Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 2 2009 10.5194/acpd-9-8341-2009 http://www.atmos-chem-phys-discuss.net/9/8341/2009/ http://www.atmos-chem-phys-discuss.net/9/8341/2009/acpd-9-8341-2009.html http://www.atmos-chem-phys-discuss.net/9/8341/2009/acpd-9-8341-2009.pdf 8341 8375 2009-03-30 Study of columnar aerosol size distribution in Hong Kong X. Yang M. Wenig Laboratory for Atmospheric Research, Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China This paper presents studies on columnar aerosol optical properties in Hong Kong with focus on aerosol volume size distribution. Long-term ground measurements in the wet season in the years of 2002, 2003, 2004 and 2008 have been performed using a sun-sky radiometer. A bimodal size distribution is found with the fine mode centering at ~0.2 μm and coarse mode centering at ~6 μm, respectively. The fine and coarse mode have close volume concentrations of nearly 50% fraction in composing local aerosols. Intercomparison of different years shows similar aerosol properties while a small increase of fine mode aerosol could be observed. A systematic shift of size distribution parameters is observed with different atmospheric conditions, where higher aerosol loadings and Angstrom exponent correspond to more fine aerosols. The fine mode is found to be more closely correlated with this shift than the coarse mode. A higher fine mode volume fraction and smaller median fine radius correspond to a larger Angstrom exponent. The fine aerosol hygroscopic growth is one of the main mechanisms for such systematic shifting. A third mode centering at ~1–2 μm could be discovered under high aerosol loading and high fine aerosol conditions. It becomes more pronounced with high aerosol optical depth and larger Angstrom exponent. 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