Abstract
Abstract. This paper presents studies on columnar aerosol optical properties in Hong Kong with focus on aerosol volume size distribution, which helps understand local aerosol properties, variation, hygroscopic growth and coagulation. 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. Data validation made using MODIS and local AERONET shows agreement. A bimodal size distribution is found with the fine mode centering at ~0.2 μm and coarse mode centering at ~3 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 mode 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 mode 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 mode aerosol conditions. It becomes more pronounced with high aerosol optical depth and larger Angstrom exponent. Investigation of its variation with corresponding optical parameters and correlation with atmospheric conditions appears to support the hypothesis that it is mainly due to the fine mode aerosol hygroscopic growth and coagulation rather than the contribution from the coarse mode. While the very humid environment facilitates the aerosol hygroscopic growth, aerosol coagulation might further produce larger aerosols under high fine aerosol conditions. The continental outflow with transported aging aerosols and biomass burning might have also contributed to this additional mode.
Highlights
Aerosols play an important role on Earth’s radiation budget due to their scattering and absorbing capability and indirectly serving as the cloud nuclei
Ground measurements allow for estimation of fine and coarse aerosol volume fraction (Eck et al, 2005; Kaufman et al, 2001; Smirnov et al, 2003) as well as their respective contributions to other optical properties like the aerosol optical depth (Dubovik and King, 2000; O’Neill et al, 2003), and comparison of fine and coarse mode aerosol contributions from
The study focuses on investigation of the aerosol size distribution and help understand the local aerosol properties, evolution, hygroscopic growth and coagulation
Summary
Aerosols play an important role on Earth’s radiation budget due to their scattering and absorbing capability and indirectly serving as the cloud nuclei. Knowing aerosol size distribution is useful to distinguish anthropogenic aerosols from urban and biomass burning and natural aerosols like sea salt and dust. Such discrimination is important since their effect on the Earth’s radiation budget depend on the aerosol type (Andreae, 2001) and helps aerosol source tracing. Such humid environment could facilitate the aerosol evolution process through hygroscopic growth as have been found in many industrial and coastal areas (Dubovik et al, 2002; Eck et al, 2003b; Reid et al, 1999; Singh et al, 2004). We analyze aerosol’s size distribution, variation and the underlying mechanism
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