THESIS
2013
xxiv, 148 pages : illustrations (some color) ; 30 cm
Abstract
The importance of aerosols study arises from their influences on the global climate, visibility as
well as human health. Atmospheric aerosol impacts the global climate via direct interaction with
solar radiation and cloud modification by acting as cloud condensation nuclei (CCN).
Hygroscopicity of aerosols governs their optical properties and CCN activity by altering their
water content, size and chemical reactivity with changing relative humidity (RH) conditions. This
study aims at characterizing the hygroscopicity of aerosols in Hong Kong and correlating the
hygroscopic properties with varying chemical compositions, which are crucial for better
prediction of aerosol climate forcing and visibility.
A new commercial Humidified Tandem Differential Mobility Analyzer (BMI HTDMA) wa...[
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The importance of aerosols study arises from their influences on the global climate, visibility as
well as human health. Atmospheric aerosol impacts the global climate via direct interaction with
solar radiation and cloud modification by acting as cloud condensation nuclei (CCN).
Hygroscopicity of aerosols governs their optical properties and CCN activity by altering their
water content, size and chemical reactivity with changing relative humidity (RH) conditions. This
study aims at characterizing the hygroscopicity of aerosols in Hong Kong and correlating the
hygroscopic properties with varying chemical compositions, which are crucial for better
prediction of aerosol climate forcing and visibility.
A new commercial Humidified Tandem Differential Mobility Analyzer (BMI HTDMA) was
deployed for hygroscopicity measurements at the HKUST Air Quality Research Supersite. The
performance of the system was first evaluated with respect to DMA sizing accuracy and
resolution, detection of bi-modal distribution and RH control for growth factor (GF)
measurements. Real-time hygroscopic growth measurements were conducted starting May 2011.
The GF at 90% RH were measured for four dry particle diameters between 75 and 200 nm in
three individual campaigns (May up to 9
th, Sep and Nov 2011), covering springtime to early
wintertime in 2011. Cluster analyses were performed to characterize the aerosol hygroscopicity
based on the air parcel trajectories computed by the HYSPLIT model and the major chemical composition (sulfate, nitrate, ammonium, and organics) measured by the High-Resolution
Time-of-Flight Aerosol Mass Spectrometer. Back trajectory analysis revealed that the air masses
were mainly originated from the South China Sea (maritime) in the springtime and late
summertime (early May and early Sep 2011). Maritime aerosol GF was higher (~1.6) than that of
the continental aerosols of higher organic fractions (between 1.3 and 1.6) observed during the
early autumn and early winter periods (late Sep and Nov 2011). GF reduction is attributed to the
high organic fraction with the enhanced fraction of less oxygenated and less hygroscopic species.
In the closure study, the measured GF values were compared against predictions based on
chemical composition. Predictions on the water-uptake by inorganic species using the simplified
ion pairing scheme and the E-AIM model were also compared. The E-AIM model was able to
predict the presence of small water content in the sulfuric acid-containing aerosols under dry
condition and gave a closer GF with the measured values than the ion pairing scheme did. For the
organic fractions, three different approximations based on the degree of oxygenation were
examined to estimate the GF of organic fraction (GF
org). However, all approximations could not
yield better closure than using a constant GF
org of 1.18. The best fit GF
org ranged from 1.1 to 1.5
over the three separate measurements campaign, reflecting the large variability of the detected
organic species in term of their hygroscopic properties.
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