[ Research Article ]
Asian Journal of Atmospheric Environment - Vol. 4, No. 1, pp.20-25
ISSN: 1976-6912 (Print) 2287-1160 (Online)
Print publication date 30 Jun 2010
Received 10 Oct 2009 Accepted 13 Mar 2010

Estimation of 222Rn Concentration in the Lower Troposphere during Precipitation Using Wet Scavenging Model for its Decay Products

Masanori Takeyasu* ; Minoru Takeishi
Radiation Protection Department, Nuclear Fuel Cycle Engineering Laboratories, Tokai Research and Development Center, Japan Atomic Energy Agency, 4-33, Muramatsu, Tokai-mura, Naka-gun, Ibaraki, Japan
Present Address: Environmental Radioactivity Monitoring Center, Safety Technology Office, Japan Nuclear Fuel limited, 4-58, Okituke, Obuchi, Rokkasho-mura, Kamikita-gun, Aomori, Japan

Correspondence to: *Tel: +81-29-282-1111, E-mail: takeyasu.masanori@jaea.go.jp

Abstract

The gaseous 222Rn concentration at the level of clouds was estimated by using the wet scavenging model of its decay products with the observed data of environmental radiation at the ground. And the origin of the 222Rn was also discussed. The estimation was done for a precipitation event on Dec. 26-27, 2003, when a large increase of the radiation was observed in Tokai-mura in Ibaraki, Japan. From a backward trajectory analysis, the origin of 222Rn atoms for that event was traced back to the northeastern part of China, and it was expected that the large amount of 222Rn emitted in the northeastern part of China was transported to Tokai-mura by the Eurasian continental air mass.

Keywords:

222Rn, Atmosphere, Precipitation, Decay products, Scavenging model, Back trajectory analysis

1. INTRODUCTION

Radon (222Rn), which has a half-life of 3.82 days, is the gaseous radionuclide that may diffuse into the atmosphere after being produced by the decay of 226Ra in soil. The 222Rn in the atmosphere disperses with advection and diffusion, and decays to radionuclides including 218Po (half-life: 3.10 min), 214Pb (half-life: 26.8 min), 214Bi (half-life: 19.9 min) and etc., according to the decay scheme. The 222Rn and its decay products are useful tracers for investigating the dynamics of harmful substances and aerosols in the atmosphere, since their half-life is in the same time scale as that of the residence time of the atmospheric substances.

The 222Rn decay products in the atmosphere are transported to the ground by the scavenging process of precipitation, which causes an increase in environmental gamma radiation at the ground surface. Ikebe et al. (1985) and Takeyasu et al. (2006) proposed scavenging models of the decay products in the atmosphere by precipitation, and these models have been validated by comparing with the observed 214Pb and 214Bi concentrations in rainwater and their dependency on the precipitation. In these models, the gaseous 222Rn concentration at the level of clouds is one of the parameters for determining the concentrations of 214Pb and 214Bi in rainwater. Few studies, however, have investigated the gaseous 222Rn concentration at the level of clouds.

In this study, an attempt was done to estimate the gaseous 222Rn concentration at the level of clouds by the wet scavenging model of the decay products proposed by Takeyasu et al. (2006) with the observed data of the increase of the environmental gamma radiation. This paper also discussed the origin of the 222Rn when a large increase of the radiation at the ground was observed.

2. MATERIALS AND METHODS

2. 1 Wet Scavenging Model of the Atmospheric 222Rn Decay Products

In general, the two scavenging mechanisms of radionuclides in the atmosphere by precipitation, namely, rainout and washout, are important (Engelmann, 1968). It is known that 222Rn decay products contained in rainwater are mainly removed by rainout (Minato, 1983; Takeuchi and Katase, 1982). In this study, a rainout model was designed, the outline of which was shown in Fig. 1. The conditions of rain clouds were set as follows:

Outline of the proposed scavenging model by precipitation for 222Rn decay products in the atmosphere.

• (a) Gaseous 222Rn is distributed uniformly in the atmosphere at the height of rain clouds.
• (b) Gaseous 222Rn in the atmosphere decays to 218Po, 214Pb, 214Bi, etc., according to the decay scheme.
• (c) The rain cloud is a homogeneous liquid water column.
• (d) A radioactive equilibrium is established between the 222Rn decay products contained in the cloud liquid water.
• (e) All the cloud liquid water removed from rain clouds is transported to the ground as rainwater.
• (f) The transport time of the liquid water from the cloud base to the ground was set in relation to the precipitation.

Under these conditions, the concentrations of 222Rn decay products in the cloud liquid water or the rainwater are given by the following.

2. 1. 1 In the Rain Clouds

The concentrations of 222Rn decay products absorbed into the cloud liquid water vary with time according to the following equations:

 $\frac{d{N}_{Po}^{W}}{dT}=\mathrm{\Lambda }{N}_{Po}^{a}-\left({\mathit{\lambda }}_{po}+\mathrm{\Psi }\right){N}_{Po}^{W}$ (1)
 $\frac{d{N}_{Pb}^{W}}{dT}={\mathit{\lambda }}_{Po}{N}_{Po}^{W}-\left({\mathit{\lambda }}_{Pb}+\mathrm{\Psi }\right){N}_{Pb}^{W}$ (2)
 $\frac{d{N}_{Bi}^{W}}{dT}={\mathit{\lambda }}_{Pb}{N}_{Pb}^{W}-\left({\mathit{\lambda }}_{Bi}+\mathrm{\Psi }\right){N}_{Bi}^{W}$ (3)

where are the number of atoms of 218Po, 214Pb and 214Bi, respectively, in the cloud liquid water (m-3 of the cloud air), Λ the transport coefficient of 218Po between the air and the liquid water in the cloud (s-1), NaPo the number of atoms of atmospheric 218Po (m-3), λi the decay constant of the 222Rn decay product i (s-1), and Ψ the removal rate of the cloud liquid water from rain clouds (s-1). Wilkening (1990) showed that 218Po that was produced by the decay of 222Rn in the atmosphere attached to cloud particles rather than to atmospheric aerosols. Therefore,

 $\mathrm{\Lambda }{N}_{Po}^{a}={\mathit{\lambda }}_{Rn}{N}_{Rn}^{a}$ (4)

where λRn is the decay constant of 222Rn (s-1), NaRn the number of atoms of gaseous 222Rn (m-3), which is constant according to the condition (a). The removal rate, Ψ, of the liquid water from various types of clouds was found to correlate well with the precipitation, according to the radar observation carried out by Takeda and Natsuki (1982).

According to the assumption (d), the Eqs. (1)-(3) are solved under the equilibrium condition, that is, $\left({dN}_{Po}^{W}/dT\right)=\left({dN}_{Pb}^{W}/dT\right)=\left({dN}_{Bi}^{W}/dT\right)=0$, and the number of atoms of 222Rn decay products in the cloud liquid water, NWi (∞) (i=Po, Pb, Bi) (m-3 of the cloud air), is obtained.

2. 1. 2 Below the Rain Clouds

During the period when the liquid water that was removed from rain clouds is being transported from the cloud base to the ground, the concentrations of 222Rn decay products in the falling liquid water are represented by the following equations:

 $\frac{d{N}_{Po}^{r}}{dt}={-\mathit{\lambda }}_{Po}{N}_{Po}^{r}$ (5)
 $\frac{d{N}_{Pb}^{r}}{dt}={\mathit{\lambda }}_{Po}{N}_{Po}^{r}-{\mathit{\lambda }}_{Pb}{N}_{Pb}^{r}$ (6)
 $\frac{d{N}_{Bi}^{r}}{dt}={\mathit{\lambda }}_{Pb}{N}_{Pb}^{r}-{\mathit{\lambda }}_{Bi}{N}_{Bi}^{r}$ (7)

where ${N}_{i}^{r}$ is the number of atoms of the 222Rn decay product i in the falling liquid water (m-3 in the air), and ${N}_{i}^{r}\left(0\right)={N}_{i}^{W}\left(\infty \right)$. According to assumption (e), the falling liquid water that was removed from rain clouds becomes rainwater, and the number of atoms of the 222Rn decay product i in the falling liquid water is converted to that in the rainwater, ${n}_{i}^{r}$ (cm-3 in the rainwater), using the following equation:

 ${\mathit{\lambda }}_{i}{n}_{i}^{r}=\frac{{\mathit{\lambda }}_{i}{N}_{i}^{r}}{M}$ (8)

where M is the liquid water content of rain clouds (cm3 m-3 (in the cloud air)), which was set to be the mean value (0.2 cm3 m-3 (in the cloud air)) (Mizuno et al., 2003).

The time, t (s), is the transport time of the falling liquid water from the cloud base to the ground, and is calculated with the cloud base height and the speed at which the liquid water falls. The cloud base height was set at 3,000 m, which was adopted as the mean height of stratiform clouds by Ikebe et al. (1985). Mason (1971) gave the relationship between the mean raindrop diameter and the precipitation and that between the raindrop diameter and the terminal speed of the raindrop. From the cloud base height and Mason’s relationships, the falling time of the liquid water can be represented as a function of the precipitation. When the precipitation is 1 mm h-1, the falling time is about 20 min, and when the precipitation is 10 mm h-1, the falling time is about 10 min.

Then, the concentrations of the decay products in rainwater that reaches to the ground surface become functional to the gaseous 222Rn concentration at the level of clouds and the precipitation. This relationship was validated by the comparison with the observation results for the precipitation events in Osaka, Japan in 1992 (Takeyasu et al., 2006).

2. 2 Estimation of the Gaseous 222Rn Concentration at the Level of Clouds from the Data of Environmental Radiation during Precipitation

On the basis of the model described in the previous section, the gaseous 222Rn concentration at the level of clouds was estimated from the observed data of the increase of environmental radiation dose rate. In this method, the increase pattern of the dose rate calculated by the model was adjusted to agree with the observed one, and the atmospheric 222Rn concentration, which is one of the model’s parameters, was estimated.

First, the gaseous 222Rn concentration at the level of clouds was assumed to be 1 Bq m-3, and the concentrations of its decay products in rainwater were calculated from the precipitation data observed continuously at the Nuclear Fuel Cycle Engineering Laboratories (Cycle Laboratory), JAEA, located in Tokai-mura in Ibaraki, Japan.

The ground deposition amounts of the decay products i (i=214Pb, 214Bi) by the rainfall during the time from tj-5 to tj+5 (min), Dji (atom cm-2 s-1) were obtained by multiplying the concentrations of the decay products in rainwater by rainfall, shown in the equation of (9). The value of j corresponds to the period of rainfall, and t1 the start time of rainfall. For example, if the rainfall period is 60 min, the value of j is from 1 to 6.

 ${D}_{ji}=\frac{2.8×{10}^{-5}{A}_{ji}{P}_{j}}{{\mathit{\lambda }}_{i}}$ (9)

where Aji is the concentrations of the decay products i in rainwater (Bq cm-3) which falled between the period of tj-5 to tj+5 (min), Pj the rainfall (mm h-1) and λi the decay constant of the decay products i (s-1).

The decay products that deposited to the ground were assumed to be not losed by surface runoff. The amount of the decay products i on the ground (Nji; atom cm-2) which falled by rainfall from the time of tj-5 to tj+5 (min) changed according to the following equations;

(1) for the time of t (tj-5<t<tj+5)

 $\frac{d{N}_{jPb}}{dt}={D}_{jPb}-{\mathit{\lambda }}_{Pb}{N}_{jPb}$ (10)
 $\frac{d{N}_{jBi}}{dt}={D}_{jBi}+{\mathit{\lambda }}_{Pb}{N}_{jPb}-{\mathit{\lambda }}_{Bi}{N}_{jBi}$ (11)

(2) for the time of t (tj+5<t)

 $\frac{d{N}_{jPb}}{dt}=-{\mathit{\lambda }}_{Pb}{N}_{jPb}$ (12)
 $\frac{d{N}_{jBi}}{dt}={\mathit{\lambda }}_{pb}{N}_{jPb}-{\mathit{\lambda }}_{Bi}{N}_{jBi}$ (13)

The amount of the decay product i (Sji; atom cm-2) at the time of t (tj+5<t) (min) resulting from the deposition amount by the rainfall until the time of tj+5 (min) is given by the following equation;

 ${S}_{ji}=\sum _{k=1}^{j}{N}_{ki}$ (14)

The conversion from the amounts of 214Pb and 214Bi on the ground to the dose rate was done using the conversion factor calculated by Minato (Minato, 1980). Minato (1980) calculated the conversion factor from the amount of 214Pb and 214Bi on the ground to the dose rate at the height of 1m above the ground, assuming that 214Pb and 214Bi on the ground formed semi-infinite plane sources. Finally, the 222Rn concentration as a parameter in the model was adjusted for the calculated increase of the dose rate to agree with the observed one at a monitoring post installed in flat ground inside the Cycle Laboratory.

The estimation of the 222Rn concentration was done for a precipitation event on Dec. 26-27, 2003 when a large increase of the dose rate was observed and for another event on Nov. 20, 2003 when no such increase was observed.

3. RESULTS AND DISCUSSION

Fig. 2 shows the results of the adjustment of the calculated increase of the dose rate to the observed one for the precipitation events on Nov. 20 and Dec. 26-27, 2003. From Fig. 2, the calculated pattern of the increase agreed well with the observed one, and the estimated gaseous 222Rn concentration at the level of clouds for the event on Nov. 20 and on Dec. 26-27 was 1.5 Bq m-3 and 6 Bq m-3, respectively. In the model used in this study, the decay products in the atmosphere deposited to the ground by rainout phenomenon, where the cloud base height was set to 3,000 m. If the decay products deposit by washout phenomenon under cloud, it is anticipated that their concentration ratio at the height of the ground changes. However, the calculated ratio of 214Bi to 214Pb by the model agreed relatively well with the observed one (Takeyasu et al., 2006). From this fact, the time from the attachment of the decay products to rainwater to the arrival of rainwater on the ground in this model seem to be correct. However, cloud water content, M, is the mean value in this model. The concentrations of the decay products in rainwater are in inverse proportion to the cloud water content, M. It is anticipated that the cloud water content differs for every rain cloud, and it becomes an error factor for estimating the concentration.

Results of the adjustment of the increase pattern of the dose rate between the calculation and the observation on (a) Dec. 26-27 and (b) Nov. 20, 2003.

Iida et al. (2003) measured vertical profiles of 222Rn concentrations up to 5,000 m in the atmosphere over central Japan during 1999 to 2000 by aircraft. The concentration was 1-2 Bq m-3 up to 5,000 m over Toyama bay on 23 March 1999. The concentration was about 1 Bq m-3 at the height of 3,000 m over Enshunada on 26 April 1999. These concentrations were the same order as the estimated one at the level of clouds on Nov. 20, 2003.

Fig. 3(a) shows the surface weather chart on 9:00 in Dec. 27, 2003 (JST). From Fig. 3(a), there was an atmospheric depression in the Pacific Ocean, east of the Japanese islands, and the typical winter atmospheric pressure pattern prevailed. A northwest wind was predominant at the meteorological tower of the Cycle Laboratory on Dec. 26-27. Fig. 3(b) shows the chart on 9:00 in Nov. 20, 2003 (JST). From Fig. 3(b), an atmospheric depression located west of the Japanese islands, and a southerly wind was dominant at the meteorological tower. On March 23 and April 26, 1999 when Iida et al. (2003) measured the vertical profile of atmospheric 222Rn concentration, the surface weather chart was more similar to that on Nov. 20, 2003. From these facts, it was concluded that the atmospheric condition over the Japanese islands on Dec. 26-27, 2003 was affected by the Eurasian continental air mass, and that the atmosphere on Nov. 20, 2003 and March 23 and April 26, 1999 were not affected by such air mass.

Surface weather chart on (a) 9:00 in Dec. 27 and (b) 9:00 in Nov. 20, 2003 (JST).

For the precipitation events in Nov. 20 and Dec. 26-27, 2003, the backward trajectory analysis was carried out from the lower troposphere at a height of rain clouds (3,000 m) over Tokai-mura in Ibaraki, Japan as the starting point. The analysis was performed using METEX online system which is offered on the web site of the Center for Global Environmental Research (CGER) in National Institute for Environmental Studies of Japan with meteorological data of the National Centers for Environmental Prediction (NCEP) of USA (National Institute for Environmental Studies of Japan; http://cgermetex.nies.go.jp/metex/index_jp.html). Fig. 4 depicts backward trajectories arrived at Tokai-mura for the events. The air mass trajectories for the events on Nov. 20 and Dec. 26-27, 2003 traced back to the Pacific Ocean and the northeastern part of China, respectively.

Backward trajectories for the transport of the 222Rn in the lower troposphere arrived at Tokai-mura on (a) Dec. 27 and (b) Nov. 20, 2003.

The backward trajectory analysis was also carried out for March 23 and April 26, 1999 when Iida et al. (2003) measured the atmospheric 222Rn concentration using airplane. As the result, the air mass above the Toyama Bay on March 23 came from northern China and the air mass above the Enshunada on April 26 from the Pacific Ocean.

Zhuo et al. (2005) measured the atmospheric 222Rn concentration at the ground level in China, and showed that the annual average concentration was 17 Bq m-3 in the interior of northeast of China. Jin et al. (1985) discussed that the atmospheric 222Rn concentration became high in winter season. They measured and discussed the concentration at the ground level, and from their discussions, the concentration on the ground level in the winter in northeastern China may become over several tens of Bq m-3. Jacobi and Andre (1963) calculated the vertical profile of 222Rn concentration in the atmosphere, and showed that the 222Rn concentration up to the height of more than 1,000 m from the ground became constant in case of the strong turbulence of the atmosphere. In such meteorological conditions, the atmospheric 222Rn concentration in the lower troposphere in that region of China might become high and the large amount of 222Rn was transported to Tokai-mura by the Eurasian continental air mass on Dec. 26-27, 2003. On the other hand, for March 23, 1999, the atmospheric 222Rn concentration in northern China was not so high because of spring season, and it was suggested that the large amount of 222Rn did not reach to Toyama Bay. In the future, further investigation will be performed in terms of the transport mechanism of the 222Rn from the continent to Japan and the wet scavenging mechanism of the decay products in the atmosphere.

4. CONCLUSIONS

The gaseous 222Rn concentration at the level of clouds was estimated by using the wet scavenging model of 222Rn decay products with the observed data of environmental radiation at the ground. The origin of the 222Rn was also discussed when a large increase of the radiation at the ground was observed.

The atmospheric 222Rn concentration for the precipitation event on Dec. 26-27, 2003, when a large increase of the radiation was observed in Tokai-mura in Ibaraki, Japan, was estimated to be 6 Bq m-3. On the other hand, the 222Rn concentration for the event on Nov. 20, 2003 when the large increase of the radiation was not observed was estimated to be 1.5Bq m-3.

The backward trajectory analysis was carried out from the lower troposphere in Tokai-mura as the starting point. From the analysis, the origin of 222Rn atoms for the events on Dec. 26-27, 2003 was traced back to the northeastern part of China. It was expected that the large amount of 222Rn emitted in the northeastern part of China was transported to Tokai-mura by the Eurasian continental air mass.

References

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Fig. 1.

Outline of the proposed scavenging model by precipitation for 222Rn decay products in the atmosphere.

Fig. 2.

Results of the adjustment of the increase pattern of the dose rate between the calculation and the observation on (a) Dec. 26-27 and (b) Nov. 20, 2003.

Fig. 3.

Surface weather chart on (a) 9:00 in Dec. 27 and (b) 9:00 in Nov. 20, 2003 (JST).

Fig. 4.

Backward trajectories for the transport of the 222Rn in the lower troposphere arrived at Tokai-mura on (a) Dec. 27 and (b) Nov. 20, 2003.