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77. Simulation of the Long-distance Atmospheric Transport of Radon-222 Using a Lagrangean-type Model
Tetsuya Sakashita, Shinji Tokonami, Masahiro Doi and Yuji Nakamura
Keywords: Rn-222, atmospheric transport, numerical simulation, eastern Asia
Atmospheric transport of radon-222 (hereafter radon) with emanation sources in a distant and region has usually been numerically simulated using an Euler-type model;such a model offers an advantage of reduced execution time. However, the Euler-type model generally has a weak point that it includes a rather large numerical coefficient of 104 m2s-1 for the horizontal diffusion process. Thus, a Lagrangean-type model, not including numerical diffusion procedure, was applied to the simulation of atmospheric transport of radon in this study. The study also evaluated exhalation rate of radon as an area-averaged value.
The atmospheric transport model used was based on a random walk method for a lot of Lagrangean particles, which has been described elsewhere. The horizontal diffusion was assumed to follow the Gifford function [Atmos. Environ., 16, 505-512, 1982]. The relational functions derived by Chino [JAERI 1334, 1995] were applied to the vertical diffusion of the particles. The area of the computational simulation covered eastern Asia, with the horizontal length reaching about 4,000 km. Radon was assumed as being emanated from the ground surface throughout the area at a flux of 1 radon atom cm-2 s-1, except for an area of 240 km2 surrounding the point at the National Institute of Radiological Sciences (NIRS) where the atmospheric concentration of radon was actually observed. The inflow flux was not taken into account, because it would give only a small contribution to radon concentration. To make a wind-field, data on grid point value (GPV) provided by the Japanese Meteorological Agency were used. The outdoor (atmospheric) concentrations of radon were measured with the Electrostatic Radon Monitor at NIRS during two periods from 19th - 27th Dec. 1997 and 20th - 30th Jan. 1998.
The observed radon concentrations, the calculated radon concentrations, and half of the calculated ones are shown in Fig. 27. A large discrepancy was found between the calculated radon concentrations (hereafter CR) and the observed. Therefore, several different radon exhalation rates were assumed to minimize the discrepancy, and the exhalation rate of 0.5 radon atom cm-2 s-1 was found as the best fit. As the result, the magnitude and the phase of the variation pattern on half of the calculated radon concentrations (hereafter HCR) seemed to be very close to those of the observed ones, especially in the term of 22th - 24th Dec. 1997. For these days, atmospheric pressure patterns typical in winter were seen in the weather charts. This mean observed radon concentrations during these days consisted mainly of radon which emanated from a remote land area, i.e. in China. Moreover, it seemed that the tracks of the radon particles transported from remote land surfaces during these days were followed well by the present model. In the second period of 20th - 30th Jan. 1998, the magnitude of HCR was almost the same level as the observed. No diurnal fluctuations were displayed in the HCR. The discrepancy seemed to be caused by a diurnal fluctuation of radon originated from an area near NIRS.
In conclusion, the simulation of long-distance atmospheric transport of radon from a remote land area was carried out by a Lagrangean-type model. The calculated radon concentrations with the exhalation rate of 0.5 radon atom cm-2 s-1 were very close to the observed ones. In addition, the results showed a Lagrangean-type model was effective for the estimation of an area-averaged exhalation rate of radon.
