Journal of Nuclear and Radiochemical Sciences, Vol. 8, No.1, pp. 5-9, 2007

Highly Time-Resolved Measurements of Airborne Radionuclides by Extremely Low
Background γ-ray Spectrometry: Their Variations by Typical Meteorological Events
Takuya Abe,* Yoshiko Yamaguchi, Kiwamu Tanaka, Yusuke Nakano, and Kazuhisa Komura
Low Level Radioactivity Laboratory, K-INET, Kanazawa University, Nomi, Ishikawa 923-1224, Japan
Received: January 16, 2007; In Final Form: March 5, 2007
In order to investigate variation of concentrations of airborne radionuclides, 214Pb, 212 Pb, 7 Be, and 210 Pb,
accompanied with a typical meteorological event, such as passage of cold front, approach of typhoon, and Asian
dust (Kosa), highly time-resolved (2–6 hours) measurements of natural radionuclides were performed by ultra low
background γ-ray spectrometry. Surface air concentrations of these nuclides showed characteristic temporalchanges by each meteorological phenomenon. In addition, variation patterns were found to be obviously different
between short-lived 214Pb and 212Pb and long-lived 7 Be and 210Pb. One of causes of the difference might be
explained by difference of their sources: local for short-lived nuclides and regional for long-lived ones.

1. Introduction
Most of the radionuclides in the atmosphere are attached toaerosol particles and behave along with these particles.
Therefore, they have been considered to be useful tracers forthe analyses of various atmospheric processes. In this reason,
observations of dynamic state of atmospheric radionuclides
have been performed to investigate the processes such as transport and/or mixing of air masses, which are characterized byresidence times of aerosol particles in the atmosphere, etc. A
number of investigations have been made for explaining
regional atmospheric processes with the use of atmosphericconcentrations and/or deposition fluxes of 210Pb (half-life: 22.3y), which is a long-lived decay product of 222Rn emanating fromground surface, and 7Be (53.3 d) produced by the nuclear spallation reaction between secondary cosmic rays and nuclei in theupper atmosphere.1- 6 On the other hand, clarifications ofadvection and diffusion processes and removal processes ofaerosol particles attaching the nuclides from the atmospherehave been attempted from variability analyses of short-lived
214
Pb and 212Pb which are decay products of airborne 222Rn and
220
Rn, respectively.7-11
The behavior of aerosols depends largely on meteorologicalfactors of ambient air, conditions of which change sensitively
with atmospheric phenomena such as Kosa events and others.
In order to use airborne radionuclides attaching to aerosol particles as tracers, it is important to perform observations withtime intervals suitable to time scale of atmospheric phenomenaof interest. In humid climate regions, wet deposition accompanied with rainfall or snowfall is a dominant removal process ofaerosols from atmosphere and it often occurs in a few hours
order. That is, observation with short time intervals is desirable to discuss the removal processes such as precipitation
scavenging. Additionally, observations on various radionuclides with different particular life-times and sources simultaneously would provide various time and spatial information on
weather changes.
Ogoya Underground Laboratory (OUL) of Low Level
Radioactivity Laboratory (LLRL), Kanazawa University, wasopened in a tunnel of former Ogoya copper mine with rockcovering of 135 m (270 m.w.e.) in 1995. Background levels of
16 Ge detectors equipped in OUL, which are shielded withthick rock cover and lead manufactured more than 200 years
ago, are about 2 orders of magnitude lower than ordinary low
background Ge detectors in ground-level laboratories. Gammaray spectrometry using 16 Ge detectors at OUL allows to measure long-lived 7Be and 210Pb along with short-lived 214Pb and212
Pb with higher time resolution (0.5–3 hours, corresponding
to only 30-180 m3 of air volume).12,13 In this paper, we reportconcentration variations of these nuclides during the times oftypical weather change events described above.
2. Experimental
2.1. Sampling. Sampling of aerosol particles was performedon the roof (10 m above ground level, and 50 m above sea level)of LLRL, Kanazawa University in Nomi, Ishikawa Prefecture,
Japan. Location map is shown in Figure 1. Aerosol particles
were collected on a weighed filter using a high volume air sampler (SIBATA HV-1000F) at a flow rate of 900 L min-1 with 2–
6 hours of sampling intervals which were varied depending on
the atmospheric condition on each occasion. Silica fiber filter(ADVANTEC QR-100, size: 203 mm × 254 mm) was chosen
for sampling because concentrations of naturally occurring
radionuclides (210Pb: 1.2 mBq g-1, 40K: 5.3 mBq g-1) were foundto be much lower than those in glass fiber filter (GB-100R810A).14 Sampling was made during the time of (1) passage ofcold front (2004/12/18 and 20 Japan Standard Time: J.S.T.), (2)approach of typhoon (2005/9/7 J.S.T.), and (3) Kosa event(2006/4/8, 18-19, and 24-25 J.S.T.). Referring informationfrom the local meteorological observatory, highly time-resolved
observations (samplings) had been carried out during a few
lat. 38˚ N

50˚

(a)

(b)

40˚

The Sea of Japan

37˚
Tokyo

Pacific Ocean
30˚

Kanazawa

LLRL
OUL
25 km
36˚

*Corresponding author. E-mail: abetaku@llrl.ku-unet.ocn.ne.jp
Fax: +81-761-51-5528

136˚

137˚

20˚
long. 138˚ E 120˚

500 km
130˚

140˚

150˚

Figure 1. Location map of the sampling point and the OUL, withoverview of near Japan.

© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 05/10/2007

Abe

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 200715

Pb activity/ Bq m-3

12
9
6
3
0
80
60
40
20
0
10
8
6
4

7

where C, λ, t1, t2, and t3 are the area of peak counts during thecounting time, the disintegration constant (s-1), the time of air
sampling (s), the time difference between the end of samplingtime and the start time of measurement (s), and the time difference between the end of sampling time and the end time of
measurement (s), respectively. And F, ε, and B denote the flowrate of air sampler (m3 s-1), the detection efficiency of detector
at the energy of γ-ray from the corresponding nuclide, and thebranching ratio of γ-ray emission, respectively. This formula
implies that the concentration a is an average value during eachsampling interval.
The average wind speed and precipitation data at every 10
min intervals of on-site and surface weather chart around Japan
were obtained from Ishikawa Prefectural Institute of Public
Health and Environmental Science and from Meteorological
Agency of Japan (http://www.jma.go.jp/jma/indexe.html),
respectively.
Ordinary observations made at sampling intervals of 1–2
day(s) for the other study were performed continuously since2003. Parts of the data from these observations (not publishedyet) were appropriately used for comparison with present study.

214(1 - eC λ2)(e-λ t2 - e-λ t3)Fε B- λ t1

Pb activity/ mBq m-3a=might be explained mainly by washout due to rainfall. It isnoteworthy that the decreases of long-lived nuclides at the passages of cold fronts were more predominant than those of
short-lived ones. This may be explained that the activities ofshort-lived nuclides are rapidly recovered by the decays of theirgaseous precursor radon isotopes, which are not removed by
precipitation. On the other hand, supplies of long-lived 7 Be
and 210Pb in the surface air require longer times. As anotherexplanation, differences of efficiencies of washout due to sizesof aerosol particles containing the radionuclides might be considered. According to Papastefanou,15,16 sizes of aerosol particles (activity median aerodynamic diameter: AMAD) attaching
long-lived 7Be and 210Pb tend to be larger than those attaching
short-lived 214Pb and 212Pb; i.e. 214Pb: 0.07–0.21, 212Pb: 0.09–
0.23, 7Be: 0.29–0.50, and 210Pb: 0.28–0.49 µm. On the other
hand, as reported by Hirose et al,17 a correlation between washout ratios (meaning efficiencies of washout) and particle diameters for aerosol particles with submicrometer size is not soclear. Therefore, differences of efficiencies of washout between
long-lived 7Be and 210Pb and short-lived 214Pb and 212Pb may becaused by differences of supply rates between them.
It is also noted that the concentrations of 7 Be and 210 Pb
showed an increasing tendency along with the approach of cold
front. A cold front transported over Japan area in winter season is provided by proceeding in the east direction of cold air
mass which originates with Siberian High pushing up warm air
mass over the Sea of Japan. Yamamoto et al18 observed
remarkably high monthly deposition in the locations of theJapan Sea side in winter season, and explained as follows. An
inversion layer, which inhibits diffusion of 222Rn and its shortlived progeny to upper atmosphere, is formed by radiation

212

days before and a few days after the each event in the case of(1) and (2). However, at the Kosa events, samplings werestarted at the time when the event was confirmed visually.
2.2. Gamma-ray measurements. In order to measure
short-lived 214Pb and 212Pb, γ-ray counting source was prepared
immediately after the sampling as follows. After few minutesof drying under an infrared lamp and weighing (except period(1)), 77% aliquot of the filter was compressed into a disk with
35 mm in diameter and 3 mm in thickness using hydraulicpress. Gamma-ray measurements of 214Pb and 212Pb were performed by using 2 coaxial-type Ge detectors installed in LLRL
for 2–12 hours. In order to measure long-lived 7Be and 210Pb,
after disintegration of short-lived nuclides, remeasurements
were performed for 2–5 days by using ultra low backgroundplaner- and well-type Ge detectors at OUL.
2.3. Data analyses. Peak counts of 46.5, 239, and 478 keV
γ-rays were used to determine 210Pb, 212Pb, and 7Be, respectively.
On the other hand, summation of the count at 242, 295, and352 keV γ-rays was used for 214Pb. Detection efficiencies ofeach detector were calibrated by the measurements of standard
sources prepared from the mixture of reference material No.
42-1 (4.04% uranium) issued from New Brunswick Laboratory(NBL), USDOE and silica fiber filter as a host material.
Activity of an airborne radionuclide a (Bq m-3) at the sampling was calculated by the following formula:

Be activity/ mBq m-3

62

3. Results and discussion0

Pb activity/ mBq m-3

2

210

3

1

Wind speed Precipitation/ m s-1/ mm h-1

3.1. Variation of concentrations of airborne radionuclides
during the passage of cold front. During period from
December 17 to 22, 2004, when two cold fronts passed overthis area, highly time-resolved sampling was performed toinvestigate the effect of the passage of cold front event.
Variations of 214Pb, 212Pb, 7Be, and 210Pb concentrations, windspeed, and precipitation are plotted in Figure 2. During thisperiod, the concentrations of all nuclides measured were varying more than an order of magnitude; 1.47±0.04–14.1±0.2 Bq
m -3 for 214 Pb, 9.28 ± 0.78 – 83.2 ± 6.5 m Bq m -3 for 21 2 Pb,
0.149±0.034–9.71±1.00 mBq m-3 for 7 Be and 0.105±0.023–
3.13±0.24 mBq m-3 for 210Pb.
According to surface weather conditions, two cold fronts had
passed over the sampling location around 9 p.m. on Dec. 18
J.S.T. and around 6 p.m. on Dec. 20 J.S.T., respectively. As canbe seen in Figure 2, all of the nuclides showed clear decreases
just at the times of the passages of cold fronts. This decrease

0
8
6
4
2
0
8
6
4
2
0
12/17

12/18

12/19

12/20

12/21

12/22

12/23

Sampling date in 2004

Figure 2. Variation of airborne radionuclides, precipitation and windspeed during the event of passages of cold fronts on Dec. 19–20,
2004.

Highly Time-Resolved Measurements of Airborne Radionuclides

7

Pb began just before the closest time when strong wind
started to blow. Because the origins of the short-lived nuclides
are neighborhood region, their concentrations hardly decrease
only by transport of maritime air mass without strong wind.
The drastic decreases of the short-lived nuclides are considered
to be caused not only by horizontal mixing with maritime air
mass but also by vertical mixing with upper atmosphere, inwhich concentrations of precursor radon isotopes are low, due
to turbulence caused by strong wind.
It is noteworthy that 210Pb/7Be activity ratio varied more than
20 times from minimum value of 0.065±0.024 to maximum
value of 1.43± 0.58 during the approach of the typhoon.
(a)

(b)

2005/9/7 9:00 JST

Figure 3. (a) The surface whether chart at the time of the closest
approach of the typhoon No. 14 in 2005 and (b) the tracks of thetyphoon. (Diverted from the website Meteorological Agency, Japan,
http://www.data.kishou.go.jp/yohou/kaisetu/hibiten/index.html).

9
6

214

Pb activity/ Bq m-3

12

3

212

Pb activity/ mBq m-3

0
120
80
40
0
3

7

Be activity/ mBq m-3

2.5
2
1.5
1
0.5

210

Pb activity/ mBq m-3

0
0.4

Wind speed Precipitation Dust conc.
-3/ m s-1/ mm h-1/ µg mcooling in Asian continent. As a consequence, cold air mass
originating from the continent in winter contains higher 210Pb
activity than in the other seasons. Additionally, a cold air mass
also comes down from arctic region of high latitude where production rate of 7Be is high to Japan region in the same season.
Therefore, convection cloud developed by transport of the cold
air masses causes rainfall and/or snowfall containing high 7Be
and 210Pb concentrations over the Japan Sea side in winter season. The increases of atmospheric concentrations of these
nuclides during the approach of the cold front observed in thisstudy may also be explained by the same mechanism.
Rapid decrease of the long-lived nuclides in the afternoon of
Dec. 22 is considered to be caused by washout due to snowfall
which started at that time. In this connection, studies19,20 suggested that washout effect of snowfall is greater than that of
rainfall because snow particles concentrate more aerosol particles due to their larger surface areas and longer flight duration
than raindrops.
3.2. Variation of concentrations during the approach of
Typhoon. In order to know effects of a typhoon on the surface
air concentrations of the nuclides, highly time-resolved observation was performed during the period of September 5 to 9,
2005. On Sep. 7, typhoon No. 14 passed over the Sea of Japan.
The closest approach of the typhoon was at about 9 a.m. J.S.T.
when it was proceeding due northeast located about 200 km
northwest of the sampling location. Figure 3 shows a surface
weather chart at that time and a track of the typhoon. From thepressure distribution, it can be confirmed that the sampling
location was involved in effective area of the typhoon.
Variations of the nuclide concentrations, mass concentration of
airborne dust, wind speed and precipitation are shown in
Figure 4. The concentrations of 214Pb, 212 Pb, 7 Be, and 210 Pb
were measured to be in the ranges of 0.272±0.014–10.4±0.1 Bq
m-3, 7.97±0.81–131±10 mBq m-3, 0.141±0.048–2.23±0.26 mBqm-3, and 0.053±0.017–0.349±0.030 mBq m-3, respectively, andthe minimum values during the typhoon period were approximately 1/20–1/10 of the corresponding values obtained in calm
weather in ordinary observations. On the contrary, the higher
mass concentrations of airborne dust than usual level of 20–40µg m-3 were observed along with the approach of the typhoon.
A maximum value of 110 µg m-3 was observed just after thetime of the closest approach of the typhoon. This increase may
be explained not by the contribution of soil components stirred
up by strong wind, but by sea salt particles generated massively
on the ocean because of no increase of natural uranium andthorium series nuclides, 40K, and artificial 137Cs which are of
typical soil particle origin.
Since occurrence of minimum concentrations of all the
nuclides coincided with the closest time of typhoon, unusual
decreases of their concentrations may be attributed to effect of
the typhoon. The decreases of concentrations of the nuclides
are caused by the fact that the typhoon originates from low latitudes Pacific Ocean. There is a report 21 which showed that
low concentrations of 7Be in surface air at low latitudes region
is explained by low production rate of 7Be at upper atmosphere
due to low geomagnetic latitude and low inflow of upper atmosphere to the lower atmosphere due to updraft of the atmospheric general circulation. In addition, on the ocean, not only
concentrations of short-lived 214Pb and 212 Pb but also that oflong-lived 210 Pb is low6 due to little dissipation of precursor
radon isotopes from seawater. Moreover, most of the aerosol
particles are scavenged by the rain during the transport oftyphoon and, consequently, radionuclides attached to aerosol
particles are effectively removed from air mass carried bytyphoon. Because a typhoon transports such maritime air mass
with low nuclide activities, it seems reasonable that the concentrations of long-lived 7 Be and 210 Pb had decreased since few
days before the closest time of the typhoon. On the other hand,the decreases of the concentrations of short-lived 214Pb andJ. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007
212

0.3
0.2
0.1
0
120
90
60
30
0
5
4
3
2
1
0
12
8
4
0
9/5

9/6

9/7

9/8

9/9

9/10

Sampling date in 2005

Figure 4. Variation of airborne radionuclides, dust concentration,
precipitation and wind speed during the event of an approach of atyphoon on Sep. 7–8, 2005.

8

Abe

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007
7

Therefore, there was no positive correlation between Be and210
Pb during this period, although positive correlation has generally been seen in ordinary observations.2,4,22,23 Such a large
fluctuation of the 210Pb/7Be ratios was also observed at the timeof passage of cold front described above. This kind of variation may be explained by a hypothesis that although both
nuclides attached on aerosol particles are once scavenged by
washout process, flow-in rates of these nuclides are different
due to the difference of their origin, i.e. 7Be from upper atmosphere and 210Pb from land surface.
3.3. Variation of concentrations during Kosa event.
Three Kosa events were observed at the sampling location in
April 2006. Variations of the concentrations of the airborne
radionuclides during these periods are shown in Figure 5
together with mass concentrations of airborne dust and meteorological data. Drastic increase of the mass concentration up
to 630 µg m-3 (value of sampling for about 3 hours), which ismore than 10 times higher than ordinary values of 20–40 µg
m-3, was observed during the Kosa event.
Short-lived radionuclides, 214Pb and 212Pb, showed essentiallythe oft-reported diurnal variation,8,11,24 i.e. the concentrations ofthese nuclides are high at first hours of morning and low atevening and decrease due to strong wind and/or precipitation.
Kosa particles were transported from the Asian continent toover Japan area for 2 days at the shortest case. Since the traveling time is 4–5 times longer than the half-life of 212 Pb, we
expect a little effect of 212Pb originating from the continent. It
is also difficult to distinguish from predominant component

Pb activity/ Bq m-3

8212

occurring from the local area. The effect of Pb derived froma long range transport may be observed if sampling is performed at higher altitude, over the sea or other locations where
the concentration of 212Pb is expected to be extremely low.
As can be seen in Figure 5, the concentrations of long-lived
7
Be and 210 Pb were high during the Kosa events in April.
Relationships between the concentrations of 7Be and 210Pb andthat of airborne dust are given in Figure 6 along with data ofordinary daily observation in April. Figure 6 suggests that theconcentrations of both 7Be and 210Pb tend to be high according
to the increase of the dust concentrations in surface air although
there seems to be no linear relationship between radionuclides
and dust concentrations. This can be explained as follows. It
is well known that concentrations of both 7Be and 210Pb in surface air are high in Japan in spring seasons.2,4 This is explainedby the inflow of air from stratosphere containing rather highamounts of cosmogenic 7Be and terrigenous 210Pb to upper troposphere due to folding of tropopause at mid-latitude region inspring season.5,25 On the other hand, Kosa particles are blown
up from land surface to upper atmosphere up to altitude of several thousands meters at dry regions, where are main origins ofKosa particles, due to upward current accompanied with strong
storm. And then, Kosa particles are transported by westerlies
and deposited on ground surface by gravitational settling and/or downward current. Therefore, it is considered that aerosol
particles attaching 7Be and 210Pb descend from the upper troposphere when Kosa phenomena are observed. However, since
the sources of Kosa particles and the aerosol particles attaching
the long-lived nuclides are different, there may not be linear
relationship between them. As a result described above, it is

6

12

214

4

7Be

(b)

210Pb

8

2

N.A.
Activity Concentration/ mBq m-3

0
100
80
60
40

212

Pb activity/ mBq m-3

(a)

20
0

4
0
1.8
1.2

0.6

0

8
6

400

600

Dust Concentration / µg m-3

Figure 6. Relationships between atmospheric concentrations of (a)7
Be and (b) 210Pb and dust concentration in Apr. 2006 including Kosaevents.

0
2
1.6

40

1.2
0.8

210

Pb activity/ mBq m-3

200

Kosa event

4
2

30

20
y = 0.052x
R = 0.93

137

0
600
400
200
0
8
6
4
2
0
10
8
6
4
2
0
4/8

Cs activity / µBq m-3

0.4

Wind speed Precipitation Dust conc.
-3/ m s-1/ mm h-1/ µg m0

Normal time7

Be activity/ mBq m-3

10

10

4/9

4/10 4/18

4/19

4/20

4/21 4/24

4/25

4/26

4/27

Sampling date in 2006

Figure 5. Variation of airborne radionuclides, dust concentration,
precipitation and wind speed during the event of influxes of Kosa on
Apr. 8, 18–19 and 24–25, 2006.

0

0

200

400

600

Dust Concentration / µg m-3

Figure 7. Relationship between atmospheric concentration of 137Cs
and dust concentration in Apr. 2006 including Kosa events.

Highly Time-Resolved Measurements of Airborne Radionuclides
7

210

considered that rather high concentrations of Be and Pb in
surface air were observed at the time of Kosa events.
It is also noted that anthropogenic 137Cs were detected insome of samples collected during Kosa events. Relationshipbetween the 137Cs and mass concentration of airborne dust is
shown in Figure 7. As is clear in this figure, a linear correlation can be seen different from the cases of 7Be or 210Pb mentioned above. There is no source of 137Cs in the atmosphereincluding stratosphere. The 137Cs deposited on land surface isconsidered to be transported together with Kosa particles due
to re-suspension process. From this result, a specific activityof 137Cs in Kosa particles obtained during the Kosa events is
estimated to be 52 mBq g-1.
4. Conclusion
Highly time-resolved measurements of the surface air concentrations of 214Pb, 212 Pb, 7 Be, and 210 Pb (+137Cs) were performed during the period of typical weather change events,
passage of cold front, approach of typhoon, and Kosa events. It
is noteworthy that variation patterns obviously differed
between short-lived 214 Pb and 212 Pb and long-lived 7 Be and210
Pb. The short-lived nuclides showed connections with windspeed which is corresponding to a local meteorological element. In contrast, concentration variations of long-lived
nuclides are influenced by larger scale meteorological phenomena. The difference is explained mainly by the difference
between origins of short-lived and long-lived nuclides, namely,
local and regional ones, respectively. Therefore, highly timeresolved measurements of long-lived nuclides in surface air arevery useful to know sources and histories of air masses including marked phenomena.
Moreover, differences between long-lived nuclides, whose
causes are assumed to be due to difference of their origins,
could be seen. This study is qualitative currently, but it isexpected to be quantitative by advance in further researches.
Acknowledgements. This work was partly supported by theEnvironment Radiation Department, Ishikawa Prefectural
Institute of Public Health and Environmental Science and was
one of the Kanazawa University 21st-Century COE Program‘Environmental Monitoring and Prediction of Long- and ShortTerm Dynamics of Pan-Japan Sea Area’.
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