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

Abstract. 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 measurements 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 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 atmosphere occurred after March 2011, and in 2012, 2013, 2014, and 2015. The data are available at https://doi.org/10.6084/m9.figshare.9757697. 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.