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The University of Toronto Continuous Flow Diffusion
Chamber (UT-CFDC): A Simple Design for Ice Nucleation
Studies
a a
Zamin A. Kanji & Jonathan P. D. Abbatt
a
Department of Chemistry , University of Toronto , Toronto, Ontario, Canada
Published online: 23 Apr 2009.
To cite this article: Zamin A. Kanji & Jonathan P. D. Abbatt (2009) The University of Toronto Continuous Flow Diffusion
Chamber (UT-CFDC): A Simple Design for Ice Nucleation Studies, Aerosol Science and Technology, 43:7, 730-738, DOI:
10.1080/02786820902889861
To link to this article: http://dx.doi.org/10.1080/02786820902889861
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www.tandfonline.com/page/terms-and-conditionsAerosol Science and Technology, 43:730–738, 2009
Copyright © American Association for Aerosol Research
ISSN: 0278-6826 print / 1521-7388 online
DOI: 10.1080/02786820902889861
The University of Toronto Continuous Flow Diffusion
Chamber (UT-CFDC): A Simple Design for Ice Nucleation
Studies
Zamin A. Kanji and Jonathan P. D. Abbatt
Department of Chemistry, University of Toronto, Toronto, Ontario, Canada
(Hobbs and Radke 1975; Cooper and Saunders 1980), a mea-
A new instrument, the University of Toronto Continuous Flow surement that is complicated by ice multiplication mechanisms
Diffusion Chamber (UT-CFDC), has been designed to study ice nu- and shattering of large ice crystals in airborne probes. There is
cleation at low temperatures. Based on previous continuous flow thus a need to better quantify the number of IN in the atmo-
instruments, it is a parallel plate model that minimizes convective
sphere.
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instabilities by operating horizontally with the warmer plate on
top. A variable position sample injector can account for effects Heterogeneous ice nucleation can take place by contact, con-
arising from gravitational settling of ice particles that form. The densation or deposition freezing onto a solid aerosol particle at
residence time in the chamber can vary between 2.6 to 25 s and ice higher Ts and lower RHs than those required for homogeneous
particle formation is monitored with a two-channel optical particle nucleation. In particular, deposition freezing has been observed
counter. Observation of homogeneous freezing of 100 nm sulfu-
to take place well below water saturation (Dymarska et al. 2006;
ric acid aerosols was used to verify the accuracy of the calculated
relative humidities (RHs) in the chamber to be ±4%, where we Kanji and Abbatt 2006; Knopf and Koop 2006; Koehler et al.
report onset RHs for 0.1% of the particles freezing in the tem- 2007). This suggests that other mechanisms beyond homoge-
perature range of 218 to 243 K. We also show that the chamber neous nucleation should be included when accounting for the
accurately establishes conditions of water saturation by conducting ice crystals observed at conditions that do not fulfill the ho-
water uptake studies onto sulfuric acid aerosol at 243 K. The two
mogenous freezing criteria.
channel OPC allows for ice and water droplet formation to be dis-
tinguished under such conditions. The chamber is a simple, cheap, Instruments have been deployed in the past few years to
and small design that can be readily assembled for laboratory bridge the gap between field observations of ice crystal con-
studies. centrations and laboratory measurements of ice nuclei (Al-
Naimi and Saunders 1985; Mizuno and Fukuta 1995; Chen
et al. 1998; DeMott et al. 2003a, 2003b; Prenni et al. 2007).
1. INTRODUCTION These field instruments use continuous flow diffusion chambers
Aerosols and clouds in the upper troposphere and lower (CFDC) based on earlier designs (Hussain and Saunders 1984;
stratosphere play an important role in the Earth’s radiation bud- Al-Naimi and Saunders 1985; Tomlinson and Fukuta 1985;
get. Cirrus ice clouds cover about 25% of the Earth and strongly Rogers 1988). The CFDCs have also been used in the labo-
affect the radiative properties of the Earth by scattering and ratory as ice nucleation counters to investigate homogeneous
absorption of incoming and outgoing radiation. Contributions freezing of sulfuric acid (Chen et al. 2000), and heterogeneous
of ice clouds to the Earth’s radiation budget still remain highly freezing onto mineral dust (Archuleta et al. 2005), silver io-
uncertain (IPCC 2007), with one issue being the identification dide (Stetzer et al. 2008), montmorillonite and kaolinite (Salam
of ice crystal formation mechanisms. For example, a common et al. 2006), and soot and soot coated with sulfuric acid (DeMott
observation is that the numbers of ice crystals in supercooled et al. 1999). While groundbreaking in their capabilities, some
clouds are larger than those of ice nuclei (IN) measured under limitations from earlier chambers include the offline manner by
similar relative humidity (RH) and temperature (T) conditions which ice crystals are observed (Tomlinson and Fukuta 1985)
and concentric cylindrical geometry which makes their design
complex for cooling the chamber and for introduction of the
sample and sheath flows (Rogers 1988; Salam et al. 2006). The
Received 2 October 2008; accepted 10 March 2009. fixed position of sample introduction also means that the particle
This work was funded by NSERC. We acknowledge the valuable residence time can only be changed by altering the total flow in
comments from the two anonymous reviewers.
Address correspondence to Jonathan P. D. Abbatt, Department of the chamber. Finally, the chambers have typically used special
Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, treatment of the inner chamber walls to minimize background
M5S 3H6, Canada. E-mail: jabbatt@chem.utoronto.ca signals. At present, there is no commercial IN counter.
730UNIVERSITY OF TORONTO ICE NUCLEATION CHAMBER 731
Other methods of measuring ice formation in the laboratory sample flow sandwiched between two particle free sheath flows
include using static diffusion chambers (Dymarska et al. 2006; set to have flow rates at least 10 times that of the sample. The
Kanji and Abbatt 2006; Knopf and Koop 2006) and differential overall flow in the chamber is in the laminar regime to ensure
scanning calorimetry (Zobrist et al. 2006; Marcolli et al. 2007) to that the sample flow is maintained in the middle of the particle-
measure ice formation in the immersion, condensation-freezing, free sheath flows. The upper and lower walls of the chamber
and deposition mode onto various mineral aerosols and organics. are maintained at different temperatures and have their inner
While the above methods can be sensitive to a very small fraction surfaces coated with ice. With a linear temperature gradient, the
of particles freezing there can be upper limits for the maximum temperature and RH conditions in the center of the chamber can
attainable RH because particles are typically deposited onto a be determined by knowing the temperature on both the cold and
supporting substrate. In the case where microscopy is used to warm walls. A steady state partial pressure of water develops
observe ice crystals, there are size limitations associated with between the cold and warm walls and is linear as a function of
detection of small particulates and ice crystals. By contrast a distance between the two walls. However, the equilibrium vapor
CFDC permits high RHs to be used, as well as smaller particles pressure as a function of T is not linear therefore establishing a
that are suspended as aerosol. The CFDC is also easy to couple supersaturated region close to the center of the chamber.
to other detection devices to study physical, chemical, and size Figure 1 illustrates this situation for a sample temperature of
effects on ice nucleation. 223 K with the warm wall at 233 K and cold wall at 213 K,
In the current study at the University of Toronto a new CFDC where the vapor pressures over ice and water were calculated
(UT-CFDC) is built based on the working principle of early con- from Murphy and Koop (2005). In this case the temperature
tinuous flow instruments (Hussain and Saunders 1984; Al-Naimi difference between the two walls is quite large and so there
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and Saunders 1985; Tomlinson and Fukuta 1985; Rogers 1988) is not only a supersaturated region with respect to ice but also
and by modifying the design of the Kumar et al. (2003) parallel- with respect to water; most work reported in this article occurred
plate CCN chamber. In previous ice nucleation studies we have with somewhat smaller temperature gradients where only ice su-
examined ice formation onto solid particulates in a static cham- persaturations prevailed. Knowing this temperature profile then
ber and used microscopy and digital photography to detect ice allows the determination of RH at any point between the two
formation (Abbatt et al. 2006; Kanji and Abbatt 2006; Kanji et plates. By passing particles through the supersaturated region,
al. 2008). The UT-CFDC was developed to further investigate some fraction of the added aerosol particles nucleate to form ice.
deposition mode nucleation onto dust particles, including the ef-
fects of chemical processing and particulate size on nucleation.
The UT-CFDC was particularly designed for low temperature 0.20 2.0
ice nucleation in the deposition mode and can also be used for
homogeneous nucleation that is relevant to cirrus regimes. How- 1.8
ever, it also has the capability to work at warmer temperatures Water Sat. 1.6
where condensation freezing at water saturation occurs. Recog- 0.15 Ice Sat.
1.4
nizing that there are no commercial ice chambers on the market,
Saturation Ratio
the specific design goals were to develop a small, compact con- 1.2
Vp (mbar)
tinuous flow chamber that is simple to build, easy to cool to 0.10 1.0
low temperatures, has inner surfaces that are easily coated with p water
0.8
ice, and yet is capable of measuring ice formation much like p ice
other continuous flow chambers. Unlike many chambers, it is 0.6
0.05
a flat parallel plate design that is horizontally oriented to min- 0.4
imize convective effects. For the first time, a variable position
0.2
sample injector is used that has the ability to account for both
potential ice crystal losses by gravitational settling as well as 0.00 0.0
210 215 220 225 230 235
to allow for ice particle nucleation and growth kinetic studies.
Finally, background signals are required to be sufficiently low Temperature (K)
that this instrument could be used at future times in field studies
FIG. 1. Supersaturation profile in a CFDC. This example illustrates a case
for ground level ambient aerosol sampling. where the cold wall is at 213 K and warm wall at 233 K (vertical dash-dotted
lines), and sample temperature of 223 K in the center. Open-symbol-line: equi-
librium vapor pressure of water with respect to water. Closed-symbol-line: the
2. GENERAL OVERVIEW OF PRINCIPLE same but with respect to ice. Black straight line: steady state partial pressure
of water across the chamber. Dash dotted curve: saturation profile in chamber
The principle used for detection of ice formation in the cur-
with respect to water. Dotted curve: same but with respect to ice. The colored
rent study is similar to that described in detail by Rogers (1988), lines refer to the left axis and the black ones to the right. For most experiments
as well as by Al-Naimi and Saunders (1985). Continuous flow the saturation ratio with respect to water did not exceed 1. (Figure provided in
thermal gradient diffusion IN counters operate with an aerosol color online.)732 Z. A. KANJI AND J. P. D. ABBATT
achieve the desired concentration of particles. The poly-disperse
aerosol from the atomizer was then passed through a scanning
mobility particle sizer (SMPS, TSI electrostatic classifier model
3080, DMA column 3081) to size select for 100 nm particles.
The flow of mono-disperse particles (typically 0.6 lpm) was
then split equally and fed into the CPC (TSI model 3010) and
the main chamber. The CPC measures particles in the size range
10 nm–3 µm and requires a sample input flow of 1 lpm. Since
the aerosol flow was 0.3 lpm, the remaining make-up flow came
from filtered room air and the concentrations recorded by the
CPC were adjusted to reflect the dilution. Typical concentrations
in the sample flow entering the CFDC were 350–1000 particles
cm−3 .
FIG. 2. Schematic showing the setup of the entire system as an IN counter. 3.2. Ice Nucleation Chamber
Directions of arrows indicate direction of flow of aerosols. Refer to text for Ice formation takes place in a rectangular-shaped chamber
acronym definitions. that has outer dimensions of 50.8 × 25.4 × 3.84 cm. The cham-
ber is made up of two horizontal flat copper plates (Figure 3)
that sandwich a TeflonR spacer (Figure 4). The inner surfaces
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3. DESIGN OF THE UT-CFDC
The apparatus used in this work consists of three components of the copper plates are lined with quartz-fiber filter paper (Pall
(Figure 2): (1) aerosol generation, conditioning, size selection, Corporation, Michigan) that is wetted prior to any experiment.
and counting, (2) ice nucleation in the UT-CFDC under con- This ensures a uniform coating of ice along the inner walls
trolled and defined temperature and supersaturation, and (3) ice of the chamber. After initial wetting, the filter paper adhered
detection using an optical particle counter (OPC). A National strongly to the copper surfaces and did not degrade with time
Instruments Inc. data acquisition board logs to a computer the as occurs with paper-fiber filter paper. The top copper plate has
temperature from thermocouples, and the particle and ice crys- five sealable ports that can be used to wet the top and bottom
tal concentrations from a condensation particle counter (CPC) plate. Three of the ports are in contact with the filter paper that
and the OPC, respectively. lines the top plate. The other two provide access to the bottom
plate filter paper by creating holes in the filter paper that lines
the top plate. While normally resting horizontally, the chamber
3.1. Aerosol Generation and Counting can be easily tilted to a 45◦ angle. In this position, the chamber
For the validation of the IN counter, aerosols are formed by is wetted by introducing approximately 26–30 ml of de-ionized
a TSIR constant output atomizer using a 70% w/w composition distilled water (Aldrich, 18.2 M) through the five ports. The
sulfuric acid prepared from 96% concentrated H2 SO4 (Fisher chamber is subsequently drained of any excess water through
Scientific, ACS grade). The sulfuric acid particles were passed the exit at the downstream end while still in the slant position.
through a silica-gel diffusion drier to remove excess moisture. The chamber is then raised back to the horizontal position and
In some cases a dilution flow of dry N2 (Figure 2) was added to the OPC is connected to the exit of the chamber.
FIG. 3. Lengthwise cross-section of chamber. Bold arrows indicate direction of particle-free sheath air that makes up about 90% of the total flow in the chamber.
The remaining 10% is made up by the sample flow indicated by thinner arrow. (Figure provided in color online.)UNIVERSITY OF TORONTO ICE NUCLEATION CHAMBER 733
FIG. 4. Top-view of the Teflon R
spacer that is sandwiched between the warm and cold copper plates. The O-ring seal between the three pieces (on either side
of the TeflonR
spacer) ensures the chamber is air tight. The injector has 6 ports that allow sample into the chamber. The minimum residence time for the aerosol
is achieved by pushing the injector in all the way so that aerosol is introduced into the chamber where the cone shape of the chamber begins. To ensure stability,
the ends of the injector slide within a notch in the Teflon
R
spacer.
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Cooling coils (0.95 cm o.d. and 172 cm length per plate) 223 K is 20 which is well below the critical value where the
soldered to the copper plates (Figure 3) have a coolant (Syltherm transition from laminar to turbulent begins to occur in parallel
XLT, Dow Corning Corporation) flowing through them from plates. For reference, if air at room temperature is entering
two re-circulating chillers, one for the warmer wall (Julabo USA the chamber, it takes about 0.3–2 seconds to achieve steady
Inc., Model FP88-MW) and the other for the colder wall (Neslab state temperature and water vapor partial pressure conditions
Instruments Inc. Model ULT-90). Holes (0.32 cm diameter) are (Hussain and Saunders 1984; Rogers 1988). The downstream
drilled into the sides of the copper plate (Figure 3) for insertion end of the chamber is funnel shaped so as to direct the flow into
of thermocouples to take temperature measurements. The holes the center as it exits into the OPC.
are centered 3.2 mm below the inner surface of the copper
plates and they are 12.7 cm in length, so that the thermocouples
are measuring the temperature in the centre of the chamber. 3.3. Ice Detection
Temperature measurements at four collinear points along the Ice is detected using an optical particle counter (OPC) that is
length of each plate are possible. directly coupled to a 0.64 cm tube exiting the chamber through
As determined by the OPC internal pump, the total flow a single 0.64 to 0.95 cm o-ring compression fitting. This ensures
through the chamber is 2.83 lpm. The sample flow was set to be that the flow exiting the CFDC does not warm sufficiently that
10% of this and the remainder of the flow, i.e., the sheath flow, the ice particles evaporate before being analyzed by the OPC.
was made up of dry particle-free N2 . The sample is injected The commercial OPC (Climet, Model CI-20) is capable of mea-
through a 0.64 cm diameter stainless steel T-shaped movable suring particles in two size bins, particles greater than 5 µm
injector that has 6 holes (1.6 mm diameter) across the width and those between 0.5 and 5 µm. It samples at a flow rate of
of the chamber as shown in Figure 4. The outermost holes on 2.83 lpm and encounters 10% coincidence errors when the par-
the injector are 6.2 cm from the walls of the TeflonR spacer so ticle concentration exceeds 3.5 × 102 particles cm–3 . Since the
as to avoid inhomogeneous relative humidity and particle loss smallest particle size detectable by the OPC is 500 nm, the par-
effects. The injector is movable so that the position at which the ticle size for validation experiments introduced into the CFDC
aerosol is introduced can be varied to change the residence time was chosen to be 100 nm so that when there is no ice formation,
of the particles in the chamber. The sheath flow is injected at the no background counts from the OPC are recorded arising from
upstream end (Figure 4) of the chamber through 4 ports (0.64 cm particle introduction. However, when ice forms, the particle al-
diameter) that are drilled into the TeflonR spacer. With the sheath ways grows to sizes larger than 500 nm and usually to larger
and sample flows, the maximum residence time in the center of than 5 µm. The applicability of each OPC channel to ice detec-
the chamber is 25 s (calculated with the bulk flow) and can be tion under specific conditions is discussed later in the article.
achieved when the injector is pulled back all the way. The sheath The background count rate in the smaller size bin was typically
flow is introduced above and below the stainless steel injector quite low, around 0.02 particles cm–3 ; in the larger size bin, the
and ensures the particle flow is kept in the center. The Reynolds background count rate was typically 0.0005 particles cm–3 . The
number for the given flows and dimensions at a temperature of source of background counts come from frost particles that may734 Z. A. KANJI AND J. P. D. ABBATT
RH i(%) Homogeneous freezing of sulfuric acid is only observed at
0
130 135 140 145 150 155 0
temperatures below 235 K (Koop 2000). Using sulfuric acid
10 10
aerosols generated from a 70% w/w solution, freezing experi-
P > 0.5 µm ments were conducted for the range 217 K < T < 244 K. The
-1
P > 5 µm
-1
10 10 RH of the chamber was increased by increasing T in the cham-
ber. The injector position, d, was chosen to be at an intermediate
Activated fraction per channel
10
-2 -2
10 position (30 cm) from the edge of the chamber (see Figure 4)
so as to give sufficient time (residence time of 12 s) for the ice
-3 -3
crystals to grow to be detected by the larger size bin, but also
10 10
short enough to avoid potential gravitational settling losses (see
below).
-4 -4
1x10 10 From a gravitational settling perspective, we note that Rogers
(1988) has described in detail that for various typical CFDC
-5
1x10
-5
10 supersaturation conditions ice particles consistently grow to a
maximum of 5–10 µm. The corresponding settling velocities for
-6 -6
this size range of ice crystals for our temperature and pressure
conditions are 0.1–0.3 cm s–1 (Finlayson-Pitts and Pitts 2000).
10 10
85 90 95 100 105
RH w(%)
For our chamber, the particle must fall 0.95 cm in order to
settle out of the flow and hit the bottom plate. So, for an ice
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FIG. 5. Activation curve for 230 K derived from a typical ice nucleation crystal 5 µm in size, it would require approximately 10 s to
experiment. The RH of the chamber is increased and concentration of ice crystals settle out of the flow. Using ice crystal and water droplet growth
monitored. The activated fraction is the ratio of OPC counts to particle counts, equations (Rogers and Yau 1989), we conducted calculations
derived from the CPC. The dashed line indicates for this temperature where to show that at 233K, RHw = 100% it would take the particle
we expect to see homogenous freezing of 100 nm H2 SO4 taken from Koop
about 6 seconds to grow to this 5 µm criterion, which is roughly
et al. (2000). Note that the large particle channel also increases indicating the
presence of ice. The particles in the smaller size channel could also be water the size at which gravitational settling would become important.
droplets after water saturation has been surpassed. However, it would then take approximately 10 s more for the
ice crystal to fully settle out of the flow, at which point it would
dislodge themselves from the ice-covered walls, and also from have been sampled by the OPC.
particles that coagulate or coalesce during their transmission Clearly, particle growth and gravitational settling are coupled
through the chamber. processes, and so this simple analysis is only an approximation
A typical experiment starts with the cold and warm walls to the true behavior. And so, for validation that gravitational set-
approximately 2 K apart in temperature (T), providing a very tling is not a major effect in our experiments we have conducted
small supersaturation with respect to ice and subsaturated with residence time experiments with Arizona Test Dust particles
respect to water. The temperature difference between the warm atomized from a suspension of ATD in water (approximately
and cold wall is then increased by changing the temperature 0.015 g/ml), passed through a diffusion dryer, and then stud-
set points on each re-circulating chiller while keeping the av- ied in the CFDC under conditions close to water saturation. In
erage temperature constant, thus increasing the supersaturation Figure 6, we plot the activated fraction of particles measured
in the chamber. The point at which ice starts forming is read- with both the smaller and larger size bins, as a function of sam-
ily apparent as the concentrations recorded by the OPC begin ple injector position. It is shown that for the residence time used
to increase from the low background level. An activation plot in this work (12 s), the smaller size bin data are converging to
(e.g., Figure 5) of the fraction of aerosol that forms ice as a a plateau value at about 30 cm injector position and higher, i.e.,
function of RH can then be produced and the RH for various there is sufficient time for particles to grow to the detectable
activated fractions can be determined. See the next section for size. For the larger size bin, corresponding to particles larger
a full discussion of Figure 5. than 5 µm, the effects of insufficient growth time are clearly
observed for positions less than 30 cm. Also, since the activated
fraction between the 14 and 25 s residence time is similar we
4. VALIDATION OF FREEZING CONDITIONS believe that for these temperature conditions that having a much
To validate the UT-CFDC we sought to determine whether longer residence time is not leading to gravitational settling.
the relative humidity conditions discussed in section 2 are ac- This is also consistent with the observations in Tomlinson and
curately achieved. For this we chose to study the homogeneous Fukuta (1985) and Rogers (1988) that despite various Ts and
freezing of 100 nm sulfuric acid aerosols since these conditions RHs ice crystals consistently grew to average sizes of 8 µm.
have been the best characterized of any particle system, both Note that the maximum activated fraction in these experiments
in the laboratory (Martin 2000) and via modeling work (Koop was somewhat below unity, possibly because of water vapor de-
et al. 2000). pletion arising from ice formation. In particular, we have notedUNIVERSITY OF TORONTO ICE NUCLEATION CHAMBER 735
0
10 180
14 25
-1 9 12 170
10
6 Homo. Freezing of 100 nm H 2SO 4
Observed activated fraction
Water Saturation
-2 160
10
RH i (%)
-3 150
10
P > 0.5 µm 140
-4
10 P > 5 µm
130
-5
10 210 220 230 240 250
10 20 30 40 50
Temperature (K)
Injector Position, d (cm)
FIG. 7. Summary of the 100 nm H2 SO4 homogeneous freezing experiments
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FIG. 6. Fraction of ATD particles that activate as ice crystals as a function of over a range of temperatures. Squares: Freezing fraction of at least 0.1% or
residence time in the UT-CFDC. These data were collected at 223 K and RHw = more of the particles. Stars: freezing of 0.1% of 50 nm H2 SO4 particles from
99%. The work presented in this article was all carried out at a residence time of Chen et al. (2000). Also, refer to text for explanation of the circle-shaped data
12 seconds, which corresponds to an injector position of 30 cm. The numbers points. The activated fraction was determined using the sum of the counts from
next to the data points indicate the corresponding residence times. the large and small particle channels. Error bars represent 1σ from at least 3
runs during an experiment.
that this maximum value for the activated fraction increases as In addition, note that even though homogeneous freezing is
the supersaturation values in the chamber are increased. This only observed at T < 235 K, there are data for 240 and 243 K
effect is due to a change in ice forming mechanism to con- in Figure 7. Even though at this temperature the data indicate
densation freezing, where the less active IN will only form ice that 0.1% of the particles activate at water saturation, we do
at water supersaturations and therefore not form ice in the de- not believe that freezing has occurred at this temperature. This
position mode. This would be consistent with not having the is illustrated in Figure 8, which shows the activation curve for
activated fraction reaching unity at water saturation. 100 nm H2 SO4 aerosols at 240 K. Note that at water saturation
For the sulfuric acid experiments used to validate the super- the smaller size channel indicates activation whereas the larger
saturation conditions in the chamber, the fraction of particles channel remains at background levels. However, we know that
that nucleated ice is plotted as a function of RH in Figure 5 for ice formation is indicated by growth in both channels since ice
an experiment conducted at 230 K. The freezing conditions for grows at faster rates than do water drops. In particular, in data
0.1% of the particles are reported as a function of temperature in not shown here, we have observed that for experiments at 243
Figure 7 juxtaposed against the homogeneous freezing curve for K that Arizona Test Dust (ATD) both OPC channels activate
100 nm H2 SO4 aerosols. Although the data are calculated from when ice formation occurs, presumably via a condensation-
counts in both the small and large size channels, it is the signal immersion freezing mode. This shows that there is sufficient
in the small size channel that dominates. Each data point in Fig- residence time in the chamber for ice particles to grow to at
ure 7 arises from a measurement where the RH is increased and least 5 µm in size at 243 K. For the case of sulfuric acid at 240
decreased in cycles so as to observe the RH conditions where K, since the growth is only in the smaller size channel it can be
the activated fraction increases to values of 0.1% or greater. concluded that the observation is due to growth of the particle
At all temperatures the values for at least 0.1% of particles ac- due to activation of water uptake to form water droplets, and that
tivated have been plotted. Since these points are close to the it is not due to ice formation. In addition, since the transition
homogeneous freezing line it shows that particles are observed to 0.1% activation coincides with the water saturation line (see
to freeze at the expected values thus validating the RH measure- Figure 7), the experiments confirm the calculated RHs in the
ments in the chamber. On the same plot, to compare our values chamber under these warmer temperature conditions.
with previous studies, we have data points for freezing of 50 nm To further validate the behavior observed in the chamber, we
sulfuric acid particles from Chen et al. (2000). These match up have conducted a number of calculations. First, using the online
quite well with our data and serve as an additional confirmation Aerosol Inorganic Model (AIM) 1 (http://www.aim.env.uea.ac.
that the chamber operates as expected in the temperature range uk/aim/aim.php) (Carslaw et al. 1995; Massucci et al. 1999;
studied. Clegg and Brimblecombe 2005) we have confirmed that uptake736 Z. A. KANJI AND J. P. D. ABBATT
RH i(%) 223 K, ice crystals just approach the 5 µm size for a RHi
0
120 125 130 135 140
0
value of 145%. Indeed, Figure 6 confirms this behavior, where
10 10
incomplete activation into the 5 µm channel is observed at
P > 0.5 µm 223 K and 12 s residence time. Figure 6 illustrates that longer
10
-1 P > 5 µm -1
10 residence times are required for more complete activation at this
low temperature.
Activated fraction per channel
10
-2 -2
10 The uncertainty in calculated chamber RH values will come
mostly from uncertainties in temperature measurements (±0.1
-3 -3
K) along the length of the copper plates. For two measurements
10 10
from the upper and lower plates, the maximum uncertainty that
could arise is ±0.2 K, which translates into an uncertainty in
RH of approximately ±4% in the center of the chamber. This
-4 -4
1x10 10
estimated uncertainty is roughly consistent with the agreement
-5
1x10 10
-5 between the measured and expected values for ice/water forma-
tion in Figure 8, which is on the order of a few percent as well.
-6 -6
10 10
85 90 95 100 105
RH w(%)
5. ATTRIBUTES AND LIMITATIONS OF THE IN
COUNTER
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FIG. 8. Activation curve of H2 SO4 aerosols at 240 K This temperature is Designed for laboratory studies, the UT-CFDC design is
above where we expect homogeneous freezing and therefore no ice formation based on a flat plate horizontal CCN chamber (Kumar et al.
should be observed. Note that once water saturation has been reached, the P > 2003). One of the biggest advantages is that the design requires
0.5 µm channel (squares) starts to increase indicating water droplet formation.
minimal machining and is easily built in house. In addition, the
However the P > 5µm channel (circles) remains at background level, indicating
no ice formation. introduction of sample and sheath flows is relatively straightfor-
ward compared to what is involved in a cylindrical geometry.
The ice detector used is a commercially available optical par-
of water by 100 nm sulfuric acid particles at 99% RHw and from ticle counter that is inexpensive ($5000) and can nevertheless
243 > T > 223 will give rise at equilibrium to 410 nm particles, distinguish between water droplets from ice particles by virtue
i.e., without water droplet activation, the conditions would be of having two different size channels. In particular, the large par-
insufficient to give rise to 500 nm particles and be detected in the ticle channel indicates ice formation exclusively because water
small OPC channel. Since sulfuric acid is the most hygroscopic drops do not grow to that large a size in the given residence
material encountered in the atmosphere, it is safe to say that if times, for temperatures up to 243 K and RHw = 104%. The
aerosol is size selected to 100 nm or smaller, growth in the 0.5 chamber also has one standard total flow rate that is determined
µm channel must be due to ice formation. by the total flow of the OPC, and no external vacuum pumps are
Second, to confirm that we indeed do not have water droplets required. For the first time to our knowledge, the design allows
that grow to 5 µm thus contaminating the large OPC channel, the residence time of particles in the chamber to be changed
we calculated the droplet growth rates using equations from by adjusting the position of the movable sample injector. This
Rogers and Yau (1989) for 12 s residence time, the maximum allows for effects of gravitational settling or insufficient crys-
saturation reached in Figure 8 (RHw = 103%) and 243 K. We tal growth time to be assessed. The icing process for both the
find that a water droplet would grow to approximately 4.6 µm cold and warm wall does not require pumping water in and out
under such conditions, and that either a higher saturation ratio of the chamber under freezing conditions as is performed by
or residence time would be required for water droplets to make other systems, nor does it require special mechanical treatment
it into the 5 µm channel. For cooler temperatures, 233 and 223 such as ebonizing (Rogers1988) or sand blasting (Stetzer et al.
K, the growth is even slower at this supersaturation. This shows 2008) of the inner walls of the chamber. All that is required is
that it would require high water saturations before the large that commercially available quartz filter paper lines the walls.
particle channel would be contaminated by water droplets. For Nevertheless, the background count rates, especially in the large
the current configuration we therefore describe our upper RHw particle size bin, are sufficiently low that measurements of IN
limit as 103% for the highest temperatures. numbers in the field may eventually be possible. Finally, the
Finally, again using the approach described in Rogers and temperature of the plates is easily controlled by two separate
Yau (1989), we calculated the conditions required for ice crystals re-circulating chillers therefore allowing precise achievement
to grow in size, for a range of temperatures. For 233 K and of sample T and RHs during a given experiment. If only one
243 K, there is no kinetic constraint for growth into the 5 µm chiller is available, then automated, rapid switching of the flow
channel down to an RHi value of 112 and 104% respectively, between the two plates with variable durations of cold flow
for a residence time of 12 s. At the lowest temperature studies, would provide a stable temperature gradient.UNIVERSITY OF TORONTO ICE NUCLEATION CHAMBER 737
On the other hand, there are limitations to the chamber im- REFERENCES
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