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71

Measurement of PM2.5 and Water-Soluble Ions at Central Tokyo,
Japan and Source Apportionment
Shiori Ota1, Junya Hanasaki2, Kazuhiro Toki2, Takuto Horigome2, Akihiro Takemasa2,
Yuri Ohkoshi3, Nami Takahashi3 and Yoshika Sekine1, 3, *1

Graduate School of Science, Tokai University, Kanagawa, Japan

2

Tokai University Bosei Senior High School, Tokyo, Japan3

Department of Chemistry, School of Science, Tokai University, Kanagawa, Japan
Abstract: Air pollution by fine particulate matter, PM2.5 has been inviting considerable concerns in East Asia. This study
then aimed to characterize PM2.5 and its water-soluble ions collected at Shibuya, central Tokyo, Japan from July 2013 to
May 2015, in order to better understand the air pollution mechanism and potential sources of PM2.5 of the city. Analyticalresults showed the PM2.5 was a mixture of aggregates of formless particles and its concentration ranged from 5.4 to332+
39µg/m , with a mean of 14 ± 6.7µg/m (n=46). SO 4 and NH 4 were abundant in PM2.5 and their presence mostly
determined the variation of PM2.5 level. High concentration episodes were found in the season when air masses camefrom Asian continent, and a long-range transport of urban aerosols and soil-derived particles was suggested by changes
in the chemical compositions.

Keywords: PM2.5, water-soluble ions, source apportionment, Tokyo.
1. INTRODUCTION
Air pollution by fine particulate matter, PM2.5(defined as PM less than 2.5µm in aerodynamic
diameter) has been inviting considerable concerns inEast Asia because of severe pollution levels in China[1, 2], and its adverse effects on human health.
Increased mortality by pulmonary and/or cardiovasculardiseases due to exposure to PM2.5 has been
documented in a number of cohort studies such as
Harvard Six Cities Study [3-5], American Cancer
Society (ACS) cohort study [6-8], Great Britain Study[9] and other valuable studies summarized in ref [10].
Association of increased daily mortality with short-term
exposure to PM2.5 has been also reported by
epidemiological studies carried out in Chinese cities[11, 12]. However, unlike other countries, noremarkable relationship has been found between fineparticles and excess mortality in Japan [10]. PM2.5 is a
mixture of various particles emitted and/or generated
from natural and anthropogenic sources, and hence
appearance of adverse health effects may depend on
the chemical composition of PM2.5 at the receptor site.
Therefore, understanding chemical components andsource apportionment of PM2.5 using the chemical
fingerprint is important for pollution control of thecomplex media.

*

Address correspondence to this author at the Department of Chemistry,
School of Science, Tokai University, Kanagawa, Japan; Tel: +81-463-58-1211;
Fax: +81-463-58-9543; E-mail: Sekine@keyaki.cc.u-tokai.ac.jp
E-ISSN: 2410-3624/15

Water-soluble components are known to account for
majority of the PM2.5 mass [13, 14]. The major ions
2such as sulfate (SO4 ), nitrate (NO3 ) and ammonium+
ion (NH4 ) have effects on the hygroscopic nature andacidity of aerosol, while their concentrations in air varywith seasons and geological locations [13]. Then, thisstudy aimed to characterize PM2.5 and its water-solubleions collected at Shibuya, central Tokyo, Japan from
July 2013 to May 2015, in order to better understandthe air pollution mechanism and potential sources of
PM2.5 of the city, employing correlation analysis andtrajectory analysis. The correlation analysis has been
widely used for source apportionment of particulatematters, because significant mutual correlations
between seasonal variations of air concentrations in
PM and its components may show a possibility ofsharing the common emission source by the paired
variables [15]. Air trajectories are also used to study
source–receptor relationship in environmental researchby calculating airflow pattern from an emission source(forward) or from a sampling site (backward) [16, 17].
In this study, the back trajectory analysis was used toinvestigate the potential source area of highconcentration episodes of PM2.5 at central Tokyo.
2. METHODS
2.1. Sampling Site
Figure 1 shows the geographical location of
Shibuya, Tokyo, Japan. Routine sampling of PM2.5 was
conducted at the open-air space of the school building© 2015 Avanti Publishers

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S-4800) at Tokai University (Kanagawa, Japan), after
coated with a thin Au film to achieve higher quality
secondary electron images. Distribution of major ionsas particles on the quartz fiber filter was also analyzed
by using Electron Probe Micro Analyzer (EPMA,
Shimadzu, EPMA-1610) at Tokai University. EPMA is a
non-destructive analytical tool used to determine thechemical composition of small volumes of solidmaterials within 10~30 cubic micrometers. The sample
is bombarded with an electron beam, and signals thatcome from the sample are collected and displayed as
like a map.
2.4. Water-Soluble Ion Analysis

Figure 1: Location of sampling site, Shibuya, central Tokyo,
Japan.

of Tokai University Bosei Senior High School located at
Shibuya, Tokyo. Shibuya is one of the biggest cities incentral Tokyo, Japan, having a population density of
13,540 people per km². The city is used to refer to a
fashion center of Japan, particularly for young people.
The sampling site locates in cultural and residential
area of the city.
2.2. Collection of PM2.5 Samples
PM2.5 was collected on a quartz fiber filter (QMA
47mmφ, Whatman) at a flow rate of 16.7L/min for 7
days by PM2.5 sampler (Thermo Fisher Scientific,®
Partisol - FRM Model 2000). Both before and after
sampling, filters were conditioned more than 24 hours
under constant temperature (20℃) and relative
humidity (50 ± 5%), then the weight of filter wasquantified by using a microbalance. The concentrationof PM2.5 was determined by the weight difference andtotal sampling volume of air. Note that routine collectionof PM2.5 and measurement of PM2.5 concentration werecarried out by high school students, and subsequentchemical analysis was conducted by staffs of Tokai
University.
2.3. FE-SEM and EPMA Analysis
Morphology of individual fine particles consisting of
the PM2.5 at Shibuya was observed by Field Emission
Scanning Electron Microscope (FE-SEM, Hitachi,

After measurement of PM2.5 concentration, thefilters were cut into 14mmφ pieces by a pierce punch.
The two pieces were transferred into a test tube
together and water-soluble ions in PM2.5 were extracted
in 10mL ultrapure water (Milli-Q) with a mild shaking for
90 min. After filtration by a disposable cellulose acetate
membrane filter having 0.20µm of pore size (Advantec,
DISMIC-25CS), the filtrate was served for ion analysis.
++
Concentrations of the water-soluble ions (Na , NH4 ,+2+2+
2K , Mg , Ca , Cl , NO3 , SO4 ) were determined byion chromatography (IC) systems.
The IC system for the determination of cationconsisted of Shimadzu LC-20AD with a conductivity
detector COD-10AVP. The following conditions wereused: column, φ 4.6×150mm, 7µm, Shim-pack IC-C4(Shimadzu); eluent, 2.5mM oxalic acid solution at
1.0ml/min (isocratic); oven temperature, 40℃; injection
volume, 20µL. The IC system for anion consisted of
Dionex ICS-90 with a chemical suppressor AMMS III.
The following conditions were used: separation column,
4.0 × 250mM, IonPac AS9-HC (Dionex) with a guard
column IonPacAG4 (Dionex); eluent, 5mM sodium
carbonate at 1.0ml/min (isocratic), scavenger for thechemical suppressor, 15mM sulfuric acid solution at
1.0ml/min; oven temperature, 40℃; injection volume,
25µL. Dilution series of reagent grade NaCl, NH4Cl,
KCl, MgCl2, CaCl2, NaCl, NaNO3 and Na2SO4 in
ultrapure water were used for calibration anddetermination. All the reagents were obtained from
Kanto Chemical, Japan.
Significant contamination by field handling andduring storage was slightly found in storage blanks foranions, limit of detection (LOD) was defined as threefold the standard deviation of the blank for anions and-1of 1.0µL ml of standard solutions for cations (3σ). The
3+
3
LODs resulted in 0.0053µg/m for Na , 0.0030µg/m forMeasurement of PM2.5 and Water-Soluble Ions at Central Tokyo
+

3

+

3

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2+

NH4 , 0.0023µg/m for K , 0.0032µg/m for Mg ,32+
3
30.013µg/m for Ca , 0.0010µg/m for Cl , 0.0029µg/m
32for NO3 and 0.0036µg/m for SO4 .
2.5. Back Trajectory Analysis
In order to characterize the general behavior of air
masses in a given sampling period and to evaluate therelative contributions by local sources and long-range
transportation to the total PM2.5, three-day backward
trajectories from the sampling site (longtitude: 139.7,
latitude: 35.6) were computed beginning every 24hduring the sampling duration using the Meteorological
Data Explorer (METEX) program (Center for Global
Environmental Research, National Institute forEnvironmental Studies) developed by Zeng et al. [16,17] and available from the METEX website. The
altitude above ground level was set to 500m.
3. RESULTS AND DISCUSSION
3.1. Morphology and Elemental Map of PM2.5
Figure 2 shows a typical FE-SEM image of fineparticles collected on the quartz fiber filter at Shibuya,
Tokyo. The aggregates of formless particles wereattached on the surface of fiber webs. These particles
seemed a mixture of different particles and the shapeand structure was not uniform, so it was very difficult to
identify the particles by visual observations.

Figure 3: Elemental distribution maps of C and S on thePM2.5 sample collected at Shibuya, Tokyo.

these elements, relatively higher abundance was foundfor C and S. Figure 3 shows elemental distribution
maps of C and S in a focus window of 480×480µm
length. Carbon (red dot) was widely distributed in allover the sample with higher content. This means fineparticles containing carbon, usually referred toelemental carbon (EC) and organic carbon (OC), aremajor components of PM2.5 collected at Shibuya,
Tokyo. Meanwhile, the picture also shows the presenceof S (red dot) with very fine and dispersed signals.
Comparing with carbon, S-containing particles weremuch finer than those containing carbon, probably
because of most of S-containing aerosols such as
water-soluble sulfate are secondary particles converted
from gases.
3.2. Concentration of PM2.5 and Water-Soluble Ions

Figure 2: A typical FE-SEM image of the PM2.5 sample
collected at Shibuya, Tokyo (x3000).

EPMA analysis showed significant abundances ofmajor elements such as Al, C, Ca, Fe, K, Mg, N, Na, O,
S, Si and Ti in the PM2.5 sample. Among the elements,
Si and O are major components of the quartz fiber
itself, and certain amount of Al, Ca, Mg and Na arecontained as impurities of the filter [18]. Excluding

Mean concentrations and standard deviations of
PM2.5 and water-soluble ions were summarized inTable 1. For the convenience in investigating seasonal
differences, the results were classified into four
seasons; spring (March to May), summer (June toAugust), autumn (September to November), and winter(December to February). The mean values andstandard deviations at each season were also

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Table 1: Concentrations of PM2.5 and Water Soluble Ions Observed at Shibuya, Tokyo, Japan from July 2013 to May
32015 (Mean ± Standard Deviation in µg/m )Annual (n=46)

Spring (n=14)

Summer (n=6)

Autumn (n=15)

Winter (n=12)14 ± 6.7

18 ± 7.6

12 ± 4.3

11 ± 3.3

14 ± 8.3

PM2.5+

0.15 ± 0.075

0.19 ± 0.068

0.067 ± 0.056

0.14 ± 0.077

0.14 ± 0.072

NH4

Na

1.2 ± 0.93

2.0 ± 1.2

1.4 ± 0.24

3.0 ± 0.39

0.87 ± 0.60+

0.17 ± 0.23

0.28 ± 0.39

0.13 ± 0.026

0.13 ± 0.04

0.12 ± 0.077

2+

0.025 ± 0.014

0.030 ± 0.015

0.036 ± 0.018

0.026 ± 0.012

0.015 ± 0.0066

2+

0.078 ± 0.032

0.092 ± 0.044

0.16 ± 0.033

0.07 ± 0.02

0.06 ± 0.021

-

+

K

Mg
Ca

0.018 ± 0.031

0.016 ± 0.023

0.0093 ± 0.0064

0.010 ± 0.007

0.031 ± 0.054

NO3

0.24 ± 0.59

0.18 ± 0.34

0.088 ± 0.0068

0.047 ± 0.036

0.63 ± 1.0

SO4

2-

3.5 ± 2.3

4.9 ± 2.4

3.8 ± 1.1

2.5 ± 1.4

3.0 ± 2.7

Others

8.6 ± 4.1

9.7 ± 4.2

7.9 ± 2.5

7.5 ± 3.0

9.1 ± 5.5

Cl

presented in the table. The 7-day average con3
centration of PM2.5 ranged from 5.4 to 39µg/m , with a3mean of 14 ± 6.7µg/m (n=46). The PM2.5 concentration
was observed higher in spring and winter due in part tohigh concentration episodes described below, andrelatively low in summer and autumn. Since the annual
mean was somewhat less than the Japaneseenvironmental quality standard (EQS) for PM2.53(15µg/m ), status of air pollution by PM2.5 is not sosevere at present.
The concentrations of water-soluble ions in PM2.5
2+++followed the order of SO4 > NH4 > NO3 > K = Na >2+
2+
Ca > Mg > Cl . However, this order slightly changed
with seasons; NO3 was relatively low in spring to+autumn, and NH4 was higher in autumn. Total mass ofthe eight kinds of ions accounted for about 40% of thePM2.5 mass in any seasons.
+-

Na and Cl are major components of sea-salt
particles. However, Cl concentration was much lower+
than that of Na with 0.058~0.095 of molar ratio of Cl+/Na in every season, probably because of “chlorineloss”. This is a phenomenon that volatile HCl isreleased by a heterogeneous reaction of sea salt
particles with acidic gases such as SO2, H2SO4 and
HNO3 in air [19, 20]. Considering the contribution fromsea salt particles, concentration of sea-salt sulfate (ss2+
SO4 ) was estimated using Na as an indicator of sea
salt particles by equation (1),2-+[ss- SO4 ] = [Na ] × 0.251

(1)
2-

+

where, 0.251 is a weight ratio of SO4 /Na in sea
2water. The results showed concentration of ss-SO4

3ranged from 0.0043 ~ 0.091µg/m with an average of
3
0.038µg/m (n=46) which corresponded to only 1.4% of
2total SO4 . Even though the sampling site locates only
5 km apart from the seashore of Tokyo bay, sea salt
2particles were not important as a source of SO4 in
PM2.5 collected at Shibuya, Tokyo.
3.3. Correlation Analysis
Figure 4 shows variations of air concentrations of
PM2.5 and water-soluble ions at Shibuya, Tokyo. The
date shown in the X-axis is start day of the 7-day
samplings. The PM2.5 concentration greatly varied with
2samplings with some peaks. The variation of SO4+
concentration was similar, whilst variations of NH4 and+K were not similar to that of PM2.5. Then, Pearson’s
correlation coefficients between paired data on thevariation of air concentrations were calculated using a®
software, IBM SPSS Statistics ver.19 and shown in
Table 2.
PM2.5 concentration significantly correlated with+
2those of NH4 (r = 0.76) and SO4 (r = 0.79). Thismeans variation of PM2.5 was mostly determined by thepresence of ammonium sulfate such as (NH4)2SO4 andNH4HSO4, typically most abundant secondary particles+
in air. Influence of sea salt particle (Na ) andcombustion effluent (NO3 ) was also suggested on thePM2.5 concentration with significant correlations.
2+

2+

A pair of Mg and Ca showed a significant
correlation with r = 0.54. The both mineral ions are wellknown to enrich in a desert soil by soil salinity, andhence they are good indicators of yellow dust storm+“Kosa” [21]. NH4 is also known to be rich in soil ofMeasurement of PM2.5 and Water-Soluble Ions at Central Tokyo

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75

Figure 4: Variations of air concentrations of PM2.5 and water soluble ions and nitrate/sulfate ratio observed at Shibuya, Tokyo,
Japan from July 2013 to May 2015 (n=46, each plot shows 7-day mean).

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Table 2: Correlation Coefficients between Paired Data of PM2.5 and Water-Soluble Ions
PM2.5
PM2.5

+

NH4

Na

+

K

+

Mg

2+

Ca

2+

Cl

-

-

NO3

1

+

.41

1

+
4

.76

.43

1

+

.27

.11

.30

1

2+

.28

.18

.52

.12

1

2+

.29

-.07

.52

.11

.54

1

-

-.19

-.05

-.15

-.09

-.17

-.14

1

3

.47

.19

.17

.06

-.23

-.04

-.02

1

24

.79

.29

.77

.23

.32

.31

-.19

.39

Na
NH
K

Mg
Ca

Cl

NO
SO

2-

SO4

1

Values expressed in bold are significant at 1% significance level.

+

China. Therefore, the significant correlation of NH4 ,2+
2+
Ca and Mg indicates a possible long-range transportation of soil dust from China to central Tokyo.
3.4. High Concentration Episodes
As can be seen in Figure 4, two remarkable peaks
2were found in PM2.5 and SO4 concentrations at the
samples collected at Feb.24~Mar.2, 2014 (#1) and May
28~June 3, 2014 (#2). Figure 5 shows 3-day backward
air trajectories corresponding to #1 and #2. As for #1,
air trajectories came from Asian continent including
2northern part of China with remarkable peaks of SO4
and NO3 concentrations. Since the mass ratio of NO3 /2SO4 has been used as an indicator of relative
importance of mobile versus stationary sources in the
air pollution [13], there were previously several reportson the ratio: 0.53 at Jinan in winter [13], 0.63 at Beijing[22], 0.60 at Shanghai [23], 0.95 at Seoul, Korea [24].
Since the mass ratio apparently increased at #1 with
0.28, this suggests #1 sample is a mixture of urban
aerosol particles transported from urban cities in Asian
continent and fine particles originated from domestic
sources of Japan.
Meanwhile, air trajectories of #2 came from Chinaor around national borders on the sea, and passed
through western part of Japan. According to Aeolian
Dust Information by Japan Meteorological Agency [25],a long-range transport of yellow sand was observed inwide areas of western Japan during the sampling.
Therefore, soil-derived particles from dessert areas in
the continent were involved in the PM2.5 sample with+2+
2+
2peaks of NH4 , Ca
and Mg . Then, SO4concentration became also high because of formationof salt with these cations.

Figure 5: Three-day backward air trajectories during 7-day
samplings, calculated for #1 (Feb.24~Mar.2, 2014) and #2(May 28~June 3, 2014).

Measurement of PM2.5 and Water-Soluble Ions at Central Tokyo

4. CONCLUSION
By investigating the concentrations of PM2.5 andwater-soluble ions at Shibuya, central Tokyo for 2
years, the following findings were obtained. Present
status of air pollution by PM2.5 was not so severe at the
sampling site with a mean concentration of
32+
14 ± 6.7µg/m during the observation. SO4 and NH4
were abundant in PM2.5 and their presence mostly
determined the variation of PM2.5 level. Highconcentration episodes were found in the season when
air masses came from Asian continent, and a longrange transport of urban aerosols and soil-derived
particles was suggested by correlations with indicator
ions.
ACKNOWLEDGEMENT
This research activity was supported by Japan
Science and Technology Agency (JST) Supporting
Science Club program and JSPS KAKENHI Grant
number 26410198. Authors awfully thank Mr. Yoshio
Yamashita and Science Club members of Bosei SeniorHigh School.
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DOI: http://dx.doi.org/10.15377/2410-3624.2015.02.02.4© 2015 Ota et al.; Avanti Publishers.
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