Wet deposition of black carbon at a remote site in the East China Sea

Mass concentrations of black carbon (BC) in air (M_BC) and rainwater (C_BC) in the East China Sea were measured at Hedo on Okinawa Island, Japan, from April 2010 to March 2013. The monthly averaged M_BC and C_BC 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 M_BC and C_BC in spring were associated with transport of air masses from the Asian continent by northwesterly winds. The BC wet deposition flux (F_BC), estimated as the product of C_BC and precipitation amount, also showed a distinct seasonal variation. The monthly average F_BC during the four spring seasons (16.8 ± 6.7 mg m^-2 month^-1) was about 3 times higher than the annual average F_BC (5.5 ± 9.9 mg m^-2 month^-1) owing to the high C_BC and precipitation amount in spring. As a result, about 76% of the annual BC deposition occurred in spring on average. The F_BC 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, C_BC values were correlated with M_BC 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 C_BC and M_BC.