Investigation of Freezing Properties of Polar
Stratospheric Cloud Particles with Optical
Microscopy
by
Huey Pin Ng
B. S., Chemistry
University of California, Los Angeles, 1993
Submitted to the Department of Chemistry
in partial fulfillment of the requirements for the degree of
Master of Science in Chemistry
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 1997
© Massachusetts Institute of Technology 1997. All rights reserved.
A uthor......................
Department of Chemistry
May 27, 1997
Certified
by............
Mario J. Molina
Lee and Geraldine Martin Professor of Environmental Sciences
Thesis Supervisor
Accepted
by...................................
Dietmar Seyferth
Chairman, Departmental Committee on Graduate Studies
1i41997
Investigation of Freezing Properties of Polar
Stratospheric Cloud Particles with Optical
Microscopy
by
Huey Pin Ng
Submitted to the Department of Chemistry
on May 27, 1997 in partial fulfillment of the requirements
for the degree of Master of Science
in Chemistry
Abstract
The goal in this investigation is to gain an understanding of the freezing behavior of
polar stratospheric cloud particles. Polar stratospheric clouds have been shown to play a
cardinal role in Antarctic ozone loss, and hence it is important to understand the nature of
the particles and their formation mechanism. In this investigation, binary H20/HNO 3,
H20/H 2SO 4 and ternary H20/HNO 3/H2SO 4 aerosols particles were generated using two
different techniques and their freezing behavior was examined using an optical microscope
equipped with a cold stage. Preliminary studies were done on the binary H20/HNO3,
H20/H 2SO 4 systems. The observed melting points of the aerosols were compared with the
melting points of bulk samples. The agreement confirmed the effectiveness of the
equilibration procedure employed to establish the composition of the aerosols. Phase
transformation of stratospheric sulfate aerosols have been studied by various research
groups because of their role as nucleation sites for PSCs, so the freezing behavior of binary
sulfuric acid was studied here. Since the stratospheric aerosols approach the composition
of binary nitric acid as the aerosols cool and deliquesce, freezing behavior for binary nitric
acid aerosols was also examined. Compositions of the ternary aerosols were chosen from
points on the deliquescence curve, corresponding to the compositions of the stratospheric
aerosols as the temperature drops to the temperature at the Antarctic. In addition, a
systematic study of the freezing of ternary aerosols was also conducted by investigating a
series of compositions with varying amounts of sulfuric acid. This method, optical
microscopy equipped with the cold stage, overcomes the typical barriers of conventional
techniques by having the advantages of controlling the temperature cycles, visually
following the freezing process, long observation times, regulating the particle size. The
freezing behavior of binary H20/HNO3 aerosols showed that formation of both NAT and
ice are possible upon cooling. The freezing behavior of binary H20/H 2SO 4 indicated that
SAT does not form upon cooling. The freezing behavior of ternary aerosols indicated that
sulfuric acid does indeed suppress the formation of NAT. We conclude from this
investigation that ice does have to form first in freezing the polar stratospheric cloud
particles if there is only homogeneous nucleation.
Thesis Supervisor: Mario J. Molina
Title: Lee and Geraldine Martin Professor of Environmental Sciences
Biographical Note
I was born and raised in Singapore, a small island in the South-east Asia. After
graduating from Dunman High School and Temasek Junior College in Singapore, I went
on to University of California, Los Angeles for my undergraduate education in 1989. At
UCLA, I worked with Prof. Richard B. Kaner and my undergraduate research project was
on synthesis of polyaniline film and investigation of its ion migration property. I also
participated in the NSF summer research program in solid state chemistry during my junior
year. After my graduation in 1993, I came to MIT for my graduate education. I have
worked on two research projects while at MIT. The first project is in the area of solid state
chemistry, my project was to synthesize new oxide materials and study the structureproperty relationship of these materials. The second project is in the area of environmental
chemistry / physical chemistry, where my project is to study the freezing properties of polar
stratospheric cloud particles with optical microscopy. Awards received include: Teaching
award for outstanding laboratory teaching at MIT, American Chemical Society Polyed
Undergraduate Research Award, Bauer Award in Inorganic Chemistry, Merck Index
Award.
Acknowledgments
I would like to thank my advisor, Prof. Mario J. Molina for his patience and
guidance during my time in graduate school. I would also like to thank Dr. Luisa T.
Molina for all her guidance and help around the laboratory.
All the members in the group, past and present, have been very helpful and friendly
and I would like to thank them for their friendship and help. They are Thomas Beiderhase,
Keith Broekhuizen, Telma Castro, Alick Chang, Yu-Min Chen, David Dai, Lukas
Gutzwiller, Danna Leard, Thomas Koop, Zhidong Li, Jennifer Lipson, Yves Mantz, Scot
Martin, Carl Percival, Ulrich Poeschl, Dara Salcedo, Geoff Smith, Deborah Sykes, Heena
Zalawadia, Renyi Zhang. I would like to express my special thanks to Alick Chang, for
his patience in teaching me a lot of things in and around the laboratory and all the things I
wanted to know about the computer. Also, I would like to thank him for his patience while
working with me on this project and I am grateful to all the advice that he has given me.
Of course, thanks to my good friends here at MIT, Kheng Leong Cheah, Klaudyne
Hong, Yuan Long Hu, Wendy Luo, Jefri Mohdzaini, Hooi Ling Soh, for being such
caring friends and good listeners. Thanks for your friendship, thanks for comforting me in
times of trouble and you all have added much joy and fun during my stay here. I would
also like to thank all my badminton buddies, it has sure been fun knowing all of you.
Thanks for all the great times and good workout we had together every weekend.
5
Most importantly, I would like to take this opportunity to thank my dearest mom for
patiently waiting for me to graduate all these years, and for her unconditioned, unlimited
love and unflagging support in both good and bad times. Also, mom, thanks for your
encouragement, guidance and your belief in me. Also, I have to thank my dearest friend,
Vinh, for always standing by me in all walks of life, for being my inspiration and
motivating me all these years. This thesis would not be possible without you all. This
thesis is dedicated to them.
Table of Contents
2
Abstract
1
11
Introduction
Ozone...........................................................
11
.......................................
13
1.1
Stratospheric
1.2
O zone D epletion
...................
1.2.1 Chlorofluorocarbons
1.3
................................................
1.2.2 A ntarctic O zone H ole ..................................................
14
1.2.3 Stratospheric A erosols ................................................
16
Polar Stratospheric Cloud & Sulfate Aerosol............................
18
1.3.1 Background Stratospheric Sulfate Aerosols....................
20
1.3.2 Overview of Formation Mechanisms Proposed..................
20
1.3.3 Aims of this work.................................
2
13
..........
Experimental Design
21
23
2.1
General
Description...........................................
23
2.2
Aerosol
Generation........................................................
23
2.2.1 Co-condensation..................................................
24
2.2.2 N ebulizer ................................................................
26
2.3
Equilibration..............................................................
28
2.4
Optical
Microscopy.........................................................
29
2.4.1 Microscope.........................................................
29
Stage..........................................................
32
2.4.2 Cold
3
4
5
2.5
Differential Scanning Calorimetry.........................................
39
2.6
Calculations................................................
40
2.6.1 Estimation of Equilibration time......................................
40
2.6.2 Volume change corresponding to concentration change........
42
Binary
3.1
Introduction...................................
3.2
Results..................................
........................
44
44
62
Binary HzSO, System
4.1
Introduction..........................................
........................
62
4.2
R esults ................................................................ ..........
62
Ternary HNO 3/ H2SO4/ H2 0 system
73
5.1
Introduction..................................................
73
5.2
Results for compositions along the deliquescence curve................
73
5.3
Results for ternary compositions with fixed amounts of sulfuric acid...
78
5.3.1 Ternary compositions with 0.5 % sulfuric acid..................
78
5.3.2 Ternary compositions with 1 % sulfuric acid....................
83
5.3.3 Ternary compositions with 5 % sulfuric acid....................
87
D iscussion..................................................
87
5.4
6
44
HNO 3 System
Conclusion
References
93
List of Figures
2-1
Schematic Set-up for the aerosol generation via Co-condensation............
25
2-2
Schematic Set-up for the aerosol generation via Nebulizer.....................
27
2-3
Schematic Set-up for the Equilibration in a closed beaker.......................
28
2-4
Schematic Set-up for Microscopy...........................
2-5
Calibration Curve for the BCS Cold Stage.................................
2-6
Heterogeneous Freezing versus Homogeneous Freezing ....................... 36
3-1
Freezing of NAT (53.85 % Nitric Acid)....................................
3-2
M elting
3-3
Freezing of 35 % Nitric Acid..................................
...........
53
3-4
Freezing of 10 % Nitric Acid..................................
...........
54
3-5
M elting of 10 % Nitric Acid..................................
............
55
3-6
Freezing of 20 % Nitric Acid..................................
...........
56
3-7
M elting of 20 % Nitric Acid..................................
............
57
3-8
Supercooling Curve (Freezing & Melting Temperature) for Binary Nitric Acid.. 58
3-9
Degree of supercooling (AT) versus the Freezing Temperatures for the Ice-regime of
............
of NAT...................................................................
the Binary Nitric Acid.....................................
34
. 35
51
52
................ 59
3-10 Freezing Temperature versus Melting Temperature for Binary Nitric Acid....... 60
3-11 Degree of supercooling (AT) versus the Freezing Temperatures for the NAT-regime
of the Binary Nitric Acid....................................
............. 61
...........
67
4-1
Freezing of 10 % Sulfuric Acid..................................
4-2
Melting of 10 % Sulfuric Acid.................................
4-3
Freezing and Melting of 1 % Sulfuric Acid...................................
4-4
Supercooling Curve(Freezing & Melting Temperature) for Binary Sulfuric Acid 70
4-5
Overlay of Freezing and Melting data with results from other research groups... 71
4-6
Degree of supercooling (AT) versus the Freezing Temperatures for the Ice-Regime
of the Binary Sulfuric Acid...................................
........... 68
69
............. 72
................. 76
5-1
Ternary Phase Diagram..................................
5-2
Crystallization on warming & Melting for 40 % Nitric, 1 % Sulfuric......... 77
5-3
Freezing of 10 % Nitric Acid, 0.5 % Sulfuric Acid Ternary..................... 79
5-4
Melting of 10 % Nitric Acid, 0.5 % Sulfuric Acid Ternary.....................
80
5-5
Freezing of 15 % Nitric Acid, 0.5 % Sulfuric Acid Ternary....................
81
5-6
Melting of 15 % Nitric Acid, 0.5 % Sulfuric Acid Ternary......................
82
5-7
Freezing of 20 % Nitric Acid, 0.5 % Sulfuric Acid Ternary..................... 84
5-8
Melting of 20 % Nitric Acid, 0.5 % Sulfuric Acid Ternary......................
85
5-9
Freezing of 20 % Nitric Acid, 1 % Sulfuric Acid Ternary.......................
86
5-10 Freezing and Melting Temperatures for Ternary acid aerosols..................
90
5-11 DSC cooling thermogram of 20 % Nitric Acid, 5 % Sulfuric Acid Ternary....... 92
5-12 DSC heating thermogram of 20 % Nitric Acid, 5 % Sulfuric Acid Ternary....... 92
5-13 DSC heating thermogram of 20 % Nitric Acid, 20 % Sulfuric Acid Ternary...... 93
5-14 DSC heating thermogram of 20 % Nitric Acid, 0.5 % Sulfuric Acid Ternary..... 93
List of Tables
2-1
Sample
Preparation................................................
25
2-2
Cooling Cycles for the Binary Nitric acid....................................
37
2-3
Cooling Cycles for the Binary Sulfuric Acid..................................
38
2-4
Cooling Cycles for the Ternary Acids......................................
38
3-1
Freezing and Melting points for Nitric Acid aerosols from Co-condensation..... 49
3-2
Freezing and Melting points for Nitric Acid aerosols from Nebulizer......... 50
4-1
Freezing and Melting points for Sulfuric Acid aerosols.........................
66
5-1
Freezing and Melting points for Ternary Acid aerosols.........................
75
5-2
Comparison of Melting points for Ternary aerosols with bulk Melting points
measured by Differential Scanning Calorimetry.............................
91
Chapter 1
Introduction
1.1 Stratospheric Ozone
Even though ozone is an undesirable product of photochemical reactions in the
troposphere, stratospheric ozone is indispensable to all living organisms on earth.
Stratospheric ozone constitutes about 90 % of the Earth's ozone, ' and it is this layer of
ozone that provides the shield against the harmful ultraviolet radiation with wavelengths >
230nm. Ultraviolet radiation with wavelengths shorter than 290 nm is capable of damaging
macromolecules such as nucleic acids and proteins, which are essential components of
living cells and the UV-B (between 290 to 320 nm) is also biologically active and can
cause skin cancer on prolonged exposure.2 Therefore, an increase in the level of ultraviolet
radiation brought by a reduction of stratospheric ozone could increase the rate of skin
cancer and cataracts in human, and it could also damage crops and phytoplankton.
Since the discovery of ozone by Schonbein, pioneering work in ozone measurements
has been done by Hartley (1881) and Dobson (1920). Beginning in 1920s, Dobson
adopted the use of a UV-spectrometer for remote measurements of ozone concentrations.
In his experiments, he found that the ozone concentration at the temperate zone is higher
than the ozone concentration at the tropics. Chapman in 1930 provided the first attempt 3to
explain the presence of ozone in the stratosphere by the following mechanism:
+
02
UV
--->
O + 0 2 + M --->
+
03
+
O
0
(1.1)
+ 0
03 + M
(1.2)
+
UV
--- >
O
02
(1.3)
03
--->
0 2 + 02
(1.4)
The number of ground stations increased substantially and was extended to
Antarctica in 1957-1958. 4 Modem era of studies began when they the ozone level was
found to be lower that the expected level. Bates and Nicolet in 1950 first proposed the
catalytic loss processes to explain the discrepancy between observed and calculated ozone
levels.5 A general expression for the catalytic loss cycle is shown as follow:
Net:
X
+
03
--->
XO
+
02
(1.5)
XO
+
O
--->
X
+
02
(1.6)
O +
03
--->
02
+
02
(1.7)
This catalytic scheme of odd-oxygen removal is a much more efficient route than
reaction (1.4). It is so effective that hundreds of thousands of ozone molecules can be
destroyed with only one single atom or molecule of the reactive species. Some of the most
important reactive species include H, OH, NO, Cl and Br. OH in the stratosphere is
formed by the reaction of excited oxygen atoms with water vapor or methane. The source
of NO is primarily from N20, which is released from the soil due to denitrification or
incomplete microbiological nitrification. The source of Cl is primarily anthropogenic.
Reactive species can also interact with each other to produce more cycles. However, since
the cycles require a high concentration of oxygen atom, they are most effective in the upper
stratosphere, where atomic oxygen is abundant.
Satellite measurements were made possible in 1970, and this coupled with the
extensive ground-based measurements enabled accurate measurements of stratospheric
ozone to evaluate the effects of anthropogenic compounds.
Ozone Depletion
1.2
Over the past 2 decades, the major concern has been about the significant ozone loss
that can be brought about by the introduction of chain carriers from human activities.
In the early 1970s, attention was focused on the possible impacts of Supersonic
Transport Aircraft (SST).6, 7, 8 SSTs would fly at an altitude of about 20 km, and would
thus inject a large amount of combustion products such as NO 2, SO 2, CO 2, CO and H20
into the lower stratosphere where there is very little vertical mixing. Since the lifetimes of
these potential ozone-depleting species in the stratosphere are quite long, ranging from 2-4
years, the impacts of the SST emissions caught the attention of scientists. The fleet of SST
did not develop due to complications in technical and economical issues, but investigations
on these aircraft emissions led scientists to look more closely at other possible
anthropogenic sources that could lead to devastating stratospheric ozone loss.
1.2.1
Chlorofluorocarbons
In the middle of 1970, attention switched from Supersonic Transport Aircraft to
chlorofluorocarbon aerosol spray cans. Chlorofluorocarbons (CFCs) were developed by
General Motors Research Laboratories in 1930 as an inert, non-toxic, non-flammable
substitute for sulfur dioxide and ammonia as refrigerant. Their chemical inertness made
them ideal candidates for many different applications, and were very soon used as aerosol
propellants, as blowing agents for plastic foam production and as solvents and cleaning
agents in the electronics industries in addition to their original use as refrigerant.
In 1973, James Lovelock and his coworkers first reported that the quantities of
chlorofluorocarbons found in the troposphere were almost equal to the total amount of
chlorofluorocarbons ever manufactured. 9 This corroborated the tropospheric inertness of
CFCs, indeed, lifetimes of CFCs can be up to hundreds of years. The only escape channel
for the tropospheric CFCs is to diffuse to the stratosphere where they can breakdown to
give chlorine radicals via ultraviolet photolysis. For example, the photolysis of CFC-12 is
shown here:
CF 2C12 +
UV
--->
CF2C1
(1.8)
C1
+
The first warning of the potential of significant stratospheric ozone depletion came
more than two decades ago.'o The ozone depleting potential of CFCs was first proposed
by Molina and Rowland. 10 It has been found that the inert chlorofluorocarbons that make
their way up to the stratosphere release chlorine radicals that in turn destroy the ozone
shield. For example:'o
CC13F
+
UV
--->
Cl
Cl
+
03
--->
C10 +
CIO
+
CC12F
+
(1.9)
and
Net: 0 3
1.2.2
+
O
O
--->
--->
Cl
02
02
+
+
02
02
(1.10)
(1.11)
(1.12)
Antarctic Ozone Hole
The first clear evidence of the stratospheric ozone loss came from Halley Bay,
Antarctica in 1985." Farman et al. from the British Antarctic Survey had been measuring
the ozone level at Halley Bay at 76oS using their ground-based Dobson spectrometer for
many years. British measurements of the ozone level went back to 1957, and so a long
term trend was well established. They discovered a drop in the springtime ozone
concentration from 1977 to 1984. The ozone level was found to be reduced from 310
Dobson Units in 1970 to 160 Dobson Units by 1980.12 Ozonesondes (balloons with
electrochemical sensors for ozone) further revealed that most of the ozone loss has
happened in the lower stratosphere, ranging from 14 to 24 km.
The Antarctic ozone loss
anomaly provided the first evidence for the connection of ozone destruction with the
anthropogenic release of chlorofluorocarbons.
There are two prominent features of the polar stratosphere: one is that the air in the
darkness of winter continues to radiate to space in the infrared region and thereby is capable
of getting very cold (below -80'C in Antarctic) ; and the second feature of polar
meteorology is the formation of polar vortex as air cools and descends during the winter.
As the autumnal equinox ends, the polar region goes into a period of darkness as solar
ultraviolet heating ends. As a result, the cooling of the stratosphere causes a pressure
gradient to develop between the pole and the mid-latitudes. This pressure gradient, in
conjunction with the Earth's rotation, creates westerly winds which circle around the pole
at speeds greater than 100 m s-' and thereby creating the "polar vortex". This vortex can
effectively segregate the polar stratosphere throughout the winter.
By taking into consideration the cold temperatures and isolation that make possible
by the polar vortex, Molina and his coworkers proposed the following mechanism to
elucidate the formation of the "ozone hole" at the poles: 13
ClO + C10 + M --->
(C10) 2 + UV
--->
Cl
+ M
--->
Cl
C1OO
M
(1.13)
C1OO
(1.14)
+ 02 + M
(1.15)
(C10) 2 +
+
2 ( Cl
Net :
03
+
+
203
UV
--- >
CIO
--- >
0
+
2
)
(1.16)
(1.17)
302
This reaction mechanism is only important at the poles, since the formation of (C10)2
(the CIO dimer) requires low temperature and high CIO concentration.
In addition, McElroy and coworkers suggested the coupling of chlorine and bromine
species in an effort to explain the "ozone hole" phenomenon. 14 The reactions they
proposed are as follows:
CIO
+
BrO
--->
Cl + Br +
02
(1.18)
Cl
+
03
--->
CO1
+
02
(1.19)
Br
+
03
--->
BrO +
02
(1.20)
--->
302
Net:
203
(1.21)
However, the extent of the loss could not be explained with the atmospheric models
at that time, which were based only on homogeneous gas phase reactions. 15 Three US
expeditions were conducted in 1986 and 1987. The expeditions, together with other
experiments provided a better understanding of the Antarctic stratosphere 16'," and an
explanation for the ozone hole phenomenon.
1.2.3
Stratospheric Aerosols
Solomon et al. 15 and McElroy et al.
18
were some of the first to unravel the mystery
of the massive ozone loss at the lower polar stratosphere. They suggested that
heterogeneous chemistry occurring on the surfaces of the Polar Stratospheric Clouds
(PSCs) and other stratospheric aerosols
19,20,21
would help to account for the abnormal and
seasonal loss of ozone. The reactive surfaces of these stratospheric aerosol particles
promote the conversion of unreactive chlorine reservoir species (HCI and CIONO 2) into
photolabile chlorine species (HOCl and Cl 2), which can then be readily converted to
photochemically active chlorine species (Cl and CIO). Secondly, the surfaces of the
clouds enable the conversion of NO x to HNO 3. Removal of NO x enhances the ozone
destruction since NO x can terminate the chlorine activation chain by reacting with the
reactive CO1 to give the inert C1ONO 2. Moreover, the HNO 3 then forms NAT (nitric acid
trihydrate), and as NAT grows in size, it falls under gravitational force and brings an
irreversible removal of nitrogen oxides. (denitrification)
These heterogeneous reactions, together with the CIO-CIO and C1O-BrO dimer
reactions proposed by Molina and Molina' 3 and McElroy et al. 14 respectively, provided a
satisfactory and reasonable explanation of the Antarctic ozone hole phenomenon."8
The following are some of the most important heterogeneous reactions:
C12
C1ONO 2 + H20 --->
HOCl + HNO 3
+
(1.23)
(1.24)
+ H2 0
HOCl
+ HC1 --->
C12
N20,
+ HCI --->
CINO 2 + HNO 3
N205
+ H20
--->
(1.22)
HNO 3
C1ONO 2 + HC1 --- >
(1.25)
(1.26)
2HNO 3
As can be seen, reactions (1.22) - (1.25) all convert inert chlorine into photolabile
chlorine species, which upon photolysis release active chlorine species that damage the
stratospheric ozone. Also, reactions (1.22), (1.23), (1.25), (1.26) all bring about nitric
acid formation. Reactions (1.22) and (1.24)
22
and reactions (1.25) and (1.26)
23
have been
shown to be effective on ice surfaces, validating the importance of heterogeneous reactions
on ozone depletion. The efficacy of NAT surfaces for reaction (1.22) has also been
demonstrated in the laboratory.
24,
25
Liquid aerosols have also been shown to serve as an
effective heterogeneous medium. The rates of the heterogeneous reactions are a strong
function of the chemical composition and phase of the aerosols.
26,27,28,29
Since the reaction rates as well as the sedimentation rates are controlled by the
formation mechanism and the nature of the stratospheric particles, it is necessary to have a
clearer understanding of the crystallization of these particles. Knowledge of their
composition can also help predict the locations and conditions which lead to polar
stratospheric cloud formation.
1.3
Polar Stratospheric Clouds & Sulfate Aerosol
With the unique feature of the polar vortex, the westerly circulation of the vortex
creates a region of very cold air which allows the formation of polar stratospheric clouds.
Polar stratospheric clouds were known more than a century ago and were called
"mother-of-pearl" clouds at that time for their bright colors. 30'
31
In the Antarctic, polar
stratospheric clouds can be observed throughout the winter and in the spring at altitudes
ranging from 12-22 km.32 It is now understood that these polar stratospheric clouds
provide the site for heterogeneous reactions that explain the ozone depletion phenomenon at
the Antarctic.1' 518
Satellite measurements provided the first evidence that polar stratospheric clouds are
prevalent during the winter season over the polar stratosphere.32 Information on the shape,
size and nature of the particles was obtained from Lidar observations. 33 Lidar
measurement indicated that there are 2 main types of polar stratospheric clouds. Type I
PSCs were seen at temperatures around 190-195 K, a few degrees warmer than the ice
frost point. The composition of type I PSCs was independently suggested by Toon et al.
and Crutzen and Arnold to be nitric acid trihydrate.34'3 5 Further Lidar experiments
showed that there are 2 subclasses of type I PSCs. Type Ia clouds have backscatter cross
sections smaller than air (backscatter ratio 0.2-0.5) but have large depolarization ratios
(0.3-0.5). Type Ib clouds have backscatter cross sections several times larger than air
(backscatter ratio 2-7) and have small depolarization ratios (0.005-0.025). The aerosol
backscatter ratio depends linearly on the size of the particles and can thus provide
information on the size of the particles, whereas the depolarization ratio is dependent on the
shape as well as the size of the particles, and is particularly sensitive to deviations from
spherical shape, for example, a small particle can have large depolarization ratio if it is
sufficiently non-spherical. Calculations from the satellite data concluded that type Ia
particles are non spherical and have sizes about 1 gm, and type Ib particles are spherical or
close to spherical and have sizes close to 0.5 gm.36 Type I PSCs were found to be the
most prevalent type of PSCs in the stratosphere. Type II PSC clouds form when
temperature drops below the ice frost point, 188 K, and Lidar backscatter cross sections
identified that they are water-ice crystals. 3 7 Type II PSC clouds were found to be much
larger than PSC I from their backscatter ratios (>10) and depolarization ratios (>0.1).33
There is also a third type of PSC clouds that are much less common. Type III PSC clouds
consist of water-ice crystals, but form only in the region where there is large wave-induced
cooling. Because of the rapid cooling rate, PSC III appear as optically thick clouds with
numerous micron-sized particles.
Since polar stratospheric clouds play a pivotal role in Antarctic ozone loss, it is
crucial to have a comprehensive understanding of these clouds. Significant progress has
been made in understanding its thermodynamics and the heterogeneous chemistry
occurring on these particles. However, the formation mechanism, the composition and the
state of type Ia PSC remains unclear.
1.3.1
Background Stratospheric Sulfate Aerosols
In addition to the polar stratospheric clouds, the stratosphere also contains aerosols
consisting of supercooled sulfuric acid droplets (Junge Layer), which can be found
globally at 15-25 km.38 Junge was the first to attempt to study the physical state and
chemical composition of these sulfate aerosols. Carbonyl sulfide is the main precursor for
the sulfur dioxide and hence, for the sulfate aerosols. These stratospheric sulfate aerosols
(SSA) can promote heterogeneous reactions and they also serve as the substrate for the
nucleation of polar stratospheric clouds.
Stratospheric sulfate aerosols are omnipresent in the stratosphere, and therefore could
be responsible for the ozone depletion in the non-polar region, where the temperature
seldom drops below 210 K. 19,20 ,21 As the SSAs move from mid-latitudes to the polar
region and the temperature drops, the sulfate aerosols continue to take up water and nitric
acid and thus the concentration of the sulfuric acid drops from 60-80 weight % to around 1
% at 185 K, to give ternary or quaternary acid solutions.
Major volcanic eruptions can significantly perturb the normal background loading of
sulfate aerosols, thereby aggravating the ozone loss. Recent AASE II investigation
indicated a major decrease in CIONO 2 and HCl levels after the Mt. Pinatubo eruption.39
Hence, it is also important to study the freezing behavior of the SSAs to further understand
the nature of the particles at stratospheric conditions.
1.3.2
Overview of Formation Mechanisms Proposed
There have been several hypothesis proposed for the formation of type I PSC. The
earliest mechanism that was suggested was a three-stage concept. In the three-stage PSC
formation mechanism, the background stratospheric aerosols are assumed to be in a
frozen state and serving as the nuclei for the condensation of nitric acid and water vapor
during the formation of type Ia PSC.40 The second mechanism proposed that the SSAs are
capable of supercooling to very low temperature and thus remain in the liquid state while
absorbing nitric acid and water as they cools, and eventually NAT freezes out from the
ternary solution. 41 In the third mechanism proposed, the ternary liquid is believed to
remain as a liquid(type Ib PSC) until its temperature drops below the ice frost point, where
the water-ice freezes out from the solution, because the sulfuric acid is believed to suppress
the formation of NAT.42 There is a fourth mechanism suggesting that the ternary liquid
transforms into a glassy state, which in turn crystallizes upon warming. 43 To reconcile
between laboratory results and field measurements, a fifth mechanism proposed a NAT
formation mechanism above the ice frost point due to a rapid temperature fluctuation (lee
wave), which can result in the stratospheric aerosols to depart from their equilibrium
concentration and have a concentration very close to that of binary nitric acid and thereby
freezing to form NAT. 44
1.3.3
Aim of this work
Most of the laboratory studies have been done either with a bulk solution sample or
with aerosols suspended in a cooled chamber. The drawback of using a bulk solution is
that the volume is much larger than that of the aerosol particles found in the stratosphere,
and hence will not provide an accurate simulation of the real freezing process in the
stratosphere. With a much larger volume characteristic of bulk solutions or films, there is a
much higher probability for heterogeneous nucleation, and the results need not be reflective
of stratospheric conditions. Also, since homogeneous nucleation is proportional to the
volume, the use of bulk solutions will also not provide an accurate understanding of
homogeneous crystallization of stratospheric aerosols. 45 As for aerosol cooling chambers,
their drawback is that that long observation times are difficult because of the sedimentation
of the suspended aerosol. Moreover, control of the cooling rate is difficult for the chamber
and again the rapid cooling will not be a good replication of stratospheric cooling. In
addition, it is also difficult to verify the composition of the suspended aerosols in the
chamber.
Hence, the goal in this investigation is to use a new approach that can circumvent to
some extent the afore-mentioned problems to study the freezing behavior of aerosol
particles that are similar to those found in the stratosphere. The idea behind this experiment
is that in the dispersed state, most of the aerosols should be free of heterogeneous nuclei,
therefore the majority of them will freeze homogeneously as temperature is lowered, and
enable us to understand the homogeneous crystallization of stratospheric aerosols.
Understanding of the freezing properties of these aerosol particles should help to elucidate
the freezing behavior of the PSCs.
In this investigation, aerosol particles of sizes comparable to those in the stratosphere
were generated via co-condensation, or via a nebulizer. The aerosols were deposited on
quartz crucibles and their freezing behavior was visually examined using an optical
microscope equipped with a cold stage. The cold stage, together with a temperature
controller, provides an excellent control of temperature and allows for temperature cycling
and for long observation times. The freezing behavior of binary H 2 0/HNO3,H 2 0/H 2 SO 4
and ternary H20/HNO 3/H2SO 4 aerosols was studied. Visual examination of the freezing
process provided valuable information on the morphology and phase change of the
particles, and also made it possible to distinguish between homogeneous freezing and
heterogeneous freezing.
Chapter 2
Experimental Design
2.1 General Description
In this experiment, we generated aerosols of sizes similar to those found in the
stratosphere. Two methods were used here: the co-condensation method and the nebulizer
method, for comparison of the consistency of our experiments. The average size of the
aerosol particles can be varied by controlling the aerosol generation process. The aerosols
were deposited on quartz crucibles that had been silanized with Prosil 28 surface treating
agent. The organo-silane coated surface enabled spherical, more 3-dimensional droplets to
be deposited and made it possible to observe for homogeneous freezing. Silanization
changed the surface chemistry of the crucibles by replacing the hydroxyl group with the
methyl group, thus creating a hydrophobic surface that can avoid heterogeneous freezing of
the aerosols. After generating the aerosols, an equilibration step was performed to make
sure that the particles were homogeneous in composition and also achieved the desired
composition. After equilibrating for an hour or longer, the particles were transferred to the
cooling stage of the microscope. The freezing process was monitored visually as well as
with a video camera that was attached to the microscope. The images were recorded on a
VCR, and could in turn be processed and analyzed using various imaging software.
2.2 Aerosol Generation
Acid solution samples were prepared by diluting concentrated nitric acid
(Mallinckroft , 70%) and concentrated sulfuric acid (Mallinckroft, 96%). The sample
preparation is shown in table 2-1. An initial check on the concentration of the acid solution
prepared was done by titrating the solution with standard sodium hydroxide solution.
Binary nitric acid solutions of 10-53.85 weight %, binary sulfuric acid solutions of 157.65 weight %, and ternary acid solutions of 10, 15, 17, 20 weight % nitric acid ( with
0.5 weight % sulfuric acid); 10, 20, 40 weight % nitric acid (with 1 weight % sulfuric
acid ); 25 weight % nitric acid (with 2 weight % sulfuric acid ); 5, 10, 20 weight % nitric
acid (with 5 weight % sulfuric acid ); 20 weight % nitric acid and 20 weight % sulfuric
acid; 1 weight % nitric acid and 50 weight % sulfuric acid were prepared.
2.2.1
Co-condensation
A schematic diagram for the setup is shown in figure 2-1. In this method, a drop of
acid solution of the desired composition was dipped from a capillary tube onto a concave
glass slide, the side of the concavity was greased with halocarbon grease for a good seal
and an inverted prosil coated quartz crucible that was used for the Linkam cold stage was
placed on top of the glass slide. The ensemble was then placed on a hotplate with the
concave glass slide facing the bottom and heated at around 150-170 0 C for a few minutes
(higher temperature may be necessary for solutions containing sulfuric acid). Due to the
temperature gradient between the glass slide and the crucible, the acid aerosols were then
collected on the prosil coated silica crucible by condensation. A few minutes were allowed
for the slide to cool down. A cover slide was greased on the side. The crucible was
quickly detached from the concave glass slide, small drops of the same acid mixtures with
diameter of approximately 0.1 mm were then introduced into the crucible as the
equilibrating droplets before it was sealed with the thin cover slide. The size of the
aerosols was fairly homogeneous, with an average radius of 10 microns.
[ Quartz Crucible
Glass Slide
I
wI Hot-Plate
Figure 2-1: Schematic set-up for aerosol generation via co-condensation.
Weight %
HNO,
10
20
25
30
32
35
38.13
40
53.85
0
0
0
0
0
0
10
15
17
20
10
20
40
5
10
20
20
1
Weight %
H2SO 4
0
0
0
0
0
0
0
0
0
1
5
10
20
30
57.65
0.5
0.5
0.5
0.5
1
1
1
5
5
5
20
50
Volume of HNO 3
(from conc. HNO,)
10.23
20.47
25.58
30.70
32.75
35.82
39.02
40.93
55.11
0
0
0
0
0
0
10.23
15.35
17.40
20.47
10.23
20.47
40.93
5.12
10.23
20.47
20.47
1.02
Volume of H2 SO 4
(from conc. H2SO 4)
0
0
0
0
0
0
0
0
0
0.58
2.89
5.79
11.57
17.36
33.36
0.29
0.29
0.29
0.29
0.58
0.58
0.58
2.89
2.89
2.89
11.57
28.94
Table 2-1 : Sample preparation.
Volume of H20
85.93
71.56
64.38
57.19
54.32
50.01
45.51
42.82
22.92
99.26
95.08
89.85
79.40
68.96
40.07
85.41
78.22
75.35
71.04
84.89
70.52
41.78
87.89
80.71
66.34
50.67
46.62
2.2.2
Nebulizer
The set-up for the apparatus is shown in Figure 2-2. In this method, the acid
solution was drawn through a 1/16 " teflon tube from an Erlenmeyer flask to the glass
capillary of the nebulizer. The primary acid aerosols were first generated by means of a
concentric glass nebulizer (Meinhard TR-30-Al) with a dry nitrogen gas pressure of 30
psi. The aerosol generation rate is about 1.1 ml/min. The aerosols were then passed
through a Scott-type spray chamber (Meinhard SPT-S02) to eliminate the over-sized
droplets (radius > 8gm) before being re-directed to a long 1/4" teflon tubing. Prior to
collection, the aerosols were directed to the exhaust for a few minutes to remove any
unwanted residue along the tube. A Prosil-coated silica crucible, with a teflon cover on the
top, was placed at a distance of less than 5mm from the open end of the teflon tubing, was
used to collect the aerosols. The time of collection was around 1.5 - 2 minutes and it was
simply controlled by a electro magnetic 3-way valve. After generating the aerosols, the
chamber is then flushed with deionized water aerosols by replacing the acid solution with a
beaker of deionized water. The pressure of the nitrogen gas, the distance where the
crucible is place and the duration of the collection can be varied to control the number
density of the aerosol particles and the size of the particles. Small drops of the same acid
mixtures with diameter of approximately 0.1 mm were then introduced into the crucible
before it was sealed with a thin glass cover slide. Halocarbon grease was applied at the rim
of the quartz crucible to ensure an air tight seal. The sealed crucible was then cooled down
to 100 C and stayed there for at least 2 hours for the equilibration of the small drops and the
aerosols. The aerosols on the crucible were found to have a rather narrow size distribution
with an average radius of approximately 5 microns.
Pressure Gauge
XT-1_1-I*'
-
Pressure
Valve
Teflon
Flow
Meter
vluvlvlv
Figure 2-2: Schematic set-up for the aerosol generation via nebulizer.
2.3 Equilibration
In the course of the aerosol generation process, the aerosol particles always lost part
of their water content since water has a much higher vapor pressure than the acids. For
example, for the nebulizer, from the point where the aerosols were generated to the point
where the aerosols were collected, since the surrounding air is usually quite dry (low
humidity), the aerosol particles would lose part of their moisture while traveling along the
teflon tube. Hence it was essential to equilibrate the aerosols to make sure that all the
particles had the desired composition.
A few methods have been attempted to equilibrate the aerosols. As shown in figure
2-3, the first method used was to sit the crucibles with the aerosol particles already
deposited in a large beaker (500 ml) , where a smaller beaker (about 25 ml) with the acid
solution of desired composition was placed. The large beaker was then closed with a latex
glove or parafilm to secure a close system for equilibration. After equilibrating for a few
hours, without opening the beaker and with careful maneuvering of the glove or the
parafilm, the crucible was covered with a cover lid that has been previously placed in the
beaker.
I
x
~czzz~
Quartz Crucible
with pre-deposited
aerosols
=czz
Figure 2-3: Schematic set-up for the equilibration in a closed beaker.
Another method is to use a micro pipette to dip a few drops of the original acid
solution onto the Prosil-coated silica crucible after the deposition of the aerosols and right
before closing the crucible with the cover slide. The large drops acted as a reservoir that
controlled the equilibrium vapor pressure in the crucible and eventually bring the small
drops back to its initial concentration. The aerosols were allowed to equilibrate with the
large drops for at least one hour before cooling down. Longer equilibration time (such as
overnight) yielded similar results, so we concluded that equilibration time of one hour is
suffice. Also, the melting point for the samples was checked against the melting data from
bulk differential scanning calorimetry experiments or previous calorimetry data from the
literature to ensure that the desired composition was achieved.
2.4 Optical Microscopy
A schematic of the apparatus is shown in figure 2-4. A Zeiss Axioskop 20
microscope equipped with a Linkam BCS 196 Biological cold stage (functional temperature
range of
-196 oC to 125 oC) was used to investigate the phase transformations of the
liquid aerosols.
2.4.1
Microscope
The Axioskop 20 microscope is fitted with 50 X and 10X objective lenses of long
working distance and a condenser with a rotatable turret disk that allows for dark field
illumination and phase contrast. A Sony CCD video camera attached to the TV camera port
on the Zeiss microscope was connected to a VCR, which in turn can record microscopic
images for observation and presentation with the use of video capturing software. A
halogen lamp is the standard light source used for optical microscopy, since it offers 2dimensional square filament and thereby allowing the optical plane to be perfectly
illuminated. The microscope is composed of four main imaging components: the collector,
the condenser, the objective and the tube lens. There are two sets of optical planes that
work jointly in the microscope: the image forming beam path consists of the luminous-field
diaphragm, the specimen plane, the intermediate image in the eyepiece and the retina of the
observer's eye; while the pupil beam path consists of the lamp filament, aperture
diaphragm, objective pupil and the pupil of the observer's eye. The pupil beam path set the
resolution of the image and the image forming beam path set the field of view and
illumination of the image. Adjustment of the pupil beam path can be done by altering the
aperture diaphragm, whereas the tuning of the illumination or field of view can be done by
controlling the luminous-field diaphragm. The numerical aperture (N. A. = sin a) of the
objective is a measure of the angle covered by the aperture, where a is half the opening
angle of the objective. The smaller the particles, the more is the diffraction and the wider
should the angle of collection be for the objective. Therefore, for optimum effect, the
tunable condenser should be at a numerical aperture close to or equal to the aperture of the
objective lens. The limit of the distance ( do ) where two small particles can be
distinguished and seen separately is the measure of the resolving power of the microscope,
and d =
(N.A.obj + N.A.cond)
. In other words, the resolution can be improved with a
shorter wavelength of incident light or a higher aperture.
As for the magnification, the overall magnification of the microscope is
the product of the magnification of the objective and the magnification of the eyepiece
(M
microscope
= M objective X M eyepiece )- Videomicroscopy, with the CCD video camera, can
further provide a fast means of recording the images on magnetic tape and allow for further
processing and storage in computer. For videomicroscopy, an adapter with a fixed
(reducing) factor was used. Normally, an adapter of 0.5X is recommended for a
sufficiently large image on the video monitor. The calculation for the magnification of TV
images from videomicroscopy is a little different from the aforementioned equation. The
overall magnification for videomicroscopy is denoted as M overall and it is defined as follow:
M
overall
M
optical
eectronic
=M optical
objective
X M electronic, where
X M
adapter
and
Diagonalmonitor
= Diagonalsensor
With a transmission microscope, in a bright background, fine structures are
sometimes hard to be seen. If the object is illuminated from the side and view in front of a
dark background, then the fine structures would be more pronounced. With a rotatable
turret disk condenser, an artificial dark background can be created in the microscope using
an annular stop in the condenser. With a hollow cone of light, the light doesn't hit the
objective but passes it by the outside, thereby providing a dark background. In the presence
of an object in the object plane, then light is diffracted and the diffracted light that strikes
the objective provide an image on the dark background. The darkfield is very useful for
ascertaining if the aerosol particle is frozen or still in the liquid state. To increase the
contrast between the sample and the background, in addition to the darkfield, polarization
contrast is also an effective technique that can be used. To achieve polarization contrast,
two filters have to be used and aligned in the beam path perpendicular to each other. The
first filter is the polarizer that only allowed linearly polarized light to pass through to
illuminate the sample and the second filter is the analyzer and is placed behind the objective
at an angles perpendicular to the polarizer and this will cancel out all the light from the
polarizer and giving a dark background. In the presence of a sample that can diffract the
light, the scattering will shift the light out of the polarized plane and the sample will appear
as a bright spot on the dark background..
2.4.2
Cold Stage
To achieve a controlled and regulated cooling process, a Linkam BCS 196 Biological
cold stage was used. The cold stage replaced the microscope table on the Zeiss
microscope and the aerosol samples were loaded on the quartz crucible which was then
placed in the cold stage via a stainless steel sample carrier. Since the cold stage came with a
built-in condenser lens, the front optics of the condenser system from the Zeiss microscope
was removed before mounting the stage, and the condenser was aligned and centered after
the assembly. The BCS cold stage came with an x-y manipulator which allowed the
sample to be moved around during cooling. The inner chamber of the stage was purged
with nitrogen gas during cooling to prevent condensation of water on the chamber and
condensation on the top of the lid was prevented with a pump connector that was placed on
the top and fanning cold nitrogen air over the top lid. Cooling of the cold stage was done
by a LNP2 cooling system which operates from a source of unpressurized liquid nitrogen
with a capillary tube drawing the liquid nitrogen to the silver block of the cold stage. The
LNP cooling system, together with the TMS 92 temperature programmer and the silver
block heater assembly, was capable of attaining cooling rates of 0.01°C/min to 130 0C/min.
Typically, a preliminary cooling run was done to determine if the sample freezes and the
ballpark of the various phase transitions. Then a step-wise cooling was conducted, with a
much slower cooling rate and longer dwelling time as the temperature approached the
approximate phase transition temperature. Tables 2-2, 2-3 and 2-4 provide examples of
the programming cycles used in the cooling. Calibration curve for the cold stage was
determined to ensure the accuracy of the temperature reading. A plot of the calibration
curve is shown in figure 2-5.
33
As mentioned earlier, aerosol particles were deposited on a quartz crucible which was
then sealed and placed on the cold stage. During the initial cooling experiments, there were
always problems with heterogeneous freezing followed by contact freezing which
ultimately lead to freezing of the entire sample, often at temperatures higher than the
maximum supercooling of water. Figure 2-6 is an example of the heterogeneous freezing
on non-Prosil coated crucible versus the homogeneous freezing on Prosil coated crucible.
Stage
SControl
Figure 2-4 : Schematic set-up for microscopy.
^^^
290
270
250
S230
E
H
210
p,
190
170
15 90
150
170
190
210
230
250
270
Observed Temperature (K)
Figure 2-5: Calibration curve for the BCS cold stage.
Standards used include water-emulsion, water,
cyclohexane, isopropyl alcohol, isopropyl
alcohol-water mixture.
290
Figure 2-6: Heterogeneous freezing on non-Prosil coated crucible versus
Homogeneous freezing on Prosil coated crucible.
The horizontal length of the rectangular box in the figure corresponds to
a dimension of 40 gm.
Weight % HNO3
53.85
38.13
38.13
35
32
30
25
20
10
Cooling Cycle
2oC/min
2oC/min
2oC/min
----------> O0C (120mins) --------- > -40'C (60mins)--------> -60'C
1oC/min
2oC/min
(30mins)---------> -800 C (60 mins) ----------> -90'C (180mins)
10oC/min
1C/min
2oC/min
1C/min
-----------> -60'C (10 mins) --------- > -50 0C ------- > -10°C ----- > 20 0C
1oC/min
50C/min
10oC/min
----------- > OoC (60mins) ----------- > -60 0 C (30mins) ----------- > -90 0 C
1oC/min
50C/min
5oC/min
0
(60mins) ----------- > -140'C (10 mins) -----------> -50 C --------> 20 0 C
1oC/min
5oC/min
5°C/min
-----------> OoC (60mins) -----------> -400 C (30mins) -----------> -800 C
1oC/min
50C/min
1oC/min
0
(60mins) ----------- > -90 C (180 mins) -----------> -40 0 C -------- > 20 0 C
5oC/min
5oC/min
10oC/min
----------> OoC (60mins) --------- > -60 0C (60mins) --------- > -70 0 C
50C/min
50C/min
(60mins) --------- > -90'C (30 mins) --------- > -110oC (240 mins)
30oC/min
1oC/min
20C/min
----------> -120 0 C (30mins)--------> -20 0 C -------- > 20 0 C
50C/min
50C/min
50C/min
0
----------> OoC (60mins) --------- > -60 C (60mins) --------- > -110 0C
10oC/min
1oC/min
5°C/min
(120mins) --------- > -140 0C (30 mins) --------- > -40C-------- > 20 0C
1C/min
50C/min
5oC/min
--------- > -100 0 C
(30mins)
>
-60'C
----------> O0C (60mins) --------0
5 C/min
1oC/min
0
(60mins) --------- > -120 C (120 mins) --------- > -140 0C (60 mins)
30oC/min
1oC/min
30C/min
20C/min
-80'C
-------- > -500C --------- > -40 0C -------- > 20 0C
---------->
1oC/min
1oC/min
5SC/min
----------- > -60 0 C (240mins) ----------- > -65 0C (120mins) ----------- >
10oC/min
50C/min
-80'C (210mins) ----------- > -100C (60 mins) ----------- > -130 0 C
1oC/min
10oC/min
-------- > -20 0C ---------- > 20 0C
1oC/min
1oC/min
50C/min
-----------> OoC (60mins) ----------- > -90 0C (120mins) ----------- >
1oC/min
50C/min
120'C (60mins) ----------- > -90 0C (180 mins) -------- > 200C
1oC/min
2oC/min
50C/min
-----------> OoC (60mins) ----------- > -40 0C (30mins) ----------- >
10C/min
1oC/min
20°C/min
-50°C (60mins) -----------> -90 0 C ----------> OoC ----------- > 20 0C
Table 2-2: Samples of cooling cycles for binary nitric acid.
Weight % H2 SO 4
Cooling Cycle
50C/min
1oC/min
1oC/min
0
---------> OoC (1 hr) --------- >-50 C (4 hr) --------- >-80 0 C (2 hr)
100C/min
30C/min
10C/min
--------> -90oC(2hr) ------- >-120 0 C (30 mins) ------- >-1400 C (30 mins)
30C/min
100 C/min
100C/min
--------->-160 0C (30 mins) ------- >-190 0C (30 mins) -------- > 200 C
20C/min
20C/min
50C/min
0
-----------> OoC (60mins) ----------- > -60 C (30mins) ----------- >
50C/min
50C/min
0
0
-80 C (60mins) ---------- > -90 C (60 mins) ---------- > -120 0C (30mins)
20C/min
10°C/min
-----------> OoC -----------> 20 0C
5oC/min
0.50 C/min
0.5 0C/min
-----------> -40'C (60mins) ----------- > -90 0C (100mins) ----------- >
1oC/min
50C/min
0
-120 C (30mins) ---------- > -140 0 C (10 mins) -------- > -5°C (30mins)
10°C/min
0.5oC/min
----------- > -40C ----------- > 20 0C
57.65
10
5
Table 2-3 : Samples of cooling cycles for binary sulfuric acid.
Weight % Acids
Cooling Cycle
1 % Sulfuric acid
loC/min
1oC/min
1oC/min
----------- > -70 0C (120mins) ----------- > -800 C (60mins) ----------- >
1oC/min
1oC/min
-90'C (60mins) --------- > -120 0C (60 mins) --------- > -140 0 C (30mins)
50C/min
2oC/min
----------- > -160'C (30mins) -----------> 20'C
1oC/min
1oC/min
1oC/min
----------- > -70 0C (60mins) ----------- > -800 C (60mins) ----------- >
50C/min
2oC/min
0
0
-90 C (60mins) --------- > -120 C (30 mins) --------- > -130'C (30mins)
20C/min
10oC/min
20C/min
0
----------- > -140 C (30mins) ----------- > -160'C ----------- > 20 0C
10C/min
5°C/min
10°C/min
0
----------- > OoC (60mins) -----------> -40 C (30mins) ----------- >
1oC/min
1oC/min
-450 C (30mins) --------- > -500C (30 mins) --------- > -550 C (30mins)
1oC/min
1oC/min
1oC/min
----------- > -60 0C (30mins) -----------> -65oC (30mins) ----------- >
1oC/min
-70oC (30mins) ----------> 20 0C
40 % Nitric acid
20 % Sulfuric
acid
20 % Nitric acid
0.5 % Sulfuric
acid
20 % Nitric acid
Table 2-4 : Samples of cooling cycles for ternary acid.
I
m
2.5 Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) was used to measure the equilibrium melting
temperatures of bulk ternary acid solutions. This is a useful technique for characterizing
both isothermal and non-isothermal crystallization behavior of a wide range of materials.
DSC is very similar to DTA (differential thermal analysis), in detecting thermal changes
corresponding to either a polymorphic transition (or change in crystal structure), melting or
solidification and decomposition. DTA is designed for maximum thermal sensitivity, but at
the expense of losing quantitative calorimetric response. For accurate calorimetric data,
DSC is much better and easier to use. With a small sample size and good thermal contact
between the sample and the sample pan, enthalpy changes can be measured precisely. In
DSC, a sample cell and a reference cell are used and both cells are maintained at the same
temperature during cooling or heating. The extra heat input provided to the reference cell
(or to the sample cell if the sample undergoes an endothermic change) in an exothermic
change to provide the balance is measured. By monitoring the cooling and the heating
curves, reversible and irreversible heat changes can be distinguished.
The Perkin-Elmer DSC-7 was used in this investigation. The maximum sensitivity
of the DSC-7 is around 0.1 mcal s-1. Typically only a few milligrams of sample are
required for each measurement. The operational temperature range for the DSC-7 is from
-175 to 725 'C. To calibrate the DSC-7, indium (99.99 % purity, AH = 28.48 J/g,
melting point 156.6 oC) , n-dodecane (purified grade, Fisher Scientific, melting point -9.65
oC),
and cyclohexane (EM Science, solid-solid transition -87.06
oC)
were used as the
calibration standards for the heat of transition and the temperature for both heating and
cooling respectively.
Since there is only limited data on the melting points of ternary acid solutions in the
literature, in order to confirm the composition of our ternary aerosols, the melting points of
the frozen ternary aerosols were checked against the melting points of bulk ternary samples
determined from differential scanning calorimetry. A platinum sample pan was used with a
thin platinum lid on top. The lid was used to prevent condensation on the side wall which
can result in unwanted signal. A micro pipette was used to fill up the platinum sample pan
with the acid sample, and this was in turned sealed and transferred to the sample cell
container. It is necessary to fill up the entire sample pan with the acid to prevent
condensation of water from the vapor, which can give rise to heterogeneous freezing and
lead to an erroneous melting point determination. The final melting point of the sample was
determined from the last melting peak of the sample.
2.6 Calculations
Several calculations were done pertaining to the experiments, to have a ball park
estimate of the equilibration time, to determine whether to place the equilibrating droplets
on the top or the bottom of the crucible, and to estimate the volume change corresponding
to a volume change and they were outlined below.
2.6.1
Estimation of Equilibration time
By the use of the simple first order Fick' s law, the equilibration time needed for the
small aerosol particles to equilibrate with the large reservoir particles was estimated.
J = -Dg (
de )
-Dg (
Ac
)
where Dg=0.239 cm 2/sec, T = 273 K
For simplicity, we assumed the distance between the aerosol particle and the
reservoir to be 1 cm, i.e. Ax = 1 cm and the size of the aerosol to be 10gm. For illustration
purposes, assume that the aerosol particle has a concentration 70 % nitric acid, and the
reservoir has a concentration of 40 % nitric acid.
For the reservoir, PH2o = 1 Torr ,
=>
(1/760)atm
-n = P- =
V
RT (0.08218latm/mol * K)(273K)(1000cm3
= 5.8 E-8 mol / cm 3
For the aerosol particle, PH20 = 0.3 Torr,
=>
n = P
V
RT
(0.3/760)atm
(0.08218latm/mol * K)(273K)(1000cm
1.7 E-8 mol / cm 3
3
AC = (5.8-1.7) E-8 mol / cm 3 = 4.1 E-8 mol / cm 3
•.
J = -Dg (
Ac
) =
0.239cm 2 /sec * 4.1E - 8mol/cm 3
1 cm
x
= 1 E-8 mol / cm2.sec
To calculate the number of moles of water needed to take up by the aerosol:
For the 70 % nitric acid aerosol particle, [H20] = 1.67 E-2 mol / cm 3
For the 40 % nitric acid reservoir, [H20] = 3.33 E-2 mol / cm 3
Volume of the aerosol particle = 4/3 Hr 3 / 2 (for hemisphere) = 2.62 E-10 cm 3
Surface Area of the aerosol particle = 2 Hr2 = 1.57 E-6 cm 2
.'. # mole of water needed for the aerosol particle to achieve an [H20] value of 3.33
E-2
mol / cm 3 = (3.33 E-2 - 1.67 E-2 ) mol / cm 3 * 2.62 E-10 cm 3
= 4.35 E-12 moles water to equilibrate with the reservoir
To calculate the time-scale for the equilibration,
4.35 E-12mol
- 1 E-8 mol/cm 2.sec* 1.57 E-6 cm 2
Time = # mole of water needed _
Flux * Surface Area
= 277 sec or 5 min
This is a lower limit from the point of view that as the 70 % nitric acid particle
becomes more dilute, its water vapor pressure increases, hence A C and consequently J
decrease with time.
2.6.2
Volume change corresponding to the concentration
change
The following calculation was done to see if the volume change corresponding to a
concentration change for binary sulfuric acid particles would be significant enough to be
observable visually.
Let Y1 and Y2 be the ratio of weight of sulfuric acid and water, M1 and M
2
be the
mass of water in the droplet, where M1 , Y, correspond to an initial droplet concentration of
30 weight % sulfuric acid, M2, Y2 correspond to a final droplet concentration of 10 weight
% sulfuric acid.
Since sulfuric acid is involatile, the mass of sulfuric acid in the droplet remain
constant as the droplet concentration change from Y1 to Y2 by the uptake of water. Let that
H2SO 4 mass be X. That is, Y,= X/M, Y2 = X /M 2 ,
.'.
Weight of sulfuric acid = X = YI (M,) = Y2 (M2)
3
1
. .(M)= - (M2) , =>M
7
9
.M
2
= 3.8 M
M
M
1
=>
1
1
-
9
2
V
V2
-
3
0.26
3.8* p 1
V,
&
7
*-
=3.8 * 1.14 = 4.3
p2
R2
2•
RI
_3.8
*p 1
p2
1.63
(p 1=1.21, p 2=1.06)
43
Therefore, from the above calculation, the volume change corresponding to a 20
weight % change in concentration should be about 4.3 times the original volume, and thus
substantial enough to be observable. Therefore, the visual observation of the swelling of
the particles can be a useful indication of the concentration change in the particle and can
help to gauge if the freezing of the big drop caused the concentration of the small drops to
depart from their original concentration via mass transport of the vapor.
Chapter 3
Binary HNO 3 / H20 system
3.1 Introduction
Binary nitric acid of compositions ranging from 0-53.85 % were studied. In order to
investigate the formation mechanism type Ia PSC, nitric acid hydrates have been
extensively studied by many research groups. 46-52 However, there have been relatively few
reports of experimental studies of phase transformations of binary nitric acid aerosols. In
our experiment, we chose binary nitric acid to be the first system to study, since the phase
diagram of binary nitric acid system is well established, and we can verify the composition
of our aerosols by checking their melting points with respect to literature values.
Furthermore, the supercooling curve for the binary nitric acid system provides us with a
baseline for the ternary acid system. Also, it has been pointed out by Meilinger et al. 44that
stratospheric aerosols may approach a binary nitric acid composition of 52 % nitric acid, 48
% water at 190K as a consequence of rapid temperature fluctuations. Therefore, the
supercooling curve of binary nitric acid may be of importance to explain the freezing
behavior of stratospheric aerosols in the presence of lee wave temperature fluctuations.
3.2 Results
The freezing points and melting points of the nitric acid aerosols are shown in tables
3-1 and 3-2. It should be pointed out that we defined the freezing point as the onset of
freezing, that is, the temperature at which a substantial portion (around 10 %) of the
aerosols is first seen frozen, and we defined the melting point as the end melting
temperature, for which the solid particles were completely melted. For compositions
between the maxima and the eutectic, melting will begin at the eutectic and the temperature
for which the particles melt completely is a signature of the composition of the particle.
The first composition tried was the 53.85 % nitric acid, corresponding to the NAT
composition. The aerosols were cooled at 20C/min, dwell at intervals for a few hours and
continue to cool at 10C/min to 183K.(see table 2-2) The freezing and melting of NAT
droplets is shown in figures 3-1 and 3-2 respectively. The aerosols began to freeze around
195 K, the solid remained spherical in shape, but the frozen particles appear to be
polycrystalline in nature. While dwelling at 183K, for those liquid droplets that remained
supercooled, an additional process took place, which appear visually as it though a
transparent film was seen sweeping over the surface in an instant, but without any change
in the physical appearance of the droplets. Upon warming up, those liquid droplets
crystallized on warming at around 218 K. It was interesting to point out that there were
two phase transitions observed prior to melting, at 230K and 248K respectively. The first
transition corresponded to the transformation of the spherical frozen solid to a dense
threadlike sphere, and during the second transition, the dense threadlike sphere crystallized
to give rod-shaped crystals.(see figure 3-2) The frozen aerosols melted completely at
254K. A similar phase transition at around 228K was also observed for the 35 %, 38.13%
and the 40 % nitric acid. Crystallization upon warming was observed for 35 % nitric acid
at 163 K and for 40 % nitric acid at 157 K. Crystallization upon warming for 35 % nitric
acid is shown in figure 3-3.
As for the compositions in the ice regime, the 10 % nitric acid froze around 223 K,
with the larger equilibrating drops freezing at 228 K; both crystalline and spherical solid
particles were seen. Figure 3-4 and 3-5 are the freezing and melting events of 10 % nitric
acid. There was contact freezing observed together with the homogeneous freezing of
individual aerosols. So, virtually all were frozen upon dwelling at 223 K. Melting began
around 230 K and all particles melted completely at 267 K. For the 20 %, 25 % and 30 %
nitric acid, there were two freezing stages giving frozen particles of different morphology.
The 20 % first froze at 210.6 K and then at around 191 K; most of the remaining aerosols
froze over. Similar to the 10 % nitric acid, virtually all particles were frozen on cooling, so
no crystallization on warming could be observed. Both types of solid particles began to
melt at the eutectic of 231 K and melted completely at 245 K. Freezing and melting events
of 20 % nitric acid are shown in figure 3-6 and 3-7. For the 25 % solution, frozen particles
were first seen at around 187 K and then at 173 K; the remaining supercooled droplets
froze to give non-crystalline looking translucent solid. They all melted at 248 K. For the
30 % nitric acid, freezing was observed at 160 K, and then at 138 K; most of the
remaining liquid aerosols transformed to translucent-looking particles. Upon warming, the
translucent particles melted at 193 K, and crystallized to give more crystalline solid
particles. The solid particles that froze at 160 K also became more crystalline. Melting
began at 225 K and ended at 230 K.
The 32 % nitric acid is the composition that is closest to the binary eutectic, since it
had a very narrow melting range. That is, the melting began at 230.2 K and ended at 231
K.(that is, the particle melted congruently) Crystallization upon warming was observed for
the 32 % nitric acid at 154 K. Upon warming, the solid particles went through the
transition to darker, denser looking particles at around 208 K before they melted.
A plot of the supercooling curve is overlaid with the binary nitric acid phase diagram
in figure 3-8. As can be seen in the figure, the melting points of the aerosol particles are in
good agreement with the expected melting points reported for bulk samples in literature. 55
So, the larger equilibration droplets were effective in ensuring that the smaller aerosols
achieved the desired composition. Also, the freezing behavior of the aerosol particles
generated via co-condensation as well as nebulizer are overlaid in figure 3-8. The freezing
and melting temperatures of the particles from co-condensation and nebulizer agreed with
each other and thereby corroborating the consistency and reproducibility of the freezing
behavior.
Also, the degree of supercooling (AT) is plotted as a function of the freezing
temperatures for the ice-regime of the binary nitric acid in figure 3-9. The data is
approximated by a linear relationship, with greater supercooling at higher acid
concentration. As mentioned in Alick Chang's thesis 54, this is in contradiction with the
hypothesis of Hallet and Lewis 56, who stated that the maximum supercooling that can be
achieved with ice-forming solutions should be equivalent to the maximum supercooling of
pure water:
Tf
=
Tm
- 40
(3.1)
A plot of the freezing temperature against the melting temperature is shown in figure
3-10. The data is fitted with a linear relationship and the relation between the freezing
temperature Tf and the melting temperature Tm is the following:
Tf
=
1.72 Tm
-
237
(3.2)
The degree of supercooling (AT) is also plotted in figure 3-11 against the freezing
temperatures for the NAT-regime of the binary nitric acid system. The data is again
approximated with a linear fit. The degree of supercoooling changes more rapidly for the
NAT forming region than the ice forming region and the NAT forming region also has a
higher degree of supercooling.
The freezing points determined for the aerosols were compared with the freezing
points determined in the emulsion experiment done in our laboratory using differential
scanning calorimetry. 54 In the emulsion system, the acid droplets are segregated by the
halocarbon oil phase and are essentially behaving like individual micro droplets and hence it
is useful to compare our experimental results with the emulsion system. For the
compositions ranging from 0-40 % nitric acid, the freezing points for the aerosols agreed
extremely well with that of the emulsion. 54 The aerosols were usually supercooled by a
few more degrees than the emulsions. This minor difference is probably due to the
interaction of the emulsion with the acid samples. However, for the compositions > 40 %
nitric acid, no freezing but only the glass transition was detected in the emulsion system.
We rationalized that this could probably due to the fact that differential scanning calorimetry
is not sensitive enough to detect stochastic freezing events, and since the freezing process
is slower and less extensive in the concentrated acid region, the heat for the phase change
may not be discernible for a portion of the micro droplets. For the aerosols, since the
remaining undercooled droplets did crystallize upon warming, we believe that this is in
agreement with the glass transition points that were observable with the differential
scanning calorimetry. In addition, our experimental results in the NAT-forming region are
in good agreement with the ongoing infra red spectroscopy experiment in our laboratory
using nebulizer generated aerosols. The infra red spectroscopy experiment in our
laboratory should be able to provide more information on the identities of the phases that
have been observed.
Weight %of
Nitric Acid
Freezing
Point (K)
End Melting
Point (K)
0
233
273
10
223
267
20
211
245
25
187
248
30
159
230
32
163
231
35
158
238
38.13
185
242.2
40
173
246
53.85
195
254
Table 3-1: Freezing and melting temperatures for binary nitric acid,
aerosols from co-condensation.
Weight % of
Nitric Acid
Freezing
Point (K)
End Melting
Point (K)
20
211
245
30
158
237
40
175
246
53.85
194
253
Table 3-2: Freezing and melting temperatures for binary nitric acid, aerosols
from nebulizer.
MMMMMMMMM"
Figure 3-1 : Freezing of 53.85 % binary nitric acid.
Freezing was observed at 195 K, the top right and bottom two figures show
the frozen aerosol particles while dwelling at 183 K.
The horizontal length of the rectangular box in the figure
corresponds to a dimension of 40 ktm.
Figure 3-2 : Melting of 53.85 % binary nitric acid.
The first two frames show the first transition at 230 K, the third and fourth
frames show the second transition at 248 K, where the frozen particles
crystallized to rod-shaped crystals and the last two frames show the melting at 254 K.
Figure 3-3 : Crystallization upon warming of 35 % binary nitric acid.
Frames 2-4 capture the crystallization of the same particle at approximately
20 seconds apart. Crystallization upon warming was observed at 163 K.
Figure 3-4 : Freezing of 10 % binary nitric acid.
Freezing was observed at 224 K. The horizontal bar in the first frame
corresponds to a length of 10 pm.
Figure 3-5 : Melting of 10 % binary nitric acid.
Melting began at 230 K and ended at 267 K.
Figure 3-6 : Freezing of 20 % binary nitric acid.
Freezing was first observed at 210.6 K.
Figure 3-7 : Melting of 20 % binary nitric acid.
Melting began at 231 K and ended at 245K.
r ]lq
Jl
SFreezing Temp (Co-condensation)
290
N Melting Temp (Co-condensation)
*
Freezing Temp (nebulizer)
-O
270
Freezing Temp (emulsion)
250
c
230
IN
-9
)
210
190
170
1 &;
0
10
20
30
40
50
60
Weight %Nitric Acid
Figure 3-8 : Freezing and melting temperatures for binary nitric acid system.
The emulsion results are from Chang. 54
70
80
70
60
Cr.•
50
O
O
Q)
CS,,
40
30
On -,-
240
230
220
210
200
190
180
170
160
150
Freezing Temperature (K)
Figure 3-9 : Degree of supercooling versus freezing temperature in the
ice regime for the binary nitric acid system.
A
=A
240
230
220
~
210
" 200
1E
90
180
N
170
160
150
14
A
220
220
230
240
250
260
270
Melting Temperature (K)
Figure 3-10 : Freezing temperature versus melting temperature for the
binary nitric acid system.
280
0
Qm)
55
5 n
200
190
180
170
160
150
140
Freezing Temperature (K)
Figure 3-11 : Degree of supercooling versus the freezing temperature in the
NAT regime for the binary nitric acid system.
Chapter 4
Binary H2SO
4
/ H20 system
4.1 Introduction
Binary sulfuric acid of compositions ranging from 0 to 57.65 %were studied.
Stratospheric sulfate aerosols, consisting of sulfuric acid and water, play a part in the
heterogeneous catalysis of nitrogen deactivation and for the more dilute solutions also of
chlorine activation, which eventually lead to the depletion of ozone. 57 The reaction rates are
57
believed to depend on the phase and composition of the SSA.
In order to ascertain the
heterogeneous reaction rates on the SSA, there should be a clear understanding of the state
of the aerosols. Moreover, since SSA is the substrate for PSC formation, it is imperative
to understand the freezing behavior of SSA. Since the sulfuric acid concentration drops
dramatically as it absorbs water and nitric acid as temperature decreases, it is necessary to
58 59 60 61 62 63
examine the freezing for both concentrated and dilute sulfuric acid. , , , , ,
It was difficult to deposit binary sulfuric acid aerosols, using the co-condensation
technique, due to the extremely low vapor pressure of sulfuric acid. So, the droplets
usually have less contrast and look less spherical due to the inevitable loss of water content.
The degree of difficulty in depositing nice droplets increases as the sulfuric acid
concentration increases.
4.2 Results
The freezing points and melting points of the sulfuric acid aerosols are shown in
Table 4-1.
For the 57.65 % sulfuric acid, corresponding to the SAT (sulfuric acid tetrahydrate)
composition, the aerosols did not freeze upon cooling and dwelling at 83 K. Different
cooling cycles were tried, both step-wise cooling and direct cooling, but no freezing was
observed. No crystallization upon warming was observed. No freezing was observed even
with only big equilibrating drops in the crucible.
Similarly, for the 30 % sulfuric acid, no freezing was observed upon cooling to 83
K. No freezing on cooling was observed even for the large equilibrating drops. However,
the aerosols and the equilibrating drops did crystallize on warming at 194 K and melted
completely at 238 K.
For the 10 % and 20 % sulfuric acid, freezing was observed at 224 K and 218 K.
Both crystals and spherical frozen particles were seen. Homogeneous freezing as well as
some extent of contact freezing was observed. Figure 4-1 and figure 4-2 show freezing
and melting for 10 % sulfuric acid droplets. More of the supercooled droplets were seen to
freeze around 183 to 193 K. Crystallization upon warming was observed for aerosols that
remained undercooled at 194 K and 191 K for 10 % and 20 % sulfuric acid respectively.
The frozen particles melted completely at 224 K for the 10% and 218 K for the 20 %
sulfuric acid. For the 1 % and 5 % sulfuric acid, freezing was observed at 230.4 K and
215 K respectively. Similarly, there was both homogeneous freezing of individual droplets
and contact freezing for droplets that were close to each other and ended touching each
other, inducing crystallization. Since most of the aerosols froze during cooling, no
crystallization on warming was observed. The particles melted completely at 270 K for the
1 % and 266 K for the 5 % sulfuric acid. Freezing and melting for 1 % sulfuric acid is
shown in figure 4-3.
A plot of the supercooling curve is overlaid with the binary sulfuric acid phase
diagram in figure 4-4. As can be seen in the figure, the melting points of the aerosol
particles are in fairly good agreement with the expected melting points reported for bulk
samples in the literature.64 So, the larger equilibration droplets were quite effective in
bringing the aerosols back to their desired compositions. The freezing and melting data
determined are also compared with the results of other research groups in figure
4-5. 58-62, 65,66
Also, similar to the binary nitric acid, the degree of supercooling with respect to the
freezing temperature is shown in figure 4-6. The data was approximated with a linear
relationship. As can be seen in the fitted equation, binary sulfuric acid has a higher degree
of supercooling than binary nitric acid.
Our results were again compared to the infra red spectroscopy experiment carried out
in our laboratory. 63 For the compositions in the SAT stability regime, the two sets of
results agree, in that no freezing was observed upon cooling. In addition, in our
experiment, no crystallization was observed upon warming for the SAT composition. As
for the ice stability regime, the results agree in that ice formation was observed upon
cooling, but we were able to eliminate heterogeneous freezing by using the Prosil-coated
crucible and supercooled our aerosol particles to a greater extent, and homogeneous
freezing of the particles could be seen visually during cooling. Also, our results are in
agreement with the calorimetry experiment conducted by Ji et al. 65'66 The freezing points
for the aerosols in the water-rich region agreed fairly well with the freezing points of their
emulsion samples. In addition, they reported that no crystallization was observed for their
emulsion samples for compositions > 27 % sulfuric acid, and our experimental results have
indicated that compositions > 30 % sulfuric acid do not freeze upon cooling. Furthermore,
recent experiments done in our laboratory with bulk drops of 32 % sulfuric acid
(equilibrated externally with acid bath) corroborated our freezing results, in that bulk 32 %
sulfuric acid drops also do not freeze upon cooling, and only crystallize upon warming.
The freezing point of our 20 % sulfuric acid agreed with the value reported by
Bertram et al.,58 although we didn't observe freezing for 30 % sulfuric acid, as they did.
Also, since their aerosol particles are much smaller (radius 0.2 gm) than the ones we had,
we would expect their aerosols to supercool to a greater extent than ours. In addition, their
aerosol particles were injected to a precooled chamber, and it is doubtful that homogeneous
freezing will occur with rapid cooling. Since it was difficult for them to determine the
composition of their aerosol particles because of the unavailability of the melting
temperatures, their particles may have a composition slightly different from their targeted
composition, and perhaps that might explain the freezing they observe for compositions
even up to about 36 % sulfuric acid.
Weight % of Sulfuric
Acid
Freezing Point
(K)
End Melting Point
(K)
1
230.4
270
5
215
266
10
228
270
20
30
218
did not freeze,
crystallize on
263
238
warming
57.65
did not freeze,
did not crystallize
-----
on warming
Table 4-1 : Freezing and melting temperatures for binary sulfuric acid aerosols.
Figure 4-1 : Freezing of 10 % binary sulfuric acid.
Freezing was observed at 228 K. Frames 4 and 5 show
an example of contact freezing.
Figure 4-2 : Melting of 10 % binary sulfuric acid.
The particles melted completely at 270 K.
The frames are approximately 1 minute apart.
Figure 4-3 : Freezing and melting of 1 %binary sulfuric acid. The first two
frames show the freezing of the particle taken at 30 seconds apart. The last
two frames show the melting of the particle taken at 30 minutes apart. It froze
around 230 K and melted completely at 270 K.
0
j
CIO
0
0
C
0
C
N
0
CN
N
0
"
C
N
0D
0
00
cJ
(i) a;nJnadwa/
0
0
freeze down to a temperature of 83 K.
0
c00
0
0O
O
0
LO
0
0
V
0
C\)
0n
C/)3
4O
00
00
C\
0>
Figure 4-4: Freezing and melting temperatures for binary sulfuric
acid.(Supercooling Curve) The arrow indicates that the particles did not
CD
2?n
*
0ý3
Lit
CD
C
-t
300
-
280
-
I ,
cD
Nc
rxl
CD
3
260
0-
_···__
-
-
Anthony, aerosol (exp. points, no freezing)
Imre, single drop (expt. line, no freezing until X)
-Clapp, aerosol (freezing line)
Bertram, aerosol (freezing line)
Petit, bulk (freezing line)
......- Petit, bulk (no freezing)
-Petit, emulsions (freezing line)
0
Il
r\
/
vl0-
240
I)
-t'
oD
PC-
S
220
200
o1
0
-t
180
CD
C-.
CD
160
.lI.-I
0
10
20
30
40
50
60
Weight % Sulfuric Acid
70
80
90
50
45
O
O
Q) 40
35
qn
240
235
230
225
220
215
Freezing Temperature (K)
Figure 4-6 : Degree of supercooling versus freezing temperature in the ice
regime for the binary sulfuric acid system.
210
Chapter 5
Ternary HNO 3 / H2SO 4 / H20 system
5.1 Introduction
Although recent laboratory studies have suggested that ternary acid droplets are
unlikely to freeze homogeneously in the stratosphere, 42 Lidar measurements have indicated
that some type I PSCs (the so called Type Ia) should be non-spherical crystalline particles.
39,67
There have been some proposed mechanisms for the formation of these crystalline
particles, but the formation and nature of type I a PSC remains a puzzle. Also, previous
studies on ternary freezing were carried out primarily on bulk solutions; even though such
studies can provide an upper limit to the freezing temperature, it would still be desirable to
learn the freezing behavior of ternary microscopic droplets, taking advantage of the ability
to regulate and control the cooling rate.
5.2 Results for compositions along the deliquescence curve
The freezing points and the melting points for the samples investigated are shown in
Table 5-1.
Four ternary compositions investigated were chosen from the deliquescence curve.
These were chosen to verify the freezing results from bulk freezing that have been
previously reported. 42 The 50 %sulfuric acid, 1 % nitric acid corresponds to the
composition of stratospheric aerosols at 195 K. The 20 %sulfuric acid, 20 % nitric acid is
approximately the composition of stratospheric aerosols at 192 K. The 1 % sulfuric acid,
40 %nitric acid is approximately the composition of stratospheric aerosols at 188 K. The
2 % sulfuric acid, 25 %nitric acid corresponds to the composition of the stratospheric
aerosols at 185 K. All of the above compositions were cooled at rates of 10C to 50C, with
step-wise as well as direct cooling, and dwelled at 83K to 93K.
As can be seen from the ternary phase diagram, figure 5-1,
68
the composition 50 %
sulfuric acid, 1 % nitric acid lies in the SAT regime and as expected, it didn't freeze on
cooling. Upon warming, the droplets crystallized at 198K, and the melting temperature
was 238 K.
The compositions 20 % sulfuric acid, 20 % nitric acid and 1 % sulfuric acid, 40 %
nitric acid lie in the NAT stability regime. And similarly, neither composition froze upon
cooling. Instead, they went through a glass transition, and crystallization upon warming
was observed at 175 K for the 20 % sulfuric acid, 20 % nitric acid ternary and at 172 K for
the 1 % sulfuric acid, 40 % nitric acid ternary. As shown on table 5-1, their melting points
are 230 K, and 244.5 K respectively. So, the presence of sulfuric acid indeed suppressed
and prevent the freezing of NAT upon cooling. Crystallization upon warming and melting
for 40 % nitric acid, 1 % sulfuric acid is shown in figure 5-2.
As can be seen from the deliquescence curve, also shown in figure 5-1, 68 the nitric
acid content decreases as temperature decreases due to denitrification of the stratosphere. In
other words, the stratospheric aerosol composition projects into the ice regime as the
temperature continues to drop. So, a composition 2 % sulfuric acid, 25 % nitric acid was
investigated. This is the composition that is fairly close to the border where the
deliquescence curve first intersects the ice-stability regime. This ternary composition did
freeze upon cooling to form ice but didn't freeze until 163 K; crystallization upon warming
was seen at 168 K and the particles began to melt around 228 K and completely melted
around 243 K. In other words, this ternary composition would not freeze under
stratospheric conditions. So, in order for stratospheric aerosols to freeze, they would have
to supercool or deliquesce further into ice stability regime of the ternary phase diagram.
10
Wt % Sulfuric
Acid
0.5
Freezing
Point (K)
213
Melting
Point (K)
261
15
0.5
201
251
17
0.5
203
250
20
0.5
198
245
10
1
221
259
20
1
186
235
40
1
*
244.5
25
2
163
243
5
5
196
244
10
5
168
235
20
5
181
232
20
20
*
230
1
50
*
238
Wt % Nitric Acid
* Did not freeze, crystallized upon warming
Table 5-1 : Freezing and melting temperatures for ternary acid aerosols.
Ternary HNO3/H2SO4/H20 Phase Diagram
0.8
0.7
0.6
0.7
0.8
0.2
0.9 A
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Weight Fraction of Sulfuric Acid
Figure 5-1 : Ternary phase diagram with projections of the
deliquescence curve for stratospheric aerosols.
Line a: 50 mbar, 5 ppm H20, 10 ppb HNO 3
Line b: 50 mbar, 5 ppm H20, 20 ppb HNO 3
U.6
Figure 5-2 : Crystallization on warming and Melting for
40 % nitric acid, 1 % sulfuric acid ternary droplets.
Crystallization on warming was observed at 172 K.
5.3 Results for ternary compositions with fixed amounts of
sulfuric acid
Upon the realization that sulfuric acid in a ternary composition prevented the freezing
of NAT upon cooling, and hence that ice would have to form first to freeze the ternary
droplets, a systematic series of freezing investigations was conducted by varying the
composition of nitric acid solutions with constant amounts of sulfuric acid. The amounts of
sulfuric acid chosen were 0.5 %, 1 % and 5 %, and 3 series of freezing experiments were
done by varying the nitric acid content. The freezing points and the melting points for the
samples investigated are also shown in Table 5-1.
5.3.1
Ternary compositions with 0.5 % sulfuric acid
For the 0.5 % sulfuric acid, compositions with 10 %, 15 %, 17 %, 20 % nitric acid
were studied. For the ternary solution containing 10 % nitric acid, the equilibrating
droplets froze at 223 K, whereas the smaller aerosol particles froze around 213 K. The
remaining supercooled aerosols all froze at 208 K, instantaneously. Upon warming, the
frozen particles began to melt at around 225 K and melted completely at 261 K. Freezing
and melting for 10 % nitric acid, 0.5 % sulfuric ternary is shown in figure 5-3 and 5-4,
respectively. For the ternary solution containing 15 % nitric acid, the aerosols froze at 201
K. Upon further cooling, most of the remaining supercooled droplets froze around 182 K;
the solid particles seen at 182 K had a different texture from those freezing out at 201 K.
Freezing and melting for the 15 % nitric acid ternary is shown in figure 5-5 and 5-6. The
aerosols that remained undercooled crystallized on warming at 161 K. All solid particles
melted completely at 251 K. For the ternary solution containing 17 % nitric acid, freezing
was observed at 203 K. On further cooling, the majority of the particles were seen to
Figure 5-3: Freezing for 10 % nitric acid, 0.5 % sulfuric acid ternary droplets.
Freezing was observed at 213 K .
Figure 5-4 : Melting for 10 % nitric Acid, 0.5 % sulfuric acid ternary droplets.
Particles began to melt at 225 K and melted completely at 261 K.
Figure 5-5 : Freezing for 15 % nitric Acid, 0.5 % sulfuric acid ternary.
Frames 1-5 show frozen particles with different morphologies. Frames 6-8 show the crystallization
of the amorphous particle on warming.
Figure 5-6 : Melting for 15 % nitric acid, 0.5 % sulfuric acid ternary droplets.
Melting began at around 231 K and particles completely melted at 251 K.
freeze out around 188K, and for the very few droplets that remained supercooled, freezing
was observed at 172 K. The particles that froze at 172 K were almost transparent and had
an amorphous appearance. Upon warming, the solid particles began to melt at 197 K and
melted completely by 250 K. For the ternary solution containing 20 % nitric acid, the large
equilibrating droplets froze around 210 K, and freezing was observed for the aerosols at
198K. All of the remaining supercooled aerosols froze at 178 K. The particles that froze at
198 K and 178 K have different morphology. They all melted completely at 245K.
Freezing and melting for 20 % nitric acid ternary is shown in figure 5-7 and 5-8.
5.3.2
Ternary compositions with 1 % sulfuric acid
For the 1 % sulfuric acid, compositions of 10 %, 20 % nitric acid were studied in
the regime where ice is the thermodynamically stable phase. For the 10 % nitric acid, the
large equilibrating drops froze around 227 K, whereas the smaller droplets froze
homogeneously at 221K. The majority of the droplets froze by 212 K. The remaining
supercooled droplets froze at 183 K. As before, the particles that froze at different stages
have distinct morphologies. The first type of frozen particles consist of crystalline ice like
particles, with distinguishable facets, whereas the latter frozen particles are spheres with
wavy or silky texture. The frozen particles began to melt at 232 K, and melting was
completed at 259 K. For the 20 % nitric acid, freezing was observed at 186 K, and the
number of frozen particles increased with time. Upon cooling to 160 K, a transition was
seen for the remaining supercooled droplets; they appeared to be enveloped with a
translucent film. Upon warming, these particles crystallized at 175 K. Melting began at
230.6 K and the frozen particles all melted completely at 235 K. Figure 5-9 shows the
freezing for 20 % nitric acid, 1% sulfuric acid ternary solution droplets.
rI
rlk~9i
)illd u
r· r ~il
%W
C
P,
Figure 5-7 : Freezing for 20 % nitric acid, 0.5 % sulfuric acid ternary droplets.
Freezing was first observed at 198 K and the remaing supercooled droplets froze at 178 K.
d
i
BIP~~
d ~pL~i~
@
iW
7
O
7
-
MW" - Iw
U •=
I
Figure 5-8 : Melting for 20 %nitric acid, 0.5 %sulfuric acid ternary droplets.
Melting began at 227 K and particles completely melted by 245 K.
Figure 5-9 : Freezing for 20 % nitric acid, 1 % sulfuric acid ternary droplets.
Freezing was observed at 186 K.
Frames 2-4 show the transformation from a translucent film to crystalline particles upon
warming.
5.3.3
Ternary compositions with 5 % sulfuric acid
For the 5 %sulfuric acid, compositions of 5 %, 10 %, 20 % nitric acid were studied.
For the 5 % nitric acid, the equilibrating drops froze around 203 K, and the smaller
droplets froze at 196 K. The remaining undercooled droplets froze almost instantaneously
at 183 K. The frozen particles melted completely at 244 K. For the 10 % nitric acid, the
equilibrating droplets froze at around 172 K; crystallization of the aerosol particles was
observed at 168 K. The number of frozen particles increased as dwell time increased.
Here again, it appeared as if a translucent film was seen sweeping across the supercooled
droplets at around 155 K, upon cooling. Crystallization upon warming was observed for
the supercooled droplets at 167 K. Melting began at 231 K and ended at 235 K. For the
20 % nitric acid, freezing was observed at 181 K. The majority of the remaining acid
droplets froze by 175 K. The morphologies of the frozen particles were different from
those seen with the 5% and 10 % nitric acid; there was one type that had a plate-like
texture, with cracks and radiating lines, and the other type was amorphous looking with
worm-like texture. Crystallization on warming was seen at 167 K for those that didn't
freeze on cooling. Melting began at 231 K and was completed at 232 K.
5.4 Discussion
A plot of the supercooling curves is overlaid with the binary nitric acid phase diagram
in figure 5-10. Differential scanning calorimetry was used to determine the melting points
for the bulk ternary solution samples. Melting points determined with the DSC served as a
reference for checking the equilibration of our ternary aerosol particles. Table 5-2 is a
comparison of the melting points of the aerosols versus the melting points of the
corresponding bulk acids. As can be seen in the table, the melting points of the aerosol
particles agree very well with the melting temperatures determined with DSC.
Therefore, the large equilibrating drops were effective in bringing the composition of the
aerosol particles to the desired values. Figures 5-11, 5-12, 5-13 and 5-14 are examples of
the DSC thermograms for bulk ternary acids.
As shown in figure 5-10, sulfuric acid does indeed suppress the formation of ice, but
it does not prevent it entirely. Even with a very small amount of sulfuric acid (for example,
0.5 %), the freezing temperatures for ice formation in the ternary acids are about 20-25 K
lower than their binary nitric acid counterparts. The degree of supercooling also increases
as the nitric acid concentration increases for the ternary acids. In addition, a comparison
can also be made between the extent of supercooling and the amount of sulfuric acid in the
ternary acids. There is no distinctive difference in the degree of supercooling between the
ternary acids containing 0.5 weight % and 1 weight % sulfuric acid, but ternary
compositions containing 5 weight % sulfuric acid do supercool to a greater extent than the
0.5 and 1 % ternary.
Even though it is visually possible to distinguish a frozen particle from a liquid
particle, it is not always possible to visually ascertain if the frozen particle is completely
frozen or partially frozen. That is, to address the question as to whether NAT and SAT
freeze after the formation of ice, the melting behavior of the particle can probably provide
some insights. Since eutectic melting was seen in all cases, it can likely that NAT and
possibly SAT crystallized after the ice formation.
Ternary compositions with 0.5 % and 1 % sulfuric acid were studied more
extensively than the 5 % sulfuric acid ternary because they resemble more the compositions
of the stratospheric aerosols that can be found around 182 - 185 K.
Since the composition 2 % sulfuric acid, 25 % nitric acid, corresponding to the
composition of stratospheric aerosol at 185 K, would not freeze at the appropriate
stratospheric temperature, the stratospheric aerosols would have to deliquesce and cool
further into the ice regime of the ternary phase diagram. In other words, the stratospheric
aerosols have to supercool fairly substantially even in the ice regime before ice will form.
The compositions investigated in section 5.3 represent a cluster of compositions in the ice
regime that are close to the projection of the deliquescence curve. The freezing
temperatures of most of the compositions studied indicated that they would freeze at the
stratospheric temperature. So, when the stratospheric aerosols cool and deliquesce below
185 K, ice will freeze first out of the ternary solution.
310
I.. I
I.I I
i
I
.
A
I
I
I
I
i
I
I
I
Melting Temp (1% H2SO4)
Freezing Temp (5% H2SO4)
290
0
Melting Temp (5% H2SO4)
O Melting Temp (0.5% H2SO4)
*
Freezing Temp (2% H2SO4)
A
O Melting Temp (2% H2SO4)
*
Freezing Temp (0.5% H2SO4)
Freezing Temp (1 % H2SO4)
270,
250
S, 2 3 01
H
210
Eu
190
170
15r0
I ",,
~''''''~''
'''''
20
30
40
Weight % Nitric Acid
Figure 5-10 : Freezing and melting temperatures for ternary acid
aerosols.
I
I
Wt % Nitric Acid
Wt % Sulfuric
Melting Point
Melting Point from
Acid
from DSC (K)
microscope (K)
10
0.5
258-261
261
15
0.5
253
251
17
0.5
249
250
20
0.5
248
245
10
1
261
259
20
1
229
235
25
2
163
243
5
5
251
244
10
5
N/A
235
20
5
242
232
40
1
243
244.5
20
20
231
230
1
50
N/A
238
Table 5-2 : Comparison of melting temperatures of ternary aerosols with bulk
melting temperatures measured by differential scanning calorimetry.
=a
50
r-----"
20-NA- 5SA.
57.SA.
__ -20%IIA.
it
lst
Run, Cool
Cool
Run.
(ter-i2c
(teri2ci)
-
----
t
37.5
+
Pea.--93.24
Peak from:-30.51
to: -25.83
Onset--27.17
J/g --3.74
Peak--27.67
i
Peak froem-109.68
to: -85.79
Onset--88.63
.,'g
-- 20.64
12.5
a
t
-
o0
00
-lo0.0o- o .oo
-:
-o0
0o
0
.o00-----
0o0
-. oo--0 -o.o--
Temperature (C)
Date: Jan 03. 1997 5: 35pm
Scanning Rate: -10.0 C/min
Sample Wt: 1.000 mgPath:c:\huey\ter
TRZ-WiCFile 1."
P
IN-ELA§ER SC7
Figure 5-11 : DSC cooling thermogram of 20 % nitric acid, 5 %sulfuric acid ternary
acid system.
ST -20-A.5%SA.
istRun.Heat
(teri2it)
T
7.5 I
T
5
J/g - 102.47
T
T
2.5
Peak from:-43.49
to:-22.00
Onset--42.07
T
cao front -84.49
to:-'4.00
Onsat--7.O80
,g - 32.62
Peak from:-109.99
to:-94.25
b~at--"j
T
Peak--104.70
0
L~*d.00 -- :io.00
--- .oo
Date:Jan 03.1997 6:01pm
Scanning Rate: 3.0 C/min
Sample Wt: 1.000 mg Path:c:\huey\ter
File I ,EP
.SC7
C
0.00oo
o.00o-...00
-10.00
.
•0.00o .
Temperature (C)
-
PEZI7R-?l A/EJ
50"(
ox2b5'/-
Figure 5-12 : DSC heating thermogram of 20 % nitric acid, 5 % sulfuric acid
ternary acid system.
~~-`~-- ~~-~
-- ~'--~~-~--~ -~ -~~
- ---- ------- ---~~~~-
2.25
1.5
0
-""-""
Date: Nov 14.1996
ScanningRate:
Temperature (C)
6.52pm
3.0 C/min
PLEk';V"-EL /ER DSC,7
Sample Ht: 1.000 mg Path:c:\huey\ter
AC
File I.-TERWA'
Figure 5-13 : DSC heating thermogram of 20 % nitric acid, 20 % sulfuric acid ternary
acid system.
-.
020NA.
S2OXNA.
5-,SA.1st
1-t Run,
Run, Heat
Heat
O.±tSA.
(terh1)
*1~
!
:5
T
- 010o0 -ioo0
0o -
...-- .0o
0
Nov 12. 1996 5: 22pm
,ing Rate:
3.0 C/m:n
.e Wt: 1.000 mg Path. c: \huey\ter
I
AER
AC
-•0.-o --
.0- -
.00
Temperature (C)
PERI2n-1
ISCC7
Figure 5-14 : DSC heating thermogram of 20 %nitric acid, 0.5 % sulfuric acid
ternary acid system.
Chapter 6
Conclusions
Binary H20/HNO 3, H20/H 2SO 4 and ternary H20/HNO 3/H2SO 4 aerosol particles
were generated using two different techniques, and their freezing behavior was examined
with an optical microscope equipped with a cold stage.
The supercooling curve for binary H20/HNO3 aerosols was established to provide a
baseline for the ternary acid system. The freezing temperatures of the aerosols are
consistent with the results of the emulsion experiment carried out in our laboratory by
Chang. 54 Meilinger et al. 44 reported that stratospheric aerosols may approach a binary
nitric acid composition of 52 % nitric acid and 48 % water at 190 K, as a consequence of
rapid temperature fluctuations. The freezing temperature of 52 % binary nitric acid can be
projected from the supercooling curve to be around 190 K.
The freezing behavior of H20/H 2SO 4 aerosols confirmed that SAT does not form
upon cooling. The freezing behavior in the SAT regime is consistent with the FT-IR
experiment in our laboratory. Furthermore, no freezing was observed upon cooling even
for the 30 % binary sulfuric acid; however, it crystallized upon warming and the melting
point confirmed that the composition was 30 % sulfuric acid.
Results from the ternary H20/HNO3/H2SO 4 system indicated that even a small
amount of sulfuric acid will indeed inhibit the formation of NAT. Only ice formation is
likely in this ternary system. Therefore, ice does have to form first in freezing polar
stratospheric cloud particles if there is only homogeneous nucleation. The presence of
sulfuric acid also causes the freezing temperatures of the ternary aerosols to be at least 2025 K below the freezing temperature of binary nitric acid. For example, a ternary
composition of 2 % sulfuric acid and 25 % nitric acid , corresponding to the stratospheric
composition at 185 K, would not freeze at the stratospheric temperature. As a result, in
order for ice to form at stratospheric temperatures, the stratospheric aerosols would have to
cool and deliquesce to around 182 K. The melting temperatures of the aerosols were
checked with the DSC melting data for the bulk acids to confirm their composition. Since
type Ia polar stratospheric cloud can be observed from satellite and Lidar measurement at
temperatures around 190-195 K, heterogeneous freezing may explain the formation of
those particles at higher temperatures. Alternatively, the particles crystallized at low
temperatures, and were subsequently observed as solids at the higher temperatures.
The aerosol particles had several distinctive morphologies observed at different
freezing stages, and there were also morphology changes during cooling and warming.
The ongoing complementary FT-IR experiment in our laboratory may provide some
insights into the phase and composition of these particles.
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