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In this dataset, the simulation from NOAA's HYSPLIT model shows a continuous release of tracer particles from 12-31 March at a rate of 100 per hour representing the Cesium-137 emitted from Fukushima Daiichi. Each change in particle color represents a decrease in radioactivity by a factor of 10. Radioactivity decreases due to removal by rainfall and gravitational settling. Decay is not a factor for Cesium in this short duration simulation compared to its 30 year long-half life. The air concentration would be computed from the particle density so it is only partially related to the color scale. The released particles are followed through the end of April using meteorological data from the 1-degree resolution NOAA global analyses.
