Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 149-153, 2007

High Resolution Measurements of Temporal Variations of Airborne 210Po, 210Pb, and 7Be
at Ishikawa Prefecture, Facing the Sea of JapanY. Yamaguchi,*,a,b T. Abe,a Y. Murata,a Muguntha Manikandan N.,a K. Tanaka,a and K. Komuraaa

Low Level Radioactivity Laboratory, K-INET, Kanazawa University, Wake, Nomi, Ishikawa 923-1224, Japan
Radioisotope Center, Osaka City University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japanb

Received: February 1, 2007; In Final Form: July 14, 2007
High-resolution measurements of temporal variations of airborne 210Pb and 7Be were performed by ultralow background γ -spectrometry at Ogoya Underground Laboratory (OUL). Moreover, high-resolution measurements of210
Po were also made by α -spectrometry at Low Level Radioactivity Laboratory (LLRL). In this paper, the results
focusing mainly on cyclic variation with ~1 week period observed in winter season, and the high resolution measurements conducted at the time of approach of typhoon and Kosa events are reported.

1. Introduction(a)Natural and artificial radionuclides have been used as versatile tracers in geochemical studies. The concentrations of these
nuclides are generally very low, and therefore a sufficientamount of samples or a longer counting period is required toobtain reliable data. However, by the use of ultralow background Ge detectors at Ogoya Underground Laboratory (OUL),
it is possible to detect extremely low levels of environmental
radionuclides.
The radioactivity levels of airborne 210Pb (half-life: 22.3 y)and cosmic-ray-induced 7Be (half-life: 53.3 d) in the surface air
are of the order of ~mBq m –3, which is three orders of magnitude lower than that of airborne 222Rn (half-life: 3.8 d). The
concentrations of these nuclides in the air are expected to varyon a time scale comparable to that of 222Rn, but no precise measurements have been performed due to the difficulty in thedetection of low levels of 210 Pb and 7 Be without using
extremely-high volume air samplers or extremely-high sensitive
detection techniques. By using ultralow background Ge detectors at the OUL (270 m.w.e.), it is possible to detect mBq levelsof 210Pb and 7Be,1,2 which correspond to only 10 min of sampling time by an ordinary high-volume air sampler. In thisstudy, we have continuously monitored the concentration of
airborne 210Po (half-life: 138.4 d), 210Pb, and 7Be at the Low
Level Radioactivity Laborator y (LLR L) in Kanazawa
University, Nomi City; we changed filter paper of air sampler
at intervals of 2–6 hours or 1 day and investigated the correlations between temporal variations of these nuclides and meteorological conditions.
Polonium-210 is daughter product of 222Rn and 210Pb, and has
been used to estimate residence time of airborne particles,3 tounderstand air mass transportation, and to evaluate the influence of volcanic activities.4–6 The concentration of airborne
210
Po increases due to forest fires, transport of continental dust
by wind, intrusion of older air masses from the stratosphere or
the high troposphere,7 etc. Measurements of airborne 210Po
have been performed to investigate the usefulness of thisnuclide as a geochemical tracer from the viewpoints mentioned
above.
In this study, daily variations and high resolution measurements (2–6 hour intervals) of temporal variations of airborne
210
Po, 210Pb, and 7Be during the approach of typhoons and Kosa

Asian Continent*Corresponding author. E-mail: yyy@mae.osaka-cu.ac.jp.
Fax: +81-6-6605-3154.

(b)

Sea of Japan
Ishikawa Pref.

LLRL

Nomi City

Pacific Ocean

OUL

Figure 1. Location maps of (a) Ishikawa Prefecture, Japan and (b)sampling point (LLRL) and Ogoya Underground Laboratory (OUL);
they are located around 136–138°E and 36–37.5°N.

events at LLRL in Ishikawa Prefecture, which faces the Sea ofJapan, are reported. The locations of LLRL and OUL are
shown in Figure 1.
2. Experimental
2.1. Sampling. We have been monitoring and analyzing
airborne 210Po, 210Pb, and 7Be at LLRL almost every day sinceNovember 17, 2005. In addition, during meteorological events
such as the approach of typhoons and Kosa events, 2–6 hour
short-interval samplings were performed to investigate theeffects of the events.
Airborne particles were collected on a silica-fiber filter(ADVANTEC QR-100, 254 mm × 203 mm) using a high-volume air sampler (Sibata HV-1000F) at a flow rate of 700–900
L min –1 on the roof (10 m a.g.l, and 50 m a.s.l) of LLRL. In
order to measure low-level 210Pb concentrations, the blank levels of natural radionuclides in the filter papers should be sufficiently low to minimize the correction to be made in theanalysis. The blank levels of the uranium and thorium series
nuclides and 40K in an ordinary glass-fiber filter (ADVANTEC
GW-100) and a silica-fiber filter (ADVANTEC QR-100) were
measured using two and four filters, respectively.8 As seen in
Table 1 and Figure 2, it is apparent that the levels of naturalradionuclides in the silica-fiber filter are found to be orders ofmagnitude lower than those in the glass-fiber filter. Thus, thesilica-fiber filter was chosen for the sampling in this work. The© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

150 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

Yamaguchi

TABLE 1: Blank levels of natural radionuclides of a glassfiber and a silica-fiber filter
mBq/filter
Glass
Silica
122
8.00
150
13.0
81.7
7.67
65.0
1.67
53.3
1.17
4990
16.7

Nuclide
210

Pb
238
U
226
Ra
228
Ra
228
Th
40
K

Silica/Glass
Ratio
0.07
0.09
0.09
0.03
0.02
<0.01

Sampling
by using a silicafiber filter

start
t0
end

Pox

t1

Chemical Separationby using 19%of the aliquot in the silica-fiber filter
209Po tracer (8.3mBq)

Extraction with 30mL of 8N-HNO 3

Filtration
Evaporation to dryness
Distilled water
Fe3+ carrier (10mg)

Concentration by evaporation

NH4OH (to adjust pH12)

Centrifugation at 2500rpm
10

40K
226

Count rate／cph keV-1

210

Ra

Glass-fiber filter × 4 by 73% well type Ge

ppt
Dissolved with 5mL of 0.5N-HCl

Pb

Evaporation to dryness
1

Dissolved with 5mL of 0.5N-HCl
Ascorbic acid

Electrodeposition on silver plate ( for 2h at 50-70˚C)

t2

0.1

α-spectrometry

0.01

Pos

Pom

Calculation of 210Po radioactivity

Silica-fiber filter × 2 by 71% well type Ge

Figure 3. Procedure for the radiochemical analysis of 210Po.
0

250

500

750

1000

1250

1500

Gamma-ray energy / keV

Figure 2. Comparison of the radioactivity levels of a glass-fiber filter
and a silica-fiber filter.

210

Pb radioactivity in the filter was subtracted from the measured 210Pb radioactivity. The 210Po radioactivity in the filter is
expected to be nearly in radioactive equilibrium with its parent
nuclide 210Pb. However, the contribution of 210Po in the filter tothe measured 210Po radioactivity was found to be negligibly
low, probably due to the fact that 210Po was firmly held within
the silica fiber and it was not easily extracted even by 8 M
HNO3 used in chemical treatment described in sec. 2.2.2.
2.2. Measurements and data analyses.
2.2.1 Radioactivities of 210Pb and 7Be. Airborne 210Pb and7
Be that were collected on the filters were measured nondestructively by a large-volume ultralow background planar-type
and/or well-type Ge detectors at OUL.9 The efficiency of thedetectors was calibrated using mock-up standards preparedfrom a mixture of the reference material No. 42-1 (4.04% uranium in radioactive equilibrium issued from New Brunswick
Laboratory (NBL, USDOE)) and a pulverized silica-fiber filter.
In order to measure γ -rays of 210Pb and 7Be, 78% of the aliquot in the filter was compressed into a disk with a diameter of
35 mm using a hydraulic press to prepare a counting source.
The measurements were carried out for one to several days
after the disintegration of the short-lived nuclides 214Pb and212
Pb. The 210Pb and 7Be radioactivities were determined by
employing the peak counts of 46.5 keV (Iγ = 4.05%) and 477.8
keV (Iγ = 10.59%), respectively, and were corrected for radioactive decay. The concentrations of these radioactivities (Bq m–3)
were calculated on the assumption that they were constant during each sampling interval.
2.2.2 Radioactivity of 210Po. In order to measure the radioactivity of 210Po collected on the filters, radiochemical separations were performed within one week (several days) after

sampling to suppress the ingrowth of 210Po from 210Pb and 210Bi(half-life: 5.0 d).
As depicted in Figure 3, 210Po was extracted from 19% of thealiquot in the silica-fiber filter by heating the filter at 250 °C
with 30 mL of 8 M HNO3 and using 209Po (8.3 mBq) as a yield
tracer. The extract was filtrated through an ordinary filter
paper and was concentrated at 250 °C. Equal amounts of distilled water and the concentrated HNO3 solution were mixed
along with 10 mg of Fe3+ ions which was added as a carrier.
Subsequently, polonium (210Po and 209Po tracer) was coprecipitated with Fe(OH)3 at pH 12 which was maintained by using
NH4OH. The Fe(OH)3 precipitate was collected after centrifuging it at 2500 rpm and redissolved in 5 mL of 0.5 N HCl,and then dried up again. The residue was dissolved in 5 mL of
0.5 N HCl, and a sufficient amount of ascorbic acid was added
to clear the solution by masking Fe3+. A 0.2-mm thick silver
plate with a size of 10 × 10 mm was immersed in the sample
solution for 2 hours at 50–70 °C for electrochemically depositing polonium on it. The polonium radioactivity deposited on
the silver plate was measured at LLRL by α -spectrometry
using a surface-barrier-type Si detector.
The concentration of 210Po was calculated by the following
process. The 210Po radioactivity at the time of measurement(Pom) was first calculated from the radioactivity of spiked 209Po
and the 210Po/ 209Po counts ratio. Then, the 210Po radioactivity
during the chemical separation (Pos) was calculated by correcting for radioactive decay for the time interval between the electrodeposition and the measurement (t2). Next, the 210 Po
radioactivity at the end of the sampling (Pox) was calculated bysubtracting the contribution of 210Po derived from 210Pb and210
Bi during the time between the end of sampling and the separation (t1). The 210Po concentration in the atmosphere was
finally calculated by considering the growth and decay of 210Pb,
210
Bi, and 210Po during sampling (t0), although, in this study,
the magnitude of this effect was negligibly low because t0 was
less than one day.

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007 151

High Resolution Measurements of Temporal Variations of Airborne

mBq m-3

L051117-060331

10

Concentration

1

0.1

0.01

7 Be
210 Pb210 Po0.001
210

Activity ratio

Po/

210

Pb

0.1

1

0.1

0.01

Activity ratio

Concentration／mBq-m3

100

10

1
0.75
0.5
0.25
0

0.05

0

mm/h

2

precipitation

1.5
1

12

s

0.5

8

Jan., 2006

Feb., 2006

Mar., 2006

Figure 4. Daily variations of airborne 7Be, 210Pb, and 210Po concentrations, and 210Po/ 210Pb radioactivity ratios at LLRL from November 17,
2005 to March 31, 2006.

mDec., 20054hNov., 2005

15

mm

0

10

0

5
0

3. Results and Discussions
3.1. Daily variations of airborne radionuclides. Figure 4
shows the variations of 7Be, 210Pb, and 210Po measured daily
from November 17 in 2005 to March in 2006 together withdaily precipitation. Measured period was only 5 months, however, general variation patterns of 210Pb and 7Be were found tobe very similar; lowest in December and gradual increase
toward March. In the previous measurements10 (May to
December, 2003) conducted simultaneously at LLRL, HeguraIsland (located 50 km from Wajima), and Shishiku Plateau (10km East of LLRL), seasonal variation patterns of 210Pb and 7Be
were found to be almost the same; decrease from spring (highest value at May) to summer (minimum in July–August), thengradual increase toward November and decrease again in winter (December–January). The lack of previous measurement
could be covered by the present study.
In the present study, characteristic variations of about a weekof cycle was first recognized both for 7 Be and 210 Pb in
December and January. As pointed by arrows, peak concentrations of 7Be and 210Pb in each cycle were observed mostly justafter the occurrence of precipitation (snow fall), therefore, thevalley shaped decrease of 7Be and 210Pb radioactivities may be
explained by being scavenged due to snow fall.
With regard to 210Po, general trend in its variation pattern issimilar to that of the parent nuclide 210Pb, because both nuclides
are long-lived daughter nuclides of 222Rn. This similarity isoften obscured on days when rain and/or snow fall, which
shorten apparent residence time of airborne particles. As
known from Figure 4, high 210Po/ 210Pb radioactivity ratio was
observed mostly at the time when 210Pb concentration showed
its minimum in each cycle. Although not confirmed yet, high210
Po/ 210Pb values over 0.1 may be explained by the effect offine particles, which may act as the nucleus of snowflake,
derived from the resuspension of the surface soil containing
high 210Po/210Pb radioactivity ratio (nearly in equilibrium).
3.2. Variations of radioactivities during the approach of
typhoon. Figure 5 shows the variations of the 7Be, 210Pb, and210
Po concentrations and the 210Po/ 210Pb radioactivity ratio during the approach and passage of typhoon No. 14 on September
5–9, 2005. The concentrations of 210 Pb and 7 Be showed adecrease when the typhoon passed over the Sea of Japan on
September 7.
The concentrations of 210Pb and 7Be were measured to be in
the ranges of 0.053–0.349 and 0.141–2.23 mBq m –3, respec-

Figure 5. Variations of airborne 7Be, 210Pb, and 210Po concentrations,and 210Po/ 210Pb radioactivity ratios during the approach and/or passage of a typhoon (September 5–9, 2005). The meteorology data were
supplied by the Environment Radiation Department, Ishikawa
Prefectural Institute of Public Health and Environmental Science.

tively, and the minimum values during the typhoon period wereapproximately 1/20–1/10 of the corresponding values in calm
weather. These minimum concentrations were observed at atime (approximately 9:00 on September 7) close to the passageof the typhoon. Since significant rainfall was not observed at
this time, the predominant removal process for airborne radionuclides was the inflow of the air masses containing low-concentration 7Be and 210Pb, and not due to washout by rainfall.
A typhoon is a tropical cyclone originating from the low-latitude Pacific Ocean. The concentration of airborne 7Be is low
in the low-latitude area. Moreover, the production rate of 210Pb
is very low on the sea because of the extremely low emanationof parent 222Rn from the ocean surface.11–14 Around the typhoon,strong winds raise sea-salt particles showing high 210Po/ 210Pb
radioactivity ratio (approximately 0.6)15 from the ocean surface.16,17 The sea-salt particles proceed northward with the air
masses containing low-concentration 7Be due to strong winds.
As a result, it is considered that the 210Pb and 7Be concentrations became low and the 210Po/210Pb radioactivity ratio showed
high during the typhoon period.
On the other hand, the concentration of cosmogenic 7 Be
showed a drastic decrease (1/20–1/10) when compared withthat of 210Pb. Consequently, the radioactivity ratio 210Pb / 7Be
showed a considerably high value (1.43 ± 0.58). Abe et al.9
reported that this kind of variation was also observed in thecase of the passage of cold fronts. These observations may be
explained under the following assumptions.
Both the nuclides 210Pb and 7Be attached to aerosol particles
were once scavenged by the washout process, but the inflow(recovering) rates of these nuclides are different due to the difference in their origin. The inflow of 7Be from the upper atmosphere is slow as compared with that of 210Pb originating fromairborne 222Rn near the ground surface.
3.3. Variations of radioactivities during Kosa events. At
least three Kosa events were observed in April 2006 (April 8,

152 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

4. Conclusion
Daily variations and highly time-resolution analysis (2–6
hours) of airborne 210Po, 210Pb, and 7Be have been investigatedat Ishikawa Prefecture, which faces the Sea of Japan.
When large-scale meteorological changes occurred, as in thecase of the typhoons and Kosa events, characteristic variation
patterns were observed in the 210Pb and 7 Be concentrations.
High resolution measurements of temporal variations of theairborne radionuclides proved to be very useful in investigating
the geochemical behaviors of airborne particles during meteo-

mBq m-3

100

Concentration

18–19, and 24–25), and sampling was performed at approximately 2–6 hour intervals around the Kosa events and at 1 day
intervals for the other days. As observed in Figure 6, the concentrations of all three radionuclides 7 Be, 210 Pb, and 210 Po
showed increases in the 2nd and 3rd events, and in all theevents 210Po/ 210Pb radioactivity ratios showed higher values(max: 0.28) compared with those in other days. The amount of
airborne particles showed extremely high values, however, nocorrelation was observed between the amount of particles andconcentrations of these nuclides. On the other hand, the concentrations of 7Be and 210Pb showed decrease in the 1st event(April 8). These phenomena may be explained as follows.
Kosa particles originating from arid zones are raised up to
2–5 km above the ground18 by convection, carried over theJapanese islands by strong westerlies, and descend gravitationally and/or by downward current of air mass to the ground surface. On the other hand, the concentrations of cosmogenic 7Be
produced in the stratosphere and upper troposphere,19 and 210Pb
in the stratosphere are greater in value as compared to the values in the upper troposphere.20,21 It is well known that the concentrations of airborne 7 Be and 210 Pb shows high values inJapan during the spring season23 due to the active inflow of air
mass from the stratosphere (which shows a high 210Po/ 210Pb
radioactivity ratio)15 to the upper troposphere in mid-latitude
regions.21,23,24 Abe et al.9 suggested that the active inflow of air
mass is one of the reasons for the high 210Pb and 7Be concentrations during Kosa events; the high concentrations should not be
due to radionuclides being attached to Kosa particles and their
being washed out together, because no correlation was observed
between the concentrations of Kosa particles and radionuclides.
The inflow flux of upper aerosol particles to the lower atmosphere is independent of the concentration of Kosa particles;
hence, the mass concentration of aerosol particles is not proportional to the 7Be and 210Pb concentrations. The decrease of7
Be and 210Pb concentrations in the 1st event may be explainedby the influence of rainfall.
Abe et al.9 reported that anthropogenic 137Cs was detected insome of samples collected during Kosa events. Contrary to thecases of 7Be and 210Pb, the relationship between 137Cs and themass concentration of airborne dust showed a linear correlation, which indicates that the 137Cs collected during Kosa events
originates from those attached to and transported together with
Kosa particles.
The suggested reasons for the high 210Po/ 210Pb radioactivity
ratios during Kosa events are the active inflow of air containing high 210Po/ 210Pb values, and also the high concentration ofKosa particles in which nuclides from 222Rn to 210Po in uranium
decay series can be assumed to be in radioactive equilibrium(210Po/210Pb = 1.0).7 However, all of 210Po in Kosa particles cannot completely be extracted by radiochemical extraction using
8 M HNO3 (sec. 2.2.2). This may be the reason why clear correlation between the increase of the weight of Kosa particles
and that of 210Po/210Pb radioactivity ratios were not observed.
It is considered that the measurements of the size distribution of 7Be, 210Pb, and 210Po is profitable in order to know if
Kosa particles have an efficiency as a scavenger of aerosol particles.

Yamaguchi

10

1

0.1

0.01
0.6

0.6

0.5
0.4

0.5
Dust

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0

0

12
9
6
3
0

Sampling date in April, 2006

Figure 6. Effect of the arrival of Kosa events on airborne 7Be, 210Pb,
and 210Po concentrations, and 210Po/ 210Pb radioactivity ratios (April,
2006). The meteorology data were supplied by the Environment
Radiation Department, Ishikawa Prefectural Institute of Public Healthand Environmental Science.

rological variations resulting from air mass movements.
With regard to 210Po, the general trend in its variation patterns was similar to that of 210Pb, which is the parent nuclide of210
Po. However, characteristic variation in the 210Po/210Pb radioactivity ratio was observed to be depending on the weather
condition.
Acknowledgment. This study was performed as a part of
the Kanazawa University 21st-Centur y COE Program“Environmental Monitoring and Prediction of Long- and Shortterm Dynamics of Pan-Japan Sea Area.”
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