Abstract

Kosa (yellow sand) aerosol affects global environment as well as human health because

nearly a billion ton of it flies from interior China to wide areas absorbing various atmospheric

elements. Investigation into individual Kosa aerosol particles, which are from sub- to several tens

of micrometers in diameter, is demanded for resolving the Kosa issue.

We installed a scanning electron microscope (SEM) equipped with an energy dispersive

X-ray spectrometer (EDX) on a synchrotron radiation (SR) beamline for application of SEM-

EDX and SR X-ray fluorescence (SR-XRF) spectrometry to individual micrometer-size Kosa

aerosol particles by introducing SR microbeam into the SEM. It should be noted that detailed

topographic observation by SEM and sensitive multi-elemental analysis by SR-XRF were first

combined in this method. Kosa aerosol particles sampled in main industrial cities in China were

measured. The results led to a hypothesis that atmospheric sulfur primarily adheres to Kosa-

intrinsic calcium in aerosol during the flight and the particle surface roughens as a consequence

of the chemical reaction between the two elements.

Other methods were carried out to examine the results and finding. Distributional

correlation between sulfur and Kosa-intrinsic elements were measured by a conventional SEM-

EDX. Obvious distributional similarity between sulfur and calcium was observed for tens of

aerosol particles and the coexistence of the two elements was verified. Further investigation into

chemical state of sulfur in Kosa aerosol was made with X-ray absorption fine structure (XAFS)

spectrometry. The simultaneous measurement of total electron yield (TEY) and X-ray

fluorescence yield (XFY) spectra revealed that sulfur in Kosa aerosol derive from atmospheric

pollutants and mainly exists in form of SO42- for both particle surface and bulk.

A synthetic interpretation of these results led to the conclusion of CaSO4 formation on

aerosol particle surface that leads to distinctive surface roughness. At the same time, the methods

mentioned above have been proved as efficient approaches especially when utilized in

conjunction. Performance of the combinational application of SEM-EDX and SR-XRF was

discussed because the technique has not been fully developed, and it was concluded that the

method is particularly suitable for soil sample analysis.

1

Contents

1. Introduction ………………………….…………………………………………. 3

1.1 Kosa …………………………………………………………………….. 3 1.2 SEM-EDX ……………………………………………………………… 4 1.3 SR-XRF ……………………………………............................................. 5 1.4 XAFS ……………………………………................................................. 6

2. Experimental ……………………………………………..………………….….. 8

2.1 Combinational application of SEM-EDX and SR-XRF ………………… 8 2.1.1 Installation of SEM on a synchrotron radiation beamline ……………… 8 2.1.2 Samples …………………………………………………………………. 8 2.1.3 Preliminary setup ………………………………………………………... 9 2.1.4 Individual particle measurement……………………………………….... 10 2.1.5 Elemental distribution analysis by SEM-EDX ………………………….. 10

2.2 XAFS measurement …………………………………………………….. 11 2.2.1 Experimental setup ……………………………………………………… 11 2.2.2 Samples …………………………………………………………………. 11 2.2.3 TEY and XFY measurement ……………………………………………. 12

3. Results and discussion ………………………………………….………….….. 13

3.1 Investigation into Kosa aerosol particles …………………………….….. 13

3.2 Evaluation of combinational application of SEM-EDX and SR-XRF...… 17

4. Conclusion ……………………………………………….………………………. 20

4. References …………………………………………………………………….….. 50

Acknowledgements ………………………….…………………………….……. 54

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1. Introduction

1.1 Kosa

Kosa (yellow sand) is the soil that originates from the desert and loess areas of interior

China. Nearly a billion ton of it per year is transported by the westerlies to eastern Asia and the

Pacific region in form of aerosol.1 The aerosol particles, absorbing atmospheric elements during

the flight, adversely affect human health by reaching the lower respiratory tract.1-3 Severe

decrease in harvest has been reported even in Japan due to direct adhesion of such contaminated

aerosol on crops. Global environment including climate is also vulnerable to the consequent mass

transportation of atmospheric elements which disturbs global radiative forcing.3-5 Recent

desertification and air pollution in China accompanied by the industrial development are

deteriorating the situation.

Research into the Kosa issue has been done with numerous methods. Elemental

composition of Kosa particles collected in several areas has been investigated by various X-ray

spectrometries and chemical analyses. Elemental composition of aerosol particles collected in

several areas has been measured with high sensitivity and accuracy for most elements by

combining more than two methods.1-5 On the other hand, approaches to surface topography and

elemental composition of individual particles have been made by SEM-EDX.2, 6, 7 The standard

Kosa soil samples were processed by National Institute for Environmental Studies, Tsukuba,

Japan as typical Kosa soil particles before atmospheric contamination to evaluate deviation in

elemental composition of the aerosol samples.1 Respective methods stated above have

contributed toward resolving the Kosa issue, realizing rapid determination of elemental

composition, for instance. However, none of the methods have succeeded to simultaneously

3

perform detailed surface topography observation and elemental analysis for trace and micro

amount of elementals with high sensitivity for individual particles. Here, “trace and micro

amount of elements” means “elements contained not only in low concentration (trace) but also in

small quantity (micro)” such as low-concentration elements in a micrometer-size aerosol particle

in this instance.

1.2 SEM-EDX

A scanning electron microscope (SEM) has been commonly used for topographic image

observation of nanometer- to micrometer-size materials. A focused electron beam, tunable down

to sub-nanometer in diameter, irradiates an analyte to excite inner shell electrons of the

constituent elements. The analyte emits characteristic X-rays, continuous X-rays, backscattered

electrons, Auger electrons, light (cathode luminescence) and secondary electrons as a

consequence of the initial inner-shell electron excitation.8 Detailed 3-dimensional-like surface

morphology is obtained by detecting the secondary electron emission, while Auger electrons and

the characteristic X-rays provide information on elemental composition.

An energy dispersive X-ray spectrometer (EDX) is used to detect the characteristic X-

rays in SEM-EDX. The SEM-EDX has been commonly used to measure individual aerosol

particles for its ability to detect elements from B to U with a detection limit of ca. 0.5 wt. % for

most elements along with the simultaneous non-destructive sub-micrometer area image

observation.6-14 Here, “non-destructive” means “without changing the elemental composition of

analyte during the measurement” in this instance. Other advantages of SEM-EDX multi-

elemental analysis are as follows; (i) sensitive to lighter elements, (ii) capable of performing

qualitative and quantitative measurement within 10 minutes for most samples, and (iii)

inapplicable under atmospheric pressure, but ultra high vacuum is not necessary.8 On the other

4

hand, low excitation efficiency for heavier elements by an electron beam as well as

bremsstrahlung that leads to stronger continuous X-ray backgrounds8,15 has been a concern when

detecting trace and micro amount of elements, which are intrinsic and crucial to soil.

1.3 SR-XRF

X-ray fluorescence (XRF) spectromicroscope is another non-destructive multi-elemental

analysis instrument also performed by detecting the characteristics X-rays. The critical difference

from the SEM-EDX is that XRF uses incident X-rays instead of an electron beam for inner shell

electron excitation and is appropriate for heavier element detection. Usage of synchrotron

radiation (SR) as a brilliant X-ray source realized SR-XRF high-sensitivity analysis, of which the

minimum detection limit is less than one femtogram. SR-XRF has the following advantages; (i)

appropriate to detect trace and micro amount of elements, (ii) the beam size is tunable down to

sub-micrometer for local measurement, and (iii) applicable under atmospheric pressure.16, 17

However, one critical problem of SR-XRF application to individual aerosol particles despite the

higher sensitivity than SEM-EDX is that optical microscopes, often used for beam position

alignment in SR-XRF, are inappropriate for micrometer-size sample observation due to the

limited resolution and too shallow focal length.18, 19 Simultaneous application of SEM image

observation has been incompatible with SR-XRF measurement due to exclusiveness of a SEM

chamber which requires a vacuum sealing.

In the present research, we succeeded to jointly, if not simultaneously, perform SEM-

EDX and SR-XRF measurement for a ca. 50-micrometer-size single Kosa aerosol particle

without changing the sample position by introducing synchrotron radiation into a SEM

chamber.20 Sensitive multi-element determination by SR-XRF and detailed surface topography

5

observation by SEM, both crucial for investigation into individual particle disposition, but had to

be carried out separately,10 have been first integrated with our method.

1.4 XAFS

X-ray absorption fine structure (XAFS) spectrometry is performed by changing energy of

X-rays irradiating a sample and observing consequent X-ray absorbance due to inner-shell

electron excitation. Thus it has close connection with other analyses relevant to electron

excitation, including XRF and SEM-EDX. Although XAFS method is not appropriate for multi-

elemental analysis, the absorption edge shift and profile change of the absorption spectra provide

information on chemical state or oxidation state, chemical environment, concentration of a

constituent element.21 XAFS spectrum consists of X-ray absorption near-edge structure

(XANES), which mainly provides electron states and symmetry information, and Extended X-ray

absorption fine structure, which furnishes information on the surrounding atoms.21 The

advantages of XAFS measurement is as follows; (i) applicable to various sample forms including

liquid and powder, (ii) small amount of sample, such as a micrometer-thick film, is enough, (iii)

capable of obtaining depth-selective information.21-24 Transmission method, which is most

fundamental to X-ray absorption observation perspective among various methods, is performed

by comparing intensities of X-rays before and after the irradiation on a sample (Fig. 2a)

In this research, the combination of Total electron yield (TEY) and X-ray fluorescence

yield (XFY) was employed. Here, TEY method, which measures current transmission between

the analyte and the earth during the irradiation (Figs. 2b, c), provides information of nanometer-

depth composition from the analyte surface, while XRY method, which measures X-ray

fluorescence emission from the analyte (Fig. 2d), furnishes micrometer-depth information. The

measurable depths of TEY and XFY derive from the escape depth of electrons and X-rays,

6

respectively and the combination of the two methods realized depth-selective analysis.

Specifically, because the sample in this instance is micrometer-size particles, the TEY and XFY

spectra account for composition of sample surface and bulk, respectively.22-24

7

2. Experimental

2.1 Combinational application of SEM-EDX and SR-XRF

2.1.1 Installation of SEM on a synchrotron radiation beamline

Experiments were carried out on the beamline BL37XU [trace element analysis] at

SPring-8 (a third-generation synchrotron radiation facility, Nishiharima, Japan). This beamline is

capable of tuning X-ray beam energy from 5 to 37 keV with hard X-ray undulator source and Si

(111) double crystal monochromator.24 A low vacuum SEM (JEOL JSM-5610LVS) mounted

with a silicon drift detector (SDD), Rontec Xflash 2000 was installed in the beamline for the

measurement. The energy resolution of the SDD is less than 160 eV at 5.9 keV (Mn-Kα) with a

possible high throughput up to 80,000 cps. The acceleration voltage of the SEM electron beam is

tunable from 0.5 to 30 kV. A beryllium window (10 µm thick, 20 mm in diameter) was equipped

on the SEM body to introduce the monochromatic synchrotron radiation microbeam inside the

SEM sample chamber. The sample stage slope was set in such a way that the synchrotron

radiation beam irradiates the sample with the incident angle of 77 degrees to the sample stage

and the characteristic X-ray take-off angle to the SDD was 5 degrees. The SEM body position

was adjustable vertically (by a pulse motor) and horizontally (manually) so that the synchrotron

radiation beam and the electron beam can cross on the SEM sample stage.

2.1.2 Samples

Two Kosa aerosol particles were prepared; one of aerosol particles sampled at a building

rooftop in Shenyang in March 2004, when severe Kosa aerosol outbreak was recorded, and one

8

of particles sampled at a rooftop of a building in Harbin University of Science and Technology in

September 2003, when Kosa phenomenon effect was moderate. Here, both samples were

expected to represent highly contaminated Kosa aerosol composition because Shenyang and

Harbin are major industrial cities located in northeastern China. Specifically, Shenyang is the

capital of Liaoning Province with a population of 6.73 million and famous for its machinery

plants and petrochemistry while Harbin, the capital of Heilongjiang Province with 3.11 million

people in the urban area, is famous for its power plant, pharmacy and automobile industry. A low

or medium volume aerosol sampler is often used for efficient aerosol collection,11, 19, 24 but we

obtained adequate amount of free-fall Kosa aerosol particles for the measurement with a mere

plastic bag settled in a container due to high aerosol concentration in the two cities. Two standard

Kosa samples CJ-1 and CJ-2 were also measured. Both the standard Kosa samples were

manufactured by National Institute for Environmental Studies, Tsukuba, Japan through screening

and refinery procedures, and are regarded as typical Kosa particles before contamination due to

flight. CJ-1 originates from typical China loess layer of 2 m depth, while CJ-2 from interior

China desert surface sand of 0 to 6 cm depth. Median diameters of CJ-1 and CJ-2 are 38.1 and

24.1 µm respectively.1 It should be noted that CJ-2 has similar particle size with Kosa aerosol in

general. In what follows, we will refer to these four samples stated above, the Shenyang aerosol

particle, the Harbin aerosol particle, CJ-1 particles and CJ-2 particles as sample A, B, C and D

respectively.

2.1.3 Preliminary setup

A thin copper wire (ca. 100 µm in diameter) placed horizontally on the SEM sample stage

was moved to the center of image observation area and the synchrotron radiation beam was

introduced into the sample chamber (Fig. 1-1). The beam size of monochromatic synchrotron

9

radiation, restricted by vertical and horizontal slits, was 50 x 50 µm and the energy was fixed at 9

keV to avoid excitation of lead used for the slits. Considering the beam size, the spatial resolution

is considered as below 100 µm, if not micrometer-scale. The SEM body was moved vertically

step by step while monitoring copper characteristic X-ray signals and the body height which

registered the highest signal intensity was employed. The horizontal position was similarly

determined by replacing the wire with a relatively large size sand particle (ca. 400 µm in

diameter). Thus the synchrotron radiation and an electron beam were arranged to cross on the

SEM observation area.

2.1.4 Individual particle measurement

Once the SEM body is most appropriately positioned as mentioned above, we proceeded

to individual sample particle measurement. Surface topography observation followed by

elemental analysis with SEM-EDX was performed by irradiating an electron beam. The

acceleration voltage was fixed at 15 kV to subdue electron charge-up on the sample. SR-XRF

elemental analysis was then carried out by irradiating the synchrotron radiation instead of an

electron beam. It should be noted that the introduction of synchrotron radiation into the sample

chamber and the SEM body position alignment mentioned above realized SR-XRF measurement

for identical particle to be carried out with a mere alternation of the excitation sources. The

duration time for each measurement was 200 s for both SEM-EDX and SR-XRF and the pressure

in the sample chamber was kept at 10-4 Pa (Fig. 1-2). Simultaneous irradiation of an electron

beam and synchrotron radiation was avoided because drain current occurrence by SR beam

irradiation disturbs SEM feedback and a consequent overload on an electron-emitting filament

leads to its shorter lifetime.

2.1.5 Elemental distribution analysis using SEM-EDX

10

SEM-EDX instrument (Hitachi S-3500H, equipped with Si-Li semiconductor, of which

energy resolution is 145 eV) was used for elemental distribution analysis of collective aerosol

particles. Two-dimensional distributional correlation between sulfur and major Kosa-intrinsic

elements were measured for tens of aerosol particles collected in Shenyang. The acceleration

voltage and the current of the electron beam were 15 keV and 100 pA, respectively. This

measurement was performed independently of the SR facility.

2.2 XAFS measurement

2.2.1 Experimental setup

Experiments were carried out on the beamline BL11-B [soft X-ray double-crystal

monochromator station] in the Photon Factory of High Energy Accelerator Research

Organization (a second-generation synchrotron radiation facility, Tsukuba, Japan).26-28 This

beamline is capable of tuning X-ray beam energy from 1.8 to 3.9 keV with a double-crystal

monochromator [Ge (111) or InSb (111)] and a bending-magnet source. A proportional counter

was used to detect X-ray fluorescence, or the characteristic X-ray emission from the sample for

XFY measurement, while a grounded pico-ammeter was connected with the sample stage to

measure TEY spectra as drain current. Another grounded pico-ammeter was connected with a Ni

mesh, which is placed in the synchrotron radiation pathway, to evaluate incident beam intensity.

2.2.2 Samples

Two types of free-fall Kosa aerosol particles in aggregate were prepared; one was

collected at a building rooftop in Shenyang in May 2003, and the other was collected at a rooftop

of a building in Harbin University of Science and Technology in September 2003. Both samples

are expected to be typical city aerosol with atmospheric contamination. It should be noted that

11

the samples in this measurement are collective particles, not individual. The two standard Kosa

samples CJ-1 and CJ-2, already mentioned in SEM-EDX & SR-XRF section, were also measured

as non-contaminated Kosa particles.

2.2.3 TEY and XFY measurement

The samples were pasted on the perpendicular stage using carbon double-adhesive tape

chips (about the size of 5 mm x 20 mm) with regular clearance in between. The synchrotron

radiation beam was introduced into the sample chamber. Fluorescent dye powder, also pasted on

the stage, was first irradiated with the synchrotron radiation, to visualize the beam imposition

area. The sample stage position was adjusted so that the imposition area centers the adhesive tape.

Sulfur derivative samples (CuS, Na2S2O3, Na2SO3, Na2SO4), and samples of interest were

then irradiated in succession by changing the vertical position of the sample stage, and the TEY

and XFY spectra were measured. The beam energy ranged from 2450 to 2550 eV, by 0.2 eV

steps, 1 sec per step, using the Ge (111) monochromator to investigate XANES spectra of sulfur

and the derivatives (K-edge, c.a. 2.5 keV29). The pressure in the sample chamber was kept at 10-4

Pa during the measurement.

12

3. Results and discussion

3.1 Investigation into Kosa aerosol particles

Figures 4A-B show secondary electron images of representative individual Kosa aerosol

particles sampled in Shenyang and Harbin, the standard Kosa samples CJ-1 and CJ-2, or sample

A through D respectively. Samples A and B particles are both at the size of 50 µm in diameter

but the surface of sample A happened to be relatively smooth compared to that of sample B.

Particles with various surface texture and size were observed in sample C and D, however, it is

obvious that average particle size of D is smaller than that of C. Thus differences in surface

topography among these four samples were clearly revealed.

Figures 5A-D correspond to SEM-EDX spectra of respective samples shown in Figs. 4A-

D and Fig. 5E indicates SEM-EDX spectrum of blank (bare carbon adhesive tape). Similarly,

Figs. 6A-E show the corresponding SR-XRF spectra. Here, SR-XRF spectral intensities of

samples A, B and E (Figs. 6A, 6B, 6E) were indicated in raw counts while those of C and D (Figs.

6C, 6D) were both normalized with respect to calcium peak intensities of sample B. This is

because calcium is a major intrinsic element in soil and was considered to be appropriate to be an

index when comparing spectrum of a single particle and that of collective particles.

Seven elements, Mg, Al, Si, Cl, K, Ca, and Fe were both detected in samples A and B by

SEM-EDX (Fig. 5A, 5B) while Ti of sample A and S of sample B were exclusively detected in

each sample. These spectral differences account for characteristics of the individual particles, if

not cities. On the other hand, both of samples C and D, which are regarded as typical Kosa

particles before atmospheric contamination, registered Mg, Al, Si, K, Ca and Fe peaks, showing

13

spectral similarity between the two samples (Figs. 5C, 5D). Spectral deviation of the individual

aerosol particle samples (Figs. 5A, 5B) from the standard Kosa samples (Figs. 5C, 5D) were then

considered supposing the deviation emerged during the flight in form of aerosol to each city. It is

clearly seen that sample A registered higher peak intensities of Mg, Cl, Ti and Fe while sample B

showed higher S and Cl peak intensities. Here, the existence of S or Cl, detected only in samples

A and B, indicates that these two aerosol particle were contaminated with atmospheric pollutants

including such acid elements. Spectral difference between the two samples may be considered as

projection of industrial differences between Shenyang and Harbin, where the aerosol particles

were sampled, because S and Cl in aerosol mainly derive from automobile exhaust, coal

combustion and iron smelter,19, 30 that are deeply related to the city industries. Stronger peak

intensities of Ti and Fe observed in the aerosol samples (Figs. 5A, 5B) also indicate absorption of

industrial pollutants, as coal-fired power plants are main sources of atmospheric Ti and Fe.21

Eleven elements, Al, Si, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni and Cu by SR-XRF (Fig. 6a, 6b)

were detected in both of samples A and B, and S was also detected in sample B. Lower excitation

efficiency by X-rays for lighter elements accounts for detection failure of Mg, which was

observed in SEM-EDX. It should be noted that Cr, Mn, Ni and Cu in the both samples and Ti of

sample B in addition were newly detected by SR-XRF. This corroborates the advantage of SR-

XRF application compared to SEM-EDX for transition metal detection, which is essential for

investigation into Kosa and other soil-originated samples. However, Ca, Cr, Mn, Fe, Ni and Cu

were also detected in blank by SR-XRF (Fig. 6E). The distinctive background radiation derives

from irradiation of scattering X-rays on stainless steel and brass materials in the sample chamber

or on particles located near the blank area. By comparing the spectral intensity counts between

blank (Fig. 6E) and the samples of interest (Figs. 6A, 6B), it was revealed that Ca peaks detected

in samples A and B mainly attribute to the particle constituents, while Ni and Cu peaks largely

14

derive from background radiation. On the other hand, Cr, Mn and Fe peak intensities partially

derive from each sample’s constituents. Although involvement of such distinctive background

radiation needs to be considered unlike SEM-EDX, which showed broad continuous X-rays (Fig.

5E), it is proved that quantitative discussion is provided by subtracting the blank spectral (Fig.

6E) from the sample spectral as mentioned above.

Results mentioned above prove that the combinational application of SEM-EDX and SR-

XRF to individual aerosol particles materialized local elemental analysis for elements from Mg to

Fe with higher sensitivity than solely using SEM-EDX.

Relation between surface topography and the elemental composition for each aerosol

particle was then considered. It was acknowledged that larger amount of Mg, Cl, K, Ti and Fe

observed in sample A (Figs. 5A, 6A) did not lead to rough particle surface (Fig. 4A). By contrast,

sulfur and higher-concentration calcium, which are both characteristics of sample B (Figs. 5B,

6B), apparently contributed to its distinctive surface roughness (Fig. 4B). One hypothesis is that

atmospheric sulfur primarily united with Kosa-intrinsic calcium in the aerosol particle and the

surface roughened as a consequence of chemical reaction of the two elements.

Conventional SEM-EDX was utilized for investigation into correlation between sulfur

and typical Kosa-intrinsic elements in aerosol. Shown in Fig. 7 is elemental mapping analysis

results of Kosa aerosol particles collected in Shenyang in April 2003 and Harbin in September

2003, followed by secondary electron images. Here, elemental mapping provides 2-dimensional

elemental distribution images. Bright parts in the sections allotted for Mg, Si, Al, S, K, Ca and Fe

represent existence of respective elements. Widely spread magnesium and aluminum aside,

which are both typically intrinsic to Kosa, definite similarity between distributions of sulfur and

calcium was confirmed. This result verified the coexistence of the two elements in tens of other

15

aerosol particles and also corroborated the importance of individual particle measurement by

revealing unevenness of elemental distribution among these particles.

XAFS method provided chemical state of sulfur in collective Kosa aerosol particles.

Shown in Fig. 8 are the TEY spectra of Na2S2O3, CuS, Na2SO3, and Na2SO4. Then the spectra of

samples of interest, Harbin aerosol, Shenyang aerosol, CJ-1, CJ-2, and blank (bare carbon

adhesive tape) are shown in Figs. 9a-e (TEY) and Figs. 10a-e (XFY). It should be noted that

detected TEY and XFY spectra intensities were divided with corresponding incident beam

intensity, which was detected as drain current when penetrating the Ni mesh, to avoid the spectral

peak emergence due to fluctuation of incident beam intensity. Spectral peaks in Figs. 8 indicate

absorption spectra profiles of the representative sulfur derivatives, determining S2-, SO32-, SO4

2-

absorption edges as 2470.6, 2477.1, 2481.4 eV respectively. The spectral profiles of Kosa

samples (Figs. 9a-d, 10a-d) were then evaluated. Shenyang aerosol registered distinct absorption

peaks near 2481 eV for TEY (Fig. 9a) and XFY (Fig. 10a), indicating that sulfur mainly exists in

form of SO42- for both nano- to micrometer-depth from the particle surface. Shenyang aerosol

also registered peaks near 2481 eV for both TEY (Fig. 9b) and XFY (Fig. 10b) but a relatively

small peak was observed in addition for TEY near 2477 eV, showing distribution of SO32- only in

nanometer-depth. On the other hand, CJ-2 registered smaller peak near 2481 eV for TEY (Fig.

7c), indicating existence of SO42- for nanometer-depth. No peak was found for CJ-1.

Thus it was concluded that sulfur in Kosa aerosol mainly exists in form of SO42-, one of

the most oxidized condition, due to adhesion of atmospheric pollutants during the transportation.

Dissimilarity in spectral profiles between TEY and XFY for Shenyang aerosol and CJ-2

designated depth-dependency of the particle composition as one of characteristics of the samples.

Such distinctive surface composition is regarded as projection of local atmospheric environment.

16

At the same time, suitability of the simultaneous measurement of TEY and XFY for Kosa aerosol

particles was also signified.

A synthetic interpretation of these results led to the conclusion of CaSO4 formation on

aerosol particle surface that leads to distinctive surface roughness due to adhesion of atmospheric

sulfur during the flight.31

3.2 Evaluation of combinational application of SEM-EDX and SR-XRF

It is of great importance that all the methods we employed in this research were

cooperatively utilized for resolving chemical composition of individual Kosa aerosol particles.

However, one obvious constraint in the combinational application of SEM-EDX and SR-XRF is

impracticality of inspection area visualization in SR-XRF unlike SEM-EDX, in which the

elemental analysis area coincides with the image observation area. SR microbeam position to the

SEM sample stage is another problem; it is expected to be consistent once the SEM body position

is settled, but possible fluctuation due to temperature change or other factors can be critical when

handling a micrometer-size sample. This leads to discrepancy in measurement areas between SR-

XRF and SEM-EDX, and thus quantitative comparison of the elemental analysis results, for

further investigation into sample characteristics, is yet to be fully developed. Further discussion

on the method’s performance is made showing other experimental results in the following.

Shown in Figs. 11a-m are secondary electron images, SEM-EDX (black) and SR-XRF

(red) spectra of other individual aerosol particles measured in similarly as already mentioned.

Both SEM-EDX and SR-XRF spectra are displayed on the same scale, though SR-XRF spectra

of some samples were not obtained due to failure of SR microbeam irradiation on the samples.

Duration times of SR-XRF measurement in some samples were extended to 2000 sec, but the

spectral intensities were converted to those of 200 sec, or virtually divided by ten when plotted. It

17

should be noted that sensitivity contrast between SEM-EDX and SR-XRF is provided in these

format. On the other hand, it is obvious that SR-XRF spectra intensities and profiles irrationally

vary for some samples, regardless of those of corresponding SEM-EDX spectra. The instability

of SR-XRF spectra and irradiation failures as stated above are considered as a consequence of SR

beam imposition condition differences, rather than the sample characteristics. It is worth noting

that possible fluctuation in positions of the SR beam or the SEM body due to room temperature

change or other factors could be critical when handling micrometer-size particles. Scattering X-

rays are also considered as one of indefinite factors.

Figure 12 shows detailed spectral comparison of spectral profiles between blank and the

sample particle. The distance between the two observation areas (blank and particle) designated

by SEM images is c.a. 500 µm, which is considered wide enough to eliminate SR beam

irradiation on the particle during blank area observation though the irradiation span is spread to

c.a. 200 µm maximum on the tilted sample stage. The spectral contrast indicates successful

constituent elemental detection of a single particle of c.a. 10 x 30 µm size with quantitative

comparison to blank for both SEM-EDX and SR-XRF. At the same time, it was proved that SR-

XRF measurement of blank was successfully performed suppressing spectral fluctuation due to

sample stage position adjustment.

Measurement results of collective particles are shown in Figs. 13a-f. The SR-XRF spectra

are plotted in different scale from SEM-EDX and it is clearly seen that sensitivity of SR-XRF for

trace elements remains higher compared to SEM-EDX even for measurement of collective

particles. It should be noted that the SR-XRF spectral intensities of particle constituent elements

relative to those of background are higher compared to the case of individual particle

measurement (Figs. 11a-m, 12). This is due to the size proportion of SR beam to the samples, and

18

consequently beam size adjustments for respective samples are proved to be a reasonable

approach for higher sensitivity.

Although quantitativeness of SR-XRF measurement has not fully developed as stated

above, the practicability of combinational application with SEM-EDX for detection of elements

from Mg to Fe in individual particles were steadily confirmed. It is of great significance that

among the detectable elements, Mg, Al, Si, K, Ca, Ti and Fe are intrinsic to soil in general, while

S and Cl are contained in typical acid pollutants. This means the method is particularly suitable

for other contaminated soil samples, because possible fluctuation in elemental composition of

original soil as well as pollutants can be sufficiently detected.

19

4. Conclusion

We reached the conclusion that atmospheric sulfur adheres to calcium of Kosa aerosol,

causing distinctive particle surface roughness as a consequent of CaSO4 formation. This finding

was provided by several approaches toward Kosa aerosol particles as mentioned below.

Combinational application of SEM-EDX and SR-XRF to individual Kosa particles was

carried out for detailed topographic observation and sensitive multi-elemental analysis. We

figured out that the individual aerosol particles were contaminated with atmospheric sulfur and

chlorine by successfully detecting deviation in elemental composition from the standard Kosa

samples. Detailed SEM images were utilized in conjunction with the sensitive elemental analysis

results and correlation between distinctive surface roughness and constituent sulfur was implied.

It should be noted that the elemental analysis results also provided sensitivity contrast between

SEM-EDX and SR-XRF.

Elemental mapping analysis by conventional SEM-EDX was carried out to investigate

distributional correlation between sulfur and major Kosa-intrinsic elements. The results verified

the coexistence of sulfur and calcium for tens of contaminated aerosol particles.

XAFS measurement was performed for depth-selective analysis of chemical state of

sulfur in aerosol particles. Information of particle surface (nanometer-depth) and bulk

(micrometer-depth) was provided by simultaneous measurement of TEY and XFY spectra,

respectively. The results showed that sulfur mainly exists in form of SO42- in contaminated Kosa

aerosol particles for both particle surface and bulk, while additional SO32- detection in surface

20

indicated depth-dependency of sulfur state, which is regarded as a projection of city industries,

verifying suitability of XAFS application at the same time.

It should be noted that all of these methods were jointly utilized for resolving the

chemical state of individual Kosa aerosol particles. Although there is still room for improvement

concerning quantitativeness of individual particle elemental analysis by SR-XRF, application of

the combinational method to other soil samples is evidently useful because of the low detection

limit and the measurable elements.

21

Fig. 1-1 Positioning of the SEM body to the synchrotron radiation.

Fig. 1-2 Schematic diagram of the combinational application of electron microscope and

synchrotron X-ray microscope.

22

Fig. 2 Schematic illustrations of XAS methods. (a) transmission method; (b, c) TEY method; (d)

XFY method.

Fig. 3 Schematic diagram of XAS measurement using TEY and XFY methods.

23

Fig. 4 Secondary electron images of samples. A, single Kosa aerosol particle sampled in

Shenyang, China in Mar. 2004. B, single Kosa aerosol particle sampled in Harbin, China in Sep.

2003. C, CJ-1 standard Kosa sample particles. D, CJ-2 standard Kosa sample particles.

24

Fig. 5 SEM-EDX spectra of samples and blank. A, single Kosa aerosol particle sampled in

Shenyang, China in Mar. 2004. B, single Kosa aerosol particle sampled in Harbin, China in Sep.

2003. C, CJ-1 standard Kosa sample particles. D, CJ-2 standard Kosa sample particles. E, blank

(bare carbon adhesive tape).

25

Fig. 6 SR-XRF spectra of samples and blank. A, single Kosa aerosol particle sampled in

Shenyang, China in Mar. 2004. B, single Kosa aerosol particle sampled in Harbin, China in Sep.

2003. C, CJ-1 standard Kosa sample particles. D, CJ-2 standard Kosa sample particles. E, blank

(bare carbon adhesive tape).

26

Fig. 7 Elemental mapping results and secondary electron images of aerosol particles collected in

Shenyang in Apr. 2003 (left), and in Harbin in Sep. 2003 (right) by SEM-EDX.

27

Fig. 8 TEY spectra of Na2S2O3, Na2SO3, CuS, and Na2SO4

28

Fig. 9 (Left) TEY spectra of; a, Kosa aerosol particles collected in Harbin in Sep. 2003. b, Kosa

aerosol particles collected in Shenyang in May 2003. c, CJ-1 standard Kosa sample particles. d,

CJ-2 standard Kosa sample particles. e, blank (bare carbon adhesive tape).

Fig. 10 (Right) XFY spectra of; a, Kosa aerosol particles collected in Harbin in Sep. 2003. b,

Kosa aerosol particles collected in Shenyang in May 2003. c, CJ-1 standard Kosa sample

particles. d, CJ-2 standard Kosa sample particles. e, blank (bare carbon adhesive tape).

29

Fig.11a SEM image and SEM-EDX spectrum of a single particle of CJ-1.

30

Fig.11b SEM image, SEM-EDX and SR-XRF spectra of a single particle of CJ-1.

31

Fig.11c SEM image, SEM-EDX and SR-XRF spectra of a single particle of CJ-2.

32

Fig.11d SEM image, SEM-EDX and SR-XRF spectra of a single particle of CJ-2.

33

Fig.11e SEM image and SEM-EDX spectrum of a single particle of aerosol collected in Kyoto on

Mar. 14, 2004.

34

Fig.11f SEM image, SEM-EDX and SR-XRF spectra of a single particle of aerosol collected in

Kyoto on Mar. 14, 2004.

35

Fig.11g SEM image and SEM-EDX spectrum of a single particle of aerosol collected in Kyoto on

Mar. 14, 2004.

36

Fig.11h SEM image, SEM-EDX and SR-XRF spectra of a single particle of aerosol collected in

Kyoto on Mar. 17, 2004.

37

Fig.11i SEM image, SEM-EDX and SR-XRF spectra of closely-located two particles of aerosol

collected in Kyoto on Mar. 17, 2004.

38

Fig.11j SEM image, SEM-EDX and SR-XRF spectra of a single particle of aerosol collected in

Kyoto on Mar. 17, 2004.

39

Fig.11k SEM image, SEM-EDX and SR-XRF spectra of a single particle of aerosol collected in

Shenyang in March, 2004.

40

Fig.11l SEM image, SEM-EDX and SR-XRF spectra of a single particle of aerosol collected in

Shenyang in March, 2004.

41

Fig.11m SEM image and SEM-EDX spectrum of a single particle of aerosol collected in Harbin

in September, 2003.

42

Fig. 12 SEM images, SR-XRF and SEM-EDX spectra for the comparison between blank and

particle. The sample particle was sampled in Harbin in September, 2003.

43

Fig. 13a SEM image, SEM-EDX and SR-XRF spectra of collective CJ-1 particles.

44

Fig. 13b SEM image, SEM-EDX and SR-XRF spectra of collective CJ-2 particles.

45

Fig. 13c SEM image, SEM-EDX and SR-XRF spectra of collective aerosol particles sampled in

Kyoto, on Mar. 14, 2004.

46

Fig. 13d SEM image, SEM-EDX and SR-XRF spectra of collective aerosol particles sampled in

Kyoto, on Mar. 17, 2004.

47

Fig. 13e SEM image, SEM-EDX and SR-XRF spectra of collective aerosol particles sampled in

Shenyang, in March, 2004.

48

Fig. 13f SEM image, SEM-EDX and SR-XRF spectra of collective aerosol particles sampled in

Harbin, in September, 2004.

49

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Acknowledgements

I would like to express my sincere gratitude to Professors Jun Kawai, Teruo Tanabe, Drs.

Hideshi Ishii, Mitsuru Nagasono and Mr. Toyohide Hayashi and Masatoshi Toyoda for their

continuous guidance and encouragement.

This work was performed under the approval of the KEK-PF Program Advisory

Committee and SPring-8 Advisory Committee. I am grateful to Yoshinori Kitajima of KEK-PF

and Yasuko Terada of JASRI for their important contributions to the experiments. I wish to thank

Mr. Teruyoshi Unesaki and Yuji Taniguchi for SEM-EDX measurement and advices.

Finally, I thank my colleagues of the Process Chemical Physics Laboratory, Z. Liu, T.

Koyama, H. Murakami, J. Itoh, D. Tanaka, N. Satoh, M. Takeda, S. Matsumoto and S. Mitsuya

for helping and supporting me.

54