Chapter 6. Chemical composition of dust particles
Chapter 6
Chemical Composition of Dust
Particles
6.1
Introduction
The potential role of mineral dust particles as heterogeneous ice nuclei is well
known (see section 1.4.1).
In order to determine their role in cloud physical
processes knowledge of on the physico-chemical properties of individual particles is
essential.
These properties of the IN surface play a major role in ice nucleation and can be
understood through Environmental Scanning Electron Microscope - Energy
Dispersive X-ray (ESEM-EDX) analysis. In this chapter the surface chemical
composition of individual and bulk dust particles from four different dust source
regions is investigated using the ESEM-EDX. Section 6.2 describes the ESEM-EDX
instrument and particle analysis procedure. The morphology and elemental analysis
of the dust particles is discussed in Section 6.3 and 6.4. The chapter is concluded
with discussion on possible chemical reactions that can occur on the mineral dust
surfaces, in Section 6.5.
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Chapter 6. Chemical composition of dust particles
In the present work, samples were collected at ground level from three different
locations in West Africa: the locations are Nigeria (1oE, 15oN), Dakar (outside the
city), Dakar (near the airport), during African Monsoon Multidisciplinary Analyses
(AMMA) campaign, and one location from Spain (South East coast). All these
samples are stored in inert, leak proof plastic bottles.
6.2
Particle elemental analysis
Particle elemental analysis is performed using an Environmental Scanning Electron
Microscope (ESEM) Philips-XL30 equipped with Energy Dispersive X-ray Analysis
(EDX) at the School of Process, Environmental and Materials Engineering,
University of Leeds, UK. The system consists of a SATW ultra-thin window which
has a spectral resolution of 137 eV at 5.9 keV, and is initially calibrated with internal
standards. The calibration of the instrument was repeatedly checked by the operator
using cobalt standard.
The typically ESEM/EDX analysis of particles was done on polycarbonate filters or
solid carbon substrates. This set up provides poor information on the carbon,
oxygen, and nitrogen content of a particle and usually elements with Z  11 are
considered in the X-ray spectra evaluation. The X-rays of unique element are
detected once the dust surface is bombarded with electrons.
The filament current was adjusted to receive 5000-6000 X-ray counts per second at
an acceleration voltage of 20 keV. The integration and acquisition time for every Xray spectrum was set at 100 s and 120 s respectively.
The quantitative accuracy of this technique is at best of the order of a few percent
(Fletcher and Small, 1993). Single-particle analysis by nature has large uncertainties
and there can be errors in the derived elemental fractions. Error can occur in the
fitting of each spectrum, and uncertainties in the spectra can arise from the
particular abundance of the element, the acceleration voltage (changes background
of the spectra mostly due to a filter influence), detector properties, dead time of the
X-ray detector and also count rate (personal communication with Research
Technician Dr. Richard Walshaw). There are also errors introduced by the three-
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Chapter 6. Chemical composition of dust particles
dimensional analysis surface. The EDX program (INCA X-SIGHT) assumes the
samples are flat and that the take-off angle is well described. However in practice
the samples are deposited on a substrate. This error could be reduced by polishing
the samples and making them perfectly flat. The overall error is difficult to determine,
but it can be in the range of 10% depending on the element and sample. Prior to the
present experiment dolomite (CaMg(CO)3) dust is used to calibrate the ESEM-EDX
system.
When the dust particles were analyzed for chemical composition they were
categorized into three groups. The first groups, approximately 20-30 particles, were
scanned for bulk composition. The second group, which involves the scanning of
single particles for single particle composition. In the third group a small area
(between 9 to 16 µm2) over the surface of the same dust particle is scanned for
composition. For each group, 5 images were obtained for image analysis. The
electronic images were saved as TIF files. The EDX data was subsequently then
downloaded into Microsoft Excel in order to compare the atomic weight percentages.
6.3
Microscopic morphology
When carrying out analysis using the ESEM/EDX, the field of view of the electron
microscope can be varied. Figure 6.1a to 6.1c shows typical high resolution
scanning electron microscope image(s) of an individual mineral dust particle. These
images are also analyzed for chemical composition, as described in the next Section
6.4
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Chapter 6. Chemical composition of dust particles
a
Fig 6.1 a
b
Fig 6.1 b
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Chapter 6. Chemical composition of dust particles
c
Fig 6.1 c
Figure 6.1 a to c: Showing the high resolution scanning electron microscope images
of individual and bulk mineral dust particles. Figure, 6.1a and 6.1b show the images
obtained for individual dust particles and 6.1c shows bulk dust particles. Different
sizes of particles are present in the bulk dust particles.
Fig 6.1a and 6.1b show the detailed surface features of the dust particles. The
irregularities of the dust surface may be due to erosion and weathering at the source
region. Such irregularities over the surface might be responsible for the change in
the surface free energy and reactivity with different chemical compounds, discussed
in Section 6.5. Over the dust surface many cracks and steps can be viewed which
might favour the deposition of different phases of chemical substances [Kärcher and
Lohmann, 2002]. In atmospheric clouds these cracks might fill up with water from
the vapour phase and enhance the activation of the dust particle.
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Chapter 6. Chemical composition of dust particles
6.4
Elemental analysis of dust particles
ESEM-EDX analysis of the mineral dust particles revealed the presence of Si, Al,
Mg, K, Ca, S and Fe elements. Along with these elements, trace amounts of Na, Cl
and Ti elements were also detected. The typical ESEM-EDX spectra of individual
dust particles and the associated spectra of small view (area between 9 to 16 µm2)
is shown in the Figure 6.2. The ESEM-EDX spectra of single dust particles shows
the particles were of mixed elemental composition. This is in agreement with the
studies by Shi et al., [2005] and Reid et al., [2003], who observed that dust particles
from arid regions were often aggregated.
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Chapter 6. Chemical composition of dust particles
Counts
a) Single Particle Spectra
Energy (keV)
Counts
b) Spectra of small view over the single particle
Energy (keV)
Fig. 6.2 (I): The spectra from Location Dakar Airport
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Chapter 6. Chemical composition of dust particles
Counts
a) Single Particle spectra
Energy (keV)
Counts
b) Spectra of small view over the single particle
Energy (keV)
Fig. 6.2 (II): The spectra from Location Dakar
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Chapter 6. Chemical composition of dust particles
Counts
a) Single Particle spectra
Energy (keV)
Counts
b) Spectra of small view over the single particle
Energy (keV)
Fig. 6.2 (III): The spectra from Location Nigeria
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Chapter 6. Chemical composition of dust particles
a) Single Particle spectra
Ca
Counts
O
Si
Al
Ca
Mg
S
2
1
K
Fe
3
4
5
Fe
6
7
Energy (keV)
b) Spectra of small view over the single particle
Ca
Counts
O
Si
Al
Ca
Mg
K
S
1
2
Fe
Ti
3
4
Energy (keV)
Fig. 6.2 (IV): The spectra from Location Spain
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Fe
5
6
7
Chapter 6. Chemical composition of dust particles
Figure 6.2: The EDX spectra of a single dust particle together with the associated
small view (area between 9 to 16 µm2) over the same dust particle from four
locations (labeled I to IV). It can be seen that the spectra in both the cases, a) and
b), are nearly similar, signifying the chemical composition homogeneity over the
entire dust surface.
After performing single particle analysis a small view is selected over the same dust
particle surface for analysis. The spectra obtained from both these provide
information about the chemical composition inhomogeneity over the entire dust
surface. From Fig 6.2 it can be seen that the spectra from both cases are identical in
terms of elements with little variation of counts, shown on the y-axis. With this
information it can be inferred that the chemical composition over the entire dust
surface is relatively homogeneous. The importance of chemical composition
homogeneity is discussed in the Chapter 7.
6.5
Average elemental composition
Approximately 400-500 particles were analyzed to determine the bulk elemental
composition of the dust samples. From each source region four samples were
randomly chosen to determine the bulk elemental composition. Each chosen sample
consists of 25-30 dust particles and their average elemental composition was
collected from four locations and is listed in the Table 6.1.
Table 6.1: The average atomic weight percent fraction of different elements in the
bulk mineral dust particles collected from the four dust source regions.
Source
Regions
Atomic Percent
% Si
% Al
% Mg
% Ca
% Na
% Fe
Dakar Airport
47
13
3
14
5
12
Dakar
51
13
3
8
5
11
Nigeria
65
15
1
3
3
8
Spain
22
7
2
60
3
4
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Chapter 6. Chemical composition of dust particles
The composition is expressed in terms of the atomic percent of Si, Al, Mg, Ca, Na
and Fe. Trace amounts (<1%) of other elements including P, K, Ti, Na and Cl were
also observed in the analysis but are not presented in Table 6.1. The data from
Table 6.1 show that variation exists in the amount of elements of dust from each
source region. Nigerian dust has more Si but less Ca compared with the Dakar and
Dakar airport dust samples. The Dakar and Dakar airport dust particles were also
associated with a higher Fe content which may explain why these particles appear
reddish in colour. Coastal dust particles have a some large percentage amount of
Na compared with other source locations, which might be due to the fact that there
may be some sea salt mixed with the dust particles. The Spanish dust had the
highest percentage of Ca (= 60 %) compared with other source locations, which
might have been derived from carbonate minerals. The significance of a larger
amount of Ca is described in Section 6.6. Mineralogical information to use in the
dust modelling studies can be derived from the bulk and single-particle analysis. For
the procedure, Reid et al., (2003) can be referred to.
6.6
Reactivity of mineral dust particles
Dust particles are sometimes sampled in the troposphere without significant sulfate
or other condensed components (DeMott et al., 2003), but these particles can also
become coated with aqueous solutions of sulfate, nitrate and sea-salt when passing
through polluted or marine regions.
Mineral dust particles dispersed throughout the atmosphere provide reactive
surfaces for a variety of chemical and physical processes. In Table 6.1 Spanish dust
is observed to contain a comparatively large amount of calcium and the dust
particles containing high levels of calcium are found to be reactive with respect to
nitric acid [Dentener et al., 1996]. Nitrates are hygroscopic in nature and deliquesce
at low relative humidity with respect to water [Tang and Fung, 1997, Al-Abadleh et
al., 2003]. Therefore the dust particles with an abundance of calcium, once
transported to high altitudes and longer distances might be neutralized with nitric
acid. These particles would then have the ability to serve as effective CCN and IN in
the appropriate atmospheric conditions. The association between calcium and
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Chapter 6. Chemical composition of dust particles
nitrate has been observed in several studies [Zhuang et al., 1999; Pakkanen, T. A.,
1996].
Table 6.1 shows that the African dust contains less calcium compared to Spanish
dust. Cziczo, et al., [2004], reported that African dust particles sampled inside cirrus
ice clouds had less soluble matter such as sulfates and nitrates. This suggests that
the African dust is less susceptible to neutralization by nitric acid or may not even
reactive with nitric acid.
Mineral dust particles also provide a reaction surface for SO2, ozone and organic
compounds. Dust particles passing over polluted areas are often coated with sulfate
due to the chemical processes which occur on their surface [Levin et al., 1996]. SO2
is the major sulfur containing anthropogenic pollutant, with urban concentrations
reaching into hundreds of parts per billion [Seinfeld and Pandis, 1998]. SO2 acts as
a precursor to sulfuric acid, which contributes to acid rain and particulate formation.
Nearly half of the global emissions of SO2 are converted to particulate sulfate, and
this sulfate is often associated with particles, either sea-salt in marine regions or
mineral dust [Levin et al., 1996]. It is well-known that SO2 can be oxidized to sulfate
by ozone and hydrogen peroxide, but the mechanisms of sulfate formation on
mineral aerosol is not completely understood [Luria et al., 1996; Fung et al., 2000].
More research is necessary to quantify the role of mineralogical elements in sulfate
chemistry.
The stratospheric ozone layer shields the surface of the Earth from the harmful UV
component of solar radiation. On the other hand, in the troposphere ozone is a
pollutant, irritant and under certain conditions can cause harmful effects to
vegetation [Fuhrer, J., 2002]. The loss mechanism for ozone in the troposphere is
mainly (~75%) by photolysis, with the rest occurring mainly by reaction with HO 2
[Seinfeld and Pandis, 1998]. However, recent modelling and field studies have
shown a correlation between ozone loss and high dust events [Dentener et al., 1996,
Prospero et al., 1995], suggesting the interaction of ozone with mineral dust
particles. Reactions with trace metals present in the mineral dust particles (e.g. Fe
and Mg) and the oxidation of organic material by ozone might explain the ozone
uptake by dust particles [Dentener et al., 1996]. It should be noted that dust contains
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Chapter 6. Chemical composition of dust particles
approx. 4 to 10% of iron by mass, depending upon the source origin and the amount
of particulate organic matter.
Organic matter is emitted from both marine and continental environments. The
organic material found in the troposphere is often associated with dust particles
[Buseck, et al., 1999, Murphy et al., 1998, Posfai 1998 and Lee 2002] observed that
particles containing elemental components (Al, Si, Fe, Ca, and other oxides) also
contained water-soluble organic acids. Falkovich et al., [2001] also observed that
organic material was associated with mineral dust.
In conclusion, the dust particle provides a reactive site for many heterogeneous
reactions involving many chemical species (including water vapour). Various
compounds can absorb mineral dust particles to form coatings and can alter the
chemical character of the particles which ultimately may enhance scattering and
absorption of solar radiation.
91