Size dependent hygroscopicity of aerosol nanoparticles
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Abstract
The hygroscopicity of aerosol nanoparticles and related physical-chemical properties
are crucial for atmospheric multiphase processes, physical chemistry, and materials
science. One of the main problems of current research on aerosol hygroscopicity is
that the most studies due to technical challenges lack measurements in the sub-10 nm
size range, which is highly relevant for research on new particle formation and its
initial growth. The goal of this thesis is to solve the technical problems and use the
advanced nano-hygroscopicity tandem differential mobility analyzer (nano HTDMA) to investigate the size dependent hygroscopicity of aerosol nanoparticles
with diameters down to 6 nm.
(1) In the first part of this thesis, the detailed information on the design of a nano HTDMA system is presented. To enable high accuracy and precision in
hygroscopicity measurements of aerosol nanoparticles, especially in the sub-10 nm
size range, systematic and comprehensive calibration criteria of the nano-HTDMA
have been developed and applied, including the calibration of nanoparticle sizing,
sheath and aerosol flow rates, DMA voltage, relative humidity (RH) sensor, and
temperature (T) sensor. After calibration, the nano-HTDMA system has been shown
to have an accurate sizing and a small sizing offset between nano-DMAs (< 1.4 %)
for aerosol nanoparticles with diameters down to 6 nm. Moreover, to maintain the
RH-uniformities that prevent the pre-deliquescence and non-prompt phase transition
of nanoparticles within nano-DMA2, the RH of sheath flow is kept as same as that
of aerosol flow at inlet of nano-DMA2. Since temperature and RH are closely linked,
the nano-DMA2 with its humidification system is placed in a well-insulated air conditioned chamber, which maintains a constant temperature. Using the nano HTDMA apparatus, we measure the hygroscopic behavior of aerosol nanoparticles
of two inorganic substances (i.e., ammonium sulfate and sodium sulfate). We find a
weak size dependence of deliquescence and efflorescence relative humidity (DRH
and ERH, respectively) of ammonium sulfate nanoparticles but a strong size
dependence of DRH and ERH of sodium sulfate nanoparticles down to 6 nm in size.
(2) The second part of this thesis is about hygroscopic properties of organic
nanoparticles with diameters down to 6 nm (i.e., levoglucosan and D-glucose)
measured by a nano-HTDMA system. Levoglucosan is a biomass burning tracer
compound and can contribute substantially (16.6–30.9% by mass) to the total
organics in PM2.5. D-glucose, a hydrolysis product of cellulose and levoglucosan, is
one of the major pyrolysis products of wood. Due to the partial evaporation of
levoglucosan with diameters smaller than 20 nm in the nano-HTDMA system, we
investigate the hygroscopicity of levoglucosan nanoparticles in the size range from
20 to 100 nm. A weak size dependence of hygroscopic growth factor is observed for
levoglucosan and D-glucose nanoparticles with diameters down to 20 nm, while a
strong size dependence of the hygroscopic growth factor is found for D-glucose
nanoparticles with diameters from 6 to 20 nm. We further compare measurements
for levoglucosan and glucose nanoparticles with modelling results of the Extended
Aerosol Inorganics Model (E-AIM) and the ideal solution theory, respectively. The
ideal solution theory well describes the hygroscopic growth factors of levoglucosan
and D-glucose nanoparticles with diameters larger than 15 nm, while the E-AIM
model prediction well describes measured growth factors of sub-15 nm D-glucose
nanoparticles.
(3) The goal of the third part of this work is to investigate the hygroscopicity of
organic surrogate compounds from biomass burning and their interaction with
inorganic ammonium sulfate aerosols using an HTDMA. The organic surrogate
compounds represent a selection of some of the most abundant pyrolysis products of
biomass burning. We find that levoglucosan and humic acid aerosol nanoparticles
release water gradually in the range from 90 % down to 5% RH. However, 4-
Hydroxybenzoic acid aerosol nanoparticles remain in the solid state and exhibit a
small shrink in size in the whole dehumidification process. Predicted growth factors
using the Aerosol Inorganic-Organic Mixtures Functional groups Activity
Coefficients (AIOMFAC) model, the E-AIM, and a fitted hygroscopicity function
are in general consistent with measured hygroscopic growth factors of levoglucosan,
respectively. However, the use of the AIOMFAC and the E-AIM models without
consideration of crystalline organic phases is not appropriate to describe the
hygroscopicity of 4-hydroxybenzoic acid. Furthermore, we observe several effects
of these organic components on the hygroscopicity behavior of mixtures containing
ammonium sulfate in relation to the different mass fractions of organic compounds:
(i) A shift of ERH of ammonium sulfate to the higher RH due to the presence of 25
wt % levoglucosan in the mixture. (ii) There is a phase transition at 25% RH for
mixtures containing 50 wt % of 4-hydroxybenzoic acid compared to the ERH (i.e.,35 %) for organic-free AS nanoparticles, and a liquid-to-solid phase transition of 4-
hydroxybenzoic acid in the mixed particles during dehydration process. (iii) The
presence of humic acid components shows no significant effects on the efflorescence
of AS in mixed aerosol nanoparticles. In addition, consideration of a solid-liquid
phase transition of AS in both the AIOMFAC and the E-AIM models leads to a
general agreement between models and measurements, as well as ERH of AS in the
mixed system. The measured diameter growth factors of aerosol nanoparticles
containing humic acid and ammonium sulfate are well predicted by Zdanovskii Stokes-Robinson (ZSR) relation. Lastly, the mixtures containing organic surrogates
(i.e., levoglucosan, 4-hydroxybenzoic acid, and humic acid) and ammonium sulfate
with increasing organics mass fractions is used to mimic, in a simplified manner,
ambient conditions in the Amazon Basin during the wet and dry season. The
measured hygroscopicity parameters (ĸdry and ĸwet) show relatively good agreement
with field data in the dry and wet seasonal period in the Amazon Basin, respectively.
This suggests that laboratory-generated mixtures of organic surrogate compounds
with ammonium sulfate can be used to represent the chemical composition of
ambient aerosols from the Amazon Basin for the purpose of RH-dependent
hygroscopicity studies