as spherical amorphous
particles externally mixed in relatively fresh biomass burning plumes
(Pósfai et al., 2003, 2004; Adachi and Buseck, 2011). Unlike soot
particles, TBs do not form chain-like aggregates of 20–50 nm spherules and
there are no turbostratic/concentrically wrapped graphitic layers in their
microstructure. Their sizes range from 30 to 500 nm in geometric diameter
(Pósfai et al., 2004). Furthermore, TBs are refractory as they can
withstand the high-energy electron beam of the transmission electron
microscopy (TEM) in vacuum (Pósfai et al., 2004; Hand et al., 2005). In
addition, elemental composition (C / O molar ratio) is also an important
characteristic of TBs. Typical C / O molar ratios of atmospheric TBs are
about 9–10, as determined by TEM with energy-dispersive X-ray spectroscopy
(TEM-EDS). It should be noted that in some studies the term “tar ball” is
used for combustion particles that are non-spherical and have a lower
C / O molar ratio (Chakrabarty et al., 2016). That is why in this paper
we apply the term “tar balls” also to non-perfectly spherical particles.
Albeit TBs are abundant in biomass burning plumes globally, very little is
known about their chemical composition mainly because in biomass smoke TBs
coexist with various other particle types (e.g. organic particles with
inorganic inclusions, soot) from which they cannot be separated physically.
Thus, the chemical properties of TBs can only be studied by single-particle
analytical techniques such as TEM-EDS or SEM-EDS (Li et al., 2003; Pósfai et
al., 2003, 2004; Hand et al., 2005; Niemi et al., 2006; Adachi and Buseck,
2011; Chakrabarty et al., 2016; Adachi et al., 2017; Cong et al., 2009, 2010;
China et al., 2013; Chakrabarty et al., 2006, 2010; Semeniuk et al., 2007),
TEM with electron energy-loss spectroscopy (TEM-EELS) (Hand et al., 2005,
Adachi and Buseck, 2011), and near-edge X-ray absorption fine-structure
spectroscopy (NEXAFS) using a synchrotron source (Tivanski et al., 2007). In
our previous works (Tóth et al., 2014; Hoffer et al., 2016) an
experimental setup had been developed for the generation of TB particles in
the laboratory without the concurrent emission of other combustion products.
The structural characteristics (homogeneity) and the elemental composition of
the TB particles generated by this experimental system were highly similar to
those of atmospheric TBs published by Pósfai et al. (2004) and Adachi and
Buseck et al. (2011) as the Lab-TBs have homogenous internal structure
without core and concentrically wrapped graphitic layers. Similarly to the
atmospheric TBs the generated Lab-TB particles were also refractory under the
electron beam of the TEM. Their particle diameter extended up to 360 nm
measured by a DMPS system. The high C / O molar ratios (see later) of the
Lab-TB particles were very similar to those found by other authors for
atmospheric TBs (C / O: 8–10; Pósfai et al., 2004; Niemi et al.,
2006). However, it should be
noted here that other authors (Tivanski et al., 2007; China et al., 2013)
reported atmospheric TBs with significantly lower C / O ratios (1–2). In
the present study the chemical properties of the TBs produced in the
laboratory were investigated by several analytical techniques which have
never been applied for the characterization of TBs. The analytical methods
deployed were direct elemental analysis (CHNSO), organic carbon / elemental carbon (OC / EC) thermal–optical
analysis (TOA), pyrolysis–gas chromatography–mass spectrometry (Py-GC-MS),
Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. The
results of the analyses were directly compared to those obtained by other
studies using the same techniques for atmospheric humic-like substances
(HULIS), or humic acid and soot (BC) with a view to locate the chemical
properties of refractory TBs (with high C / O ratio) in the continuum of
carbonaceous aerosol constituents.
Experimental section
The generation of TB particles in the laboratory was carried out as
described in Hoffer et al. (2016). Briefly, Lab-TBs were produced from the
liquid tarry condensates obtained by dry distillation of wood chips of Norway
spruce (Picea abies), European turkey oak (Quercus cerris)
and black locust (Robinia pseudoacacia), separately. The
concentrated aqueous phase of the tarry condensates (wood tars) was nebulized
to produce tar droplets which were first exposed to a “thermal shock” by
passing them through a heated (at 650 ∘C) quartz tube, then cooled
and dried with dry filtered air. The optical and the morphological properties
of the particles generated from Norway spruce and black locust were described
by Hoffer et al. (2017). The size distribution of the generated particles was
measured by a DMPS system (Hoffer et al., 2017). The shapes of the particles
(investigated by TEM) generated from European turkey oak were mostly
distorted spheres. In this case the majority of the particles were likely
deliquescent upon collision with the collection surface. The morphologies of
these particles were very similar to those of the freshly formed atmospheric
TBs (or to the precursor particles of the TBs) collected from biomass burning
smoke 3–4 km away from fire, presented by Adachi and Buseck (2011) in their
Fig. 4c. Although the shape of the particles generated from European turkey
oak was not perfectly spherical on the TEM grids, we assume that prior to
impaction they were likely spherical. Based on this assumption and the fact
that their chemical properties (e.g. elemental composition and IR spectra; see
later) were similar to those of the other laboratory-generated TB particles,
we refer to these particles also as Lab-TBs. The generated particles were
collected on different sampling substrates: on TEM grids (lacey
Formvar/carbon TEM copper grid of 200 mesh, Ted Pella Inc., USA), on
pre-baked double (front and backup) quartz filters (QMA, ∅ 47 mm, Whatman) and on pre-cleaned aluminium foils using a Berner cascade
impactor (Wang and John, 1988). In the case of the impactor samples we used
the samples collected on stage 2 (aerodynamic diameter between 125 and
250 nm), representing about half (based on the DMPS measurements
∼ 37 %, ∼ 47 %, ∼ 59 %, for the turkey oak,
black locust and Norway spruce, respectively) of the mass of the generated
particles. The morphologies and the elemental compositions (C, O, N, S) of
individual TB particles from different wood species were studied in
bright-field TEM images obtained using a Philips CM20 TEM operated at 200 kV
accelerating voltage. The electron microscope had an attached
ultra-thin-window Bruker Quantax X-ray detector that allowed the
energy-dispersive X-ray analysis (EDS) of individual particles. The relative
concentrations of C and O were determined using sensitivity ratios
(k factors) derived from EDS spectra acquired from a standard (a
stoichiometric dolomite (CaMg[CO3]2) sample). Nitrogen was not
determined quantitatively. We took great care both in this and all former
studies to analyse the compositions of only those particles that had
sufficient volumes over holes, avoiding any contribution from the support
film in the EDS spectra.
The elemental (C, H, N, S, O) compositions of the aqueous phases of the
concentrated wood tar samples (the starting material for Lab-TB generation)
and those of the Lab-TBs were determined using a EuroVector EA3000 CHNS/O
elemental analyser. The carrier gas was helium (He) (purity: 4.6; Messer)
with a flow rate of 110 L min-1, the temperature of the reactor tube
and that of the GC oven were 980 and 70 ∘C, respectively. The
instrument was equipped with a thermal conductivity detector (TCD). The
measurements of CHNS and oxygen content of samples were carried out
separately from quartz filters, and all analyses were performed in duplicate.
The portions of filters with area of 1 cm2 were packed in double tin
(∅ 5×9 mm) capsules in the case of the CHNS analysis
while in the case of the oxygen analysis the samples were wrapped in double
silver capsules (∅ 5×9 mm). The data were corrected for
blanks taken from backup quartz filters of the same size in double tin/silver
capsules. The calibration for these elements was carried out using reference
standard materials (acetanilide, SOIL#5, SOIL NCS–2) from EuroVector,
Italy.
The OC / EC thermal–optical analysis of Lab-TBs was performed by a
Model-4 Semi-Continuous OC-EC Field Analyzer (Sunset Laboratory Inc., USA).
The aerosol samples on quartz filters (∅ 13.06 mm) were analysed
following the EUSAAR_2 protocol (Cavalli et al., 2010). The data were
corrected for blanks taken from backup quartz filters of same size.
The Lab-TB samples collected on quartz filters were investigated using
Py-GC-MS. The analyses were performed with a Pyroprobe 2000 pyrolyser (CDS
Analytical) interfaced directly to a gas chromatograph–mass spectrometer
(Agilent 6890A/5973). The portions of sample filters with areas of
0.5 cm2 were heated from 250 to 600 ∘C at a heating rate of
1 ∘C ms-1 and held for 20 s in the pyrolyser. High-purity He
(purity: 5.0; Linde) as a carrier gas was used at a controlled flow rate of
20 mL min-1 to flush the pyrolysis products into a DB-1701capillary
column (30 m × 0.25 mm ID, 0.25 µm film thickness,
Agilent). The GC injector was set in splitless mode with an inlet temperature
of 250 ∘C. The temperature of the column was kept at 40 ∘C
for 2 min, then increased at a heating rate of 10 ∘C min-1 to
280 ∘C and held there for 5 min. Temperatures of the GC-MS interface
and the detector were 280 and 230 ∘C, respectively. The mass
spectrometer was operated at 70 eV with a mass detection in the m/z range
of 15–350.
The characteristic functional groups of wood tar and Lab-TB samples collected
on aluminium foils were examined using specular reflection FT-IR technique.
The spectra of the samples were recorded on a Bruker Vertex 70 FT-IR
spectrometer coupled with a Hyperion 2000 IR microscope with 15×
(NA = 0.4) specular reflection objective. Spectra were recorded over the
range of wave number 4000–400 cm-1 at room temperature using 128 scans
at 2 cm-1 resolution.
The wood tar and Lab-TB samples collected on aluminium foils were also
investigated by Raman spectrometry. Raman spectra were recorded with a Thermo
Scientific DXR Raman microscope at excitation wavelength (λ0) of
532 nm, applying maximum 10 mW laser power, with the laser beam focused using
a 50× objective lens, resulting in a spot size of
∼ 1 µm. Typically, 20 scans were recorded and averaged with
4 cm-1 resolution in the 200–1800 cm-1 range. The peak fittings
of Raman spectra (in the range between 1000 and 1800 cm-1) were
executed after multi-point baseline correction using by the GRAMS/AI
(Version 7.02) software.
Results
Elemental composition of laboratory-generated TBs
According to the CHNSO analysis the mean C, H, N, and O contents of TB samples
(n=3) on quartz filter were 82 % (RSD: 0.5 %), 4 % (RSD:
6.7 %), 3 % (RSD: 39 %), and 11 % (RSD: 9.2 %) by mass,
respectively. Sulfur was below the detection limit.
Table 1 summarizes the average O / C and H / C molar ratios of wood
tars and Lab-TBs produced from the three wood types, as determined by CHNSO
elemental analysis. For comparison, the O / C molar ratios obtained for
individual particles by TEM measurements are also given. The O / C molar
ratios of the Lab-TB particles obtained from three different tree species
were very similar to each other and to the values obtained from TEM-EDS
analyses. The H / C molar ratio was relatively low (between 0.51 and
0.58), indicating that the Lab-TBs consist mostly of unsaturated, aromatic
and oxygenated organic compounds. It should be noted that wood tars (starting
material for TB generation) exhibited significantly higher O / C and
H / C molar ratios (0.182 and 1.215, respectively), which strongly
suggests that the “thermal shock” employed during Lab-TB generation (as
described in Hoffer et al., 2016) has markedly increased the degree of
aromatization (Francioso et al., 2011). It is also possible that some
residual water is present in the raw tar, which is then removed in the thermal
process. Its presence may also be a plausible explanation of the markedly
different shapes and C / O ratios reported for atmospheric TB particles
in the literature.
Oxygen to carbon (O / C) and hydrogen to carbon (H / C)
molar ratios of laboratory-generated TBs and wood tar samples measured by
TEM-EDS (from analysis of 12 particles from each sample) and by CHNSO
elemental analyser. The numbers in parentheses represent the relative
standard deviation (RSD%) of the parameters. * Data from Hoffer et
al. (2017).
O / C molar ratio
O / C molar ratio
H / C molar ratio
(by TEM-EDS)
(by CHNSO)
(by CHNSO)
Lab-TB – black locust
0.110 (12 %)*
0.094 (6 %)
0.584 (3 %)
Lab-TB – Norway spruce
0.108 (7 %)*
0.109 (8 %)
0.511 (22 %)
Lab-TB – turkey oak
0.111 (9 %)
0.094 (5 %)
0.543 (1 %)
Lab-TB sample average
0.110 (10 %)
0.099 (10 %)
0.546 (7 %)
Wood tar sample average
no data
0.182 (9 %)
1.215 (4 %)
In order to compare the elemental composition of Lab-TBs with those of soot,
HULIS and atmospheric TBs, a van Krevelen diagram is plotted (Fig. 1). For
the atmospheric TBs measured in earlier studies using TEM-EDS (Pósfai et
al., 2004), SEM-EDS (Chakrabarty et al., 2010; China et al., 2013), and NEXAFS
(Tivanski et al., 2007), only the available O / C molar ratios are
presented in the diagram. It can be clearly seen that the average O / C
molar ratio of our Lab-TB particles is very similar to that of atmospheric
TBs examined by Pósfai et al. (2004), whereas it is lower than those
obtained by some other authors (Tivanski et al., 2007; Chakrabarty et al.,
2010; China et al., 2013). This difference in compositions may result from
differences in the formation temperatures and/or atmospheric processing of
the TBs. China et al. (2013) collected slightly aged (1–2 h) particles from
the smoldering phase of the Las Conchas fire in northern New Mexico, USA.
Chakrabarty et al. (2010) investigated particles from the smoldering
combustion of dry duffs, whereas Tivanski et al. (2007) observed aged TBs
during episodes characterized by high particle light-scattering coefficients.
In contrast to these authors, Pósfai et al. (2003, 2004) measured TB
particles from both flaming and smoldering savanna fires, although the
authors mention that the distinction between different burning stages was
often not straightforward, since flaming and smoldering stages of the burn
could be present simultaneously in adjacent areas. The uncertain
identification of burning stages notwithstanding, these observations suggest
that several types of TBs may exist with different O / C molar ratios,
depending on the formation temperature and the temperature and duration of
the heat shock that the particles are exposed to, and/or on the degree of
atmospheric processing.
Van Krevelen diagram of different soot (Akhter et al., 1985; Clague
et al., 1999; Collura et al., 2005), Lab-TB, wood tar and HULIS (Krivácsy
et al., 2001; Kiss et al., 2002; Salma et al., 2007) samples. The elemental
compositions were measured by energy-dispersive X-ray spectroscopy (EDS),
scanning transmission X-ray microscopy with near-edge X-ray absorption fine
structure spectroscopy (STXM/NEXAF), or different elemental analysis
techniques with or without direct oxygen measurement (EA, EA w O, EA w/o O).
However, when comparing the elemental compositions of our Lab-TBs to that of
HULIS, both the O / C (0.094–0.109) and the H / C (0.511–0.584)
molar ratios of our Lab-TBs were substantially lower than those reported for
HULIS samples (O / C: 0.455–0.563; H / C: 1.431–1.537)
(Krivácsy et al., 2001; Kiss et al., 2002; Salma et al., 2007). The
O / C molar ratio of other atmospheric TBs varies widely and in some
cases compares better with the O / C molar ratio of HULIS. The O / C
and H / C molar ratios of TBs identified by Pósfai et al. (2004) from
savanna fires, as well as our Lab-TBs (O / C: 0.094–0.109; H / C:
0.511–0.584) are close to the upper limit of those characteristic of soot
(O / C: ∼ 0.12; H / C: ∼ 0.38).
As expected from the above results, the mean carbon to mass conversion factor
of our Lab-TBs (1.21; RSD: 0.5 %) is between that of HULIS (1.81–1.93;
Krivácsy et al., 2001; Kiss et al., 2002; Salma et al., 2007) and soot
samples (1.04–1.15; Akhter et al., 1985; Clague et al., 1999).
Characterization by FT-IR spectroscopy
The FT-IR spectra of wood tar and Lab-TB samples are characterized by broad
and overlapping bands (Fig. 2). The figure also shows that the FT-IR
spectra of Lab-TBs produced from the three different wood species are much
more similar to one another than the spectra from different wood tars. Large
differences in the IR spectra of wood tars can be observed particularly in
the fingerprint region (1400–500 cm-1); in contrast, being exposed to
a heat shock, the Lab-TBs from different sources became chemically similar to
one another.
FT-IR spectra of (a) wood tars and
(b) laboratory-generated TBs (Lab-TBs) produced from different wood
species.
The FT-IR spectra of wood tars and Lab-TBs show a very broad band between
3600 and 3000 cm-1 (might be assigned to OH-stretching of phenol and/or
hydroxyl groups) and a smaller band in the region between 3000 and 2780 cm-1,
which can be attributed to asymmetric and symmetric C–H stretching of
methyl and methylene aliphatic groups (Coates, 2000; Graber and Rudich, 2006;
Yang et al., 2007). In the spectra of Lab-TBs the sp2-aromatic C–H
stretching at 3060 cm-1 (Coates, 2000; Cain et al., 2010;
Santamaría et al., 2006) is more pronounced than in the spectra of wood
tars, indicating the increased aromaticity. The spectra of wood tars and
Lab-TBs are dominated by two strong bands at ∼ 1700 and at
∼ 1605 cm-1, assigned to C=O stretching and C=C stretching of
aromatic rings (with overlapping C=O stretching), respectively (Coates,
2000; Graber and Rudich, 2006; Santamaría et al., 2006; Cain et al.,
2010). The ratio of these two bands is the opposite in the two sample types;
the intensity of the aromatic C=C stretching increases relative to the
C=O stretching in Lab-TBs.
The region between 1450 and 1380 cm-1 can be assigned to aliphatic or
aromatic methyl and methylene bending (Craddock et al., 2015; Coates, 2000;
Santamaría et al., 2006; Cain et al., 2010).
The possible aromatic C–C and C–H plane deformation bands in the region
between 1300 and 1000 cm-1 overlap with the band of the C–O single bond.
The broad band at 1220 cm-1 probably belongs to the C–O stretching of
phenolic hydroxyl groups in FT-IR spectra of wood tar and Lab-TB samples
(Coates, 2000; Yang et al., 2007), whereas the peaks at ∼ 920;
∼ 1040, ∼ 1110, and ∼ 1321 cm-1 might correspond to
the C–H bending of carbohydrate, to C–O stretch in the C–OH in
carbohydrate structure, to stretching of the C–O of the C–O–C linkage, and
O–H bending of C–OH group, respectively (Santamaría et al., 2006; Yang
et al., 2007; Cain et al., 2010; Carletti et al., 2010; Anjos et al., 2015).
By comparing the IR spectra of Lab-TBs with those of HULIS it can be
concluded that they show large-scale similarity, since the characteristic
bands, the aliphatic and aromatic C–H, aromatic C=C, hydroxyl and keto
groups (Krivácsy et al., 2001; Kiss et al., 2002; Duarte et al., 2005;
Graber and Rudich, 2006; Kristensen et al., 2015) are present, but the
intensity ratios of the C=O (∼ 1700 cm-1) and the C=C
(∼ 1605 cm-1) bands are the opposite. This implies that Lab-TBs
might have a higher proportion of aromatic structure than HULIS, and the
composition of HULIS is probably more similar to the wood tars in this
respect. Another difference between the spectra of Lab-TBs and HULIS is that
the HULIS spectra contain a very broad band (assigned to O–H stretching in
carboxyl group), which occurs at 3400 to 2400 cm-1 and often overlaps
with C–H stretching. Since this characteristic broad band is missing in the
spectra of both wood tar and Lab-TB samples, the presence of the carboxylic
groups in the samples was not confirmed.
The FT-IR spectra of Lab-TBs also differ from those of soot: the band
representative of the acetylenic group at 3300 cm-1 is absent in the
spectra of TBs and the spectra of soot do not contain the bands of
OH-stretching (Cain et al., 2010; Santamaría et al., 2006; Santamaria et
al., 2010).
Curve fit with five bands for the first-order Raman spectra
(excitation wavelength: λ0=532 nm) of
laboratory-generated tar ball (Lab-TB) particles, produced from (a) black
locust and (b) Norway spruce, using as proposed by Catelani et al. (2014) for
carbonaceous materials.
Raman spectroscopy
Raman spectroscopy was used to characterize the short-range order in the
molecular structure of Lab-TBs. Whereas Raman activity was not detected
(either because of the lower amount of the substances or because of their
chemical composition) in the wood tar samples in the range of 1000–1800 cm-1,
the Raman spectra of laboratory-generated TBs were dominated by
two pronounced bands with intensity maxima near 1580 and 1350 cm-1.
This double peak was deconvoluted by the five-band fitting procedure first
proposed by Sadeczky et al. (2005), but it was found that instead of using
four Lorentzian (G, D1, D2, D4) and one Gaussian (D3)
peaks, the best fit was obtained with five Voigt functions, similarly to
Catelani et al. (2014). The Raman spectra and examples for the peak
deconvolution (in the range between 1000 and 1800 cm-1) of the Lab-TBs
are shown in Fig. 3. The Raman spectra of the Lab-TBs generated from turkey
oak were not evaluated, since the peak fitting was uncertain due to a reduced
signal-to-noise ratio. The presence of the G band in the Raman spectra of
Lab-TBs indicates that Lab-TBs contain an aromatic layer built up from
sp2-hybridized carbon atoms, whereas the existence of the D bands points
to the presence of poorly organized carbonaceous materials. Kristensen et
al. (2015) investigated the Raman and IR spectra of different HULIS samples.
The Raman spectra of HULIS exhibited sloping backgrounds, and the presence of
a small peak at 1630 cm-1 was attributed to the stretching of
aromatics. The height of this peak was somewhat higher in the case of a
fulvic acid standard indicating the higher aromaticity of this compound
compared to the HULIS extracted from urban and rural samples. Ivleva et
al. (2007) investigated the Raman spectra of a humic acid standard and those
of soot samples. The obtained G and D bands were more pronounced in the
spectra of these components than in the spectra of the HULIS. The Raman
spectra of the macromolecular humic acid (purified, Carl Roth GmbH,
Karlsruhe, Germany) investigated by Ivleva et al. (2007) were very similar to
those of our Lab-TBs.
Py-GC-MS measurements
With Py-GC-MS of the Lab-TB samples, approximately 40 compounds were
identified (Table 2). The pyrolysis products were identified by comparison of
their mass spectra with the standard mass spectra in the NIST 02 Library
(NIST/EPA/NIH Mass Spectral Library, 2002).
Identified components from Py-GC-MS chromatogram of Lab-TBs
produced from different wood species, compared with the identified pyrolytic
products of hexane, gasoline, diesel, and wood soot (Song and Peng, 2010), and
humic acid extracted from particulate matter (Subbalakshmi et al., 2000).
Examined samples
Reference samples
Black locust
Norway spruce
Turkey oak
Hexane
Gasoline
Diesel
Wood
Humic
Name of pyrolytic compounds
Lab-TB
Lab-TB
Lab-TB
soot
soot
soot
soot
acid
Aromatic hydrocarbons
1-ring aromatic hydrocarbons
Benzene
×
×
×
×
o/m/p-Dimethylbenzene
×
×*
×
×
×
×
×
×
α-Methylstyrene
×
×
×*
×
Styrene
×
×*
×*
×
×
×
×
×
Toluene
×
×
×
×
×
×
×
2-ring aromatic hydrocarbons
Biphenyl
×
×
×
×
×
×
×
×
1,6-Dimethylnaphthalene
×
×
×
×
2,3-Dimethylnaphthalene
×
×
×
×
2,7-Dimethylnaphthalene
×
×
×
×
Indene
×
×
×
×
×
2-Methylindene
×
×
×
1-Methylnaphtalene
×
×
×
×
×
×
2-Methylnaphtalene
×
×
×
×
×
×
3-Methyl-1H-indene
×
×
×
Naphthalene
×
×
×
×
×
×
×
3-ring aromatic hydrocarbons
Acenaphthylene
×
×
×
×
Anthracene
×
×
×
Fluorene
×
×
×
×
×
×
1-Methyl-9H-fluorene
×
4-Methyl-9H-fluorene
×
Phenanthrene
×
×
×
×
4-ring aromatics hydrocarbons
Fluoranthene
×
×
×
Pyrene
×
×
×
×
5-ring aromatic hydrocarbons
Benzo[mno]fluoranthene
×
×
×
Oxygenated aromatics
1-ring oxygenated aromatics
Acetophenone
×
×
×
Benzaldehyde
×
×*
×*
×
2,4-Dihydroxy-3,6-dimethylbenzaldehyde
×
×
2,3-Dimethylphenol
×
×
×
×
2,4-Dimethylphenol
×
×
×
×
2,5-Dimethylphenol
×
×
×
×
2,6-Dimethylphenol
×
×
×
3,4-Dimethylphenol
×
×
2,6-Dimethoxyphenol
×
2-Ethylphenol
×
×
×
2-Methoxy-4-methylphenol
×
2-Methoxyphenol
×
2-Methylphenol
×
×
×
×
×
4-Methylphenol
×
×
×
×
×
Phenol
×
×
×
×
×
×
×
×
2-ring oxygenated aromatics
2,3-Dihydro-1H-inden-1-one
×*
×
×*
Phthalic acid anhydride
×
×
×
×
Continued.
Examined samples
Reference samples
Black locust
Norway spruce
Turkey oak
Hexane
Gasoline
Diesel
Wood
Humic
Name of pyrolytic compounds
Lab-TB
Lab-TB
Lab-TB
soot
soot
soot
soot
acid
Oxygen-containing heterocyclic aromatics
1-ring oxygen-containing heterocyclic aromatics
3-Furancarboxaldehyde
×
×
5-Methyl-2-furaldehyde
×*
×
×*
×
2-ring oxygen-containing heterocyclic aromatics
2-Methylbenzofuran
×
×
×
7-Methylbenzofuran
×
×
×
Benzofuran
×
×
×
×
×
×
3-ring oxygen-containing heterocyclic aromatics
Dibenzofuran
×
×
×
×
* The concentration of the given component is the same as on the back-up filter in this sample.
In the pyrograms of the Lab-TB samples, mainly aromatic compounds have been
identified in accordance with the results of Raman and FT-IR spectroscopy,
and indirectly with the results of the elemental analysis. Aromatic
hydrocarbons (benzene, alkyl-, alkenyl-substituted benzenes) and smaller
(2–3 aromatic rings) polycyclic aromatic hydrocarbons (PAHs), oxygenated
aromatics (phenol, alkyl-substituted phenols) and heterocyclic aromatics
(phthalic anhydride, furan, benzofuran, dibenzofuran and their derivatives)
were identified. Many of the above-mentioned components have been identified
using the same analytical technique from humic acid (extracted by sodium
hydroxide solution and precipitated with hydrochloric acid from urban
aerosol) and different (hexane, gasoline, diesel and wood) soot samples (see
Table 2) in previous studies (Subbalakshmi et al., 2000; Song and Peng,
2010). Comparing the chromatograms of humic acid, Lab-TBs and soot,
significant differences occur in the quality (different numbers of aromatic
rings) and quantity (number of compounds) of PAH components. The pyrograms
of humic acid contain only few small (2-ring) PAHs, e.g. naphthalene and
its derivatives (methyl- and dimethyl-naphthalene), whereas in the pyrograms
of the Lab-TBs larger (3-ring) PAHs can be also found. On the other hand, in
the pyrograms of soot samples (Song and Peng, 2010) both smaller (2–3-ring)
and larger (4–5-ring) PAHs were also found, but the latter compounds were
not detected in the pyrograms of Lab-TBs.
OC / EC thermal–optical analysis
Since TBs belong to the BrC fraction of carbonaceous aerosol (Hoffer et al.,
2016), their EC content is expected to be very small or even negligible.
Since the results of Raman spectroscopy indicated some structural
similarities with atmospheric soot, we determined the apparent EC content of
Lab-TBs by standard OC / EC analysis. The results of OC / EC
thermal–optical analysis of Lab-TBs which were produced from three different
wood species are given in Table 3.
Organic carbon (OC), elemental carbon (EC), total carbon (TC)
content and ratio of elemental carbon to total carbon content (EC / TC)
(RSD% of 3 Lab-TB samples) of laboratory-generated tar balls (Lab-TBs)
on quartz filters (spot ∅ 13.06 mm) obtained by the EUSAAR_2 protocol.
OC
EC
TC
EC / TC
[µg cm-2]
[µg cm-2]
[µg cm-2]
Lab-TB – black locust
9.0
4.2
13.2
0.32
Lab-TB – Norway spruce
14.1
2.9
17.1
0.17
Lab-TB – turkey oak
14.2
2.9
17.1
0.17
Lab-TB sample average
0.22 (RSD: 39 %)
The EC / TC (total carbon) ratio for Lab-TBs varied from 0.17 to 0.32 (on average: 0.22; RSD:
39.1 %), which is far from being negligible, contrary to expectations. It
is also important to note that there is an uncertainty in the position of the
split point in the OC / EC measurement. In the case of the TBs, the
criterion that OC should be non-absorbing is not met; thus, the absorption of
BrC lowers the baseline of the transmittance. Consequently, the split point
is set earlier and a larger EC signal is measured (Chen et al., 2015). The
thermogram of TBs produced from Norway spruce shows that the detector signal
returns to the baseline neither after the fourth OC peak nor after
the pyrolytic carbon peak, and thus a notable fraction of pyrolytic carbon
(PC) is identified as EC (see Fig. 4). According to Piazzalunga et
al. (2011) the EC / TC ratio of the water-soluble fraction of urban background
aerosol (which contains the HULIS fraction as well) measured with the
EUSAAR_2 protocol was 0.02. On the other hand Han et
al. (2007) investigated the EC / TC ratio of different SRM soot and chars with
the IMPOOVE TOR method. For the soot samples EC / TC ratios of 0.68–0.96 were
obtained, whereas values for the char samples ranged between 0.53 and 0.85.
Thermogram from analysis of Lab-TBs produced from Norway spruce,
measured by thermal–optical analysis (TOA) obtained by the
EUSAAR_2 protocol.