Measurements of atmospheric radioactivity dose rates over the North Paciﬁc after the Fukushima Daiichi nuclear power plant accident during the period March 2011 - March 2015

. On 11 April 2011, a magnitude 9.0 earthquake occurred about 154 km northeast of the Fukushima Daiichi Nuclear Power Plant (FNPP1; 37 . 420 ◦ N and 141 . 033 ◦ E ). Here we present continuous measurements of the atmospheric dose rates after the Fukushima accident over North Paciﬁc atmosphere by a ﬂeet of thirteen in-service global container cargo ships and another at a Tokyo port site from the Taiwanese PGGM (Paciﬁc Greenhouse Gases Measurements) project. The continuous measure- 5 ments of atmospheric dose rates were collected from a total of 294 cruises with 41,485 measurements during the period March 2011 to 2015. In this work, we identify three key aspects of the impacts of the radioactive materials following the Fukushima accident: The altitude effect, the land surface effect, and the transported effect. We showed measurements of air dose rates over the land surface areas and over the oceanic atmosphere. The striking differences in the air dose rates measured during the 2011-2015 period clearly identify the deposition effects and the effects associated with radioactive materials transported 10 from the FNPP1. Air dose rates measured over the land surface areas were resulted from a combination of the effects from the deposition of radioactive materials on the surface and the radioactive materials contained in the airborne particles. Air dose rates measured over the oceanic atmosphere contains radioactive effects from airborne particles. The data of air dose rates over the North Paciﬁc atmosphere show the eastward transport of radioactivity. Eastward transport of radioactive materials had been observed after 11 Mar 2011. Monitoring data show that the export of radioactive materials to the Paciﬁc 15 after March 2011, and in 2012, 2013, 2014, and 2015. All measurment raw data reported in this work are available at https://doi.org/10.6084/m9.ﬁgshare.9757697 (Wang et al., 2019), and the calibrated data ﬁles are available at http://doi.org/10.5281/zenodo.3468896 (Wang, 2019). Our data can help to further develop and verify model for the atmospheric dispersion of nuclear materials from the FNPP1 over the land surface areas and over the North Paciﬁc atmosphere. dose rates measured over the northeastern Paciﬁc. As such, the ship-based dose rate measurements reveal for the ﬁrst time that the northeastern Paciﬁc are particularly susceptible to the long-range transport of radioactive materials from Asian land areas.

sensor. We then build a lookup table of calibration factors, Ai, for each day of measurements by linearly interpolating A1 and A2 in time. This look-up table of Ai is then used in the calibrations of measured dose rates (Wang et al., 2019) for each B20 85 and G10 sensors (Wang, 2019). The results and distribution of Ai for each sensor are shown in Fig. 1.

The Monitoring Sites
The sensor number G01 measured the radioactivity dose rates at a land-based site located at the Tokyo Port (Fig. 2) and southwest to the FNPP1. The rest of thirteen sensors were deployed onboard the container ships operated over the North Pacific ( Fig. 3). Table 1 shows a list of air dose sensors and measurement period.

Data Collection and Validation
Measurements were taken 6-hourly onboard the ship. These measurements were made manually by officers of the ship at the compass deck, which is located at the top of the ship. The readings were taken from 6 directions: east, west, north, south, 95 upward, and downward directions. Measurements at Tokyo Port were also manually taken at 00 UT and 06 UT hours during the working day. The readings were also taken at six directions as those onboard the ships. In order to test the importance of sensitivity of measurements as a function of distance from the ground, two additional measurements were taken: at 50 cm and 10 cm above the ground at the attic of the office (Wang et al., 2019).      measurements at the attic of the office and the A4 gate. The measurements at the attic office were higher than the measurements made at the A4. The discrepancies between these two data set are found in the time close to 11 March 2011 and become close to each other at the time of September 2015. The main reason for these discrepancies is due to the altitudes of measurements taken in the attic and the A4 gate. Two additional measurements were taken in the attic of the office. Elevated dose rates from 135 the attic measurements than at the A4 gate indicate the presence of deposited radioactive materials on the ground. Highest radioactivity levels were measured at 10-cm altitude from the ground; followed by 50-cm and 1-m from the ground. Elevated results were measured close to the surface, indicating the effect of radioactivity from deposited radioactive materials on the surface.   Most of these variabilities had occurred during the autumn-winter-spring months when Tokyo Port was downwind from the inland area. As such, the calling ships berthed at the Tokyo Port intercepted radiative dust materials transported from land 155 toward the ocean, giving rise to the radioactive dose measurements over calling ships.

Effect of the Ground Level Deposition of Radioactive Materials: The Pacific Sailing Ships
In order to further compare measurements made over the land surface to the measurements made over the ocean water surface,  The differences between the land-surface measurements at Tokyo Port and the Pacific sailing ships (Fig 9) are smaller than the differences between the land-surface measurements and the ocean-surface measurements on the calling ships at Tokyo Port     plant. This comparison indicates that radioactive materials in Tokyo followed closely to that of the radioactive materials in the surface water of the canal of the Fukushima nuclear power plant.
The 75th, the 50th and the 25th percentiles, and mean of the dose rates also shows decay patterns but the reduction rates are 200 less than those seen in the reduction rates of the monthly maximum levels of dose rates. Interestingly, the monthly lowest levels of dose rates were persistently close to 0.07 µSv/h, except in September 2012. These lowest dose rates were not perturbed by the Fukushima accidents, indicating the background levels of radioactive dose rates over the Tokyo area.   Fig. 11 and in Aoyama et al., 2016). Hence, These data indicate that sporadic waves of radioactive materials being deposited into the water and the air. The measurements were able to pick up signals from these unusual events.

Temporal-Spatial Distribution of Radioactivity Levels over the North Pacific
Blue dots indicate all data; red lines are monthly statistics.
The impact of radioactive materials over the northwestern Pacific can also be seen when the average of 6-hourly measure-240 ments are plotted against time (Fig. 13a) (Fig. 13b). For the northwestern Atlantic, the highest monthly mean dose rates were observed in August 2011 (Fig. 13c). With air dose rates reported in this work, and also data reported in previously published results, we will be able to use atmospheric models to test and verify the amount of the radioactive materials been emitted into the atmosphere following the Fukushima accident occurred on 11 March 2011 (Saunier et al., 2013.).

Data availability 270
All raw data files reported in this work are available for public downlad at https://doi.org/10.6084/m9.figshare.975769 (Wang et al., 2019), and the calibrated data files are available at http://doi.org/10.5281/zenodo.3468896 (Wang, 2019). The data we submitted is reachable with one click (without the need for entering login and password), and a second click to download the data, consistent with the two-click access principle for data published in ESSD (Carlson and Oda, 2018).
Author contributions.

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KYW and NH designed the experiments and KYW carried them out. KYW, PN,and HC performed the analysis. KYW and PN prepared the manuscript with contributions from all co-authors.
The authors declair that they have no conflict of interest.