Size distributions of elemental carbon in the atmosphere of a coastal urban area in South China: characteristics, evolution processes, and implications for the mixing state X.-F. Huang1,2 and J. Z. Yu1,3 1Department of Chemistry, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China 2Shenzhen Graduate School, Peking University, Shenzhen, 518055, China 3Atmospheric Research Center, HKUST Fok Ying Tung Graduate School, Nansha, Guangzhou 511458, 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 China's ambient environments. We report size
distributions of EC particles in the urban area of Shenzhen in Southern
China. In our samples, EC was consistently found in two modes, a fine mode
and a coarse mode. The majority of EC mass (~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
the region, the mode at 0.75 μm was likely a result of particle growth
from smaller EC particles. We theoretically investigated the particle growth
processes that caused the 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 was not produced through either coagulation or H2SO4
condensation; both processes are too slow to lead to significant EC growth.
Hygroscopic growth was also determined to be insignificant. Instead,
addition of sulfate through in-cloud processing was found to cause
significant growth of the EC particles and to explain the EC peak at 0.75 μm.
We also estimated the mixing state of EC from the EC size
distributions. In the droplet size, at least 45–60% of the EC mass in the
summer samples and 68% of the EC mass in the winter samples was
internally mixed with sulfate as a result of in-cloud processing. This
information on EC should be considered in models of the optical properties
of aerosols in this region. Our results also suggest that the in-cloud
processing of primary EC particles could increase the light absorbing
capacities through mixing EC with sulfate.
Citation: Huang, X.-F. and Yu, J. Z.: Size distributions of elemental carbon in the atmosphere of a coastal urban area in South China: characteristics, evolution processes, and implications for the mixing state, Atmos. Chem. Phys., 8, 5843-5853, doi:10.5194/acp-8-5843-2008, 2008.