제주 지역 에어로졸의 40K, 238U, 232Th 농도 분석 및 연간 유효선량 평가

Abstract

The atmospheric total suspended particulates (TSP) aerosols were collected at Gosan site of Jeju Island, which is one of the background sites of Korea, during January to April 2013. This study analyzed using Inductively Coupled Plasma - Dynamic Reaction Cell - Mass Spectrometer (ICP-DRC-MS) the concentrations of potassium, uranium and thorium, and evaluated the annual effective dose by breathing from the results. A radium equivalent index (Raeq), which is known as a radiation risk coefficient through the radioactive concentrations of naturally occurring potassium, uranium and thorium, was added. Also, the backward trajectory analyses were conducted to check the inflow of the aerosols collected. As a result, it was observed during this study that the radioactive concentrations of 40K, 238U and 232Th were within the scope of 1.341 ~ 46.483 μBq/㎥ (mean: 14.188 ± 11.451 μBq/㎥), 0.006 ~ 1.032 μBq/㎥ (mean: 0.263 ± 0.226 μBq/㎥), 0.001 ~ 2.579 μBq/㎥ (mean: 0.226 ± 0.282 μBq/㎥), respectively. In the UNSCEAR 2000 Report, the radioactive concentration baselines of 238U and 232Th among the air particles were 1.0 and 1.5 μBq/㎥, respectively. In this study, an average of low concentration scope was shown except the specific meteoric phenomenon. During Asian dust periods, the concentrations of 40K (33.221 ± 12.333 μBq/㎥), 238U (0.788 ± 0.260 μBq/㎥), and 232Th (0.865 ± 0.252 μBq/㎥) were highly as 3.59, 5.51 and 7.86 times, respectively, compared to the non-event periods. For the haze event days, 40K (34.729 ± 10.792 μBq/㎥), 238U (0.730 ± 0.261 μBq/㎥), and 232Th (0.608 ± 0.475 μBq/㎥) were highly as 3.75, 5.10 and 5.53 times, respectively, compared to the non-event days. For the fog-mist event days, 40K (19.970 ± 15.824 μBq/㎥), 238U (0.371 ± 0.374 μBq/㎥), and 232Th (0.329 ± 0.451 μBq/㎥) were highly as 2.16, 2.59 and 2.99 times, respectively, compared to the non-event days. The correlations between the studied natural isotopes is a good positive correlation between 232Th and 238U (R2 = 0.9012), supporting the conclusion that they originated from the same source, mostly the crust. The average calculated 232Th/238U ratio was 1.01, which is close to the world average of unity (UNSCEAR, 2000). On the other hand, weak positive correlations between 232Th and 40K and between 238U and 40K were found, supporting the conclusion that 40K originated from other sources besides the crust. The inhalation annual effective radiation dose (default mode F, Male) to the public due to natural isotopes of the airborne TSP was in the range 16.2 ~ 77.89 nSv/y, depending on the age group. Because of the variations in air breathing rate, the total annual dose due to the natural radioactivity in airborne TSP increases for the older age groups. It is obvious that 232Th is the main contributor to the inhalation annual effective dose. 232Th was found to be responsible for 98.4 ~ 99.1% of the total dose. On the other hand, 40K was found to slightly contribute to the total dose. Since radium equivalent index is not efficient in expressing the inhalation annual effective dose, it was not included in the calculation of the total inhalation annual effective radiation dose. In order to check the migratory routes of the air particles, the backward trajectory analyses were carried out using the HYSPLIT 4 model provided by the NOAA(National Oceanic and Atmosphere Administration) in the U.S. As a result of classifying into 3 zones and comparing the moving route of air currents by zone, the inflow routes of air currents were 48.3 % in zone I (China), 10.3 % in zone II (Korean Peninsula), 3.4 % in zone III (Japan & North Pacific Ocean), and 37.9 % in the other overlap zone, which showed that the inflow route of air currents from China was the highest during this study. It was observed that a Radium equivalent index (Raeq), which is known as a radiation risk coefficient through the radioactive concentrations of naturally occurring potassium, uranium and thorium, was calculated as 0.111 ~ 6.263 μBq/㎥(mean: 1.679 ±1.516 μBq/㎥). Jeju Island with less pollution source and low population density is also one of the best places as a background area in Asia. It is judged that the results become a preliminary data on the impact of fine dust from China, which has been recently intensified, on the Korean Peninsula. Furthermore, unlike the previous weather in which the Asian dust blows throughout the peninsula only during the spring season, but now frequently occur even in the autumn. Therefore, it is judged as necessary to observe such Asian dust phenomenon in the long term through collecting a large number of the TSP aerosol filters.