Influences of Source Term on Long-Range Transport of Radionuclides from the Fukushima Daiichi Nuclear Accident with FLEXPART Model
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摘要: 对包括拉格朗日粒子模式在内的大气扩散模式，提供准确的源项有助于获取更好的模拟结果。以日本福岛核电站2011年3月发生的核泄漏事故为研究对象，采用日本原子能机构Terada源项以及挪威大气研究所Stohl源项，利用FLEXPART（FLEXible PARTicle dispersion mode）模拟了放射性物质137Cs在全球大气中扩散传输的过程，并利用大气辐射监测数据讨论了基于两种源项模拟烟云的时空分布特征，探讨了源项对模拟结果的不确定性影响。结果显示：Terada源项及Stohl源项之间释放总量、释放速率、释放高度等虽然略有差别，但总体趋势描述相似，使得基于两源项的模拟烟云的扩散过程及影响区域基本一致。两模拟烟云在中纬度西风带作用下，均表现为自西向东扩散，经过太平洋、美洲大陆、欧洲，最后在整个北半球传输。基于两源项在亚洲-太平洋及北美大陆等近距离的模拟烟云的首次到达时间与首次监测时间吻合度较好，在全球尺度上基于Stohl源项的模拟在首次到达时间方面表现更优。其次，基于两源项进行全球尺度的模拟，近距离站点的模拟效果优于远距离站点模拟效果，且基于Stohl源项的模拟精度较好，Terada源项可能存在低估。另外模式进行全球尺度的模拟时，针对不同粒子数目对模拟结果的影响进行了分析，发现粒子数目的多寡对模拟精度有所影响，也影响模拟烟云扩散后期的疏密程度。
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关键词:
- 福岛核事故 /
- 放射性物质 /
- 大气扩散 /
- FLEXPART模式 /
- 源项
Abstract: The lagrangian particle dispersion models require an accurate source term as input to obtain better simulation results. In this paper, two different 137Cs source terms of Fukushima accident from Terada and Stohl were utilized. The FLEXPART (FLEXible PARTicle dispersion mode) was used to simulate global atmospheric dispersion and transport of radionuclides released from the Fukushima Daiichi nuclear accident. The atmospheric radiation monitoring data enable us to assess the spatial-temporal distribution of the two radioactive plumes and explore the uncertainty in results due to the different source terms. The results showed that despite the differences in the total emission, emission rate, and height of emission between the source terms of Terada and Stohl, the two source terms were alike in the whole developing trend. Thereby the two radioactive plumes had similar diffusion processes and reached similar regions. Under the influence of mid-latitude westerlies, the two radioactive plumes moved eastward across the Pacific Ocean, the American continent and Continental Europe, and eventually spread over the entire Northern Hemisphere. For short-range stations in the Asia-Pacific and North American regions, the simulated time of first detection of radioactive plume arriving at these stations with the two source terms both agreed well with the observed time. For long-range stations, the simulated time of first detection of the radioactive plume at these stations agreed better with the observed time when using the Stohl source term as input compared to that using the Terada source term as input. Second, using the two different source terms as input respectively, FLEXPART was applied to simulate global atmospheric dispersion of radionuclides. Results indicated that the simulated radioactivity concentrations were more accurate at the short-range stations than at the long-range stations. In addition, the simulated radioactivity concentrations were more accurate with Stohl's source term as input than that with Terada's source term as input. Terada's source term appeared to be lower than observations. Finally, the influence of the number of particles in the global model simulation was evaluated. It was found that differences in the number of particles could affect the statistical indexes of long-range transport of pollutants and the density of the diffusive radioactive plume in its late stage. -
图 1 计算网格浓度场的示意图。粒子的位置被标记为“＋”
Figure 1. Illustration of the method used to calculate gridded concentration.The particle position is marked by “＋”
图 2 Terada 源项的释放速率、释放源高度随时间变化图
Figure 2. Estimated temporal changes in the release rate and height of 137Cs by Terada
图 3 Stohl 源项的释放速率随时间的变化图
Figure 3. Estimated temporal changes in the release rate of 137Cs by Stohl
图 4 CTBTO 的38 个站点以及伯克利大学的UCB 站点及日本国立环境研究所Tsukuba 站点分布
Figure 4. Locations of the 38 CTBTO(Comprehensive nuclear-Test-Ban Treaty Organization)measurement stations, the UCB(University of California, Berkeley)station and the Tsukuba station of National Institute for Environmental Studies
图 5 EXP-Terada 试验获得的大气边界层内(2000 m 以下)的放射性物质137Cs 浓度：(a)2011 年3 月14 日08:00(协调世界时，下同)；(b)2011年3 月17 日01:00；(c)2011 年3 月21 日17:00；(d)2011 年3 月28 日00:00；(e)2011 年4 月17 日06:00；(f)2011 年4 月24 日00:00
Figure 5. Temporal changes in radioactivity concentration in ABL(Atmospheric Boundary Layer)(below 2000 m)from EXP-Terada:(a)0800 UTC 14 Mar2011;(b)0100 UTC 17 Mar 2011;(c)1700 UTC 21 Mar 2011;(d)0000 UTC 28 Mar 2011;(e)0600 UTC 17 Apr 2011;(f)0000 UTC 24 Apr 2011
图 6 EXP-Stohl 试验获得的大气边界层内(2000 m 以下)的放射性物质137Cs 浓度：(a)2011 年3 月14 日08:00；(b)2011 年3 月16 日14:00；(c)2011 年3 月21 日09:00；(d)2011 年3 月26 日14:00；(e)2011 年4 月16 日11:00；(f)2011 年4 月24 日00:00
Figure 6. Temporal changes in radioactivity concentration in ABL(Below 2000 m)from EXP-Stohl:(a)0800 UTC 14 Mar 2011;(b)1400 UTC 16 Mar 2011;(c)0900 UTC 21 Mar 2011;(d)1400 UTC 26 Mar 2011;(e)1100 UTC 16 Apr 2011;(f)0000 UTC 24 Apr 2011
图 7 EXP-Terada和(b)EXP-Stohl 模拟获得CTBTO 站点的137Cs 模拟值与观测值的散点图
Figure 7. Scatter plots of the observed and simulated 137Cs radioactivity concentration of CTBTO stations from(a)EXP-Terada and(b)EXP-Stohl
图 8 EXP-Stohl 及EXP-Terada 试验中相关站点137Cs 浓度模拟值与观测值的时间序列对比：(a)KIP39；(b)UCB；(c)USP71；(d)PHP52；(e)FRP28；(f)DEP33；(g)CAP17；(h)MNP45
Figure 8. Comparisons of time series of the observed and simulated 137Cs radioactivity concentration from EXP-Stohl and EXP-Terada:(a)KIP39;(b)UCB;(c)USP71;(d)PHP52;(e)FRP28;(f)DEP33;(g)CAP17;(h)MNP45
图 9 EXP-Stohl 及EXP-Terada 试验在(a)JPP38、(b)Tsukuba 观测站点137Cs 模拟值与观测值的时间序列对比
Figure 9. Comparisons of time series of the observed and simulated 137Cs radioactivity concentration at(a)JPP38 and(b)Tsukuba from EXP-Stohl and EXP-Terada
图 10 2011 年3 月(a)17 日与(b)19 日850 hPa 的位势高度(单位：gpm)及风场示意图
Figure 10. 850-hPa geopotential height(gpm)and wind on(a)17 Mar and(b)19 Mar 2011
图 11 EXP- Terada2 试验获得的大气边界层内(2000 m 以下)的放射性物质137Cs 浓度：(a)2011 年3 月14 日08:00；(b)2011 年3 月17 日14:00；(c)2011 年3 月21 日17:00；(d)2011 年3 月28 日00:00；(e)2011 年4 月17 日06:00；(f)2011 年4 月24 日00:00
Figure 11. Temporal changes in radioactivity concentration in ABL(Below 2000 m)from EXP-Terada2:(a)0800 UTC 14 Mar 2011;(b)1400 UTC 17 Mar 2011;(c)1700 UTC 21 Mar 2011;(d)0000 UTC 28 Mar 2011;(e)0600 UTC 17 Apr 2011;(f)0000 UTC 24 Apr 2011
表 1 试验方案
Table 1. Experimental design
试验 释放日期 释放总量/pBq pBq 时间段 粒子总数 EXP-Terada 3 月11 日至5 月1 日 8.86 30 886660 EXP-Stohl 3 月11 日至4 月20 日 36.60 324 3663606 
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表 2 模拟的烟云首次到达时间与观测结果相差天数的站数统计分析
Table 2. Statistical analysis of the differences in time(number of days)of first detection of radioactive plumes at stations between simulations and observations
Terada 站数 Stohl 站数 相差天数0 d 6 5 相差天数1 d 8 16 相差天数2 d 4 4 相差天数3 d 3 3 相差天数4 d 7 5 相差天数5 d 4 0 相差天数＞5 d 3 2 无监测数据 4 4 总站数 38 38
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