| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 97 === Aerosol particles are a major air pollutant that affects human health, play
important roles in climate by scattering or absorbing solar radiation, and can modify
cloud properties by acting as cloud condensation nuclei and ice forming nuclei. The
significance of these effects varies with their composition and mixing state as well as
the spatial distribution which unfortunately are not well understood.
With the advantage of high temporal and spatial resolutions, LIDAR (Light
Detection and Ranging system) is a powerful tool for aerosol and boundary layer
analysis. This study applies the RCEC/NTU-AS Raman lidar to analyze the
composition and vertical distribution of aerosol particles, and deduce from them the
boundary layer characteristics at Taipei. The parameters analyzed include (1) the
backscattering signals which are representative of aerosol concentration; (2) the
depolarization ratio which reflects the irregular shape of aerosols as non-spherical
aerosols give higher depolarization ratio than spherical ones; and (3) the
backscatter-related Ångstrom exponent, α, which indicates the size of aerosol particles
as it approximately decreases with increasing particle size. Seasonal variation of these
parameters over Taipei during 2006-2007 are analyzed in this study.
In addition, we focus on the cases on 5 May 2007, 28 Jan 2007 and 4 Mar 2008,
when Taipei was situated at the outskirt of a high pressure system. The 5 May 2007 case
shows the swelling process of aerosol, and the cases of 28 Jan 2007 and 4 Mar 2008
were Asian dust incursion events. Besides mineral dust, the lidar data also revealed
other types of aerosols in these two cases. By combining the analyses of
backscattering intensity, Angstrom exponent and depolarization ratio, we identified
several important phenomena such as deliquescence-dehydration, swelling, cloud drop
activation, aerosol nucleation and boundary layer turbulence mixing, as well as aerosol
mixing states. The findings are summarized and reduced to a conceptual diagram
which could be a useful tool for facilitating further analysis of lidar signals.
Keywords: aerosol, lidar, depolariztion, Ångstrom exponent, deliquescence, swelling
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