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

Abstract:

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 (M_BC) and in rainwater (C_BC) simultaneously at Cape Hedo on Okinawa Island, Japan, in the East China Sea, from April 2010 to March 2013. M_BC was measured by a Continuous Soot Monitoring System (COSMOS) and C_BC was measured by a combination of an ultrasonic nebulizer and a Single-Particle Soot Photometer (SP2). The monthly averaged M_BC, C_BC, and BC wet deposition flux (F_BC) showed marked seasonal variations, being highest in spring. The F_BC during the four spring seasons (51 mg m^-2 (3month)^-1) was about 77% of the annual average of F_BC (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, C_BC values were correlated with M_BC (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 M_BC and C_BC.