CONCENTRATIONS OF 222 RN, ITS SHORT-LIVED DAUGHTERS AND
212
PB AND THEIR RATIOS UNDER COMPLEX ATMOSPHERIC
CONDITIONS AND TOPOGRAPHYTOSHIO KATAOKA, EIJI YUNOKI, MITSUO SHIMIZU and TADASHIGE MORI
Okayama Prefectural Institute for Environmental Science and Public Health, 739-1, Uchio,
Okayama 701-0298, JapanOSAMU TSUKAMOTO, SATOSHI TAKAHASHI and HIRONORI FUDEYASU
Faculty of Science, Okayama University, 3-1-1, Tsushimanaka, Okayama 700-0082, JapanYUKITAKA OHASHI
Department of Geophysics, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto
606-8502, JapanKEN SAHASHI
Okayama University, 3-1-1, Tsushimanaka, Okayama 700-0082, JapanTOSHIHIKO MAITANI and KOH’ICHI MIYASHITA
Research Institute for Bioresources, Okayama University, 2-20-1, Chuhoh, Kurashiki 710-0046,
Japan

TORU IWATA
Environmental Science and Technology Department, Okayama University, 2-1-1, Tsushimanaka,
Okayama 700-0082, Japan

TAKAYUKI SASAKI, YOKO FUJIKAWA and AKIRA KUDO
Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-Gun, Osaka 590-0451, Japan

ROGER H. SHAW
Land, Air and Water Resources, University of California, One Shields Avenue, Davis, California
95616-8627, U.S.A.

(Received in final form 14 June 2002)

Abstract. Atmospheric activity concentrations of 212 Pb and short-lived 222 Rn daughters, together
with meteorological elements, have been observed continuously at three sites at Kamisaibara Village
in Japan. In addition, atmospheric activity concentration of 222 Rn, equilibrium-equivalent concentration of 222 Rn and conditions of the lower atmosphere were observed for three intensive observation
periods at Akawase, one of the three sites in Kamisaibara Village. The equilibrium-equivalent concentration of 222 Rn is almost the same as the atmospheric activity concentration of short-lived 222 Rn
daughters.
The activity concentrations of 212 Pb and the short-lived 222 Rn daughters and their ratio were low
in the daytime owing to convective mixing, and high at night owing to the surface-based inversion
during periods of no precipitation. Their variations have several patterns corresponding to the scaleof the drainage wind or weak mixing.
Mechanical mixing due to strong winds through both day and night during the first and second
observation periods made the atmospheric activity concentrations of 212 Pb and the short-lived 222 Rn
daughters continuously low. However, their ratios were continuously high during the first period
yet continuously low during the second period. This difference can be explained by the effect ofBoundary-Layer Meteorology 107: 219–249, 2003.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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TOSHIO KATAOKA ET AL.

extraction of 220 Rn and 222 Rn due to strong winds and snow cover. There were also cases in which
the ratio of the atmospheric activity concentration of 212 Pb to that of the short-lived 222 Rn daughters
at night was equal to or less than the ratio in the daytime. This inverse trend, as in the periods of no
precipitation mentioned above, is considered to be due to near-neutral conditions on these nights.
We find a difference in the ratio of the equilibrium-equivalent concentration of 222 Rn (the activity
concentration of short-lived 222 Rn daughters) to the activity concentration of 222 Rn during the firstobservation period and that during the second. The difference can be explained by snow cover on
the ground. We also find differences among the ratios of the activity concentration of the short-lived
222 Rn daughters to that of 222 Rn during the three observation periods. These differences can be
explained by the submergence of paddy fields.
Keywords: Concentration of 212 Pb, Concentration of 222 Rn, Concentration of short-lived 222 Rn
daughters, Snow depth, Submergence of paddy fields.

1. Introduction
Airborne radon (222 Rn), thoron (220 Rn) and their decay products have been usefulas suitable tracers in studies of the atmospheric boundary layer. Many reports havebeen published relating conditions of the lower atmosphere (especially meteorological elements) and concentration of airborne 220 Rn, 222 Rn and/or its daughters(e.g., Moses et al., 1963; Pearson and Moses, 1966; Ikebe, 1970; Druilhet andFontan, 1973; Beck and Gogolak, 1979; Guedalia et al., 1980; Robé et al., 1992).
Reports relating meteorological elements and exhalation rate of 220 Rn and/or 222 Rnfrom soil have also been published (e.g., Megumi and Mamuro, 1973; Clements
and Wilkening, 1974; Guedalia et al., 1970; Schery et al., 1984).
With the above as background, ‘A Study of Local Meteorology at Ningyotoge and Its Vicinity’ program was designed in 1994 and performed from 1995
to 1998 as part of ‘Monitoring of Radiation around Ningyo-toge Environmental
Engineering Center, Japan Nuclear Cycle Development Institute’. This monitoringof radiation has been sponsored by the Japanese Ministry of Education, Culture,
Sports, Science and Technology. The focus of the above program was to makeclear the relation between atmospheric concentrations of total α-activity (this is due
to α-particles from two of 220 Rn daughters (212 Bi and 212 Po) and can be reduced
to atmospheric activity concentration of 212 Pb, one of the 220 Rn daughters) andlocal meteorology. Included in the program were simultaneous observations of theatmospheric boundary layer and atmospheric activity concentrations of 222 Rn andits short-lived daughters at a location near one of the three observatories.
In a previous paper (Kataoka et al., 2000), we simply related time variations ofatmospheric activity concentrations of 222 Rn, its short-lived daughters and 212 Pb
and their ratios with some meteorological parameters such as wind speed, net radiative flux and snow depth. Following that early work, we have obtained additional
examples including observations of the atmospheric boundary layer, and have usedother methods for data analysis. In this paper, we present in detail the atmosphericCONCENTRATIONS OF 222 RN

221

activity concentrations of 222 Rn, its short-lived daughters and 212 Pb and their ratios
and relate them to conditions within the atmospheric boundary layer.
We use the following terminology. Atmospheric activity concentrations of212
Pb, 222 Rn, and of short-lived 222 Rn daughters (which is assumed to be inradioactive equilibrium with 222 Rn) will hereafter be denoted by, in order, concentrations of 212 Pb, 222 Rn and 222 Rn daughters. Also, the ratio of the concentrationsof 212 Pb and 222 Rn daughters and the ratio of the concentration of 222 Rn daughters to the concentration of 222 Rn will hereafter be designated as the 212 Pb/222 Rn
daughters ratio and 222 Rn daughters/222 Rn ratio, respectively. The equilibriumequivalent concentration of 222 Rn (ECRn ) is defined in ICRP (1981) as follows: theequilibrium-equivalent 222 Rn concentration of a non-equilibrium mixture of shortlived 222 Rn daughters in air is the activity concentration of 222Rn in radioactive
equilibrium with its short-lived daughters that has the same potential α-energy
concentration as the non-equilibrium mixture to which the equilibrium-equivalentconcentration of 222Rn refers. It is almost the same as the concentration of 222 Rn
daughters (Shimo, 1986) and will hereafter be designated as the concentration of
222
Rn daughters (ECRn ) to avoid confusion. Furthermore, the ratio of the concentration of 222 Rn daughters (ECRn ) and the concentration of 222 Rn will hereafter be
designated as the 222 Rn daughters (ECRn )/222 Rn ratio.
The rate of decay of a radionuclide is described by its activity, namely, by the
number of atoms that decay per unit time. The unit of activity is the bequerel (Bq),defined as one disintegration per second: 1 Bq = 1 s−1 .

2. Study Sites and Instrumentation
Since 1 April 1993, concentrations of 212 Pb and 222 Rn daughters have beenmeasured simultaneously every three hours at observatories at three locations in
Kamisaibara Village, Okayama Prefecture, Japan (35◦ 18 N, 133◦ 35 E). These
locations are identified by the letters A, B and C in Figure 1a and are, in order,
Tohge, Akawase and Tennoh. Tohge (A in Figure 1a) is at the top of a ridge in the
Chuhgoku Mountains and is at an elevation of 740 m above sea level. At Tohge,
the observatory is at the north side of a parking zone, which is 75 m long by 60 m
wide and is laid with asphalt. Vegetation near this site is Rubus crataegifoliusAralia elata community, Fagus crenata-Quercus mongolica var. grosseserrata
community, plant communities in clear cut areas and a Cryptomeria japonica
plantation. Ningyo-toge Environmental Engineering Center, Japan Nuclear Cycle
Development Institute is situated close to the parking zone. Its site is about 750 m
from east to west and about 500 m from north to south. Akawase (B in Figure 1a)
is located in a basin (1200 m by 700 m) at the mid-point of Akawase Valley, andis at an elevation of 710 m. A detailed description of the Akawase site is given in
Kataoka et al. (1998). The location of the observatory at Akawase is designated as
B1 in Figure 1b (A in Kataoka et al., 1998). Tennoh (C in Figure 1a) is situated at

222

TOSHIO KATAOKA ET AL.

the junction of Ikegoh Valley and Akawase Valley, and has an elevation of 540 m.
The axis of this area is oriented from north to south. The floor of the area is 1100 mlong by 400 m wide, and falls 240 m per km to the east and 60 m to the south.
The observatory at Tennoh is at the northeast side of the area and is surroundedby paddy fields. On both sides of the area are walls having heights of 80–220 m
and slopes of 35-37◦ , whose vegetation consists of plant communities in clear cut
areas, Quercus serrata community, Miscanthion sinensis, a Cryptomeria japonica
plantation. From the junction of the two valleys, a brook runs along the east side of
the area at the base of the wall.
Some uranium deposits exist in this region, especially near Tohge. However,they contain low-grade ores and do not influence the concentration of 222 Rn
daughters measured at these three sites (Kataoka et al., 1999).
The measurements of 212 Pb and short-lived 222Rn daughters have been carriedout with devices equipped with two α-scintillation counters. Details of the instruments are given in Kataoka et al. (1996). The reported concentration of 212Pb isthe average of a three-hour collection. However, as the effective half-life of shortlived 222Rn daughters is about 40 min, the latter half of the three-hour collection
has a major impact on the measurement of the concentration of 222 Rn daughters.
Meteorological variables such as wind speed and direction have also been observed
continuously at the three observatories. Net radiative flux, atmospheric pressure
and snow depth have been observed continuously only at the Akawase observatory
as a representative of the three sites.
At location B2 in Figure 1b (B in Kataoka et al., 1998), intensive observations of
the atmospheric boundary layer were carried out during three periods. The first observation period was from 6 to 13 October 1995, the second was from 27 February
to 1 March 1998 and the third was from 22 July to 2 August 1998. These intensive
observation periods will hereafter be designated as, in order, IOP-1, IOP-2 andIOP-3. Table I provides a summary of the instrumentation used to obtain the dataof 222 Rn, its daughters and meteorology during the three periods. The methodologyof the measurements was provided in Kataoka et al. (1998).

3. Exhalation Rates of 220 Rn and 222 Rn and Atmospheric Pressure
Since the variation of atmospheric pressure was not large (the variation was 6 hPa)in work presented in a previous paper (Kataoka et al., 1998), exhalation rates of220
Rn and 222 Rn were treated as constant. However, in this paper, we are concernedwith the concentrations of 222 Rn, its daughters and 212 Pb (one of the daughters of
220Rn) during a period in which the variation of atmospheric pressure is relatively
large. We discuss here the influence of atmospheric pressure on exhalation rates of220
Rn and 222Rn.
Reports concerning the relation between exhalation rates of 220 Rn and/or 222 Rn
and atmospheric pressure include the following: Guedalia et al. (1970), Clements

CONCENTRATIONS OF 222 RN

223

Figure 1. Topographic map around (a) three observatories (A: Tohge observatory, B: Akawase observatory, C: Tennoh observatory) and (b) Akawase observatory (B1: Akawase observatory, B2: A sitewhere three intensive observations were carried out).

Concentration of 222 Rn
Equilibrium-equivalentconcentration of 222 Rn
Exhalation rate of 222 Rn
Vertical profile of meteorologicalelements up to about 1000 m
Vertical profile of meteorologicalelements up to about 600 m
Inversion layer up to 400 m
Continuous wind profile up to 600 m
Sensible and latent heat
fluxes at ground level
Profile of slope temperatureof basin floor
Meteorological elements
at ground level

222 Rn monitor

: Data were obtained for more than half of the period.
: A few data were obtained.
×: No data were obtained.

Meteorological observation
stationInfrared thermal imagery

Monostatic sodar
Doppler sodar
Surface eddy flux system

Tethersonde system

ER system
Low-level sonde

WL monitor

Objective of measurement

Instrumentation



×

×
×
×
×
×
×







×




Intense observation period
First (IOP-1)
Second (IOP-2)
6–13 October 1995 27 February–1 March 1998

TABLE I
Instrumentation for measurement at Akawase (Location B2).





×









×

Third (IOP-3)
22 July–2 August 1998

224
TOSHIO KATAOKA ET AL.

CONCENTRATIONS OF 222 RN

225

and Wilkening (1974), Schery et al. (1984), Ishimori et al. (1998). We describe
the relation between the exhalation rate of 222Rn and atmospheric pressure firstly
and the relation between the exhalation rate of 220 Rn and atmospheric pressure
next. The experiments of Clements and Wilkening (1974) and Schery et al. (1984)
were carried out at sites about 1 and 2 km west of the centre of the New Mexico
Institute of Mining and Technology campus, respectively. Although soils at thetwo sites are slightly different, they are gravelly sandy loam, with a water table
about 30 m below the surface. Since the climate is semi-arid (rainfall averages
200 mm y−1 ), soil moisture is low. In accordance with Clements and Wilkening(1974), when changes in the atmospheric pressure are +14.5 hPa and −15.6 hPa
after being relatively constant for at least 24 h, changes in the exhalation rates of222
Rn are about −40% and about +60%, respectively. A report by Schery et al.
(1984) shows that, when the change in atmospheric pressure is slow and small,the exhalation rate of 222 Rn can be treated as constant, but that, when the change
in atmospheric pressure is sharp or large, the exhalation rate of 222 Rn cannot be
considered constant.
At our site, volcanic ash soil exists over decomposed granite soil, and the thickness of the soil layers varies within the experimental area. This pattern is typical,
although there are several spots having sedimentary rocks of the Tertiary period.
Since the depth of any private-dug well is 2–3 m, it is considered that there areshallow water veins in the area. The climate is humid temperate (rainfall averages
2600 mm y−1 ). The rainfall and shallow water veins being taken into consideration, soil moisture is likely to be substantially higher at our experimental area
than that at the sites used by Clements and Wilkening (1974) and Schery et al.
(1984). Further, the soil in our area is also different from that at the sites used in
the referenced studies. Therefore, the relation between the exhalation rate of 222 Rn
and atmospheric pressure obtained by Clements and Wilkening (1974) and Scheryet al. (1984) cannot be directly adopted here.
According to Ishimori et al. (1998), the exhalation rate of 222 Rn varies within±12% and is considered to be constant during the period of 2–9 October 1994. The
site used by Ishimori et al. (1998) is about 200 m east-northeast of the Tohge observatory. The height of the site above the sea level is almost the same as the heightof the Akawase observatory. Therefore, the change in the atmospheric pressure isalmost the same as that at Akawase observatory, which was 13 hPa for this period,
with no rain. Therefore, the exhalation rate of 222 Rn can be assumed to be only
weakly dependent on atmospheric pressure in the case of no rain, for the site andperiods of data collection. As such we consider the exhalation rate to be dependent
upon processes other than atmospheric pressure.
Atmospheric pressure varied between 934 and 945 hPa, between 928 and 939
hPa and between 927 and 938 hPa during the three periods of data analysis, from
7 to 18 October 1995, from 27 February to 1 March 1998 and from 22 July to 2
August 1998, respectively, as shown in Figure 2. Therefore, it is reasonable that

226

TOSHIO KATAOKA ET AL.

the exhalation rate of 222 Rn is independent of the atmospheric pressure within eachperiod for the reason above.
Table II gives exhalation rates of 222 Rn measured for the IOP-1 and IOP-3 at
Akawase. The uncultivated lands (1, 2 and 3 in the table) are within 20 m of one
another; the Akawase observatory is also within 20 m. Since the exhalation rates of222
Rn were measured more than two days after rainfall, they are not influenced by
the precipitation. Measurements of the 222 Rn exhalation rate were carried out on
9 and 12 October 1995 for the IOP-1. The difference in the atmospheric pressure
was 3 hPa between the two measurements of the 222 Rn exhalation rate during the
IOP-1. Errors being taken into account, the exhalation rates of 222 Rn agree at eachpoint. The atmospheric pressure at 0503–0616 JST on 2 August 1998 was lower
than that at 1352–1437 JST on 9 October 1995 by 3 hPa and was lower than that at
1108–1147 JST on 12 October 1995 by 6 hPa. The above explanation being taken
into account, the difference between the 222 Rn exhalation rates at the uncultivatedlands 1, 2 and 3 is due to the fact that they are different sites. Furthermore, theaverage of the 222 Rn exhalation rate at the uncultivated land 1 is about one thirdof the average of those at the paddy fields and at the ridge between the paddy
fields on 9 and 12 October 1995. In addition, the average of the 222 Rn exhalation
rates at the uncultivated lands 2 and 3 is also about one third of that at the forest
area (uncultivated land 4 in Table II) on the opposite side of the paddy fields on
2 August 1998. Hence, it is reasonable to assume that the exhalation rate of 222 Rn
at the area comprising the uncultivated lands 1, 2 and 3 near Akawase observatory
is approximately one third of those at the paddy area and the forest area on the
opposite side of the paddy fields during the three observation periods.
Next, we describe the relation between atmospheric pressure and the exhalation
rate of 220 Rn. Guedalia et al. (1970) indicate that the exhalation rate of 220 Rn is
independent of atmospheric pressure. Schery et al. (1984) showed that the exhalation rate of 220 Rn exhibited only one-fourth of the amplitude of 222 Rn for the same
variation of atmospheric pressure. From these two facts and very weak pressuredependence of the 222 Rn exhalation rate at our experimental area as mentioned
above, it is reasonable that the exhalation rate of 220 Rn is also independent of
atmospheric pressure at the area.
4. Concentrations of 212Pb and 222Rn Daughters and their Ratio
4.1. A STANDARD NORMAL PERIOD – CYCLE OF CONVECTIVE MIXING AND
SURFACE - BASED INVERSION
In a previous paper (Kataoka et al., 2000), we briefly introduced the time variationof concentrations of 212 Pb and 222 Rn daughters and 212 Pb/222 Rn daughters ratio
during the IOP-1. In the paper, we showed a trend such that the concentrations andthe ratio were low in the daytime and high at night, except for a day having precipitation and for a cloudy day. Adding data having the same trend, we show here

CONCENTRATIONS OF 222 RN

227

Figure 2. Time variation of atmospheric pressure for the periods (a) from 7 to 18 October 1995, (b)from 27 February to 1 March 1998 and (c) from 22 July to 2 August 1998.

Third period

Paddy field

First period

Uncultivated land 2
Uncultivated land 3
Uncultivated land 4

Ridge between
paddy fields
Uncultivated land 1

Soil type

Period0546–0616, 2 August 1998
0503–0533, 2 August 1998
0628–0658, 2 August 1998

1352–1423, 9 October 1995
1120–1151,12 October 1995
1400–1431, 9 October 1995
1116–1147, 12 October 1995
1406–1437, 9 October 1995
1108–1139, 12 October 1995

Date/Time(JST)

Atmospheric pressure
(hPa)
939
942
939
942
939
942
936
936
936

Exhalation rate of 222 Rn(Bq m−2 s−1 )0.0103 ± 0.0016
0.0097 ± 0.0014
0.0187 ± 0.0018
0.0209 ± 0.0017
0.0063 ± 0.0009
0.0055 ± 0.0015
0.0043 ± 0.0004
0.0035 ± 0.0004
0.0139 ± 0.0010

TABLE II
Exhalation rate of 222 Rn.

228
TOSHIO KATAOKA ET AL.

CONCENTRATIONS OF 222 RN

229time variations of the concentrations of 212 Pb and 222 Rn daughters and 212 Pb/222 Rn
daughters ratio at the three sites for the period of 7–18 October 1995 in Figure 3.
Kataoka et al. (1999) showed statistically that the levels of the concentrations of212
Pb and 222 Rn daughters and 212 Pb/222 Rn daughters ratio at the three sites differ
from each other. This reveals the difference of the 220 Rn and 222 Rn exhalation rates
between the three sites. Time variations of the concentrations of 212 Pb and 222 Rn
daughters, and of the 212 Pb/222 Rn daughters ratio indicate almost the same tendency
for the three sites. We notice that the 212 Pb/222 Rn daughters ratio is small in thedaytime when a convective mixing layer develops over the experimental area, while
the ratio is large in the nighttime when a surface-based inversion layer develops.
The decrease of the 212Pb concentration with height is larger than that of the shortlived 222 Rn daughters over the duration of the surface-based inversion layer due tothe difference in their half-lives (212 Pb: 10.64 h, 222 Rn: 3.82 days). Mixing after
sunrise may cause the 212 Pb/222 Rn daughters ratio to decrease. During the daytime,the mixed-layer depth is mainly controlled by thermal convection and mechanicalmixing. Thermal convection can be represented by solar radiation or simply, cloudamount, while, mechanical mixing is represented by wind speed. On sunny andwindy days, the mixed layer develops to higher altitudes. At night, cloud amountand wind speed are both controlling factors for the development of a surface-based
inversion. Unlike the daytime situation, high wind speeds lead to suppression of
the inversion due to forced mixing. A well developed surface-based inversion canbe observed during a clear and calm night. Diurnal ranges of the variations of theconcentrations of 212 Pb and 222 Rn daughters and the 212 Pb/222 Rn daughters ratio at
the three sites differ from each other. However, it can be summarized that they aresmall in the daytime and large at night. The cycle of this pattern is considered tobe typical for continuous days with no rain.
Examining thoroughly the time variations of the concentrations of 212 Pb and222
Rn daughters and 212 Pb/222 Rn daughters ratio for periods with no rainy days, wenote that they have several different patterns at night. Examples of the patterns are
picked up from the periods 7–18 October 1995 and 16–26 October 1997, and areshown in Table III. Since the examples are obtained more than two days after rain,
and since the variation of the atmospheric pressure for each period is within 13 hPa,it may be assumed that the exhalation rates of 220 Rn and 222 Rn are constant. The
observatory at Tohge is at the pass, and the observatories at Akawase and Tennoh
are in the basin. The observation of the atmospheric boundary layer, including themeasurement of 222Rn, was carried out at the basin (Akawase), but was not done at
the pass. Therefore, we here narrow down discussions to Akawase and Tennoh andleave the discussion about Tohge for the future.
Let us discuss each case from the top of Table III based on the accumulation of212
Pb, 222 Rn and its daughters near the ground, the decrease of their concentrations
with height, the surface-based inversion, and the mixing condition. The concentrations of 212 Pb and 222 Rn daughters, and 212 Pb/222 Rn daughters ratio, continued toincrease from 1800 JST on 20 October to 0600 JST on 21 October 1997. This is a

230

TOSHIO KATAOKA ET AL.

Figure 3. Time variations of (a) the concentration of 212 Pb, (b) the concentration of 222 Rn daughtersand (c) the 212 Pb/222 Rn daughters ratio, at three sites for the period from 7 to 18 October 1995.

Akawase
AkawaseAkawaseAkawaseAkawaseTennohAkawaseAkawaseAkawaseTennohAkawaseTennoh1800 JST 20 Oct.–0600 JST 21 Oct. 1997
0300–0600 JST 11 Oct. 1995
0000–0300 JST 12 Oct. 1995
2100–2400 JST 11 Oct. 1995

0300–0600 JST 12 Oct. 1995

0300–0600 JST 12 Oct. 1995

2100–2400 JST 9 Oct. 1995

0300–0600 JST 25 Oct. 1997

0300–0600 JST 14 Oct. 1995

2100–2400 JST 21 Oct. 1997

0300–0600 JST 15 Oct. 1995

0300–0600 JST 25 Oct. 1997+: Increase, −: Decrease.

Location

PeriodConcentration of
222 Rn daughters
+
+
+
+
−

−
−
−

−
−
+
+

Case
212 Pbconcentration
+
+
+
+
−

−
−
−

+
+
−
−

−

−

+

+

−

−

−

+

+
−
−
−

ratio

212 Pb/222 Rn daughters

Surface-based inversion
Surface-based inversion
Surface-based inversion
Middle accumulation
Small-scale of drainage wind
Large accumulation
Strong surface-based inversion
Large-scale of weak mixing
Large accumulation
Small-scale of weak mixing
Poor accumulation
Drainage wind
Large accumulation
Weak surface-based inversion
Large-scale of drainage windIntermediate-scale ofdrainage wind or weak mixing
Intermediate-scale ofdrainage wind or weak mixing
Intermediate-scale ofdrainage wind or weak mixing
Intermediate-scale ofdrainage wind or weak mixing

Conditions

TABLE III
Changes of the concentrations of 212 Pb and 222 Rn daughters and 212 Pb/222 Rn daughters ratio and meteorological conditions.

CONCENTRATIONS OF 222 RN

231

232

TOSHIO KATAOKA ET AL.

good example to indicate the accumulation of 212Pb, 222Rn and its daughters near
the ground by a surface-based inversion.
The concentrations of 212Pb and 222 Rn daughters increased but the 212 Pb/222 Rn
daughters ratio decreased at Akawase during the periods of 0300–0600 JST on
11 October and 0000–0300 JST on 12 October 1995. The 222 Rn concentration
increased less during the period of 0255–0355 JST on 11 October and much lessduring the period of 0055–0155 JST on 12 October 1995 (Kataoka et al., 1998).
Surface-based inversion layers were caught by a sodar during these two periods.
Since the 222 Rn concentrations increased, there must have existed surface-based
inversion layers lower than 25 m, which is the lowest level of detection by the sodar.
To our deep regret, it is difficult to explain why the smaller increases happened
during the two nights using the meteorological data simultaneously obtained. The
concentrations of 212 Pb and 222 Rn daughters and 212 Pb/222 Rn daughters ratio during
2100–2400 JST on 11 October 1995 behaved the same as those during the two
periods. However, the 222 Rn concentration decreased a little during the period of2155–2255 JST on 11 October (Kataoka et al., 1998). The sodar caught a surfacebased inversion layer during the period. The analysis of the meteorological data
indicates that a small-scale drainage wind occurred. The decrease of the 222 Rnconcentration may have been provoked by the small-scale drainage wind.
222
Rn, its daughters and 212Pb accumulated near the ground due to a surfacebased inversion by 0300 JST on 12 October 1995. Cloud cover at Akawase for theperiod of 0300–0530 JST on 12 October resulted in a net radiative flux that wasless negative than usual and an atmospheric surface layer that was less stable thannormal. Combined with slightly higher than normal wind speeds for the period of
0400–0600 JST, this is likely to have resulted in weak mixing, and the observed
decreases in concentrations of 212 Pb and 222 Rn daughters. The scale of the weakmixing is presumed to be fairly large from the large decrease of the concentrationsof 212 Pb and 222 Rn daughters. This weak mixing also produced an extremely large
value of the 212 Pb/222 Rn daughters ratio at 0600 JST. At the same time, at Tennoh,
the concentration of 212 Pb decreased by 25% while the concentration of 222 Rn
daughters decreased by only 5%. Considering the accumulation of 212 Pb and shortlived 222 Rn daughters near the ground, the mixing is likely to have been much
weaker than at Akawase. This resulted in a more normal value to the 212 Pb/222 Rn
daughters ratio at Tennoh.
In the period immediately after the formation of the surface-based inversion
layer, 222 Rn, its daughters and 212 Pb are not greatly accumulated near the ground.
Therefore, the decrease of the concentration of 212 Pb with height is not much larger
than that of the 222Rn daughters. Weak mixing or drainage winds may decrease the212
Pb/222 Rn daughters ratio, since the difference of the concentrations of 212 Pb and222
Rn daughters with height is small. The case of 2100–2400 JST on 9 October
1995 at Akawase (see Figure 3) is an example of a drainage wind situation.
After sufficient accumulation of 222 Rn, its daughters and 212Pb near the groundsurface, both concentrations of 212 Pb and 222 Rn daughters and their ratio decrease

CONCENTRATIONS OF 222 RN

233

considerably due to developed mixing. The case of 0300–0600 JST on 25 October
1997 at Akawase corresponds to this phenomenon; strong winds were observed
during the period of 0300–0500 JST. During the nighttime, the net radiative flux
ranged from −72 to −78 W m−2 , under clear skies and large radiative cooling.
Under such conditions, the strong wind is symptomatic of a drainage wind. With
the degrees of decrease of the concentrations with time taken into account, thisphenomenon is considered to be almost the same scale as that at Akawase for theperiod of 0300–0600 JST on 12 October 1995. However, the 212 Pb/222 Rn daughters
ratio of the former decreased while that of the latter increased. This may be due tothe difference between the two cases in the vertical profiles of the concentrationsof 212 Pb and 222 Rn daughters provoked by the formation of the surface-based inversion layer. At Tennoh, moderate winds blew from 0300 to 0600 JST on 25 October(this case will be mentioned in the next paragraph).
The other two cases happened at night according to the degrees of decreaseof the concentrations of 212 Pb and 222 Rn daughters with height and the scale ofmixing. One is a case in which the concentration of 212 Pb increased and the concentration of 222 Rn daughters decreased (0300–0600 JST on 14 October 1995,
Akawase; 2100–2400 JST on 21 October 1997, Tennoh). The other is a case inwhich the concentration of 212 Pb decreased and the concentration of 222Rn daughters increased (0300–0600 JST on 15 October 1995, Akawase; 0300–0600 JST on
25 October 1997, Tennoh). There might have existed an intermediate scale of drainage wind or weak mixing during the period of these four events. However, sinceobservations of the atmospheric boundary layer were not carried out during these
periods, it is difficult to determine the meteorological cause of these concentration
changes.
The patterns of the concentrations of 222 Rn and 222Rn daughters (ECRn )(Kataoka et al., 2000) measured at location B2 for the IOP-1 are similar to theconcentration of 222 Rn daughters for the same period shown in Figure 3b. We
discuss their relations in detail in Section 5.
4.2. P ERIODS OF METEOROLOGICAL AND SURFACE CONDITIONS DIFFERINGFROM A STANDARD NORMAL PERIOD

Figure 4 shows time variations of the concentrations of 212 Pb and 222Rn daughtersand 212 Pb/222 Rn daughters ratio at the three sites during the IOP-3. The time variation of the concentration of 222 Rn measured at location B2 is also shown in the
figure. Although the concentration of 222 Rn was measured at the submerged paddy
field, its pattern of variation resembles that of the concentration of 222 Rn daughters(Figure 3b) measured at location B1. However, since further analysis of the datashows the effect of submergence, we describe the effect in Section 5.2.
The observation period has two interesting features. One is that the concentrations of 222 Rn, its daughters and 212Pb are low through the day and night during
the periods from 0800 JST on 24 July to 1800 JST on 27 July and from 1000 JST

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Figure 4. Time variations of (a) the concentration of 212 Pb, (b) the concentration of 222 Rn daughters,(c) the 212 Pb/222 Rn daughters ratio at three sites and (d) the concentration of 222 Rn at Akawase(location B2) for the period from 22 July to 2 August 1998.

CONCENTRATIONS OF 222 RN

235

Figure 5. Time variations of precipitation at three sites for the period from 22 July to 2 August 1998.

on 1 August to 1400 JST on 2 August. Another is that the 212 Pb/222 Rn daughters
ratios at night during the period from 27 to 31 July are equal to or less than those in
the daytime. This trend is the inverse of the situation in which the ratios are low inthe daytime and high at night, as in the standard normal period. We explain these
phenomena respectively in the following (Sections 4.2.1 and 4.2.2).
Furthermore, the IOP-2 with a snow depth of 0.2–0.3 m is interesting. Since
we discussed the time variation of the concentrations of 222 Rn daughters and 212 Pb
and 212 Pb/222 Rn daughters ratio in detail for this period previously (Kataoka et al.,2000), we present only a summary in the following (Section 4.2.3).
4.2.1. Period of Continuous Low Concentrations of 222 Rn Daughters and 212Pb
Figure 5 shows the occurrence of precipitation for the IOP-3. At Akawase, theamounts of rainfall were 10 mm, 2 mm and 0.5 mm during the periods of0500–0800 JST and 1400–1500 JST on 22 July, and 0400–0500 JST on 23 July,
respectively. There was no rain at that location from 0500 JST on 23 July to 2400
JST on 26 July. Precipitation at Tohge and Tennoh was almost the same as that at
Akawase. The exhalation rate of 222 Rn largely recovers one day after rain (Ishimori
et al., 1998), whilst the exhalation rates of 220 Rn and 222 Rn reveal almost the same
trends of recovery after rain (Megumi and Mamuro, 1973). Hence, the continuously low concentrations of 212Pb and 222 Rn daughters and the continuously high212
Pb/222 Rn daughters ratios for the period of 0800 JST on 24 July to 2400 JST
on 26 July at the three sites are considered to be due to meteorological conditions
other than precipitation.
Strong winds blew continuously from 0800 JST on 24 July to 1800 JST on 27
July because of a stationary front near the Japanese Island. In the daytime of 24
and 25 July, mixing layers developed due to the strong winds and intense solar
radiation. In the daytime of 26 July, the strong winds again led to the development

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of a mixing layer. During the nighttimes of 24–25 and 25–26 July, the net radiativeflux was small and negative, and the wind was strong. Therefore, it is likely that amixing layer existed during the nighttime. The wind was weak at Tennoh for theperiods of 0100–0500 JST and 1800–2100 JST on 25 July, and a surface-based
inversion layer is considered to have formed for these two short periods. This is
supported by the increase in the concentrations of 212 Pb and 222 Rn daughters.
On 27 July, rainfall amounted to 4 mm from 0000 to 0600 JST at Akawase and2.5 mm from 0000 to 0600 at Tennoh. The strong winds continued for the periodof 0800 JST on 24 July to 1800 JST on 27 July. Therefore, the low concentration of212
Pb and 222 Rn daughters may have been caused by the strong winds and rainfall(mainly the strong winds) from 0000 to 1800 JST on 27 July.
As mentioned in Section 4.1, a cycle such that the 212Pb/222 Rn daughters ratio
is large in the nighttime and small in the daytime was repeated in the case of thecontinuous days with no rainy and no cloudy days. In the case of the continuous
windy days, the 212 Pb/222 Rn daughters ratio may be conjectured to retain its daytime value. However, in reality, the 212Pb/222 Rn daughters ratios were larger thanthose of the daytime. This shows that 220 Rn is mixed up more from the groundinto the atmosphere by the strong winds than is 222 Rn. Therefore, the 212 Pb/222 Rn
daughters ratio, which normally decreases by mixing, may have increased due tothe extraction of 220 Rn and 222 Rn from the ground. Furthermore, the fact that the212
Pb/222 Rn daughters ratio increased to a slightly larger value and became almost
constant, indicates that the effect of the extraction of 220 Rn and 222 Rn by the strong
winds and the convective mixing became stationary.
The observations with the Doppler sodar as shown in Figure 6 indicate upper
level strong winds from 0800 JST on 24 July to 1800 JST on 27 July. The wind wasmoderate (0.5–2 m s−1 ) near the ground from the night of 1 August to the daytimeof 2 August, but the wind in the upper air was not strong, as shown in Figure 7.
Under these conditions, the concentrations of 212Pb and 222 Rn daughters did notincrease and the 212Pb/222 Rn daughters ratio remained at a middle value (about0.02) during the period from the night of 1 August to the daytime of 2 August.

4.2.2. Period of Low 212 Pb/222 Rn Daughters Ratio at Night
We note from Figures 4 and 5 that the period exhibiting the inverse trend of the212
Pb/222 Rn daughters ratios (27–31 July 1998), when the ratios at night are equal
to or less than those in the daytime, corresponds to a period of rainfall. With
this in mind, the inverse trend is thought to be caused by the following possible
factors: (1) a variation of exhalation rates of 220 Rn and 222Rn from the grounddue to the permeation of rain water into the soil, (2) washout of 212 Pb and 222 Rn
daughters from the atmosphere by precipitation, (3) a degree of mixing by winds
simultaneously occurring with precipitation, and (4) an increase or decrease of theexhalation rates of 220Rn and 222Rn due to a change in atmospheric pressure. We
examine these factors in order as follows:Figure 6. An example of vertical and temporal variations of (a) horizontal component (U) and (b) vertical component (W) of wind speed observed by
Doppler sodar on 24 July 1998.

CONCENTRATIONS OF 222 RN

237

Figure 7. An example of vertical and temporal variations of (a) horizontal component (U) and (b) vertical component (W) of wind speed observed by
Doppler sodar for the night of 1 August 1998.

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CONCENTRATIONS OF 222 RN

239

Figure 8. Time variation of the concentration of aerosols.

1. According to Megumi and Mamuro (1973), Ishimori et al. (1998) and Koarashi
et al. (2000), the exhalation rates of 220 Rn and 222 Rn considerably decrease
due to rain. Although the 220Rn concentration in soil air becomes constant at
a much shallower depth than does the 222Rn concentration in soil air (Israël,1962), the exhalation rate of 220 Rn decreases and recovers, having almost thesame trend as that of 222 Rn (Megumi and Mamuro, 1973). Therefore it is difficult to believe that this is the reason for the inverse trend of the 212 Pb/222 Rn
daughters ratio at night.
2. In Figure 8, we show the time variation of the concentration of aerosols at
Akawase (location B2) for the IOP-3. Since the concentration of aerosols was
lower during the rainy period than that with very little or no rain, as shownin Figures 5 and 8, some of the aerosols may have been eliminated from theatmosphere due to the rain. It is reported that rainout (the short-lived 222 Rn
daughters attach to the raindrops during the process of formation of the raindrops in the upper air) is much more important than washout (the raindrops
collide with the aerosols containing the short-lived 222Rn daughters) (Jacobi,
1961; Bhandari and Rama, 1963)). Therefore it is difficult to evaluate the shortlived 222Rn daughters removed by washout using the ground-based measuringsystem. According to Jacobi (1961), the change in the concentration of 222 Rn
daughters in the atmosphere by washout is at most 5%. The rate of eliminationof 220 Rn daughters by washout may be almost the same as that of the 222 Rn
daughters. Even if it is much different, the half-lives of 220 Rn, 222 Rn and theirdaughters being taken into account, the decrease of the concentrations of 212 Pb
and 222 Rn daughters recover near the ground within a few hours. Therefore it
is unlikely that washout is the cause of the inverse trend of the 212 Pb/222 Rn
daughters ratio at night.
3. From the night of 27 July to the dawn of 31 July, the weather was variablewith periods of rain. The net radiative flux ranged between −25 and −19 W

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TOSHIO KATAOKA ET AL.

m−2 for the period of 0200–0500 JST on 29 July. Unfortunately, there were noobservations of the vertical profile of temperature during this period. However,
it is considered that a surface-based inversion formed, since the concentrationsof 212 Pb and 222Rn daughters increased substantially. During the nights of 27–
28 and 29–31 July, it was rainy or cloudy, and the net radiative flux was nearlyzero. From the time variation of the vertical profiles of temperature obtainedwith the tethered instrumented balloon, it was observed that surface-based inversion layers were not present during the nights of 29–31 July. The wind was
weak and the atmospheric conditions were near neutral (weakly stable) during
these four nights. This near-neutral condition in the nighttime is considered tobe the cause of the inverse trend of the 212 Pb/222 Rn daughters ratio at night.
4. Since the atmospheric pressure increased by 8 hPa during the period of 27–30
July and maintained the same level until the evening of 2 August, the exhalation rate of 222 Rn may have been independent of atmospheric pressure for theperiod, as mentioned above. Therefore, it is very unlikely that the inverse trendof the 212 Pb/222 Rn daughters ratio at night is due to an increase of atmosphericpressure.
From the above discussions, the inverse trend of the 212Pb/222 Rn daughters ratio
at night is considered to be due to the near-neutral condition.
4.2.3. Period of Snow Cover
The pattern of variation of the concentrations of 222Rn daughters and 212Pb and the212
Pb/222 Rn daughters ratio for the first half of the IOP-2 is similar to that during
the standard normal period. The snow depth was 0.2–0.3 m for the IOP-2. The
ground in the forest area having very little snow cover, the atmospheric condition
near the surface may have been almost the same as that with no snow cover.
In the latter half of the period, the concentrations of 222 Rn daughters and 212 Pb
and the 212 Pb/222 Rn daughters ratio was continuously low through both day andnight from 0300 JST on 1 March. The 212 Pb/222 Rn daughters ratio during thisperiod was much lower than those during the period of strong winds (from 24 to 27July) for the IOP-1. Snow cover existed on the paddy fields, while there was very
little in the forest area. Considering the half-life of 220 Rn (55.4 s), it is thought thatthe snow decreased the effect of the extraction of 220Rn. Hence, it is believed thatthe decrease of 212 Pb concentration was due to convective mixing provoked by thestrong winds and due to the decrease of the extraction of 220 Rn from the ground bysnow. With a snow depth of 0.2–0.3 m, a large number of 222 Rn atoms that reachthe snow-ground interface pass through the snow and exhale from the air-snow
interface into the atmosphere. It is considered that the decrease of concentrationof 222 Rn daughters was due only to the convective mixing provoked by the strong
winds. Considering these factors, the difference between the 212 Pb/222 Rn daughters
ratios in the latter half of the IOP-2 and during the period of strong winds ofthe IOP-1 may be due to the fact that a snow cover of 0.2–0.3 m decreased theextraction of 220 Rn by the strong winds but did not decrease that of 222 Rn.

CONCENTRATIONS OF 222 RN

241

5. Concentrations of 222Rn and its Daughters and their Ratio
Figure 9 shows the relations between the concentration of 222 Rn above the paddy
field (location B2) and that of 222 Rn daughters above the uncultivated land (location
B1) during the three observation periods. Figure 10 illustrates the time variations of222
Rn daughters/222 Rn ratio for the three periods; the concentration of 222 Rn daughters (ECRn ) could not be measured for the IOP-3 owing to instrument problems.
Therefore, we compare the concentrations of 222 Rn and 222 Rn daughters (ECRn ) andtheir ratios for the IOP-1 and IOP-2 only. Figure 11 shows the relations between
the concentrations of 222 Rn and 222 Rn daughters (ECRn ) for the IOP-1 and IOP-2,
and Figure 12 shows time variations of the 222 Rn daughters (ECRn )/222 Rn ratio forthe two periods.
We also treat here the IOP-1 as a standard normal period as in Section 4. The
snow depth was 0.2–0.3 m at location B for the IOP-2; the paddy fields were
submerged for the IOP-3.
5.1. P ERIOD OF SUBMERGENCE OF PADDY FIELDS
Figure 13 shows time variations of the sensible and latent heat fluxes on one of the
paddy fields (location B2) during the IOP-1 and IOP-3. The sensible and latent heat
fluxes were both about 100 W m−2 for the IOP-1, but the latent heat flux was about
200 W m−2 , and the sensible heat flux only about 50 W m−2 for the IOP-3. The
former period represents a case of no water over the paddy fields, while the latter
represents water cover on the paddy fields and a large amount of transpiration fromthe rice plants. This is representative of all the paddy fields at Akawase. The paddy
fields were covered with water for the IOP-3, and therefore the exhalation rate of222
Rn is considered to have been much less than that for the IOP-1. The 222 Rnconcentration measured at the ridge in the centre of the paddy fields became not so
low because of the advection of 222 Rn from the forest area due to the moderate tostrong winds. From this, it is reasonable to assume that the concentrations of 222 Rn
and its daughters in such a condition were almost the same for these two periods,
as shown by the low concentrations in Figures 9a and 9c.
222
Rn was not supplied from the submerged paddy fields during the IOP-3,
but was supplied only from the ridges between the paddy fields. There existed
a surface-based inversion or near neutral (weakly stable) condition in the nighttime for the period. The concentrations of 222 Rn at the submerged paddy fields
were lower than those when the paddy fields had no surface water. At location
B1, the concentrations of 222 Rn daughters at night for the IOP-1 were a little
lower than those for the IOP-3. It may be due to these conditions that the 222 Rn
daughters/222 Rn ratios were more than 0.4 at night for the IOP-3 (see Figure 10c),
contrary to the observation that they were less than 0.4 for the IOP-1 (see Figure
10a).

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TOSHIO KATAOKA ET AL.

Figure 9. Relations between the concentration of 222 Rn at location B2 and the concentration of
222 Rn daughters at location B1. Correlation coefficient (r), regression equation and regression line
are shown in each figure. (a) 6–13 October 1995; (b) 27 February–1 March 1998; (c) 23 July–2
August 1998.

CONCENTRATIONS OF 222 RN

243

Figure 10. Time variations of the ratio of the concentration of 222 Rn daughters at location B1 to theconcentration of 222 Rn at location B2 (222 Rn daughters/222 Rn ratio). (a) 6–13 October 1995; (b) 27
February–1 March 1998; (c) 23 July–2 August 1998.

244

TOSHIO KATAOKA ET AL.

Figure 11. Relations between the concentration of 222 Rn and the concentration of 222 Rn daughters(ECRn ). Correlation coefficient (r), regression equation and regression line are shown in each figure.
(a) 6–13 October 1995; (b) 27 February–1 March 1998.

5.2. P ERIOD OF SNOW COVER
We find from Figures 9a and 9b that the range of concentrations of 222 Rn and 222 Rn
daughters (ECRn ) during the IOP-2 is almost the same as that during the IOP-1. No
snow was observed over the paddy fields and forest area for the IOP-1, but there
was a snow depth of 0.2–0.3 m during the IOP-2. With a snow depth of 0.2–0.3 m, alarge number of 222 Rn atoms that reach the snow-ground interface pass through thesnow and exhale from the air-snow interface into the atmosphere. Therefore, theexhalation rate of 222Rn is considered to have been almost the same for these two
periods. The small amount of snow over the forest area during the IOP-2 made thesurface meteorological condition similar to that with no snow cover. This may beCONCENTRATIONS OF 222 RN

245

Figure 12. Time variations of the ratio of the concentration of 222 Rn daughters (ECRn ) to the concentration of 222 Rn (222 Rn daughters (ECRn )/222 Rn ratio). (a) 6–13 October 1995; (b) 27 February–1
March 1998.

the cause of the fact that the range of concentration of 222 Rn daughters for the IOP2 was almost the same as that for the IOP-1. Consequently, 222Rn daughters/222 Rn
ratios during the IOP-2 were almost the same as those during the IOP-1, as shownin Figures 10a and 10b.
From Figure 11, we see that the concentrations of 222Rn at location B2 during
the IOP-1 and during the IOP-2 are within the same range. We also find from thefigure that when the concentration of 222 Rn daughters (ECRn ) at location B2 is low,
the IOP-1 and IOP-2 are similar, but when the concentration is high, the concentration during the IOP-2 is about one fourth of that during the IOP-1. The range ofthe 222 Rn daughters (ECRn )/222 Rn ratio for the IOP-1 was 0.3–0.9 at location B2(see Figure 12a). However, the range of the 222 Rn daughters (ECRn )/222 Rn ratio for246

TOSHIO KATAOKA ET AL.

Figure 13. Comparison of sensible heat flux (continuous curve) and latent heat flux (dashed curve)
between (a) 6–13 October 1995 and (b) 23 July–2 August 1998.

the IOP-2 was 0.1–0.3 at location B2 (see Figure 12b) and outside the range for
the IOP-1. The exhalation rate of 222Rn at location B1 is about one third of thatat the other area (see Table II). We surmise that the 222Rn daughters/222 Rn ratio
at location B1, where the meteorological condition near the ground was similar tothat with no snow, varies from 0.2 to 0.9 from Figure 10b. Thus, the equilibriumCONCENTRATIONS OF 222 RN

247

factor in the case of snow cover is different from that in the case of no snow cover.
This may be due to small differences in meteorological conditions caused by thedifference between the snow of the paddy and forest areas.

6. Conclusions
We presumed in prior statistical analysis (Kataoka et al., 1999) that the 212 Pb/222 Rn
daughters ratio is a good supplemental tracer for a study of detailed atmosphericconditions that cannot be elucidated using concentrations of 222 Rn, 212 Pb and/or
short-lived 222 Rn daughters alone. The observations described in this paper clearlysupport the assumption.
There are very few studies in which the influence of meteorological conditions
on a lack of radioactive equilibrium between 222 Rn and its short-lived daughters is
discussed in detail. From the observations described in this paper, it is suggested
that the ratio of the concentration of 222 Rn daughters to the concentration of 222 Rn
measured at the same location is useful as a supplemental tracer to their concentrations for a study of detailed atmospheric conditions. It is also suggested that theratio of the concentration of 222 Rn daughters at a given location to the concentrationof 222 Rn in a nearby area is useful as a supplemental tracer to their concentrations
in order to study the difference in atmospheric conditions and topography between
the two locations.
Recently, new methods have been developed for estimating variations of concentrations of airborne 222 Rn and 212Pb (one of the 220 Rn decay products), but
the results of computation do not always agree with observations (Segawa et al.,
1993; Chino and Yamazawa, 1996; Sakashita et al., 1997). Such methods involve
diffusion and transport models of 220 Rn, 222 Rn and their daughters using constant
exhalation rates of 220Rn and 222 Rn from the ground. The data and discussion
presented in this paper may be fundamental to improving computational models,such that they accommodate a wider range of weather related variations in 220 Rn
and 222 Rn exhalation rates.

Acknowledgements
This work was funded by the Japanese Ministry of Education, Culture, Sports,
Science and Technology. The authors wish to thank Mr. Akinori Matano regarding
negotiations with the Ministry to acquire funding for this work.

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