dc.description.abstract | III
Abstract
Hygroscopic aerosols play a significant role in atmospheric phenomena
such as aerosol mass change, acidic precipitation, formation of clouds and fogs,
changes in visibility, climate change, and human health assessment. This work
presents a gas chromatographic method that uses a thermal conductivity detector
(GC-TCD) to measure the liquid water mass (LWM) of collected aerosols. The
method is a modification of the previously developed EA-TCD method (Lee and
Hsu, 1998).
The aerosol LWMs of NaCl, Na2SO4, NH4NO3, (NH4)2SO4 and NH4Cl
were measured at 25℃ under relative humidities (RHs) varying between 20%
and 90%, in both deliquescence and efflorescence conditions. The precision of
this method is within ±3%. In addition, the deviation and average error of
GC-TCD method compared with other literature’s data are within ±6% and ±
5%, respectively. In this work, the performance of the ISORROPIA, AIM2 and
GFEMN thermodynamic models are compared with the measurements. The
results indicate ISORROPIA model, a thermal equilibrium model for predicting
the hygroscopic characteristics of most pure-salt inorganic aerosols, is the best
and is adopted for investigating the hygroscopic characteristics of mixed
aerosols. The mutual deliquescence relative humidity (MDRH) of various mixed
fractions of NaCl-Na2SO4 and NH4NO3-NH4Cl binary-salt aerosol systems is
found at 74% and 54%, respectively. In contrast, the MDRH of
NaCl-Na2SO4-NaNO3 and NaCl-(NH4)2SO4-NH4NO3 tri-salt aerosol systems is
70% and 50%, respectively. The complete deliquescence relative humidity in
these mixed aerosol systems is demonstrated to be dependent on the original
aerosol composition. The deliquescence relative humidity (DRH) of NH4NO3
aerosol is shifted to higher value by decreasing the ambient temperature, as is
evidenced by the DRH obtained at 54, 62, and 68% from the measurement at 35,
25, and 15℃, respectively.
For the first time, the DRH and crystallization relative humidity of
Na2CO3 aerosol presented by this method is 78% and 39%, respectively.
Moreover, this work originally demonstrates the eutonic composition of
NaCl-Na2SO4 binary aerosol is close to the composition of a solution with 90%
NaCl and 10% Na2SO4. To date experimental data from complicated tri-salt
aerosol systems have been very scarce. The data from this work and the
comparison with model estimates facilitate the understanding of this aerosol
system. Finally, an application of GC-TCD method to the measurement of LWM
of atmospheric aerosol shows the fraction of aerosol water in PM2.5 can be
ranged from 19 to 41% for ambient humid condition varying between 62 to 77%
RH. | en_US |