Laser Induced Breakdown Spectroscopy of Diesel Particulate

Matter Exhaust Emissions Generated from on Road Diesel Engine:

Light Duty Vehicles

Richard Viskup, Christoph Wolf and Werner Baumgartner

Institute of Biomedical Mechatronics, Johannes Kepler University Linz, Altenberger str. 69, 4040 Linz, Austria

Keywords: Laser Induced Plasma, Laser Induced Breakdown Spectroscopy, LIBS, Laser Induced Plasma Spectroscopy,

LIPS, Optical Emission Spectroscopy, Particulate Matter, PM, DPM, Soot, Black Carbon, Carbon Black,

Diesel Combustion Engine, Engines, Emissions, Diesel Emissions, Diesel Exhaust, Diesel, Diesel Engine,

WHO, Air Quality.

Abstract: In this research we apply Laser Induced Breakdown Spectroscopy (LIBS) technique for high resolution

spectrochemical analysis of Diesel Particulate Matter - DPM exhaust emissions. DPM has been collected

from real, on road - Light - Duty Vehicles, driven by combustion Diesel engine. We have been concerned

with the main chemical elements, presents in various type of real Diesel particulate matter. From LIBS

measurements, it has been shown, that the plasma electron density can be use for the basic classification of

different types of DPM matrices. The excitation temperatures of atoms and ions in plasma can be use for

further quantitative analyses of diverse Diesel Particulate Matter. The aim of this study is to reveal the

compounds, which are mostly dominant in the Diesel engine exhaust emissions and can affect the overall

composition of the DPM. The presence of these elements in exhaust emission may point to different

processes, mainly to fuel quality, insufficient engine combustion process, incomplete catalytic reaction,

inefficient Diesel particulate filtering technique, or failure of the Diesel engine.

1 INTRODUCTION

Diesel combustion engine driven vehicles are

currently failing to follow the Euro 6 vehicle

emission standards in real driving environment, due

to the strict emission norms (Ntziachristos, 2016;

Zacharof, 2016; Commission Regulation EU

2016/646). The current existing emission standards

Euro 6 (Commission Regulation EC 692/2008;

Regulation EC 715/2007), Tier 3 (United States

Environmental Protection Agency, Regulations) or

LEV III (California Environmental Protection

Agency), for Diesel engine passenger vehicles are

the norms for hydrocarbons, carbon monoxide,

nitrogen oxides and for particulate matter (PM) from

Diesel exhaust emissions, as the total number of all

particles.

However, there are no other emission standards

for additional compounds or chemical elements

contained in the exhaust gas, Diesel particulate

matter, PM, or in the soot formed from the Diesel

combustion engine. Even though these chemical

elements additional to Carbon, present in the

particulate matter, forms very significant fraction of

the total DPM or the soot emissions content.

In this research we apply Laser Induced

Breakdown Spectroscopy technique (Noll, 2012;

Miziolek, 2006; Cremers, 2006) for diagnostics of

DPM, formed from combustion Diesel engine

exhaust emissions, mainly concerning the detection

of main chemical elements presents in various DPM

matrices.

Laser Induced Breakdown Spectroscopy is an

emerging measurement technique (Hahn, 2012) for

rapid qualitative (Noll, 2014) and sensitive

quantitative compositional analysis (Fortes, 2013;

Wang, 2016) of various forms of materials like

solids (Viskup, 2012), liquids (Samek, 2000), gases

(Effenberger, 2010), powders (Stehrer, 2009) or

nanoparticles (Viskup, 2008).

The aim of this study is to measure the main

compounds, that are present in these exhaust

emissions and can mostly affect the chemical

composition of the DPM. The presence of these

elements in exhaust emissions may point to different

processes, mainly to insufficient engine combustion

308

Viskup, R., Wolf, C. and Baumgartner, W.

Laser Induced Breakdown Spectroscopy of Diesel Particulate Matter Exhaust Emissions Generated from on Road Diesel Engine: Light Duty Vehicles.

DOI: 10.5220/0007618203080314

In Proceedings of the 7th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2019), pages 308-314

ISBN: 978-989-758-364-3

process, unburned Diesel, incomplete catalytic

reaction, inefficient Diesel particulate filtering

technique, or failure of the Diesel engine.

2 EXPERIMENTAL SECTION

LIBS Setup

For Laser Induced Breakdown, the Nd:YAG solid

state laser from Quantel has been used. It has been

operated at the fundamental laser wavelength

1064nm with 8.5ns pulse duration and laser energy

300mJ per pulse. The laser radiation has been

focused with 10cm focusing lens into the plane solid

target surface to create plasma. Optical emission

from plasma has been collected perpendicularly via

optical telescope into the high resolution Echelle

spectrograph model Aryelle Butterfly from LTB

Berlin equipped with ICCD detector. Spectrometer

consists of two separate spectrographs, one part for

UV range from 190nm to 440nm and the second part

for VIS optical spectrum in range 440nm to 800nm.

Spectral resolution capability is from 3pm to 7pm

for VUV part and from 4pm to 8pm for VIS part,

thus providing spectral information of a broad

spectral range with very high resolution and

variability. Optical emission from plasma has been

collected from VUV as well as from VIS parts, thus

the total spectral window from 190nm to 800nm

wavelength has been recorded. The delay time 1μs

after the laser trigger and gate width 2μs were

always kept constant, as well as all experimental

parameters during the measurements. In earlier delay

time as 1μs the black body radiation is dominating

in laser plasma, while in time later than 3μs the

atomic and ionic emissions are decaying. The LIBS

emission has been recorded in open air atmosphere

under atmospheric pressure and at room

temperature.

Figure 1: Layout of LIBS experimental setup.

Sample Preparation and Collection

More than 60 different samples from real Diesel

engine passenger vehicles of major brand car

producers in Europe have been analysed by LIBS.

Diesel Particulate Matter has been collected from the

tail pipe at the end of the exhaust manifold, after the

Diesel Particulate Filter (DPF), if it was applied.

Selections of the vehicles were performed randomly

and no company was given preference. The results

presented here are the selections of eight diverse

DPM matrices. Laser induced plasma spectroscopy

reveal optical emission lines that are characteristic

for UV and VIS spectral region. The collected

particulate matter from Diesel engine Light - Duty

vehicles exhaust has been mechanically pressed into

pellets with flat disc shape. Each displayed LIBS

spectrum has been averaged over twelve laser shots.

3 RESULTS AND DISCUSSION

3.1 Identification of the Main Matrix

Elements in DPM

Optical emission spectras from Laser Induced

Breakdown Spectroscopy measurement of Diesel

particulate matter obtained from selected eight

matrices, are shown in the Figure 2(a-h).

Diesel particulate matter is characterised by

strong optical emission from a) Carbon, b) Iron, c)

Magnesium, d) Aluminium, e) Chromium, f) Zinc,

g) Sodium and h) Calcium. Spectra shown here are

characteristic optical emission lines, dominating in

the LIBS spectral signal from 200nm to 800nm.

From figures 2(a-h) it is evident that the

chemical composition of selected eight matrices

differ considerably from each other. This is due to

the different origin of each DPM sample, and due to

the unique composition of the exhaust emissions

from Diesel engine vehicles. In fact, the source of

different compositions is the combination of the

Diesel fuel quality, composition of the intake air,

quality of the combustion process, type of the

engine, or performance of the engine. Other parts

that influence the total composition of DPM are

applied aftertreatment devices, like Diesel particle

filters (DPF) or catalysts like Selective Catalytic

Reduction devices. All count to the final chemical

composition of DPM.

Laser Induced Breakdown Spectroscopy of Diesel Particulate Matter Exhaust Emissions Generated from on Road Diesel Engine: Light Duty

Vehicles

309

Figure 2: Optical emission spectras generated from Diesel

particulate matter measured by high resolution laser

induced plasma spectroscopy shows high content of: a)

Carbon, b) Iron, c) Magnesium. d) Aluminium, e)

Chromium, f) Zinc, g) Sodium and h) Calcium species.

3.2 Comparison of Different Diesel

Particulate Matter

From the optical emission spectra shown in the

Figure 2(a-h) we selected atomic and/or ionic

spectral lines that have a major impact to the line

intensity for each Diesel particulate matter matrix. In

the Figure 3(a-h) comparison of DPM samples with

high measured content of 1. Cr, 2. Ca, 3. Zn, 4. C, 5.

Na, 6. Fe, 7. Mg, 8. Al and calculated spectral peak

area of: a) Carbon, b) Iron, c) Magnesium, d)

Aluminium, e) Chromium, f) Zinc, g) Sodium, h)

Calcium - atomic or ionic lines are shown. Here, an

individual bar represents calculated peak area of

selected spectral atomic or ionic line. These have

been obtained after base line correction and

calculation of the fitted peak area under the spectral

line. From the bar graphs Figure 3, it is possible to

obtain relative values of the concentration of

chemical elements, presents in the DPM samples.

Two types of information can be obtained, by either

horizontal or vertical reading of this bar graph.

From horizontal reading of bar graph - Figure

3(a-h) it is possible to observe that Carbon (a)

content is not constant in DPM samples, but its

concentration rather change in the individual

samples (1-8). Iron (b) concentration also varies

from low to high in different matrices. Magnesium

matrices ( 6.Fe, 8.Al ) posses high value of

Aluminium (d). Chromium (e) as well as Zinc (f)

content plays important role within the DPM

matrices too. Diesel particulate matter contains also

Sodium (g) and Calcium (h), and its concentration

can be relatively high too.

From vertical reading of this bar graph Figure

3(a-h) it is possible to obtain information about

relative concentration of different elements in each

DPM matrix. Particularly, sample 1 consists of

relatively high level of Iron (b) and Chromium (e).

Sample 2: high level of Carbon (a), Zinc (f) and

Calcium (h). Sample 3: high level of Carbon (a),

Magnesium (c) and Zinc (f). Sample 4: higher level

of Carbon (a) and Magnesium (c). Sample 5

relatively high level of Iron (b), Magnesium (c),

Sodium (g) and Calcium (h). Sample 6: higher level

of Iron (b) and Aluminium (d). Sample 7: higher

level of Carbon (a), Iron (b) and Magnesium (c).

Sample 8: higher level of Iron (b) and Aluminium

(d).

Figure 3: Comparison of eight Diesel particulate matter

samples with mostly pronounced content of 1. Cr, 2. Ca, 3.

Zn, 4. C, 5. Na, 6. Fe, 7. Mg, 8.Al. Number indicate the

sample #, and element name indicate the main element

content in DPM matrix.

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3.3 Calculation of the Plasma Electron

Density

In the case of thermal plasma, and in first

approximation, the total width of the line profile

mainly depends on electron density (Griem, 1997).

Thus a direct measurement of line profile, for which

the Stark effect is predominant, leads to electron

density, independent of the local thermal equilibrium

condition. Calculation of the plasma electron density

can be obtained from Stark broadening of H(α)

line by applying following formula (Gigosos, 2003):

(

)

0.67965

23 3

0.549

FWHA nm

−

=×

(1)

where FWHA shows the full width high amplitude

of the H(α) line broadening at 656.27 nm. Profiles

of H alpha spectral lines obtained from individual

DPM matrices with high C, Fe, Mg, Al, Cr, Zn, Na,

Ca content are shown in Figure 4.

Figure 4: Comparison of H alpha lines for various DPM

matrices.

From H(α) line broadening in the Figure 4 and

equation (1), the electron concentration n

has been

calculated in interval from 6.6 x 10

-3

to 8.1 x

-3

. Highest electron concentration n

= 8.11

x 10

-3

and n

= 7.97 x 10

-3

has been

obtained from sample with high content of Calcium

and Sodium respectively. Moderate electron density

from plasma were measured in samples with high

content of Magnesium n

= 7.46 x 10

-3

, Carbon

= 7.49 x 10

-3

, Aluminium n

= 7

.39 x

-3

, Iron n

= 7.07 x 10

-3

and Chromium

= 7.22 x 10

-3

. Low electron concentration in

plasma was obtained from sample with high content

of Zinc n

= 6.62 x 10

-3

. The comparison of

reached electron density in laser induced plasma

from Diesel particulate matter is shown in Figure 5.

Figure 5: Comparison of electron density n

from laser

produced plasma obtained from eight different DPM

matrices. (numbers shown in bar graph are in 10

-3

From the Figure 4 and 5 we can observe that

Diesel particulate matter respond to the same laser

irradiation conditions with different electron density.

This point to distinct type of plasma property for

each DPM matrix. However, very similar value of

electron density has been measured in case of 1.Cr,

6.Fe and 8. Al samples (shown with dotted line in

Figure 5). These are the samples, with measured

high concentration of Fe content. Here we can

conclude that electron density in laser-produced

plasma is alternating according to the matrix type

and chemical composition of DPM. Therefore, it can

be use for basic classification of different DPM

matrices.

3.4 Calculation of the Excitation

Temperature

If we assume the local thermal equilibrium in laser

plasma, excitation temperature T

exc

can be obtained

from the slope of the Boltzmann plot by calculating

the ratio of the relative atomic line intensities,

emitted from different excited energetic levels by

using the following formula:

ln ln

... ()

i u ul ul i exc

gAh ZT kT



=−





(2)

where I

u ,l

is optical emission line intensity, g

i,u

the statistical weight of the upper excited state of the

chemical species i, A

u,l

is the corresponding

transition probability per unit time, h is the Planck’s

constant; ν

u,l

is the frequency of the photons emitted

due to transition from upper excited level u to the

Laser Induced Breakdown Spectroscopy of Diesel Particulate Matter Exhaust Emissions Generated from on Road Diesel Engine: Light Duty

Vehicles

311

lower level l; F is the factor depending upon

experimental setup; n

is the concentration of the

chemical species i; Z

is the partition function of the

chemical species i calculated at T

exc

, E

i,u

is the

energy of the upper excited state of the chemical

species i; k is the Boltzmann constant. For

calculation of excitation temperature T

exc

the

background corrected relative intensity of iron

atomic lines, emitted at three different excited

energetic levels have been used. In Table 1

spectroscopic parameters of atomic iron used for

construction of Boltzmann plot are given. Data have

been obtained from NIST atomic spectra database

(Kramida, 2015). In Figure 6, different Boltzmann

plots for DPM samples with high content of Cr, Zn,

Na, Fe, Mg and Al are shown.

Table 1: Spectroscopic parameters used for Boltzmann

plot.

Figure 6: Boltzmann plots and linear fit for determination

of T

exc

for Iron atoms, from DPM samples with high Cr,

Zn, Na, Fe, Mg and Al content.

From Boltzmann plot, in Figure 6 we can

observe that one sample with increased Zinc content

(3. Zn), data point lie outside of the linear curve.

This is due to the relatively weak iron spectral line,

measured at this wavelength. With lower

concentration of iron species in this sample, the

spectral line intensity decreases and became less

pronounced. Therefore, it was not possible to

construct the Boltzmann plot and calculate the

excitation temperature for the 2. Ca and 4. C sample,

where the concentrations of iron species are even

lower. It would be necessary to consider different

spectral lines. Figure 7 shows the comparison of

plasma excitation temperatures obtained for iron

atoms from DPM samples. From the linear fit and

slopes of the Boltzmann plots, plasma excitation

temperature T

exc

for Fe atoms has been calculated in

interval from 6774 K to 7953 K. Samples with

higher concentration of Fe species have lower

excitation temperature e.g. T

exc

(6. Fe) = 6774 K

compared to samples with low concentration of Fe,

where temperature needed for excitation of these

atoms was T

exc

(3. Zn) = 7953 K. The excitation

temperature of Fe atoms in plasma is related to the

Iron concentrations in the Diesel Particulate Matter.

Therefore T

exc

parameter can be use for quantitative

measurements of DPM. However calculation of the

Boltzmann plots and calibration functions are

necessary preconditions for each element present in

the DPM matrix.

Figure 7: Comparison of the calculated excitation

temperature of iron atoms from eight DPM matrices with

high measured Cr, Zn, Na, Fe, Mg and Al content.

4 CONCLUSIONS

We have performed laser induced breakdown

spectroscopy (LIBS) measurements from more than

60 different samples of Diesel Particulate Matter.

DPM were obtained from real, on road - Light -

Duty Diesel engine vehicles. Selections of on road

passenger vehicles were performed randomly from

major brand car producers in Europe. We found that

DPM does not consist of purely / mainly carbon

particles. However DPM contains many additional

compounds - chemical elements with various

concentrations. Indeed, we can classify Diesel

Particulate Matter into samples with high

concentration of Carbon, Iron, Magnesium,

Aluminium, Chromium, Zinc, Sodium and Calcium

content. These elements, form major compounds of

DPM matrix. With the use of laser induced

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breakdown spectroscopy, we can very precisely

measure elements that are majorly presents in

different DPM. The major compounds that are well

presents in the DPM are Carbon, Magnesium,

Sodium and Calcium. The other major compounds

that are also presents in the DPM are Iron,

Aluminium, Chromium and Zinc. The

concentrations of these elements are changing

according to the Diesel engine vehicle. In this paper,

quantitative elemental analysis of DPM was not an

object. Instead rather qualitative, showing the major

chemical elements of different DPM matrices. We

have shown individual LIBS spectra's from eight

matrices. These are characterised with high

concentration of C, Fe, Mg, Al, Cr, Zn, Na and Ca

content. We have shown the basic laser plasma

properties obtained from various DPM matrices, and

we found that electron density n

in laser induced

plasma varies according to the DPM matrix.

Therefore it can be use for basic classification of

different types of DPM. This has been confirmed by

the calculating of the excitation temperature T

exc

iron atoms in DPM plasma from Boltzmann plots.

The excitation temperatures of atoms and ions in

plasma can be use for further quantitative analyses

of diverse Diesel Particulate Matter.

Here we have revealed the main chemical

elements presents in the various DPM matrices.

However further research is necessary to obtain

detail picture about the quantitative composition of

these elements. Understanding the chemical

composition of DPM can help to better control the

engine, as well as combustion process and thus

reduce unwanted emissions generated from Diesel

engine vehicles to meet future environmental

emission standards.

ACKNOWLEDGEMENTS

Authors would like to thank to the Austrian Science

Fund - FWF for providing financial support under

the project number [FWF, P 27967]. Additionally

authors would like to thank to Dr. Maria Rusnak for

the proofreading and for the corrections.

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