A recent earthquake and the subsequent tsunami have extensively damaged the Fukushima nuclear power plant, releasing harmful radiation into the environment. Despite the obvious implication for human health and the surrounding ecology, there are no quantitative estimates of the neutron flux leakage during the weeks following the earthquake. Here, using measurements of radioactive ~(35)S contained in sulfate aerosols and SO_2 gas at a coastal site in La Jolla, California, we show that nearly 4 × 10 ~(11) neutrons per m~2 leaked at the Fukushima nuclear power plant before March 20, 2011. A significantly higher ~(35)SO_4~(2-) activity as measured on March 28 is in accord with neutrons escaping the reactor core and being absorbed by the coolant seawater ~(35)Cl to produce ~(35)S by a (n, p) reaction. Once produced, ~(35)S oxidizes to ~(35)SO_2 and _(35)SO _4~(2-) and was then transported to Southern California due to the presence of strong prevailing westerly winds at this time. Based on a moving box model, we show that the observed activity enhancement in ~(35)SO_4~(2-) is compatible with long-range transport of the radiation plume from Fukushima. Our model predicts that ~(35)SO_4~(2-), the concentration in the marine boundary layer at Fukushima, was approximately 2 × 10~5 atoms per m~3, which is approximately 365 times above expected natural concentrations. These measurements and model calculations imply that approximately 0.7 of the total radioactive sulfate present at the marine boundary layer at Fukushima reached Southern California as a result of the trans-Pacific transport.
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