ALBERT

Description: Morphological features and aging processes of soot-containing aerosol particles are important information for the simulation of climatic effects of aerosols in the atmosphere. To elucidate morphological features and aging effects of soot-particle mixing states after their transport from megacities, we obtained aerosol samples of continental outflow at Cape Hedo, Okinawa, Japan, in March 2008. Six aerosol samples were obtained for air masses from similar source regions (eastern central China) at different times (1–5 days). Before and after extraction of water-soluble materials from the samples, 0.2–0.7 μm diameter particles were analyzed using transmission electron microscopy. Results show that most soot-containing particles were mixed with large amounts (〉80% particle volume) of water-soluble materials. The number proportions of soot-containing particles to total particles for 0.4–0.7 μm (14%–59%) were higher than those for 0.2–0.4 μm (2%–25%). For samples consisting mainly of ammonium sulfate and organics, most soot-containing particles were spheroids of 0.2–0.4 μm, clustered in multiple small (∼0.3 μm diameter) units of 0.4–0.7 μm. The number proportions of coagulated particles to the total were estimated using a simple coagulation model with an initial unit diameter of 0.3 μm. Results for polluted conditions in windward areas showed reasonable agreement with the number proportions of observed clustered particles, which suggests that coagulation clusters might also be important for aerosol size evolution in a polluted atmosphere. The formation of larger soot-containing cluster particles through coagulation must be considered when assessing climatic effects of aerosols from megacities.