Wet Deposition of Black Carbon at a Remote Site in the East China Sea

Black carbon (BC) is produced by incomplete combustion of fossil fuels and biomass, and removed by wet deposition. Long-term measurements of BC wet deposition are very important to understand BC loss because wet deposition is major sink of BC. We measured BC mass concentrations in air (MBC) and in rainwater (CBC) simultaneously at Cape Hedo on Okinawa Island, Japan, in the East China Sea, from April 2010 to March 2013. MBC was measured by a Continuous Soot Monitoring System (COSMOS) and CBC was measured by a combination of an ultrasonic nebulizer and a Single-Particle Soot Photometer (SP2). The monthly averaged MBC, CBC, and BC wet deposition flux (FBC) showed marked seasonal variations, being highest in spring. The FBC during the four spring seasons (51 mg m-2 (3month)-1) was about 77% of the annual average of FBC (65.5 mg m-2 yr-1). This flux is comparable to the average BC net flux (emission - deposition) over North China, indicating the importance of precipitation amount over the East China Sea as a sink of BC transported from North China. In summer, CBC values were correlated with MBC (r2 = 0.47) for rain events associated with local convective activity, as identified by the convective available potential energy (CAPE). A-one dimensional thermodynamic model successfully explained the relation between MBC and CBC.