Volume 15 (2015) Issue 2 Pages 2_1-2_12
In this study, we compiled a comprehensive dataset of hourly atmospheric 134Cs and 137Cs radioactivity concentrations in suspended particulate matter (SPM; aerosols less than 10 μm in diameter) that was systematically collected from March 12 to 23, 2011, in eastern Japan. In Japan, mass concentrations of SPM are measured using samples automatically collected on filter tapes at the air quality monitoring stations managed by the local governments. Most of the SPM monitoring stations in eastern Japan were operated during and/or after the Great East Japan Earthquake and tsunami that occurred on March 11, 2011, which triggered severe accidents at the Fukushima Daiichi nuclear power station (FDNPS). And the used filter tapes were sent to Tokyo Metropolitan University by the local governments through the Ministry of the Environment, Japan. The radionuclides in the hourly collected SPM samples at 99 of the more than 400 SPM stations were measured using Ge detectors. Because SPMs were collected hourly, the time series of the radiocesium concentration from March 12 to 23, 2011, has been described in detail. During this time, several radioactive plumes were observed in eastern Japan. Thus, precise description of these plumes (the peak time and the maximum radiocesium concentration in each plume at each SPM station) was our primary concern in this study. To confirm that our data were consistent with data independently measured by a research institution in Tokyo, we compared the hourly data by the institution with the hourly data for March 15-16 at an SPM station located near the institution. Although total suspended particulates were collected using a high-volume air sampler at the institution with a different sampling system, the data were highly consistent regarding the time series variations and radioactivity concentrations of 134Cs and 137Cs. This finding clearly indicates that the data presented in this study precisely reveal the 134Cs and 137Cs radioactivity concentrations in the atmosphere during March 2011. In addition to the radioactivity measurements, identifying the time and date when individual SPM samples were collected is equally important for producing a reliable database, and monthly reports of hourly SPM mass concentrations and SPM recording charts were used for confirmation. Cross-contamination is possible due to the apposition of radioactive materials from the SPM spot to the backside of the contacting tape. This cross-contamination is particularly likely for filter tapes made of polytetrafluoroethylene rather than glass fiber.