The Mixing State of Black Carbon in Asian Outflow (Invited)

Black carbon (BC) aerosol efficiently absorbs visible sunlight and heats the atmosphere. The magnitude of the photo-absorption of BC depends on its particle size distribution and mixing state (coating thickness or shell/core diameter ratio). In addition, the efficiency of wet removal of BC is sensitive to mixing state. Therefore, it is critically important to understand the evolution of the mixing state of BC, especially in Asia, where BC emissions are largest. The size distribution and mixing state of BC were measured by an SP2 instrument over the East China Sea and Yellow Sea in March-April 2009 during the A-FORCE (Aerosol-Radiative Forcing in East Asia) aircraft campaign. Aircraft sampling was made in air masses strongly influenced by emissions from China, Korea, and Japan in the latitude region between 27 and 37N. In the boundary layer, the mass median diameter of BC was stable at about 200 nm in air masses with ages exceeding 1 day. BC in these air masses was thickly coated even over the Yellow Sea at distances of only a few hundred km from the coastlines of China and Korea. Thinly coated (shell/core diameter ratio < 1.2) BC was observed only in the vicinity of urban areas of Korea and Japan. The dominance of thickly coated BC in the moderately aged outflow is consistent with our previous ground-based and aircraft measurements showing that BC coating proceeds within 24 hours in the outflow of urban air. The variability of the observed BC mixing state is interpreted in terms of available volatile aerosol compounds.