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Takemura et al. (2011) show that the negative anomaly of a 500-hPa height over the Okhotsk Sea area along 145° E made the westerly jet stronger than the climatological mean during mid-March; consequently, 70 to 80% of the radioactive material from the FDNPS was driven to the Pacific Ocean and the rest of the globe (Takemura et al. 2011; Stohl et al. 2012; Mészáros et al. 2016). The remaining material spread over and deposited onto the land area of Japan, producing characteristic hot spot patterns (Yasunari et al. 2011; JAEA 2012; SCJ 2014). The total emission of ¹³⁷Cs into the atmosphere until the end of April was estimated to be 14.6 ± 3.5 PBq (SCJ 2014). The ratio of the total deposition over the Japanese land area to the total atmospheric emission was estimated as 20 ± 6%, according to the airborne monitoring conducted by the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT 2011), whereas the ratio was calculated as 27 ± 10% based on the multi-model intercomparison by the Science Council of Japan (SCJ 2014). To date, this inconsistency has not been fully understood, owing to the lack of observation data, which is attributable to instrumental damage and electric outages as well as modeling uncertainties. In addition, there is still great uncertainty in the emission time series of the radioactive material, as shown in Fig. 1. Yumimoto et al. (2016) conducted an inverse analysis to optimally estimate the emission rate using the time series of the deposition map, but the result is very different from that of Katata et al. (2015).

Time series of the ¹³⁷Cs emission rate from the FDNPS, as estimated by Terada et al. (2012), Katata et al. (2015), and Yumimoto et al. (2016)

Recently, Tsuruta et al. (2014) developed a method to directly measure the hourly time series of the atmospheric ¹³⁷Cs concentration at surface level, from the aerosol sampling tapes of the national suspended particulate matter (SPM) network. The SPM network monitors air pollution by employing beta-ray attenuation counters. Four laboratories, namely, those of Tokyo Metropolitan University, the Nuclear Professional School of the University of Tokyo, the Japan Atomic Energy Agency, and the Japan Chemical Analysis Center, retrieved the atmospheric loading from the hourly aerosol spots on the SPM tape. This method offers the potential for studying the atmospheric transport of ¹³⁷Cs, although the data is from surface level, during the entire post-accident period; the SPM dataset has high temporal and spatial sampling, with observations every hour at 90 out of 400 sites (Fig. 2). In Fig. 2, it can be seen that the Nakadori region is a channel basin area between the Ou and Abukuma mountains, while the Hamadori region is a coastal region to the east of the Abukuma mountains. The FDNPS is located in the northern part of the Hamadori region. In this report, we compare the ensemble results of two aerosol transport models with SPM data. An important purpose of the comparison is to investigate the validity of the combined use of SPM data and multi-model simulations to depict the transportation of atmospheric ¹³⁷Cs over the Japan land area. Once validated, further analysis can be performed on a larger volume of SPM data, such as the most recent data from 99 SPM sites, which has recently been made available to the public (Oura et al. 2015). In addition, the results could be a useful input for our second model intercomparison, which is intended as a follow-up to the first comparison, which was made by the SCJ (SCJ 2014), and this can contribute to future discussions of the use of models in emergency protocols.

Names of key regions and locations of SPM sites at the time of accident for the present study. The Tohoku region is the northeastern part of the Japanese islands and includes the Fukushima and Miyagi prefectures; the Kantou region is the area that includes the Tokyo, Saitama, Chiba, Kanagawa, and Ibaraki prefectures. The FDNPS is located in the northern part of the Hamadori region, a coastal area to the east of the Abukuma Mountains. The Nakadori region is a channel basin area between the Ou and the Abukuma mountains. Open circles are SPM monitoring sites managed and maintained by local governments in eastern Japan before the accident. The base map was modified by using the original map in Fig. 1 of Tsuruta et al. (2014)