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Temperature-dependent hygroscopic properties of atmospheric carboxylic acid salt aerosols using ATR-FTIR spectroscopy

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Temperature-dependent hygroscopic properties of atmospheric carboxylic acid salt aerosols using ATR-FTIR spectroscopy
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Roberts, Jason Eric ( author )
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Atmospheric modelling requires extensive knowledge of aerosols' interaction with water, including abundant carboxylic acid salt aerosols. The hygroscopic behaviors of atmospheric relevant monoprotic and diprotic carboxylic acid salts were studied at different tropospheric relevant temperatures using ATR-FTIR spectroscopy. The deliquescent relative humidity (DRH) and efflorescent relative humidity (ERH) of all studied salts were determined by changes in spectral peak locations and spectral shapes of the carbonyl symmetrical [&ngr; s(COO-)] and asymmetrical [&ngr;as(COO -)] stretching vibrational modes. Distinct peak shifts were observed for sodium formate, sodium acetate, and sodium succinate as the relative humidity (RH) was increased to 100% RH and then decreased to 0% RH, indicating DRH and ERH values for these atmospheric salts. Sodium oxalate showed no spectral evidence of deliquescence. The DRH and ERH of these salts vary due to their differing solubilities and enthalpy of solutions which, then, vary their susceptibility to act as cloud condensation nuclei and scatter incoming solar radiation. The hygroscopic study of aerosol salts provides insight into their physicochemical, light scattering, and reactivity properties as well as their influences on climate and weather.
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Thesis (M.S.)- University of Colorado Denver, Department of Chemistry
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Includes bibliographic references
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by Jason Eric Roberts.

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Full Text
TEMPERATURE-DEPENDENT HYGROSCOPIC PROPERTIES OF ATMOSPHERIC
CARBOXYLIC ACID SALT AEROSOLS USING ATR-FTIR SPECTROSCOPY
by
JASON ERIC ROBERTS
B.S., United States Air Force Academy, 2011
A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillment
of the requirements for the degree of
Master of Science
Chemistry Program
2015


This thesis for the Master of Science degree by
Jason Eric Roberts
has been approved for the
Chemistry Program
by
Yong Liu, Chair
Xiaotai Wang
Marta Maron
Date: 13 November 2015
ii


Roberts, Jason Eric (M.S., Chemistry)
Temperature-Dependent Hygroscopic Properties of Atmospheric Carboxylic Acid Salt
Aerosols Using ATR-FTIR Spectroscopy
Thesis directed by Assistant Professor Yong Liu
ABSTRACT
Atmospheric modelling requires extensive knowledge of aerosols' interaction with
water, including abundant carboxylic acid salt aerosols. The hygroscopic behaviors of
atmospheric relevant monoprotic and diprotic carboxylic acid salts were studied at different
tropospheric relevant temperatures using ATR-FTIR spectroscopy. The deliquescent relative
humidity (DRH) and efflorescent relative humidity (ERH) of all studied salts were
determined by changes in spectral peak locations and spectral shapes of the carbonyl
symmetrical [vs(COO~)] and asymmetrical [vas(COO")] stretching vibrational modes. Distinct
peak shifts were observed for sodium formate, sodium acetate, and sodium succinate as the
relative humidity (RH) was increased to 100% RH and then decreased to 0% RH, indicating
DRH and ERH values for these atmospheric salts. Sodium oxalate showed no spectral
evidence of deliquescence. The DRH and ERH of these salts vary due to their differing
solubilities and enthalpy of solutions which, then, vary their susceptibility to act as cloud
condensation nuclei and scatter incoming solar radiation. The hygroscopic study of aerosol
salts provides insight into their physicochemical, light scattering, and reactivity properties as
well as their influences on climate and weather.
The form and content of this abstract are approved. I recommend its publication.
Approved: Yong Liu


ACKNOWLEDGEMENTS
I would like to acknowledge my family and friends for pushing me to follow my
dreams. Also, I would like to acknowledge Dr. Yong Liu for being a great mentor and pushing
me to my full potential.
IV


TABLE OF CONTENTS
CHAPTER
I. INTRODUCTION.........................................................1
II. LITERATURE REVIEW....................................................5
III. METHODOLOGY..........................................................7
IV. RESULTS AND DISCUSSION..............................................11
4.1.1 DRH and ERH of Sodium Formate at 298 K........................11
4.1.2 Temperature-dependence of Sodium Formate DRH and ERH Values...16
4.2.1 DRH and ERH of Sodium Acetate at 285 K........................20
4.2.2 Temperature-dependence of Sodium Acetate DRH and ERH Values...24
4.3.1 DRH and ERH of Sodium Succinate at 298 K.....................29
4.3.2 Temperature-dependence of Sodium Succinate DRH and ERH Values.32
4.4.1 DRH and ERH of Sodium Oxalate................................36
V. CONCLUSION..........................................................40
REFERENCES.....................................................................42
v


CHAPTER I
INTRODUCTION
Aerosols are ubiquitous in the atmosphere, yet there are many uncertainties in their impact
toward climate change.1 They scatter, reflect, and absorb incoming solar radiation; act as cloud
condensation nuclei (CCN); and affect weather, including precipitation. Along with their climate
affecting potential, aerosols undergo heterogeneous reactions, further changing the atmosphere's
chemical composition.2 Understanding aerosols is a growing topic in order to reduce this uncertainty
and understand their interactions in a complex system. One of aerosols' largest interaction is with
atmospheric water. These hygroscopic properties affect aerosols' size and phase, which then affects
many physicochemical characteristics of the particles. Also, hygroscopic properties affect
respiratory tract deposition,3'4'5 aerosol optical properties,6 atmospheric removal rates, and chemical
reactivity, notably heterogeneous chemistry.7
When the concentration of water in the atmosphere, or relative humidity (RH), reaches a
certain point, particular aerosols will absorb water from the atmosphere and deliquesce through a
thermodynamic process known as the deliquescence relative humidity (DRH). The process of an
aerosol particle transforming from its solid phase to its aqueous phase is paramount because
heterogeneous reaction rates are increased and light scattering properties are heightened from its
previous crystalline, solid phase. ' ' As the saturated relative humidity decreases, certain
aqueous aerosols effloresce water into the atmosphere and undergo the opposite phase change
where the aqueous particle turns into a crystalline, solid aerosol. This value of relative humidity is
called the efflorescence relative humidity (ERH). DRH and ERH are temperature dependent and are,
therefore, changing at different heights in the troposphere. The DRH can be predicted theoretically
as it is a thermodynamic process, but this calculation requires temperature-dependent solubility
data and enthalpy of solution of aerosols which are not available for many carboxylic acid salts and
1


can vary considerably when temperature is changed.13 Therefore, experimental data of DRH and
ERH values over different tropospheric temperatures are more significant and accurate than
theoretical values. Comprehending aerosols deliquescence and efflorescence values at varying
tropospheric temperatures provides further knowledge on the atmosphere's direct and indirect
radiative forces,14 provides evidence towards their ability to serve as a cloud condensation nucleus
(CCN),5 and further strengthens tropospheric modeling.
Carboxylic acid salt aerosols are prevalent in the troposphere. However, there is little
literature pertaining to the DRH and ERH of carboxylic acid salt aerosols, and no data on their
temperature-dependent properties. The sources of these inorganic salt aerosols are from biogenic
as well as anthropogenic processes. A variety of monoprotic and diprotic carboxylic acids are
released in high concentrations from the burning of biomass, especially the combustion of wood
from wildfires.15 Also, they are produced from the ozonolysis of volatile organic compounds (VOCs),
approximately two-thirds of which are released from Earth's vegetation.16 Anthropogenic sources
include the release of carboxylic acids from the grilling of meat and the exhaust from automobiles.17
From these sources, carboxylic acids such a formic (HCOOH) and acetic (CH3COOH) acids have been
calculated to contribute between 16% and 35% of the free acidity in North American precipitation
and up to 98% in remote areas.18 Once these weak acids have been released into the troposphere,
they undergo an acid displacement reaction with abundant airborne cations and anions. They
transform into carboxylic acid salts in a water medium, such as a cloud or evaporating rain puddle as
seen in the following reaction.19
RCOOH(g, aq) + NaCl(aq) ^ RCOO~Na+(aq) + HCl(g, aq)
The carboxylic acid salt is favored due to Le Chatelier's principle. The HCI gas diffuses from the
water medium, thus, forming more products through the equilibrium process. This process happens
extensively near coastal regions where sea salt aerosols have been recorded in high concentrations
2


hundreds of kilometers inland in North America and begin to decrease rapidly at 500 km inland.20 A
previous study showed these weak acids deplete 20-30% of atmospheric chloride from sea salt
aerosols in Northern Finland.21 After reacting, the carboxylic acid salts are then carried inland and
even transcontinental^.22 With the risks of wildfires across North America increasing significantly
due to global climate change, notably in California and the Northwest United States, the risk of high
concentrations of these inorganic salts transported into North America will begin to rise as well.23
An additional reason for hygroscopic analysis of these carboxylic acid salts is their health
concerns. Respiratory exposure of aerosol salts depends on hygroscopic behavior, for example their
size and phase.24 The diprotic carboxylic acid salt, sodium oxalate, has adverse health effects when
inhaled including, throat and esophagus pain and even cardiovascular collapse.25 Sodium formate
aerosols, a monoprotic carboxylic acid salt, also cause respiratory tract irritation, including coughing
and sore throat.26 Investigating the DRH and ERH of carboxylic acid salt aerosols can provide further
insight on outdoor air quality and its human health impacts on one's exposure under dry or humid
conditions.
The DRH and ERH of carboxylic acid salts were analyzed using Attenuated Total
Reflection Fourier Transform Infrared spectroscopy (ATR-FTIR). This process of analysis was
authenticated in Zeng et al., 2014 as the produced data closely matched previous sodium chloride
temperature-dependent ERH and DRH values. ATR-FTIR spectrometer is a FTIR spectrometer with
an ATR flow accessory in which the IR beam from the FTIR enters the ATR crystal at an incident angle
larger than the crystal's critical angle. In this experiment, an incident angle (0) of 45 enters a
germanium crystal with a critical angle (0 c) of 22.27 The reflection of the IR beam inside the ATR
crystal creates an evanescent wave. The produced evanescent wave infiltrates the aerosol molecule
0.5 pm 5 pm into the crystal surface,28 and the studied salt will absorb this energy at its particular
vibrational modes. This method provides many advantages in studying hygroscopicity compared to
3


previous measuring techniques, including a rapid response of water absorption on the surface of the
studied aerosols with a continuous infrared beam and, also, high reproducibility in measurements.
As a carboxylic acid salt crystal begins to deliquesce, there is a noticeable red-shift in the carbonyl
peaks which can be concisely observed in the ATR-FTIR spectra. Also, there is a pronounced blue-
shift by the same peaks in the spectra when water is effloresced from the aqueous aerosol. This is
lacking in other methods such as the electrodynamic balance technique where hygroscopic
transitions are determined by a constantly changing aerosol weight.21
In this work, the hygroscopic properties of sodium acetate, sodium formate, sodium
succinate, and sodium oxalate were analyzed. The aerosols of these carboxylic acid salts were
deposited onto Ge ATR slides and investigated with ATR-FTIR spectroscopy. The DRFI and ERH were
recorded at varying temperatures between -2 C and 25 C, relevant to the troposphere, by
analyzing changes in peak position and peak shape of the carbonyl functional group's vibrational
modes.
4


CHAPTER II
LITERATURE REVIEW
There are currently only two published journal articles on the hygroscopicity of atmospheric
carboxylic acid salts, and no articles pertaining to their temperature dependence. Peng and Chan,
2000 and Wu et al., 2011 experimented to find DRH and ERH values for common monoprotic and
diprotic carboxylic acid salt aerosols at 298 K. In Peng and Chan's article, "The water cycles of
water-soluble organic salts of atmospheric importance," electrodynamic balance (EDB) is used to
observe DRH and ERH values by finding the relative mass of the salt with DC voltage as the relative
humidity is increased.21 The phase change between the solid phase and aqueous phase is
determined when there is a rapid increase in the mass fraction of solute (mfs) in the plotted graph
of mfs vs. water activity. Though this change is obvious, this technique cannot accurately pinpoint
when the DRH occurs and ends. Each measurement takes 40 minutes to enable the EDB to attain
the pre-set RH,21 which may makes it difficult to obtain accurate results. This work determines the
DRH and ERH for all of the carboxylic acid salts used in this experiment at 298 K, and it will be used
for quantitative comparison.
In "Hygroscopic behavior of atmospherically relevant water-soluble carboxylic salts and their
influence on the water uptake of ammonium sulfate," Wu uses Hygroscopicity Tandem Differential
Mobility Analyzer (H-TDMA) to find only the DRH of many carboxylic salts.29 H-TDMA measures the
size of the aerosol, and a rapid increase in size can represent a phase change.30 The DRH values
differ in some areas when compared to the DRH values obtained in Peng and Chan, 2000. This
provides evidence that determining the DRH with precision using a method that measures the
aerosol weight or size is difficult.
The method used in this paper that can pinpoint when the DRH or ERH transition occurs is
from Zeng et al., 2014. This article discusses the temperature-dependent hygroscopic properties of
5


methanesulfonate sodium aerosols using ATR-FTIR spectroscopy.2 First, this method was
authenticated by measuring the temperature-dependent hygroscopic properties of sodium chloride
particles and comparing these values to previous literature values. Not only did the DRFI and ERH
values closely match, but this method also provided further insight into when the DRFI and ERH
transition occurs through spectral changes. Next, this method was used to find methanesulfonate
sodium DRFI and ERH values. Zeng et al., 2014 presents the advantages of this method's ability to
detect minor particle phase changes as well as water content changes. This makes this technique an
effective and accurate method in finding the hygroscopic properties of carboxylic acid salt particles.
6


CHAPTER III
METHODOLOGY
Sodium acetate, sodium formate, sodium succinate, and sodium oxalate aerosols were
deposited onto a germanium ATR crystal from their corresponding aqueous solutions to be analyzed
by an ATR-FTIR spectrometer. Sodium acetate (99.0%, Fisher Scientific) was dissolved in deionized
water to create a 0.5 M C2H3Na02 solution. The solution was atomized and vacuumed through two
diffusion driers to produce crystalized aerosols where they were deposited onto a New Era
Enterprise Ge ATR crystal (1 cm X 5 cm), based on size and phase, with a MSP, INC., Model 110,
Micro-Orifice Uniform Deposition Impactor (MOUDI). The fifth stage of the MOUDI was used, which
deposits aerosols onto the crystal of a diameter of approximately 0.7pm, observed using a scanning
electron microscope, which was confirmed from a previous study.2 The aerosol plated Ge crystal
was mounted into a flow-through stainless steel reactor with a multi-reflection sampler and secured
to the top of a HARRICK ATR accessory in order to pass humid air over the aerosol while collecting
ATR-FTIR spectra. This was repeated with sodium formate (98%, Alfa Aesar), sodium succinate
(99%, Alfa Aesar), and sodium oxalate (100%, J.T. Baker Chemical).
The overall flow system included multiple parts in order to achieve an accurate relative
humidity flow at a specific temperature. The flow system, seen in Figure 1, comprises of incoming
purified air passing through a diffusion dryer (LABCLEAR) to remove any humidity. The air splits into
flow a and flow (B, and each air flow passes through an Alicat Scientific mass flow controller. Flow a
is humidified as it passes through a V-gen, InstruQuest dew point/RH generator. The V-gen,
InstruQuest dew point/RH generator outputs an air flow at an accurate relative humidity and can be
used as a relative humidity calibrator.49 Flow a and flow (B are combined to dilute the humid air and
create a flow rate of 1.000 L/min. The relative humidity of the flowing air is modulated by adjusting
the temperature inputs on the dew point/RH generator and/or the mass flow controllers. The
7


combined humid air enters a flow-through stainless steel reactor which is temperature-controlled by
a NESLAB, RTE 740 circulation bath. The circulation bath circulates Dynalene HC heat-transfer fluid
through a stainless steel cooling block surrounding the reactor. The cooling block was insulated with
silicon, and two thermocouples were installed to read the inlet and outlet temperatures. The
temperature of the reactor was calculated from the average of the inlet and outlet temperatures,
which had relatively small differences at room temperature (approximately 0.2 C) and larger
differences at lower temperatures (approximately 1.5 C at 271 K). The flow-through stainless steel
reactor was attached to the HARRICK ATR flow accessory that is installed in a Nicolet Thermo 6700
FTIR spectrometer with a liquid nitrogen cooled mercury cadmium telluride detector (Picture 1).
Incorrinfl
Pulflc-d AJr
>
Figure 1. Diagram of the overall flow system
8


Picture 1. Silicone covered reaction chamber on top of the ATR accessory
for the FUR spectrometer
After the RH was changed and the flow system was allowed to reach equilibrium, a FTIR
spectrum was acquired. A minimum of a five minute wait period was allowed before a spectrum of
the salt was collected, which is sufficient time to allow the system to reach equilibrium and allow
the salt to deliquesce or effloresce.31,32,33 Equilibrium of this flow system is achieved nearly lOx
quicker than the flow system using EDB analysis. A spectrum of 64 scans at a resolution of 4 cm"1
was obtained using OMNIC software, and the peak locations were recorded. The scans ranged from
4000 cm"1 to 1000 cm"1 to identify red or blue-shifts in the group frequency region from the carbonyl
peaks for each carboxylic acid salt. The fingerprint region was not analyzed because unique
9


absorption bands are not needed to identify DRH and ERH points.34 The different temperatures
used in this experiment to measure temperature-dependent DRH and ERH values ranged from 271 K
(-2 C) to 298 K (25 C) in order to represent temperatures relevant to the troposphere. The
experiment was repeated 3 times at each temperature and then averaged. The relative humidity
was increased from 0% to 100% to observe a DRH point, and the RH was decreased from 100% to
0%to observe a ERH point. The experimental data was compared with available DRH and ERH
literature values.
10


CHAPTER IV
RESULTS AND DISCUSSION
4.1.1 DRH and ERH of sodium formate at 298 K
Sodium Formate was deposited onto a Ge ATR slide, and the DRH and ERH points were
observed at 298, 288, 278, and 271 K. Absorbance spectra of sodium formate as relative humidity is
increased at 298 K are shown in Figure 2a, and detailed spectra of the carboxylate peaks are shown
in Figure 2b. The spectra were stacked for clarity. The spectrum at 0%RH represents sodium
formate in the crystalized phase, with a peak at 1622.8 cm"1,attributing to the carboxylate
asymmetric [vas(COO")] stretch; a peak at 1357.2 cm"1,attributing to the carboxylate symmetric
[vs(COO")] stretch, and smaller peaks at 2920 cm"1 and 2852 cm"1 attributing to the alkyl C-H stretch.
From 3200 cm"1 to 3500 cm"1 there is no observed broad peak in the 0%RH spectra because no
hydrogen bonded O-H stretch is present at 0%RH. These observed peaks and absence of peaks
indicate a crystalline and water free sodium formate salt, which match literature IR spectra
peaks.35'36
11


Absorbance
a.) rh (%)
Figure 2 a.) ATR-FTIR absorbance spectra of sodium formate as RH increases at 298 K. b.) Spectra of carbonyl peaks of
sodium formate at 298 K. Spectra were stacked for clarity.
As the RH is increased at 298 K, there is a distinct peak shift from 63%RH to 65%RH. The
asymmetric carboxylate stretch peak red-shifted from vas(COO")=1623.1 cm_1to vas(COO")=1585.1
cm'1. This observed red-shift is due to the increased hydrogen bonding between water and the
carboxylate functional group. The presence of the hydrogen bonding decreases the bond order of
the C-0 bonds and decreases the stretching force constant k as shown in Hooke's Law Equation 1,
therefore decreasing the vibrational frequency.37
12


V
2.7
k
m
(1)
The symmetric carboxylate stretch peak at vs(COO")=1357.26 cm_1transforms into a doublet at
vs(COO")=1382.1 cm"1 and vs(COO")=1351.4 cm"1. This may occur due to different types of
intermolecular hydrogen bonding interactions with water in the symmetric stretch. The carboxylate
group is hydrogen bonding with one water molecule at the higher wavenumber, and interacting
with two separate water molecules at the lower wavenumber.38 Also, this may occur due to the
formation of an aqueous formic acid complex from the deprotonation of water.50 The crystalized
salt absorbs water from the atmosphere at 63%RH and transitions to its aqueous phase as seen in
the reaction below.39 At 65%RH, sodium formate has changed to its aqueous phase and a droplet is
observed, and, as a result, the peak no longer shifts to the right as the relative humidity is increased.
H20(g) ----H20(aq)
H20(aq) + nHCOONa(s) '=--nHCOONa(aq)
The observed DRH point for this run began at 63%RH and ended at 65%RH. The carboxylate
peak locations are shown in Figure 3. The alkyl peaks at 2920 cm"1 and 2852 cm"1 are unaffected by
the increase in RH and remain unchanged from 0%RH to 100%RH. The hydrogen bonded O-H peak
rapidly grows after the DRH point due to the salts ability to absorb the atmospheric water, and the
water stretching vibrations are interacting with the ATR's incoming infrared radiation. This can be
observed after 63%RH, which shows further evidence of the DRH of sodium formate at this
humidity. The integrated water absorbance can be used to observe the DRH and ERH points, but it
is not as responsive as observing the carbonyl peak change and, therefore, was not used in this
study. The water absorbance peak continues to grow after the DRH point as the water droplet is
increasing in size, and thus, sodium formate demonstrates its ability to act as a cloud condensation
13


nuclei.40 Also, this property makes it susceptible to cloud droplet nucleation scavenging, where the
aerosol can be removed from the atmosphere relatively quickly under higher relative humidity.41
Figure 3 a.) Carbonyl symmetric stretch peak location as RH increases at 298 K. b.)
Carbonyl asymmetric stretch peak location as RH increases at 298 K.
In the opposite method, the carbonyl peaks blue-shift as the relative humidity is decreased
from 100%. The salt's spectra of decreasing RH at 298 K can be seen in Figure 4. At 43% the
aqueous salt begins to release water into the atmosphere and recrystallize. The spectra at 42%
shows a transition of the salt changing to the solid phase as the asymmetric stretch peak shifts to
higher wavenumbers. Once the RH reaches 41%, the sodium formate is fully recrystallized and has
14


Absorbance
effloresced all water as the peak shifts are complete. This is indicated by the asymmetric stretch
peak blue-shifting back to its original position at 1622.9 cm"1 and the doublet returning to a single
peak at 1357 cm"1 (Figure 4), as the water is no longer decreasing the k constant in Hooke's Law.
The salt remains unchanged as the relative humidity is decreased further, as seen in the ATR
Figure 4 ATR-FTIR absorbance spectra of sodium formate as RH decreases at 298 K.
The DRH and ERH of sodium formate, which can be seen in Table 1, are presented as a range
because the salt changes phase over multiple RH values. The DRH at 298 K takes place at 63%-
65%RH, and the ERH takes place at 41%-44%RH. These values are higher than Peng and Chan, 2001
DRH and ERH values at 50.5%-52.1%RH and 26.8%-29.1%RH, respectively. The discrepancy is due to
the different methodology used. Peng and Chan, 2001 used an electrodynamic balance that
measures the change in total mass fraction of solute (mfs) as the RH is changed.21 This method
requires 40 minutes for each measurement to enable the EDB to attain the pre-set RH, which can
lead to variability. Also, Peng and Chan's DRH may be lower than the actual DRH as water can coat
the aerosol's surface and increase its weight before dissolving it. The ATR-FTIR can rapidly record
any changes in the salts phase after 5 minutes, and the phase change is distinct with a peak shift.
15


Table 1: Sodium Formate (HCOONa) at 298 K
Begin DRH 63%RH (0)
End DRH 65%RH (0)
Begin ERH 44%RH (1)
End ERH 41 %RH (0)
4.1.2 Temperature-dependence of sodium formate DRH and ERH Values
The DRH and ERH at 288 K are determined in the same method as above. The salt begins to
absorb water from the atmosphere at 55%RH until fully deliquesced at 57%RH (see Figure 5a).
There is the same red-shift in the asymmetric carbonyl stretch peaks, and the carbonyl symmetric
stretch peak split occurs as in the spectra at 298 K. The aqueous salt releases water into the
atmosphere at 39%RH until fully crystalized at 35%RH at this temperature. The averaged DRH and
RH (%
65
57
55
50
25
Wavenumbers (crn-t)
FUR spectra of sodium formate as RH decreases at 288 K. Spectra was stacked for clarity
16


ERH ranges can be seen in Table 2, where these changes occur at lower RH values than at 298 K
(Figure 5b). There is no current literature on the DRH or ERH of carboxylic acid salts at temperatures
other than 298 K, so literature comparisons of temperature-dependence cannot be made.
Table 2: Sodium Formate (HCOONa) at 288 K
Begin DRH 56%RH (1)
End DRH 5 7%RH (1)
Begin ERH 39%RH (1)
End ERH 36%RH (1)
The DRH is a thermodynamic process, and a temperature-dependent expression of the DRH
can be derived from the Clausius-Clapeyron equation below,42
d \n(DRH/100) nAHs
dT = ~~ RT2 ^
Where n is the solubility in moles of solute per moles of water, and AHs\s the enthalpy of solution
of the salt. Wexler and Seinfeld, 1991 assumed solubility and enthalpy of solution to be constant
over the change in temperature, which produces equation 2.43
1
DRH(T) = DRH(298 K)exp
nAHs (1
R \T 298
(2)
This equation can provide a rough estimation of the DRH temperature-dependence when the
temperature is changed in very small increments because the solubility and enthalpy of solution of
many salts deviate with changes in temperature. The solubility for sodium formate at 298 K is 0.271
mol HCOONa/mol H2O.44 The enthalpy of solution of sodium formate significantly differs at different
temperatures and, in some cases, is procedurally unattainable in literature. Chawla and Ahluwalia,
1975 derived the AHS to be 0.74 KJ/mol at 298 K and changes greatly to AHS = -0.10 KJ/mol at 308
C.45 Due to this high fluctuation, the assumption of constant AHS with changing temperature
cannot be used in this equation, and this equation is invalid for estimating sodium formate's DRH
temperature-dependence.
The same ATR-IR spectral analysis was used to assign the DRH and ERH of sodium formate at
278 K and 271 K, and the spectra are compiled in Figure 6 and Figure 7, respectively.
17


a.)
RH (%)
Figure 7 a.) ATR-FTIR absorbance spectra of sodium formate as RH increases at 278 K. b.) ATR-
FTIR spectra of sodium formate as RH decreases at 278 K.
spectra of sodium formate as RH decreases at 271 K.


The DRH and ERH values can be seen in Table 3 and 4. The peak shift occurs over a lower range of
relative humidities in the efflorescence process when the temperature is at the two lower
temperatures (5 C and -2 C) and when the water is susceptible to freezing. From this decrease in
water movement and activity, there is a relatively increased variability when efflorescence occurs.
Table 3: Sodium Formate (HCOONa) at 278 K
Begin DRH 46%RH (1)
End DRH 48%RH (0)
Begin ERH 20%RH (3)
End ERH 17%RH (2)
Table 4: Sodium Formate (HCOONa) at 271 K
Begin DRH 40 %RH (1)
End DRH 41 %RH (1)
Begin ERH 15 %RH (1)
End ERH 13 %RH (1)
Graph 1 shows the DRH and ERH of sodium formate at temperatures pertinent to the
troposphere. Both DRH and ERH values decrease with decreasing temperature. This provides
evidence that sodium formate is more receptive to change to its aqueous phase at higher altitudes
where temperatures are lower. In the aqueous phase, the aerosols can grow to large droplets and
eventually clouds as they act as cloud condensation nuclei. This property can benefit Earth's
radiation budget as the formation of clouds, especially in the lower troposphere, reflect a portion of
incoming solar radiation, especially higher energy radiation, due to clouds high albedo.46
19


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DRH
ERH
0 _________i_________i________i_________i________i_________i________i
265 270 275 280 285 290 295 300
Temperature (K)
Graph 1 DRH and ERH values of sodium formate over tropospheric-relevant temperatures
4.2.1 DRH and ERH of sodium acetate at 285 K
The DRH and ERH values of sodium acetate, in Table 5, were recorded using the same
methodology as sodium formate at 271 K, 278 K, 285 K, and 298 K.
Table 5: Sodium Acetate (CH3COONa) at 285 K
Begin DRH 3 7%RH (1)
End DRH 40 %RH (1)
Begin ERH 30%RH (2)
End ERH 2 7%RH (1)
A detailed analysis of hygroscopic properties of sodium acetate at 285 K is studied first. These
values are determined from the red-shift of the asymmetric carbonyl stretch peaks and carbonyl
symmetric stretch peak in their corresponding spectra (Figure 8a, 8b). In Figure 8c, the asymmetric
carbonyl stretch peak is at [vas(COO")]= 1570.6 cm"1 at 0%RH and remains there until the RH is
increased to 36%RH. Sodium acetate's asymmetric carbonyl stretch peak is 50 cm"1 lower than
20


sodium formate's peak. This is due to the electron donating methyl group attached to the
carboxylate group decreasing the stretching force constant K in Hooke's Law and the
wavenumber.47 Sodium acetate (vas) peak shifts to a lower wavenumber until the RH reaches 42%,
where the peak remains at [vas(COO")]= 1558.9 cm"1. This shift occurs as the water begins to dissolve
the sodium acetate, and the water in the atmosphere reaches a concentration high enough to
undergo the following reaction.
H20(g) H20{aq)
H20(aq) + nCH3COONa(s) " nCH^COONa(aq)
n is the solubility of sodium acetate in moles of the salt per moles of water. The water interacts
with the carbonyl functional group through hydrogen bonding and decreases the strength of the
C=0 bond, thereby, decreasing the carbonyl's vibrational frequency. This movement of the carbonyl
symmetric stretch can also be observed, shifting from [vs(COO")]= 1419.4 cm"1 to [vs(COO")]= 1409.7
cm"1 once the water can infiltrate the sodium acetate aerosol (Figure 8d). The peaks at 2921 cm"1
and 2852 cm"1 are attributed to -C-H stretch in the alkyl group. These peaks are more prominent in
spectra at lower temperatures, and remain unchanged as the relative humidity is increased or
decrease (see figure 12, spectra at 271 K) because water has no effect on -C-H stretching. The
broad -OH stretch peak at approximately 3400 cm"1 begins to grow significantly after the DRH of the
salt. This indicates the droplet growing larger with increasing humidity. From this growth, sodium
acetate can act as cloud condensation nuclei and can be more quickly removed from the
atmosphere due to its susceptibility to cloud droplet nucleation scavenging.40,41
21


RH (%)
55
42
40
39
38
37
36
30
0
A
C.)
d.)
%RH
0 10 20 30 40 50 60
%RH
Figure 8 a.) ATR-FTIR absorbance spectra of sodium acetate as RH increases at 285 K. b.) Spectra of carbonyl peaks of
sodium formate at 285 K. c.) Carbonyl asymmetric stretch peak location as RH increases at 285 K. d.) Carbonyl symmetric
stretch peak location as RH increases at 285 K.
22


As the RH is decreased after the salt has fully dissolved, the peaks return to their original
position (Figure 9). The salt begins to dry as the hydrogen bonds between the water, and the
carboxylate group separate until the sodium acetate is fully solidified at 30%RH and 28%RH,
respectively. The broad -OH stretch peak at approximately 3400 cm"1 is nearly gone after the ERH
values, further signaling that the salt has effloresced all water. There is no current literature on the
DRH and ERH values at this temperature for comparison. There are only literature values on the
DRH and ERH at 298 K, which will be discussed in the next section.
RH (%)
47
33
31
30
29
28
26
21
Figure 9 a.) ATR-FTIR spectra of sodium acetate as RH decreases at 285 K. b.) Carbonyl asymmetric stretch peak
location as RH decreases at 285 K. c.) Carbonyl symmetric stretch peak location as RH decreases at 285 K.
23


4.2.2 Temperature-dependence of sodium acetate DRH and ERH Values
The humidity was increased at 298 K to find the DRH of sodium acetate aerosols and
decreased to find the ERH using an ATR-FTIR spectrometer. The spectra from the instrument are
stacked in Figure 10. The asymmetric and symmetric carbonyl stretch peak shift undergo the same
red-shift as seen in the previous section when the relative humidity reaches 49%RH, and continues
RH (%)
54
52
51
49
45
40
30
0
RH (%)

v
Figure 10 a.) ATR-FTIR spectra of sodium acetate as RH increases at 298 K. b.) ATR-FTIR spectra of sodium acetate as RH decreases
at 298 K.
24


to shift until the RH reaches 51%. The large peak at 1220 cm"1 is the C-H bend mode in the alkyl
group.47 The RH is decreased from 100%RH, and the peaks begin to blue-shift to their original
position at 46%RH until the salt is in its solid phase at 44%RH. The average DRH and ERH values at
298 K are seen in Table 6.
Table 6: Sodium Acetate (CH3COONa) at 298 K
Begin DRH 48%RH (1)
End DRH 51 %RH (2)
Begin ERH 44%RH (2)
End ERH 39%RH (2)
Two literature values exist for the DRH and ERH of sodium acetate at 298 K. Peng and Chan,
2000 state that DRH for sodium acetate begins at 43.5%RH and ends at 45.2%RH using a
electrodynamic balance method. Wu et al., 2011 records a DRH of 39-42%RH using H-TDMA.29 The
two literature values differ from each other indicating that different methods can provide varying
results when studying hygroscopic properties. The DRH values are lower than the ones obtained in
this experiment. These authors may observe the initial weight and size increase of the aerosols as
water begins to stick to the surface. The water may not be dissolving the aerosols at this low RH.
Both of the techniques by Peng and Chan, 2000 and Wu et al., 2011 use methods of measuring size
and weight to observe the deliquescent property. It can be difficult to pinpoint when the aerosol
has begun/finished absorbing water because an aerosol continue to change as this is happening.
Also, water can stick to the surface of the aerosol and increase the size and weight before the DRH
occurs. For this reason, these literature DRH values may be recorded before the transition begins.
ATR analysis can better see these exact moments by rapid vibrational mode changes that only
happen during the phase change. Peng and Chan, 2000 have the only literature value of the ERH of
sodium acetate at 298 K, which begins and ends at 40.6% and 36.7%RH respectively. This ERH is
slightly lower than the ERH value obtained in this experiment indicating that the aerosols
effloresced sooner in this study than in the experiment using EDB.
25


The DRH and ERH values for sodium acetate at 278 K can be seen in Table 7, and the stacked
spectra are in Figure 11. The standard deviation is relatively higher in the ERH than the DRH
because the water's kinetic energy is reduced at this temperature making it more difficult to be
released from the crystalizing salt.
Table 7: Sodium Acetate (CH3COONa) at 278 K
Begin DRH 33 %RH (1)
End DRH 35%RH (2)
Begin ERH 29%RH (3)
End ERH 26%RH (2)
26


RH (%)
Figure 11 a.) ATR-FTIR absorbance spectra of sodium acetate as RH increases at 278 K. b.) ATR-FTIR spectra of sodium acetate as RH
decreases at 278 K.
Table 8 shows the DRH and ERH values of CH3COONa below freezing at 271 K. The peaks in
the carbonyl stretching area are noisier than spectra at different temperature due to superimposed
rotational energy level absorptions of water vapor in the reaction chamber.37 The stacked spectra as
the relative humidity is decreasing in Figure 12, show that the blue-shift is less distinct at the lower
temperature. This is due to the water's susceptibility to crystalize at this temperature, which slows
down the water releasing process.
27


Table 8: Sodium Acetate (CH3COONa) at 271 K
Begin DRH 23 %RH (1)
End DRH 24%RH (1)
Begin ERH 15 %RH (1)
End ERH 12 %RH (1)
Figure 12 a.) ATR-FTIR spectra of sodium acetate as RH increases at 271 K. b.) ATR-FTIR spectra of sodium acetate as RH
decreases at 271 K.
28


60
I
o
40
E
3
30
0)
>
*3
V
ec
20
10
I
l
i I
i
I
I
DRH
ERH
g ___________i_________i_________i_________i_________i_________i_________i
265 270 275 280 285 290 295 300
Temperature (K)
Graph 2 DRH and ERH values of sodium acetate over tropospheric-relevant temperatures.
In Graph 2, the DRH and ERH values at different temperatures are plotted. The DRH and
ERH decrease as the temperature decreases. This correlation is not necessarily linear as seen with
methanesulfonate sodium aerosols in Zeng et al., 2014. Sodium acetate aerosols will change to
their aqueous phase at relatively low humidity and at higher elevations where the temperature is
lower. This provides evidence that this salt is more responsive in forming clouds which reflect
incoming solar radiation at higher elevations.
The following equation, described in Section 4.1.2, can thermodynamically predict the
change in DRH with a change in temperature.43
DRH(T) = DRH(298K)exp
nAHs
R
f
The AHs and n for sodium acetate is 17.32 and 0.09961 ml CH3C00Na respectively at 298 K.43
5 mol mol H20
This enthalpy of solution changes slightly over a small change in temperature, so this equation can
be used for this chemical. The theoretically predicted DRH at 285 K begins at 46%RH and ends at
29


49% RH. The experimental DRH values at 285 K are about 10% lower (See Table 5). However, this
does predict that the DRH will be lower at lower temperatures, which agrees with DRH data in
Graph 2.
4.3.1 DRH and ERH of sodium succinate at 298 K
Sodium Succinate is a diprotic carboxylic acid salt and dissociates two sodium ions in the
deliquescence phase, thus, requiring a higher humidity than sodium acetate or sodium formate to
RH (%)
a.)
100
90
84
82
78
70
60
40
0
/V.
Figure 13 a.) ATR-FTIR absorbance spectra of sodium succinate as RH increases at 298 K. b.) Spectra of carbonyl peaks of sodium
succinate at 298 K. Spectra was stacked for clarity.
30


fully deliquesce. The spectra of sodium succinate as the RH is increased can be seen in Figure 13a. In
Figure 13b, the carbonyl peaks begin to shift after the RH is increased more than 78%RFI. The
asymmetric carbonyl stretch peak is at [vas(COO")]= 1551.5 cm"1 at 0%RH. This peak shifts to the
right and splits into two peaks at 1562.9 cm"1 and 1551.5 cm"1 (Figure 14a). The transition ends when
the RH reaches 84%RFI, which represents the end of the DRFI. The split peak is most likely due to
different intermolecular hydrogen bonding of the two carbonyl functional groups with water.38 The
symmetric stretch peak, [vs(COO")], at 0%RH is a split peak at 1434.2 cm"1 and 1402.1 cm"1. This may
also be due to different ionic interactions with sodium slightly increasing or decreasing the
a.)
1564 -
1562 f *
| 1560 -
S 1558
.Q
i 1556
c
4)
% 1554 L
5
1552 , , , . ______ __
1550 ------------'-----------:-----------!----------------------
0 20 40 60 80 100
%RH
b.)
1440
Figure 14 a.) Carbonyl asymmetric stretch peak location as RH increases at 298 K. b.)
Carbonyl symmetric stretch peak location as RH increases at 298 K.
31


symmetric carbonyl stretch frequency. As the RH is increased past 78%RH, the peaks begin to
merge into one peak at 1397.5 cm"1 at 84% RH (Figure 14b). The intermolecular interactions are
nearly equal in the symmetric carbonyl stretch and, thus, form one broad peak. In Figure 13a, the
broad -OH stretch peak at approximately 3400 cm"1 begins to grow rapidly at 78%RH, indicating that
the solid salt is absorbing water and continues to grow after a RH of 84%. Also, the H20 bending
vibration peak at 1640 cm"1 appears after the DRH point, which evidences that water infiltrated the
system. This shows that this salt can act as a CCN after the DRH, but at a much higher RH than the
previous salts.
The peaks of the carbonyl functional group return to their original positions and shape as
the RH is decreased to 0%. When the RH is decreased to 72%RH, the hydrogen bonds with water
and the aqueous salt begin to dissociate, and water is released into the atmosphere. The C-0 bonds
shorten, and the carbonyl peaks blue-shift until 67%RH. The broad -OH stretch peak at
approximately 3400 cm"1 and the H20 bending vibration peak at 1640 cm"1 nearly disappear after
this efflorescence transition (Figure 15).
RH (%)
84
75
72
71
69
67
65
0
v
32


The average DRH and ERH values at 298 K can be seen in Table 9. Peng and Chan, 2000
experimental DRH and ERH values are 63.5-66.0%RH and 46.7-47.9%RH, respectively. Wu et al.,
2011 did not observe a DRH point using the H-TDMA method. The DRH and ERH values fluctuate
significantly from the different methods. This fluctuation may be caused by sodium succinates
highly temperature-dependent solubility48 and may have a strong effect on the long experimental
durations used in Peng and Chan, 2000.
Table 9: Sodium Succinate (C4H4 Na204) at 298 K
Begin DRH 78%RH (1)
End DRH 82%RH (1)
Begin ERH 71 %RH (1)
End ERH 68%RH (1)
4.3.2 Temperature-dependence of sodium succinate DRH and ERH Values
In Figure 16, the DRH and ERH transition is at a lower relative humidity at 288 K than at 298
K. The peaks transition the same as above, but the lower temperature causes this transition earlier
when the relative humidity is increased. The average DRH and ERH values can be seen in Table 10.
33


RH (%)
Figure 16 a.) ATR-FTIR spectra of sodium succinate as RH increases at 288 K. b.) ATR-FTIR spectra of sodium succinate as RH
decreases at 288 K.
Table 10: Sodium Succinate (C4H4 Na2Q4) at 288 K
Begin DRH
End DRH
Begin ERH
End ERH
67%RH (1)
73%RH (1)
62%RH (1)
59%RH (1)
34


Once again, the DRH and ERH values decrease with a decrease in temperature (Figure 17
and Figure 18). The average values at 278 K and 271 K can be seen in Table 11 and Table 12.
Figure 17 a.) ATR-FTIR spectra of sodium succinate as RH increases at 278 K. b.) ATR-FTIR spectra of sodium
succinate as RH decreases at 278 K.
Table 11: Sodium Succinate (C4H4 Na2Q4) at 278 K
Begin DRH
End DRH
Begin ERH
End ERH
56%RH (1)
60%RH (1)
53 %RH (1)
49 %RH (1)
35


RH (%)
90
80
56
54
50
40
0
RH (%)
90
50
40
33
31
29
28
'V
Figure 18 a.) ATR-FTIR spectra of sodium succinate as RH increases at 271 K. b.) ATR-FTIR spectra of sodium succinate as
RH decreases at 271 K.
Table 12: Sodium Succinate (C4H4 Na2Q4) at 271 K
Begin DRH
End DRH
Begin ERH
End ERH
49 %RH (1)
56%RH (1)
33 %RH (1)
29%RH (1)
36


In Graph 3, the DRH and ERH values at different temperatures are plotted. As the
temperature decreases, the DRH and ERH decrease. This graph shows that this aerosol can be
beneficial in reflecting incoming solar radiation at high elevations because it can transition to its
aqueous phase and form light scattering water droplets at relative low humidity. Acquiring
experimental data on the DRH of sodium succinate is more important than using theoretical
Temperature (K)
Graph 3 DRH and ERH values of sodium succinate over tropospheric-relevant temperatures.
calculations, such as Equation 1 in Section 4.1.2 because sodium succinate's solubility changes
considerably with a change in temperature, and the incorrect temperature correlation would be
made using theoretical means.48
4.4.1 DRH and ERH of sodium oxalate
Sodium Oxalate (Na2C204) is a diprotic carboxylic acid salt with a low solubility of 0.0380 mol
Na2C204/mol H20.48 This solubility is lower than sodium succinates at ( n =0.04413 mol C4H4Na204
/ mol H20), and the solubility of sodium formate is over 7 times greater. The same method was
used to observe DRH and ERH values for this salt. When the RH was increased from 0%RH to
37


100%RH at 298 K, there was no peak shift or spectral change. The salt did not deliquesce at this
temperature, and the salt remained in the crystalline phase throughout the entire experiment.
Since the salt never converted to the aqueous phase, the method for determining the ERH of
sodium oxalate could not be tested. The RH must be reduced from 100% and the salt must be in the
aqueous phase in order to test for an ERH value using this technique. The experiment was run at 30
C to see if the salt would deliquesce at a higher temperature. The spectra for this experiment are in
Figure 19. Notice the spectra do not change when the RH is increased to 100%. The carbonyl
asymmetric stretch peaks remain at 1635.5 cm"1 for the entire run, and, also, the split symmetric
stretch peaks remain at 1338.1 cm"1 and 1321.2 cm"1. The broad -OH stretch peak at approximately
3400 cm"1 does not grow like it does with the other salts. There is a small broad peak in this region
starting at 60%RH, indicating that some water was sticking to the surface of the salt but was not
dissolving it.
RH (%)
/f.
100
85
60
40
30
20
0
Figure 19 ATR-FTIR absorbance spectra of sodium oxalate at 303 K.
38


There was no DRH observed for any of the tropospheric related temperatures. Even at the
lowest tested temperature 271 K (See Figure 20), there is no spectral change. The lack of a DRH for
sodium oxalate provides evidence that this salt cannot act as CCN and does not reflect as much
incoming solar radiation as the other carboxylic acid salts studied. The salt remains in its crystalline
phase in the troposphere and may remain airborne for longer periods of time than the other salts
because it is not susceptible to cloud droplet nucleation scavenging41 and will only be removed
through gravitational settling and surface deposition.
Figure 20 ATR-FTIR spectra of sodium succinate at 271 K.
The asymmetric carbonyl stretch peak for sodium oxalate is approximately 80 cm"1 higher
than the asymmetric carbonyl stretch peak for sodium succinate in the crystalline phase (0%RH).
This is due to the electron withdrawing carbonyl group adjacent (in the a position) to each other,
increasing the peak's wavenumber.47 Sodium succinate's carbonyl groups are protected by the two
electron donating CH2groups.
39


The data obtained for sodium oxalate matches with literature. Peng and Chan, 2000 do not
observe a DRH point using EDB analysis; however, they found the ERH (72-75.2%RH) of the salt
through the levitation of previously aqueous particles in the EDB.21 Wu et al., 2011 did not observe
a DRH value for sodium oxalate and reported no hygroscopic growth below 90%RH using H-TDMA.29
Both articles attribute the absence of a DRH to sodium oxalate's low solubility which agrees with the
findings in this study.
40


CHAPTER V
CONCLUSION
The temperature-dependent deliquescent and efflorescent relative humidity points of
sodium formate, sodium acetate, sodium succinate, and sodium oxalate were determined using
ATR-FTIR spectroscopy. This work provided much insight into these carboxylic acid salt interactions
with water at lower tropospheric temperatures and their ability to become cloud condensation
nuclei and reflect incoming solar radiation. Though many of the DRH and ERH values at 298 K were
higher than literature values, this difference is attributed to the different methodology used.
Excluding sodium oxalate, these carboxylic acid salts' DRH and ERH values decrease with decreasing
temperature. With this information, these common atmospheric aerosols will deliquesce at low
humidity when they float to higher altitudes in the troposphere. At these higher altitudes, these
salts can rapidly grow into water droplets and eventually clouds, and reflect a greater portion of
incoming solar radiation. Therefore, the formation of carboxylic acid salts from the acid
displacement of atmospheric carboxylic acids with airborne cations may be beneficial in managing
the Earth's radiation budget.
This study pointed out the significance of collecting experimental data in atmospheric
studies. The theoretical thermodynamic calculation of the temperature-dependent DRH values was
inadequate in predicting DRH values when the temperature was changed. This is due to the salts'
solubility and enthalpy of solution being temperature dependent. Also, there is no theoretical
calculation for temperature-dependent ERH values, making experimentation the only way for ERH
determination. Collecting experimental data for the hygroscopicity of carboxylic acid salt aerosols
has shown to be beneficial for atmospheric modeling.
ATR-FTIR spectroscopy revealed its advantages in studying the hygroscopicity of carboxylic
acid salts in this experiment. This technique pinpoints when the DRH and ERH transition occurs by
41


its ability to study 0.5 nm 5 pm into the salt crystal surface. Further, equilibrium of the ATR-FTIR
flow system is nearly lOx quicker than the flow systems in studies using EDB analysis because the
reaction chamber is much smaller. Reproducibility is another advantage of this method, as many
DRFI and ERH values have a standard deviation of plus or minus one percent relative humidity. ATR-
FTIR spectroscopy has proven to be a highly effective hydroscopic tool and should be used to further
study atmospheric interactions.
42


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41 Svenningsson, B.; Hansson, H.C.; Martinsson, B.; Wiedensohler, A.; Swietlicki, E.; Cederfelt,
S.I.; Wendisch, M.; Colvile, R.N. Cloud droplet nucleation scavenging in relation to the size and
hygroscopic behaviour of aerosol particles. Atmospheric Environment, 1997, 31 (16) 2463-2475.
42 Tang, I.; Munkelwitz, H. Aerosol Phase Transformation and Growth in the Atmosphere. Journal of
Applied Meteorology. 1993. 33, 791-796.
43 Wexler, A.; Lurmann, F.; Seinfeld, J. Modeling urban and regional aerosols: I. Model development,
Atmos. Environ. 1994. 28, 531-546.
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44 CRC Handbook of Chemistry and Physics. 96th ed. CRC Press: Boca Raton, FL, 2015-2016.
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Sussex. 2009.
48 Zul, M; Rozaini, M; Brimblecombe, P. The solubility measurments of sodium dicarboxylate salts;
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Full Text

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TEMPERATURE DEPENDENT HYGROSCOPIC PROPERTIES OF ATMOSPHERIC CARBOXYLIC ACID SALT AEROSOLS USING ATR FTIR SPECTROSCOPY by JASON ERIC ROBERTS B.S., United States Air Force Academy, 2011 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Master of Science Chemistry Program 2015

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ii This thesis for the Master of Science degree by Jason Eric Roberts has been approved for the Chemistry Program by Yong Liu, Chair Xiaotai Wang Marta Maron Date: 13 November 2015

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iii Roberts, Jason Eric (M.S., Chemistry) T emperature Dependent H ygroscopic P roperties of A tmospheric C arboxylic A cid S alt A erosols U sing ATR FTIR S pectroscopy Thesis directed by Assistant Professor Yong Liu ABSTRACT Atmospheric modelling requires extensive knowledge of aerosols' interaction with water, including abundant carboxylic acid salt aerosols. The hygroscopic behaviors of atmospheric relevant monoprotic and diprotic carboxylic acid salts were studied at different tropospheric relevant temperatures using ATR FTIR spectroscopy The deliquescent relative humidity (DRH) and efflorescent relative hu midity (ERH) of all studied salts were determined by changes in spectral peak location s and spectral shapes of the carbonyl s (COO )] and asymmetrical as (COO )] stretching vibrational modes. Distinct peak shifts were observed for sodium f ormate, sodium acetate, and sodium succinate as the relative humidity (RH) was increased to 100% RH and then decreased to 0% RH indicating DRH and ERH values for these atmospheric salts S odium oxalate showed no spectral evidence of deliquescence. The D RH and ERH of these salts vary due to their differing solubilities and enthalpy of solutions which, then, vary their susceptibility to act as cloud condensation nuclei and scatter incoming solar radiation The hygroscopic study of aerosol salts provides insight into their physicochemical, light scattering, and reactivity properties as well as their influences on climate and weather. The form and content of this abstract are approved. I recommend its publiclation. Approved: Yong Liu

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iv ACKNOWLEDGEMENTS I would like to acknowledge my family and friends for pushing me to follow my dreams. Also, I would like to acknowledge Dr. Yong Liu for being a great mentor and pushing me to my full potential.

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v TABLE OF CONTENTS CHAPTER I. INTRODUCTION ................................ ................................ ................................ ............. 1 II. LITERATURE REVIEW ................................ ................................ ................................ ..... 5 III. METHODOLOGY ................................ ................................ ................................ ............ 7 IV. RESULTS AND DISCUSSION ................................ ................................ ......................... 11 4.1.1 DRH and ERH of Sodium Formate at 298 K ................................ .................... 11 4.1.2 Temperature dependence of Sodi um Formate DRH and ERH Values ........... 16 4.2.1 DRH and ERH of Sodium Acetate at 285 K ................................ ..................... 20 4.2.2 Temperature dependence of Sod ium Acetate DRH and ERH Values ............ 24 4.3.1 DRH and ERH of Sodium Succinate at 298 K ................................ .................. 2 9 4.3.2 Temperature dependence of Sodi um Succinate DRH and ERH Values ......... 32 4.4.1 DRH and ERH of Sodium Oxalate ................................ ................................ ... 36 V. CONCLUSION ................................ ................................ ................................ ............... 40 REFERENCES ................................ ................................ ................................ ................................ ... 42

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1 CHAPTER I INTRODUCTION Aerosols are ubiquitous in the atmosphere yet there are many uncertainties i n their impact toward climate change. 1 They scatter, reflect, and absorb incoming solar radiation; act as cloud condensation nuclei (CCN); and affect weather including precipitation. Along with their climate affecting potential, aerosols undergo heterogeneous reactions chemical composition. 2 Understanding aerosols i s a growing topic in order to reduce this uncertainty and understand their interactions in a complex system. atmospheric water which then affect s many physicochemical characteristics of the particle s Also, hygroscopic properties affect respiratory tract deposition 3 4 5 aerosol optical properties 6 atmospheric removal rates, and chemical reactivity, notably heterogeneous chemistry. 7 When the conc entration of water in the atmosphere, or relative humidity (RH), reaches a certain point particular aerosols will absorb water from the atmosphere and deliquesce throug h a thermodynamic process known as the deliquescence relative humidity (DRH). The proc ess of an aerosol particle transforming from its solid phase to its aqueous phase is paramount because heterogeneous reaction rates are increased and light scattering properties are heightened from its previous crystalline, solid phase. 8 9 10 11 12 As the saturated relative humidity decreases, certain aqueous aerosols effloresce water into the atmosphere and undergo the opposite phase change where the aqueous particle turns into a crystalline, solid aerosol. This value of relative humidity is call ed the efflorescence relative humidity (ERH). DRH and ERH are temperature dependent and are, therefore, changing at different heights in the troposphere. The DRH can be predicted theoretically as it is a thermodynamic process, but this calculation requir es temperature dependent solubility data and enthalpy of solution of aerosols which are not available for many carboxylic acid salts and

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2 can vary considerably when temperature is changed. 13 Therefore, experimental data of DRH and ERH values over different tropospheric temperatures are more significant and accurate than theoretical values. Comprehending aerosols deliquescence and efflorescence values at varying radi ative forces, 14 provides evidence towards their ability to serve as a cloud condensation nucleus (CCN), 5 and further strengthen s tropospheric modeling. Carboxylic acid salt aerosols are prevalent in the troposphere. However, t here is little literature pert aining to the DRH and ERH of carboxylic acid salt aerosols, and no data on their temperature dependent properties. The sources of these inorganic salt aerosols are from biogenic as well as anthropogenic processes. A variety of monoprotic and diprotic car boxylic acids are released in high concentrations from the burning of biomass, especially the combustion of wood from wildfires. 15 Also, they are produced from the ozonolysis of volatile organic compounds (VOCs) approximately two thirds of which are released from Earth's vegetation. 16 Anthropogenic sources include the release of carboxylic acids from the grilling of meat and the exhaust from automobiles. 17 From these sources, carboxylic acids such a formic (HCOOH) and acetic (CH 3 COOH) acids have been calculated to contribute between 16 % and 35 % of the free acidity in North American precipitation and up to 98% in remote areas. 18 Once these weak acids have been released into the troposphere, they undergo an acid displacement reaction with abundant a irborne cations and anions They transform into carboxylic acid salts in a water medium such as a cloud or evaporating rain puddle as seen in the following reaction. 19 The carboxylic acid salt is favored due to T he HCl gas diffuses from the water medium, thus, forming more products through the equilibrium process. This process happens extensively near coastal regions where sea salt aerosols ha ve been recorded in high concentrations

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3 hundreds of kilometers inland in North America and begin to decrease rapidly at 500 km inland. 20 A previous study showed these weak acids deplete 20 30% of atmospheric chloride from sea salt aerosols in Northern Finl and. 21 After reacting, the carboxylic acid salts are then carried inland and even transcontinentally. 22 With t he risks of wildfires across North America increasing significantly due to global climate change notably in California and the Northwest United States, the risk of high concentrations of these inorganic salts transported into North America will begin to rise as well. 23 An additional reason for hyg roscopic analysis of these carboxylic acid salts is their health concerns. Respiratory exposure of aer osol salts depends on hygroscopic behavior, for example their size and phase. 24 The diprotic carboxylic acid salt, sodium oxalate, has adverse health effects when inhaled including throat and esophagus pain and even cardiovascular collapse. 25 Sodium form ate aerosols, a monoprotic carboxylic acid salt, also cause respiratory tract irritation including coughing and sore throat. 26 Investigating the DRH and ERH of carboxylic acid salt aerosols can provide further insight on outdoor air quality and its human conditions. The DRH and ERH of carboxylic acid salt s were analyzed usi ng Attenuated Total R eflection F ourier Transform I n frared spectroscopy (ATR FTIR) This process of analysis wa s authenticated in Zeng et al., 2014 as the produced data closely ma tched previ ous sodium chloride temperature dependent ERH and DRH values. ATR FTIR spectrometer is a FTIR spectrometer with an ATR flow accessory in which the IR beam from the FTIR enters the ATR crystal at a n incident angle larger than the crystal's critical angle. In this experiment, a ) of 45 enters a g ermanium C ) of 22 27 The reflection of the IR beam inside the ATR crystal creates an evanescent wave. The produced evanescent wave infiltrates the aerosol molecule 0.5 m 5 m into the crystal surface 28 and the studied salt will absorb this energy at its particular vibrational modes. This method provides many advantages in studying hyg roscopicity compar ed to

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4 previous measuring techniques including a rapid response of water absorption on the surface of the studied aerosols with a continuous infrared beam and, also, high reproducibility in measurements. As a carboxylic acid salt crystal begins to delique sce, there is a noticeable red shift in the carbonyl peak s which can be concisely observed in the ATR FTIR spectra. Also there is a pronounced blue shift by the same peaks in the spectra when water is effloresced from the aqueous aerosol. T his is lackin g in other methods such as the electrodynamic balance technique where hygroscopic transitions are determined by a constantly changing aerosol weight 21 In this work, the hygroscopic properties of sodium acetate, sodium formate, sodium succinate, and sodiu m oxalate were analyzed. The aerosols of these carboxylic acid salts were deposited onto Ge ATR slide s and investigated with ATR FTIR spectroscopy The DRH and ERH were recorded at varying temperature s between troposphere by analyzing c hanges in peak position and peak sha pe of the carbonyl vibrational modes.

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5 CHAPTER II LITERATURE REVIEW There are currently only two published journal articles on the hygroscopicity of atmospheric carboxylic acid salts and no articles pertaining to their temperature dependence Peng and Chan, 2000 and Wu et al., 2011 experimented to find DRH and ERH values for common monoprotic and diprotic carboxylic acid salt aerosols at 298 K water observe DRH and ERH values by finding the relative mass of the salt with DC voltage as the relative humidity is increased. 21 The phase change between the solid phase and aqueous phase is determined when there is a rapid increase in the mass fraction of solute (mfs) in the plotted graph of mfs vs. water activity. Though this change is obvious, this technique canno t accurately pinpoint when the DRH occurs and ends. Each measurement takes 40 minutes to enable t he EDB to attain the pre set RH, 21 which may make s it difficult to obtain accurate results. This work determines the DRH and ERH for all of the carboxylic aci d salts used in this experiment at 298 K and it will be used for quantitative comparison. soluble carboxylic salts and their influence on the water uptake ygroscopi city T andem D ifferential M obility A nalyzer (H TDMA) to find only the DRH of many carboxylic salts. 29 H TDMA measures the size of the aerosol, and a rapid increase in size can represent a phase change. 30 The DRH values differ in some areas when compared to the DRH values obtained in Peng and Chan, 2000. This provides evidence that determining the DRH with precision using a method that measures the aerosol weight or size is difficult. The method used in this paper that can pinpoint when the DRH or ERH tran sition occurs is from Zeng et al., 2014. This article discusses the temperature dependent hygroscopic properties of

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6 methanesulfonate sodium aero sols using ATR FTIR spectroscopy 2 First, this method was authenticated by measuring the temperature dependent hygroscopic properties of sodium chloride particles and comparing the se values to previous literature values. Not only did the DRH and ERH values closely match, but this method also provided further insight in to when the DRH and ERH transition occurs thr ough spectral changes. Next, this method was used to find methanesulfonate sodium DRH and ERH values. Zeng et al., 2014 presents the advantages of this method detect minor particle phase changes as well as water content changes. This makes t his technique an effective and accurate method in finding the hygroscopic properties of carboxylic acid salt particles.

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7 CHAPTER III METHODOLOGY Sodium acetate, sodium formate sodium succinate, and sodium oxalate aerosols were deposited onto a germanium ATR crystal from their corresponding aqueous solutions to be analyzed by an ATR FTIR spectrometer. Sodium acetate (99.0%, Fisher Scientific) was dissolved in deionized water t o create a 0.5 M C 2 H 3 NaO 2 solution. The solution was atomized and vacuumed through two diffusion driers to produce crystalized aerosols where they were deposited onto a New Era Enterprise Ge ATR crystal (1 cm X 5 cm) based on size and phase with a MSP, INC., Model 110, Micro Orifice Uniform Deposition I mpactor (MOUDI). The fifth stage of the MOUDI was used which deposits aerosols onto the crystal of a diameter of approximately 0.7 observed using a scanning electron microscope which was confirmed from a previous study. 2 The aerosol plated Ge crystal was mounted into a flow through stainless steel reactor with a multi reflection sampler and secured to the top of a HARRICK ATR accessory in order to pass humid air over the aerosol while co llecting ATR FTIR spectra. This was repeated with sodium formate (98%, Alfa Aesar), sodium succinate (99%, Alfa Aesar), and sodium oxalate (100%, J.T. Baker Chemical). The overall flow system included multiple parts in order to achieve an accurate relati ve humidity flow at a specific temperature. The flow system, seen in Figure 1, comprises of incoming purified air passing through a diffusion dryer (LABCLEAR) to remove any humidity. The air splits into passes through an Alicat Scientific mass flow controller. Flow is humi dified as it passe s through a V gen, InstruQuest dew point/RH generator The V gen, InstruQuest dew point/RH generator outputs an air flow at an a ccurate relative humidity and can be used as a relative humidity calibrator 49 dilute the humid air and create a flow rate of 1.000 L/min. The relative humidity of the flowing air is modulated by adjusting the temperature inputs on the dew point/RH gener ator and/or the mass flow controllers. The

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8 combined humid air enters a flow through stainless steel reactor which is temperature controlled by a NESLAB, RTE 740 circulation bath. The circulation bath circulates Dynalene HC heat transfer fluid through a s tainless steel cooling block surrounding the reactor. The cooling block was insulated with silicon, and two thermocouples were installed to read the inlet and outlet temperatures. The temperature of the reactor was calculated from the average of the inle t and outlet temperatures which had relatively small differences at room temperature (approximately 0.2 C) and larger differences at lower temperatures (approximately 1.5 C at 271 K). The flow through stainless steel reactor was attached to the HARRICK ATR flow accessory that is installed in a Nicolet Thermo 6700 FTIR spectrometer w ith a liquid nitrogen co oled mercury cadmium telluride detector (Picture 1). Figure 1 Diagram of the o verall flow s ystem

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9 After the RH was changed and the flow system was allowed to reach equilibrium, a FTIR spectrum was acquired. A minimum of a five minute wait period was allowed before a spectrum of the salt was collected which is sufficient time to allow the system to reach equilibrium and allow the salt to deliquesce or effloresce. 31 32 33 Equilibrium of this flow system is achieved nearly 10x quicker than the flow system using EDB analysis. A spectrum of 64 scans at a resolution of 4 cm 1 was obtained using OMNIC software, and the peak locations were recorded. The scans ranged from 4000 cm 1 to 1000 cm 1 to identify red or blue shifts in the group frequency region from the carbonyl peaks for each carboxylic acid salt. The fingerprint region was not ana lyzed because unique Picture 1 Silicone covered reaction chamber on top of the ATR accessory for the FTIR spectrometer

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10 absorption bands are not need ed to identify DRH and ERH points. 34 Th e different temperature s used in this experiment to measure temperature dependent DRH and ERH values ranged from 271 K ( in order to represent temperatures relevant to the troposphere. The experiment was repeated 3 times at each tem perature and then averaged. The relative humidity was increas ed from 0% to 100% to observe a DRH point, and the RH was decrease d from 100% to 0% to observe a ERH point. The experimental data was compared with available DRH and ERH literature values.

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11 CHAPTER IV RESULTS AND DISCUSSI ON 4.1.1 DRH and ERH of s odium f ormate at 298 K Sodium Formate was deposited onto a Ge ATR slide, and the DRH and E RH points were observed at 298 288, 278, and 271 K. Absorbance spectra of sodium formate as relative humidity is increased at 298 K are shown in Figure 2a, and detailed spect r a of the carboxylate peaks are shown in Figure 2b. The spectra were stacked for clarity. The spectrum at 0%RH represents sodium formate in the crystalized phase, with a peak at 1622.8 cm 1 attributing to the carboxylate as (COO )] stretch ; a peak at 1357.2 cm 1 attributing to the carboxylate symmetric s (COO )] stretch, and smaller peaks at 2920 cm 1 and 2852 cm 1 attributin g to the alkyl C H stretch. From 3200 cm 1 to 3500 cm 1 there is no observed broad peak in the 0%RH spectra because no hydrogen bonded O H stretch is present at 0% RH These observed peaks and absence of peaks indicate a crystalline and water free sodium formate salt which match literature IR spectra peaks. 35 36

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12 As the RH is increased at 298 K, there is a distinct peak shift from 63%RH to 65%RH. The asymmetric carboxylate stretch peak red shift ed as (COO )=1623.1 cm 1 as (COO )=1585.1 cm 1 This observed red shift is due to the increased hydrogen bonding between water and the carboxylate functional group The presence of the hydrogen bonding decreases the bond order of the C O bonds and decreases the stretching force constant as shown in E quation 1 therefore decreasing the vibrational frequency. 37 a.) b.) Figure 2 a.) ATR FTIR absorbance spectra of sodium formate as RH increases at 298 K. b.) Spectra of carbonyl peaks of sodium formate at 298 K. Spectra were stacked fo r clarity.

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13 s (COO )=1357.26 cm 1 transforms into a doublet at s (COO )=1382.1 cm 1 s (COO )=1351.4 cm 1 This may occur due to differ ent types of intermolecular hydrogen bonding interactions with water in the symmetric stretch. The carboxylate group is hydrogen bonding with one water molecule at the higher wavenumber, and interacting with two separate water molecules at the lower wavenumber 38 Also, this may occur due to the formation of an aqueous formic acid complex from the deprotonation of water. 50 The crystalized salt absorb s water from the atmosphere at 63%RH and transitions to its aqueou s phase as seen in the reaction below 39 At 65%RH, s odium formate has changed to it s aqueous phase and a droplet is observed and, as a result, the peak no longer shifts to the right as the relative humidity is increased. The observed DRH point for this run began at 63% RH and ended at 65%RH. The carboxylate peak locations a re shown in Figure 3. The alkyl peaks at 2920 cm 1 and 2852 cm 1 are unaffected by the inc rease in RH and remain unchanged from 0%RH to 100%RH. The hydrogen bonded O H peak rapidly grows after the DRH poi nt due to the salt s ability to absorb the atmospheric water, and the water s This can be observed after 63%RH which shows further evidence of the DRH of sodium formate at this humidity. The integrated water absorbance can be used to observe the DRH and ERH points, but it is not as responsive as observing the carbonyl peak change and, therefore, was not used in this study. The water absorbance peak continues to grow after the DRH point as the water droplet is increasing in size, and thus, sodium form ate demonstrates its ability to act as a cloud condensation

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14 nuclei. 40 Also this property makes it susceptible to cloud droplet nucleation scavenging where the aerosol can be removed from the atmosphere relatively quickly under higher relative humidity. 41 I n the opposite method, the carbonyl peak s blue shift as the relative humidity is decreased from 100%. The salt's s pectra of decreasing RH at 298 K can be seen in Figure 4. At 43% the aqueous salt begins to release water into the atmosphere and recrystallize. The spectra at 42% shows a transit ion of the salt changing to the solid phase as the asymmetric stretch peak shifts to higher wavenumbers. Once the RH reaches 41%, the sodium formate is fully re crystallized and has 1580 1590 1600 1610 1620 1630 0 10 20 30 40 50 60 70 80 Wavnumber (cm 1 ) %RH 1340 1350 1360 1370 1380 1390 0 10 20 30 40 50 60 70 80 Wavenumber (cm 1 ) %RH Figure 3 a.) Carbonyl symmetric stretch peak location as RH increases at 298 K. b.) Carbonyl asymmetric stretch peak location as RH increases at 298 K. a.) b.)

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15 effloresced all water as the peak shifts are complete This is indicated by the asymmetric stretch peak blue shifting back to its original position at 1622.9 cm 1 and the doublet ret urning to a single peak at 1357 cm 1 (Figure 4) as the wat er is no longer decreasing the constant in Hooke's Law. The salt remains unchanged as the relative humidity is decreased further, as seen in the ATR spectra. The DRH and ERH of sodium formate which can be seen in Table 1, are presented as a range because the salt changes phase over multiple RH values. The DRH at 298 K takes place at 63% 65 %RH, and the ERH takes place at 41% 44 %RH. These values are higher than Peng and Chan, 2001 DRH and ERH values at 50.5% 52.1% RH and 26.8% 29.1%RH respectively. The discrepancy is due to the different methodology used. Peng and Chan, 2001 used an electrodynamic balance that measures the change in total mass fraction of solute (mfs) as the RH is changed. 21 This method requires 40 minutes for each measurement to enable the EDB to attain the pre set RH which can lead to variability. ore dissolving it. The ATR FTIR can rapidly record any changes in the salts phase after 5 minutes, and the phase change is distinct with a peak shift. Figure 4 ATR FTIR absorbance spectra of sodium formate as RH decreases at 298 K.

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16 Table 1: Sodium Formate (HCOONa) at 298 K Begin DRH End DRH Begin ERH End ERH 4.1.2 Temperature dependence of s o dium f ormate DRH and ERH Values The DRH and ERH at 288 K are determined in the same method as above. The salt begins to absorb water from the atmosphere at 55%RH until fully deliquesced at 57%RH (s ee Figure 5a). There is the s ame red shift in the asymmetric carbonyl stretch peaks, and the c arbonyl symmetric stretch peak split occurs as in the spectra at 298 K. The aqueous salt releases water into the atmosphere at 39%RH until fully crystalized at 35%RH at this temperature. Th e averaged DRH and Figure 5 a.) ATR FTIR absorbance spectra of sodium formate as RH increases at 288 K. b.) ATR FTIR spectra of sodium formate as RH decreases at 288 K. Spectra was stacked for clarity

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17 ERH ranges can be seen in Table 2, where these changes occur at lower RH values than at 298 K (Figure 5b). There is no current literature on the DRH or ERH of carboxylic acid salts at temperatures other than 298 K, so literature compari sons of temperature dependence cannot be made. Table 2: Sodium Formate (HCOONa) at 288 K Begin DRH End DRH Begin ERH End ERH The DRH is a thermodynamic process, and a temperature dependent expression of the DRH can be derived from the Cl ausius Clapeyron equation below, 42 Where is the solubility in moles of solute per moles of w ater, and is the enthalpy of solution of the salt Wexler and Seinfeld, 1991 assumed solubility and enthalpy of solution to be constant over the change in temperature, which produces e quation 2. 43 This equation can provide a rough estimation of the DRH temperature dependence when the temperature is changed in very small increments because the solubility and enthalpy of solution of many salts deviate with change s in temperature. The so lubility for sodium formate at 298 K is 0.271 mol HCOONa/mol H 2 O 44 The enthalpy of solution of sodium formate significantly differs at different temperatu res and, in some cases, is procedurally unattainable in literature. Chawla and Ahluwalia, 1975 deri s to be 0.74 KJ/mol at 298 K s = 0.10 KJ/mol at 308 C. 45 s with changing temperature cannot be used in this equation, and this equation is invalid for estimatin temperature dependence. The same ATR IR spectral analysis was used to assign the DRH and ERH of sodium formate at 278 K and 271 K, and the spectra are compiled in Figure 6 and Figure 7, respectively.

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18 Figure 6 a.) ATR FTIR absorbance spectra of sodium formate as RH increases at 271 K. b.) ATR FTIR spectra of sodium formate as RH decreases at 271 K. Figure 7 a.) ATR FTIR absorbance spectra of sod ium formate as RH increases at 278 K. b.) ATR FTIR spectra of sodium formate as RH decreases at 278 K. a .) b .) a .) b .)

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19 The DRH and ERH values can be seen in Table 3 and 4. The peak shift occurs over a lower range of relative humidities in the efflorescence process whe n the temperature is at the two lower temperatures (5 C and 2 C) and when the water is susceptible to freezing. From this decrease in water movement and activity, there is a relatively increased variability when efflorescence occurs. Table 3: Sodium Formate (HCOONa) at 278 K Begin DRH End DRH Begin ERH End ERH Table 4: Sodium Formate (HCOONa) at 271 K Begin DRH End DRH Begin ERH End ERH Gr aph 1 shows the DRH and ERH of s odium formate at temperatures pertinent to the troposphere. Both DRH and ERH values decrease with decreasing temperature. This provides evidence that sodium formate is more receptive to change to its aqueous phase at h igher altitudes where temperatures are lower. In the aqueous phase, the aerosols can grow to large droplets and eventually clouds as they act as cloud condensation nuclei radiation budget as the formation of clouds, espe cially in the lower troposphere reflect a portion of incoming solar radiation, especially higher energy radiation, due to clouds high albedo. 46

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20 Graph 1 DRH and ERH values of sodium formate over tropospheric relevant temperatures 4.2.1 DRH and ERH of s odium a cetate at 285 K The DRH and ERH values of sodium acetate, in Table 5, were recorded using the same methodology as sodium formate at 271 K, 278 K, 285 K, and 298 K Table 5: Sodium Acetate (CH 3 COONa) at 285 K Begin DRH End DRH Begin ERH End ERH A detailed analysis of hygroscopic properties of sodium acetate at 285 K is studied first. These values are determined from the red shift of the asymmetric carbonyl stretch peaks and carbonyl symmetric stretch peak in their corresponding spectra (Figure 8a, 8b). In Figure 8c, the asymmetric carbonyl stretch p eak is at as (COO )]= 1570.6 cm 1 at 0%RH and remains there until the RH is increase d to 36%RH. 1 lower than 0 10 20 30 40 50 60 70 265 270 275 280 285 290 295 300 Relative Humidity Temperature (K) DRH ERH

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21 This is due to t he electron donating methyl group attached to the wavenumber 47 Sodium acetate as ) peak shifts to a lower waven umber until the RH reaches 42%, as (COO )]= 1558.9 cm 1 This shift occurs as the water begins to dissolve the sodium acetate, and the wa ter in the atmosphere reaches a concentration high enough to undergo the following reaction. is the solubility of sodium acetate in moles of the salt per moles of water. The water interacts with the carbonyl functional group through hydrogen bonding and decreas es the strength of the C=O bond, thereby This movement of the carbonyl symmetric stretch can also be observed, shifting from s (COO )]= 1419.4 cm 1 s (COO )]= 1409.7 cm 1 once the water can infiltrate the sodium acetate aerosol (Figure 8d). The peaks at 2921 cm 1 and 2852 cm 1 are attributed to C H stretch in the alkyl group. These peaks are more prominent in spectra at lower temperatures, and remain unchanged as the re lative humidity is inc reased or decrease ( s ee f igure 12 spectra at 271 K ) because water has no effect on C H stretching. The broad OH stretch peak at approximately 3400 cm 1 begins to grow significantly after the DRH of the salt. Th is indicates the dr oplet growing larger with increasing humidity. From this growth, sodium acetate can act as cloud condensation nuclei and can be more quickly removed from the atmosphere due to its susceptibility to cloud droplet nucl eation scavenging 40, 41

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22 Figure 8 a.) ATR FTIR absorbance spectra of sodium acetate as RH increases at 285 K. b.) S pectra of carbonyl peaks of sodium formate at 285 K. c.) Carbonyl asymmetric stretch peak location as RH increases at 285 K. d.) Carbonyl symmetric stretch peak location as RH increases at 285 K. a.) b .) c .) d .) a .)

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23 As the RH is decreased after the salt has fully dissolved, the peaks return to their original position (Figure 9). The salt begins to dry as the hydrogen bonds between the water and the carboxylate group separate until the sodium acetate is fully solidif ied at 30%RH and 28%RH, respectively. The broad OH stretch peak at approximately 3400 cm 1 is nearly gone after the ERH values further signaling that the salt has effloresced all water. There is no current literature on the DRH and ERH values at this temperature for comparison. There are o nly literature values on the DRH and ERH at 298 K which will be discussed in the next section. Figure 9 a.) ATR FTIR spectra of sodium acetate as RH decreases at 285 K. b.) Carbonyl asymmetric stretch peak location as RH decreases at 285 K. c.) Carbonyl symmetric stretch peak location as RH decreases at 285 K. a.) b .) c .)

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24 4.2.2 Temperatur e dependence of s odium a cetate DRH and ERH Values The humidity was increased at 298 K to find the DRH of sodium acetate aerosols and decreased to find the ERH using an ATR FTIR spectrometer. The spectra from the instrument are stacked in Figure 10. The asymmetric and symmetric carbonyl stretch peak shift undergo the sam e red shift as seen in the previous section when the relative humidity reaches 49%RH and continues Figure 10 a.) ATR FTIR spectra of sodium acetate as RH increases at 298 K. b.) ATR FTIR spectra of sodium acetate as RH decreases at 298 K. b .) a .)

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25 to shift until the RH reaches 51%. The large peak at 1220 cm 1 is the C H bend mode in the alkyl group. 47 The RH is decreased from 100%RH, and the peaks beg in to blue shift to their original position at 46%RH until the salt is in its solid phase at 44%RH. The average DRH and ERH values at 298 K are seen in Table 6. Table 6: Sodium Acetate (CH 3 COONa) at 298 K Begin DRH End DRH Begin ERH End ERH Two literature values exist for the DRH and ERH of sodium acetate at 298 K. Peng and Chan, 2000 state that DRH for sodium acetate begins at 43.5%RH a nd ends at 45.2%RH using a electrodynamic balance method. Wu et al., 2011 records a DRH of 39 42%RH using H TDMA. 29 The two literature values differ from each other indicating that different methods can provide varying results when studying hygroscopic properties. The DRH values are lower than the ones obtained in this experiment. These authors may observe the initial weight and size increase of the aerosols as water begins to stic k to the surface. The water may not be dissolving the aerosols at this low RH. Both of the techniques by Peng and Chan, 2000 and Wu et al., 2011 use methods of measuring size and weight to observe the deliquescent property. It can be difficult to pinpoint when the aerosol has begun / finished absorbing water because an aerosol continue to change as this is happening Also, water can stick to the surface of the aerosol and increase the size and weight before the DRH occurs For this reason, these literature DRH values may be recorded before the transition begins. ATR analysis can better see these exact moments by rapid vibrational mode changes that only happen during the phase change Peng and Chan, 2000 have the only literature value of the ERH of sodium ac etate at 298 K which begins and ends at 40.6 % and 36.7%RH respectively. This ERH is slightly lower than the ERH value obtained in this experiment indicating that the aerosol s effloresced sooner in this study than in the experiment using EDB.

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26 The DRH an d ERH values for sodium acetate at 278 K can be seen in Table 7, and the stacked spectra are in Figure 11. The standard deviation is relatively higher in the ERH than the DRH because the water kinetic energy is reduced at this temperature making it more difficult to be released from the crystalizing salt. Table 7: Sodium Acetate (CH 3 COONa) at 278 K Begin DRH End DRH Begin ERH End ERH

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27 Table 8 shows the DRH and ERH values of CH 3 COONa below freezing at 271 K. The peaks in the carbonyl stretching area are noisier than spectra at different temperature due to superimposed rotational energy level absorptions of water vapor in the reaction chamber. 37 The stacked spectr a as the relative humidity is de creasing in Fig ure 12 show that the blue shift is less distinct at the lower temperature. This is due to the water s susceptibility to crystalize at this temperature which slow s down the water releasing process. Figure 11 a.) ATR FTIR absorbance spectra of sodium acetate as RH increases at 278 K. b.) ATR FTIR spectra of sodium acetate as RH decreases at 278 K. b .) a .)

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28 In Graph 2, the DRH and ERH values at different tempe Table 8: Sodium Acetate (CH 3 COONa) at 271 K Begin DRH End DRH Begin ERH End ERH Figure 12 a.) ATR FTIR spectra of sodium acetate as RH increases at 271 K. b.) ATR FTIR spectra of sodium acetate as RH decreases at 271 K. a .) b .)

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29 In Graph 2, the DRH and ERH values at different temperatures are plotted. The DRH and ERH decrease as the temperature decreases. This correlation is not necessarily linear as seen with methanesulfonate sodium aerosols in Zeng et al., 2014. Sodium acetate aerosols will change to their aqueous phase at relatively low humidity and at higher elevations where the temperature is lower. This provides evidence that this salt is more responsive in forming clouds which reflect incoming solar radiation at higher elevati ons. The following equation, described in Section 4.1.2, can thermodynamically predict the change in DRH with a change in temperature. 43 The and for sodium acetate is and respectively at 298 K. 43 This enthalpy of solution changes slightly over a small change in temperature, so this equation can be used for this chemical The theoretically predicted DRH at 285 K begin s at 46%RH and ends at Graph 2 DRH and ERH values of sodium acetate over tropospheric relevant temperatures.

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30 49% RH. The experimental DRH values at 285 K are about 10% lower (See Table 5). However, this does predict that the DRH will be lower at lower temperature s, which agrees with DRH data in Graph 2. 4.3.1 DRH and ERH of sodium s uccin ate at 298 K Sodium Succinate is a diprotic carboxylic acid salt and dissociates two sodium ions in the deliquescence phase, thus, requiring a higher humidity than sodium acetate or sodium formate to Figure 13 a.) ATR FTIR absorbance spectra of sodium succinate as RH increases at 298 K. b.) Spectra of carbonyl peaks of sodium succinate at 298 K. Spectra was stacked for clarity.

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31 fully deliquesce. The spectra of sodium succinate as the RH is incre ased can be seen in Figure 13a. In Figure 13b, the carbonyl peaks begin to shift after the RH is increased more than 78%RH. The asymmetric carbonyl stretch peak is at as (COO )]= 1551.5 cm 1 at 0%RH. This peak shifts to the right and splits into two pea ks at 1562.9 cm 1 and 1551.5 cm 1 (Figure 14a). The transition ends when the RH reaches 84%RH which represents the end of the DRH. The split peak is most likely due to different intermolecular hydrogen bonding of the two carbonyl functional groups with wa ter. 38 The symmetric stretch peak, s (COO )], at 0%RH is a split peak at 1434.2 cm 1 and 1402.1 cm 1 This may also be due to different ionic interactions with sodium slightly increasing or decreasing the Figure 14 a.) Carbonyl asymmetric stretch peak location as RH increases at 298 K. b.) Carbonyl symmetric stretch peak location as RH increases at 298 K. a.) AT R FTIR spect ra of sodiu m succi nate at 298 K. b.) AT R FTIR spect ra of sodiu m succi nate at 298 K.

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32 symmetric carbonyl stretch frequency. As the RH is increased past 78%RH, the peaks begin to merge into one peak a t 1397.5 cm 1 at 84% RH (Figure 14b). The intermolecular interactions are nearly equal in the symmetric carbonyl stretch and, thus, form one broad peak. In Figure 13a, the broad OH stretch peak at approximately 3400 cm 1 begins to grow rapidly at 78% RH indicating that the solid salt is absorbing water and continues to grow after a RH of 84%. Also, the H 2 O bending vibration peak at 1640 cm 1 appears after the DRH point which evidence s that water infiltrated the system This shows that this salt can act as a CCN after the DRH but at a much higher RH than the previous salts The peaks of the carbonyl functional group return to their original positions and shape as the RH is decreased to 0%. When the RH is decreased to 72%RH, the hydrogen bonds with water and the aqueous salt begin to dissociate and water is released into the atmosphere. The C O bonds shorten, and the carbonyl peaks blue shift until 67%RH. The broad OH stretch peak at approximately 3400 cm 1 and the H 2 O bending vibration peak at 1640 cm 1 nearly disappear after this efflorescence transition (Figure 15). Figure 15 ATR FTIR spectra of sodium succinate as RH decreases at 298 K.

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33 The average DRH and ERH values at 298 K can be seen in Table 9. Peng and Chan, 2000 experimental DRH and ERH values are 63.5 66.0%RH and 46.7 47.9%RH, respectively. Wu et al., 2011 did not observe a DRH point using the H TDMA method. The DRH and ERH valu es fluctuate significantly from the different methods. This fluctuation may be caused by sodium succinates highly temperature dependent solubility 48 and may have a strong effect on the long experimental durations used in Peng and Chan, 2000. 4.3 .2 Temperature dependence of s odium s uccinate DRH and ERH Values In Figure 16, the DRH and ERH transition is at a lower relative humidity at 288 K than at 298 K. The peaks transition the same as above, but the lower temperature causes this transition earlier when the relative humidity is increased The average DRH and E RH values can be seen in Table 10 Table 9 : Sod ium Succinate (C 4 H 4 Na 2 O 4 ) at 298 K Begin DRH End DRH Begin ERH End ERH

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34 Table 10 : Sodium Succinate (C 4 H 4 Na 2 O 4 ) at 288 K Begin DRH End DRH Begin ERH End ERH Figure 16 a.) ATR FTIR spectra of sodium succinate as RH increases at 288 K. b.) ATR FTIR spectra of sodium succinate as RH decreases at 288 K. a .) b .)

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35 Once again the DRH and ERH values decrease with a decrease in temperature (Figure 17 and Figure 18). The average values at 278 K and 271 K can be seen in Table 11 and Table 12 Table 11 : Sodium Succinate (C 4 H 4 Na 2 O 4 ) at 278 K Begin DRH End DRH Begin ERH End ERH Figure 17 a.) ATR FTIR spectra of sodium succinate as RH increases at 278 K. b.) ATR FTIR spectra of sodium succinate as RH decreases at 278 K. a .) b .)

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36 Figure 18 a. ) ATR FTIR spectra of sodium succinate as RH increases at 271 K. b.) ATR FTIR spectra of sodium succinate as RH decreases at 271 K. Table 12 : Sodium Succinate (C 4 H 4 Na 2 O 4 ) at 271 K Begin DRH End DRH Begin ERH End ERH a .) b .)

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37 In Graph 3, the DRH and ERH values at different temperatures are plotted. As the temperature decreases, the DRH and ERH decrease. This graph shows that this aerosol can be beneficial in reflecting incoming solar radiation at high elevations because it can transition to its aqueous phase and form light scattering water d roplets at relative low humidity Acquiring experimental data on the DRH of sodium succinate is more important than using theoretical Graph 3 DRH and ERH values of sodium succinate over tropospheric relevant temperatures. calculations considerably with a change in temperature, and the incorrect temperature correlation would be made using theoretical means. 48 4.4.1 DRH and ERH of sodium o xalate Sodium Oxalate ( ) is a diprotic carboxylic acid salt with a low solubility of 0.0380 mol /mol H 2 O 48 This solubility is lower than sodium succinates at ( 0.04413 mol C 4 H 4 Na 2 O 4 / mol H 2 O ) and the solubility of sodium formate is over 7 times greater. The same method was used to observe DRH and ERH values for this salt. When the RH was increased from 0%RH to 0 10 20 30 40 50 60 70 80 90 265 270 275 280 285 290 295 300 Relative Humidity (%) Temperature (K) DRH and ERH of C 4 H 4 Na 2 O 4 DRH ERH

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38 100%RH at 298 K, there was no peak shift or spectral change. The salt did not deliquesce at this temperature, and the s alt remained in the crystalline phase throughout the entire experiment. Since the salt never converted to the aqueous phase, the method for determining the ERH of sodium oxalate could not be test ed The RH must be reduced from 100% and the salt must be i n the aqueous phase in order to test for an ERH value using this technique The experiment was run at 30 C to see if the salt would deliquesce at a higher temperature. The spectra for this experiment are in Figure 19. Notice the spectra do not change wh en the RH is increased to 100%. The carbonyl asymmetric stretch peaks remain at 1635.5 cm 1 for the entire run, and, also, the split symmetric stretch peaks remain at 1338.1 cm 1 and 1321.2 cm 1 The broad OH stretch peak at approximately 3400 cm 1 does not grow like it does with the other salt s There is a small broad peak in this region starting at 60%RH indicating that some water was sticking to the surface of the salt but was not dissolving it. Figure 19 A TR FTIR absorbance spectra of sodium oxalate at 303 K.

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39 There was no DRH observed for any of the tropospheric related temperatures. Even at t he lowest tested temperature 271 K (See Figure 20), there is no spectral change. The lack of a DRH for sodium oxalate pro vides evidence that this salt cannot act as CCN and does not reflect as much incoming solar radiation as the other carboxylic acid salts studied. The salt remains in its crystalline phase in the troposphere and may remain airborne for longer periods of ti me than the other salts because it is not susceptible to cloud droplet nucleation scavenging 41 and will only be removed through gravitational settling and surface deposition. Figure 20 ATR FTIR spectra of sodium succinate at 2 71 K. The asymmetric carbonyl stretch peak for sodium oxalate is approximately 80 cm 1 higher than the asymmetric carbonyl stretch peak for sodium succinate in the crystalline phase (0%RH). This is due to the electron withdrawing carbonyl group adjacent to each other 47 two electron donating CH 2 groups.

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40 The data obtained for sodium oxalate matches with literature. Peng and Chan, 2000 do not ob serve a DRH point using EDB analysis; however, t hey found the ERH (72 75.2%RH) of the salt through the levitation of previousl y aqueous particles in the EDB. 21 Wu et al., 2011 did not observe a DRH value for sodium oxalate and reported no hygroscopic g row th below 90%RH using H TDMA. 29 Both articles findings in this study.

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41 CHAPTER V CONCLUSION The temperature dependent deliquescent and efflorescent relative humidity points of sodium formate, sodium acetate, sodium succinate, and sodium oxalate were determined using ATR FTIR spectroscopy. This work provided much insight into these carboxylic aci d salt interactio ns with water at lower tropospheric temperatures and their ability to become cloud condensation nuclei and reflect incoming solar radiation Though many of the DRH and ERH values at 298 K were higher than literatur e values, this differenc e is attributed to the different methodology used. E xcluding sodium oxalate, DRH and ERH values decrease with decreasing temperature. With this information, these common atmospheric aerosols will deliquesce at low humidity w hen they float to higher altitudes in the troposphere. At these higher altitudes, these salts can rapidly grow into water droplets and eventually clouds, and reflect a greater portion of incoming solar radiation. Therefore, the formation of carboxylic ac id salts from the acid displacement of atmospheric carboxylic acids with airborne cations may be beneficial in managing This study pointed out the significance of collecting experimental data in atmospheric studies. The theoretical thermodynamic calculation of the temperature dependent DRH values was inadequate in predicting DRH values when the temperature was changed. This is due to the salts solubility and enthalpy of solution being temperature dependent Also t here is no theoretical calculation for temperature dependent ERH values making experimentation the only way for ERH determination. Collecting experimental data for the hygroscopicity of carboxylic acid salt aerosols has shown to be beneficial for atmosph eric modeling. ATR FTIR spectroscopy revealed its advantages in studying the hygroscopicity of carboxylic acid salts in this experiment. This technique pinpoints when the DRH and ERH transition occur s by

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42 its ability to study 0.5 m 5 m into the salt crystal surface Further, equilibrium of the ATR FTIR flow system is nearly 10x quicker than the flow systems in studies using EDB analysis because the reaction chamber is much smaller. Reproducibility is another advantage of this method as many DRH and ERH values have a standard deviation of plus or minus one percent relative humidity. ATR FTIR spectroscopy has proven to be a highly effective hydroscopic tool and should be used to further study atmospheric interactions.

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46 44 CRC Handbook of Chemistry and Physics. 96th ed. CRC Press: Boca Raton, FL, 2015 2016. 45 Chawla, B.; Ahluwalia, J. Enthalpies and Heat Capacities of Dissolution of Some Sodium Carboxylates in Water and Hydrophobic Hydration. Solution Chem. 1975. 4, 383. 46 Graham, S. Clouds and Radiation. Nasa: Earth Observatory. March 1999. 47 Socrates, G. Infrared and Raman Characteristic Group Frequencies: Table and Charts. 3 rd ed. Wiley, Sussex. 2009. 48 Zul, M; Rozaini, M; Brimblecombe, P. The solubility measurments of sodium dicarboxylate salts; sodium oxalate, malonate, succinate, glutarate, and adipate in water from t=(279.15 to 358.15) k, Journal of Chemical Thermodynam ics, 2009, 41, 980 983. 49 InstruQuest. V Gen Dew Point/Relative Humidity Generator Operational &Software Manuals, 1 st ed.; InstruQuest Inc.; Coconut Creek. 2004. 50 Max, J. J.; Chapados, C. J. ison between Different Acids and Their Salts Phys. Chem. A 2004 108, 3324 3337