Mass concentrations of black carbon (BC) in air (MBC) and rainwater (CBC) in the East China Sea were measured at Hedo on Okinawa Island, Japan, from April 2010 to March 2013. The monthly averaged MBC and CBC showed marked seasonal variations, being highest in spring (0.32 ± 0.13 μgm-3 and 92 ± 76 μgL-1, respectively) and lowest in summer (0.06 ± 0.03 μgm-3 and 8.0 ± 4.1 μgL-1, respectively). The high MBC and CBC in spring were associated with transport of air masses from the Asian continent by northwesterly winds. The BC wet deposition flux (FBC), estimated as the product of CBC and precipitation amount, also showed a distinct seasonal variation. The monthly average FBC during the four spring seasons (16.8 ± 6.7 mg m-2 month-1) was about 3 times higher than the annual average FBC (5.5 ± 9.9 mg m-2 month-1) owing to the high CBC and precipitation amount in spring. As a result, about 76% of the annual BC deposition occurred in spring on average. The FBC in spring is comparable to the average BC net flux in North China, indicating the importance of precipitation over the East China Sea as a sink of BC transported from North China. In summer, CBC values were correlated with MBC for rain events associated with local convective activity, as identified by the convective available potential energy. A one-dimensional thermodynamic model successfully explained the relation between CBC and MBC.
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