Atmos. Chem. Phys. Discuss., 7, 10743-10766, 2007
www.atmos-chem-phys-discuss.net/7/10743/2007/
doi:10.5194/acpd-7-10743-2007
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Size distributions of elemental carbon in a coastal urban atmosphere in South China: characteristics, evolution processes, and implications for the mixing state
Xiao-Feng Huang and Jian Zhen Yu
Atmospheric, Marine and Coastal Environment Program and Department of Chemistry, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China

Abstract. Elemental carbon (EC), as one of the primary light-absorbing components in the atmosphere, has a significant impact on both regional and global climate. The environmental impacts of EC are strongly dependent on its particle size. Little is known about the size distribution characteristics of EC particles in the ambient environments of China. We here report size distributions of EC in the urban area of Shenzhen in South China. EC consistently exhibited two modes, a fine and a coarse mode. The majority of EC (~80%) in this coastal metropolitan city resided in particles smaller than 3.2 μm in diameter. The fine mode peaked at around either 0.42 μm or 0.75 μm. While the mode at 0.42 μm could be ascribed to fresh vehicular emissions in this region, the mode at 0.75 μm had to be a result of particle growth from smaller EC particles. We made a theoretical investigation of the particle growth processes that were responsible for EC particles to grow from 0.42 μm to 0.75 μm in the atmosphere. Our calculations indicate that the EC peak at 0.75 μm could not be produced through either coagulation or H2SO4 condensation; both were too slow to lead to significant EC growth. Hygroscopic growth was also calculated to be impossible. Instead, addition of sulfate through in-cloud processing was found to be able to significantly grow EC particles to explain the EC peak at 0.75 μm. We also estimated from the EC size distributions the mixing state of EC. In the droplet size, at least 45–60% of EC mass in the summer samples and 68% of EC mass in the winter samples was internally mixed with sulfate as a result of in-cloud processing. Such information on EC needs to be considered in modeling aerosol optical properties in this region. Our results also suggest that the in-cloud processing of primary EC particles could enhance light absorbing capacities through mixing EC and sulfate.

Citation: Huang, Xiao-Feng and Yu, Jian Zhen: Size distributions of elemental carbon in a coastal urban atmosphere in South China: characteristics, evolution processes, and implications for the mixing state, Atmos. Chem. Phys. Discuss., 7, 10743-10766, doi:10.5194/acpd-7-10743-2007, 2007.
 
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