THESIS
2015
viii, 81 pages : illustrations (some color) ; 30 cm
Abstract
Aerosol size distribution as a key property of ambient aerosols, plays a very important role in
understanding properties, formation process and visibility degradation. Humic-like substances
(HULIS) as the hydrophobic part (carbon in humic-like substances, HULIS-C) of water
soluble organic carbon (WSOC) has been increasingly concerned recently due to its
abundance and associated importance in atmospheric activities. From August 2013 to August
2014, one-year size-segregated aerosol samples in the range of 0.056-18 μm were collected
using an impactor sampler at an urban site in Tsuen Wan, Hong Kong. HULIS, WSOC and
other major aerosol components - organic carbon (OC), elemental carbon (EC) and ion
species were measured in the study. A tri-modal size distribution with a dominant dro...[
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Aerosol size distribution as a key property of ambient aerosols, plays a very important role in
understanding properties, formation process and visibility degradation. Humic-like substances
(HULIS) as the hydrophobic part (carbon in humic-like substances, HULIS-C) of water
soluble organic carbon (WSOC) has been increasingly concerned recently due to its
abundance and associated importance in atmospheric activities. From August 2013 to August
2014, one-year size-segregated aerosol samples in the range of 0.056-18 μm were collected
using an impactor sampler at an urban site in Tsuen Wan, Hong Kong. HULIS, WSOC and
other major aerosol components - organic carbon (OC), elemental carbon (EC) and ion
species were measured in the study. A tri-modal size distribution with a dominant droplet
mode was observed for OC, EC, WSOC, HULIS and several ion species (NH
4+, SO
42-, K
+ and
oxalate) with mass median aerodynamic diameter (MMAD) of 0.6-0.9 μm. For HULIS, the
coarse mode (MMAD 4.2-5.2 μm) made up a larger fraction (26%) than the condensation
mode (13%, MMAD 0.16-0.23 μm) on year average. The droplet-mode HULIS was more
predominant in winter (77%, 3.6 μg m
-3) when total HULIS mass concentration was highest.
The hydrophilic part of WSOC (WSOC_h) was obtained by subtracting HULIS-C from
WSOC and was more evenly distributed in spring and summer compared with HULIS. The
correlation coefficient results between HULIS, WSOC_h and other species in three modes
suggested secondary formation involving in-cloud processing and particle growth from condensation mode to droplet mode through heterogeneous reactions were the major
formation pathways for both droplet-mode HULIS and WSOC_h. Ship emission and fresh
sulfate formation were the possible major sources for the condensation-mode HULIS.
Coarse-mode HULIS was probably formed from coagulation, resuspension with soil particles.
While the formation processes and possible sources of WSOC_h were more complicated and
multiple than HULIS and need further investigation. A second objective of this thesis work is
to take advantage of the size distribution data to estimate light the extinction coefficient (b
ext)
using a modified Mie calculation method. The results showed that estimated b
ext agreed well
with the observed b
ext and particles ranging from 0.32 to 1.8 μm contributed most to the total
b
ext.
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