Variations of the Environmental Gamma Absorbed Dose Rate in Air in Hong Kong. H.Y. Mok, M.C. Wong, Y.H. Kwok and H.T. Poon

Transcription

1 Abstract Variations of the Environmental Gamma Absorbed Dose Rate in Air in Hong Kong H.Y. Mok, M.C. Wong, Y.H. Kwok and H.T. Poon Hong Kong Observatory, 134A Nathan Road, Kowloon, Hong Kong, China hymok@hko.gov.hk The Hong Kong Observatory has been continuously monitoring the environmental gamma absorbed dose rates in the air in Hong Kong since the late 1980 s. Spatial and seasonal variations in the environmental gamma absorbed dose rate in the air in Hong Kong have been evaluated. The spatial variations are found to be related to the variation in the geological structure across the territory and the seasonal variations are found to be attributable to changes in the activity concentration of radon in air which in turn may be affected by the direction of the prevailing surface winds, the temperature difference between soil and the overlying air and the moisture content of the soil. 1. Introduction The Hong Kong Observatory (HKO) operates a Radiation Monitoring Network (RMN) to continuously monitor the environmental gamma absorbed dose rates in air (hereafter called environmental gamma dose rates ) in Hong Kong since The HKO also conducts routine radiological surveys to characterize the population exposure to natural terrestrial radiation. The first comprehensive territory-wide radiological survey of environmental gamma dose rate in Hong Kong was conducted in 1999 on both open fields and built-up areas [1]. Sites at 42 open fields and 61 builtup areas were selected for the survey, according to the population distribution and land use. As urbanization continues to take place in Hong Kong, the survey is conducted on an on-going basis. 2. Methods 2.1. The Radiation Monitoring Network The RMN consists of 10 fixed monitoring stations (FIG 1) each equipped with a high pressure ionization chamber (HPIC) for measuring the environmental gamma dose rates (γ e ) continuously round the clock. Eight of the stations, namely, Kat O, King s Park, Ping Chau, Sha Tau Kok, Tai Mei Tuk, Tap Mun, Tsim Bei Tsui and Yuen Ng Fan are located on open fields. (Note: the station at King s Park, although located at the city centre, can also be classified as an open field station as it is on the top of a small hill and relatively far away from high-rise buildings.) Among these open field stations, the HPICs at Ping Chau, Kat O and King s Park are installed on ground surface of soil or rock, while those for the other stations are installed on concrete ground surface. The two remaining stations, at Kwun Tong and Sai Wan Ho respectively, are located at built-up areas with high-rise buildings nearby. In addition, the activity concentrations of gross alpha/beta and gamma emitters in air are also continuously monitored at the Ping Chau station by means of an Automatic Gamma Spectrometry System. This system utilizes the pseudocoincidence technique to analyze the alpha/beta transfer of radon, which enables the determination of activity concentrations of radon in air through gross alpha/beta counting [2]. The rainfall values at the RMN stations correspond to those measured by either co-located or nearby automatic rain gauges. Soil temperature values refer to those measured at King s Park at 20 cm below ground surface. 1

2 2.2. Territory-wide Radiological Survey For the purpose of survey, the territory was divided into 42 grid boxes of 5 km x 5 km each for open fields (FIG 2) and 61 grid boxes of 2.5 km x 2.5 km for built-up areas (FIG 3) according to the population distribution and land use. A survey site was selected within each grid box. A high pressure ionization chamber was used to measure γ e continuously for a period of 30 minutes at each of the survey sites. Measurements at built-up areas were conducted at street level. While most of the streetlevel survey sites are surrounded by high-rise buildings, all open field survey sites have good exposure without any man-made obstacles in the vicinity Cosmic Ray Measurements In order to estimate the terrestrial component of γ e, it is necessary to deduct the contribution from the cosmic rays. The cosmic ray component (γ c ) was measured for a period of 45 minutes once every quarter at a central spot on a freshwater reservoir using a high pressure ionization chamber. The measurement results obtained at the same quarter of the year were used for subtracting γ c from γ e to obtain the terrestrial environmental gamma dose rates (γ n ). The uncertainties in the γ c measurements depend on the uncertainties in the amount of gamma emitters in air at measurement time. 3. Results and Discussion Significant spatial variations of γ n were observed in the RMN and territory-wide radiological survey measurement results. In addition, seasonal variations of γ n with lower values in summer months (April to September) and higher values in winter months (October to January) were also observed at the open field RMN stations. In order to study the underlying factors that caused the observed spatial and seasonal variations, the measurements obtained in 2001 are used as an illustration Variations of Cosmic Ray Intensity Results of the cosmic ray measurements conducted in 2001 range from 0.031µGy/h to 0.034µGy/h with an average of 0.032µGy/h. The average value and the variation observed agreed well with other reported values [3]. It has been known that the temporal variation of cosmic ray intensity was caused by the temporal variation of solar activity [3]. The measurement results were subtracted from γ e to obtain γ n, the terrestrial environmental gamma dose rates Spatial Variations of the Terrestrial Environmental Gamma Dose Rates The annual average γ n recorded by the RMN in 2001 showed significant spatial variations (FIG 1). From the territory-wide radiological survey results (FIG 2 & FIG 3), similar spatial variations of γ n were observed. γ n values were in general higher at built-up areas than in open fields. The proximity of high-rise buildings at the survey sites was believed to be a major cause for higher γ n at built-up areas. The geological map of Hong Kong [4] in FIG 4 shows a complex geological pattern of soil and rocks over the territory, which primarily is composed of intrusive igneous rocks, volcanic rocks and sedimentary rocks of different levels of natural radioactivity. Areas of relatively high γ n (coloured in pink and deep blue) and low γ n (coloured in yellow and green) in FIG 2 coincide well with areas comprising soil or rocks of relatively high (e.g. granite) and low (e.g. quartz) level of natural radioactivity respectively in FIG 4 except at some grid boxes where the survey sites were close to sea areas (Grid Box A and B), in between two or more types of soil or rock (Grid Box C) or disturbed by human activities in the past (Grid Box D). Comparison of the distribution of γ n and the geological composition are highlighted below : (a) Relatively high γ n were measured over the Lantau Island (southwestern part of the territory) except Grid Box E (FIG 2) where γ n was relatively low. FIG 4 shows that the 2

3 (b) (c) (d) areas at the Lantau Island are largely composed of relatively high level of natural radioactivity (e.g. rhyolitic vitric tuff and java) except the area at Grid Box E which comprises quartz type rocks of relatively low level of natural radioactivity [5]. Higher γ n were measured at the southern part of Sai Kung (Grid Box F and G) than that at the northern part of Sai Kung (Grid Box H). This coincides with the difference in the types of rock at the two areas (FIG 4). Relatively low γ n were measured at the southeastern part of the territory (Grid Box I, J & K). FIG 4 shows that the areas are largely composed of trachytic tuff rock which is of the same classification as the type of rock at the northern part of Sai Kung. Low γ n were measured at the northwestern part of the territory (Grid Box L and M in FIG 2) which correspond to an area covered by superficial deposits of silt, sand and gravel (FIG 4) which have relatively low level of natural radioactivity [5]. Higher γ n was measured at Grid Box N (FIG 2), which is just to the west of Grid Box L. FIG 4 shows that the area at Grid Box N is composed of granite of relatively high level of natural radioactivity [5]. The above observations indicate that there is a connection between the spatial variations of the terrestrial environmental gamma dose rates and the different geological structures across the territory Seasonal Variations of the Terrestrial Environmental Gamma Dose Rates FIG 5 shows the monthly average γ n at the open field RMN stations in Seasonal variations of γ n were observed with lower values in summer months and higher values in winter months. Among these stations, the seasonal variations at the King s Park, Ping Chau, Kat O, Tai Mei Tuk and Yuen Ng Fan station were more significant than those at the Sha Tau Kok, Tap Mun and Tsim Bei Tsui station Radon Activity Concentration in Air and Terrestrial Environmental Gamma Dose Rates The radon activity concentrations measured at the Ping Chau station, a station which had no anthropogenic activities, showed similar seasonal variations as γ n for the station (FIG 6), with a correlation factor of about 0.8 (FIG 6(a)). Also, gamma spectroscopic analysis results of the air as measured by the Automatic Gamma Spectrometry System showed that there were no other gamma emitting radionuclides in air apart from the daughter radionuclides of radon [7]. These results together indicated that the variation of radon concentrations in air was a major cause for the seasonal variation of γ n Weather Elements Affecting Radon Concentration in Air There were two sources of radon in air at the monitoring stations: (a) radon originating from upstream being carried to the stations by the prevailing winds, and (b) radon exhaled from the soil, rock and building materials in the vicinity of the monitoring stations Weather Conditions and the Terrestrial Environmental Gamma Dose Rates in Hong Kong For the first source of radon in air (radon originating from upstream), Wong et al discussed in earlier studies that in winter, the northeast monsoon blowing from the mainland China carried with it radon from natural sources while in summer, the maritime airstream which originated from the sea was relatively deficient in radon [8]. Regarding the second source of radon in air (radon exhaled from nearby areas), Dushe et al s study indicated the exhalation rate of radon from soil depended on the temperature difference between the soil and the air and the moisture content of the soil [9]. A higher soil temperature than the overlying air could lead to a convective flow of radon from soil to air. The larger the temperature difference, the higher will be the exhalation rate. In respect of soil moisture, water could fill up the pore space in the soil and suppress the exhalation of radon. Thus, the higher water content in the soil, the lower will be the exhalation rate. In 2001, the monthly mean soil temperatures were higher than the monthly mean 3

4 air temperatures at the King s Park station with larger differences in the winter months (FIG 7). According to Dushe et al [9], this larger difference supported a larger exhalation rate of radon from soil in winter than in summer. This is exacerbated by the much lower rainfall in winter (FIG 5). Assuming the high correlation between the activity concentration of radon in air and γ n observed at the Ping Chau station was representative of the situation at the other open field RMN stations, the above discussion indicates that three factors, namely the northeast monsoon, larger temperature difference between soil and the overlying air and the drier weather in winter were favourable for higher γ n in winter than in summer in Hong Kong. 4. Conclusion Significant spatial variations in the environmental gamma absorbed dose rates in air on open fields and at built-up areas have been observed in Hong Kong. The spatial variation on open fields is found to be connected to the variation of the geological structure across the territory. In addition, noticeable seasonal variations in the environmental gamma absorbed dose rates in air on open fields with higher values in winter months than in summer months have also been observed. The seasonal variations can be attributed to changes in the activity concentration of radon in air. Three factors, namely the northeast monsoon, larger temperature difference between soil and the overlying air and the drier weather in winter are found to be favourable for higher activity concentration of radon in air. This, in turn, is found to be related to the higher environmental gamma absorbed dose rates in air observed in winter. References 1. Hong Kong Observatory, Environmental Gamma Absorbed Dose Rates in Air in Hong Kong 1999, Technical Report No.17. Hong Kong Observatory, Hong Kong (1999). 2. Mattsson, R., Paatero, J. and Hatakka, J., Automatic Alpha/Beta Analyzer for Air Filter Samples Absolute Determination of Radon Progeny by Pseudo-Coincidence Techniques. Radiation Protection Dosimetry 63(2), pp (1996). 3. UNSCEAR, Sources and Effects of Ionizing Radiation - UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes, Volume I: Sources. United Nations, New York (2000) 4. Civil Engineering Department, Geological Map of Hong Kong, Edition 2. Geotechnical Engineering Office, Civil Engineering Department, Hong Kong (1999) 5. National Council on Radiation Protection and Measurements, Exposure of the population in the United States and Canada from natural background radiation, NCRP Report No.94. National Council on Radiation Protection and Measurements (1987). 6. Mok, H.M., Cheng, K.M., Kwok, Y.H. and Chan, Y.K, A case study on the effect of high rise buildings on the environmental gamma absorbed dose rates in air. Proceedings of Symposium of Natural Radiation Exposure and Control, Beijing 2000 p (2000). 7. Hong Kong Observatory, Environmental Radiation Monitoring in Hong Kong: Annual Report 2001, Technical Report No.21. Hong Kong Observatory, Hong Kong (2002). 8. Wong, M.C., Lam, H.K. and Mok, H.Y., Effects of Weather on the Ambient Gamma Radiation Level in Hong Kong. Proceedings of the 1996 International Conference on Radiological Protection, Vienna, April 1996 (1996). 9. Dushe,C., Kummel, M. and Schulz, H., Investigations of Enhanced Outdoor Radon Concentration in Johanngeorgenstadt (Saxony). Health Physics 84(5): (2003). 4

5 FIG. 1. The terrestrial environmental gamma absorbed dose rates in air at the monitoring stations of the RMN in

6 FIG. 2. The terrestrial environmental gamma absorbed dose rates in air γ n at open fields in 2001 FIG. 3. The terrestrial environmental gamma absorbed dose rates in air γ n at built-up areas in

7 FIG. 4. Geological map of Hong Kong [4] 7

8 8

9 FIG. 6. Terrestrial environmental gamma absorbed dose rates in air γ n and radon activity concentrations at the Ping Chau station in 2001 FIG. 6(a). Correlation between γ n and radon activity concentrations at the Ping Chau station in

10 10 FIG. 7. Monthly mean air temperature, soil temperature and difference between air and soil temperatures at the King s Park station in 2001.