ABSTRACT

We present a numerical study conducted using a regional Lagrangian model to account for the transport, deposition and radioactive decay of ³⁵S in sulfur dioxide and sulfate aerosols emitted into the atmosphere during the Fukushima Dai-ichi Nuclear Power Plant incident. The model is a Eulerian-Lagrangian hybrid system that accounts for chemical conversion of SO₂ into SO₄²⁻ in a Eulerian manner. The simulations were compared to field measurements of atmospheric ³⁵S in sulfate collected at Kawamata, Tsukuba, Kashiwa, Fuchu and Yokohama, Japan. The ³⁵S emission scenario that best replicated the field measurements followed the same temporal variation pattern as the ^134/137Cs emissions. These results suggest that ³⁵S and ^134/137Cs follow a similar release pattern. Among the considered emission scenarios, a maximum flux of emitted chemical compounds was assumed to be either 100% ³⁵SO₄²⁻ or 100% ³⁵SO₂, with values of 4.0 × 10¹⁹ molecules/hour and 4.0 × 10²⁰ molecules/hour, respectively on March 14th. These emission scenarios reflect the findings reported in the literature, where traces of ³⁵SO₂ were measured along with ³⁵SO₄²⁻, so the actual emission is expected to be a combination of both chemical forms. The Kawamata measurements (Figs. 5 and 6) presented a large concentration in the July–August period, several months after emissions decreased by more than an order of magnitude. To explain this anomaly, re-suspension ratios were calculated for the Kawamata site, which ranged between 0.1 and 1.5% and partially, but not fully, explain the large measured concentrations. Furthermore, they show large discrepancies with ^134/137Cs re-suspension values for measurements at the town of Namie. This situation indicates a lack of understanding of the transformations of ³⁵S that occurs after deposition and the mechanisms involved in the ³⁵S re-suspension process.