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Role of Organic Hydrocarbons in Atmospheric Ice Formation via Contact Freezing Kristen N. Collier, and Sarah D. Brooks J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11890 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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The Journal of Physical Chemistry

Role of Organic Hydrocarbons in Atmospheric Ice Formation via Contact Freezing Kristen N. Collier and Sarah D. Brooks*

Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, United States

*Corresponding author.

ABSTRACT: An optical ice microscope apparatus equipped with a sealed cooling stage and CCD camera was used to examine contact freezing events between a water droplet and ice nucleating particles (INP) containing organic hydrocarbons including octacosane, squalane, and squalene. Sample viscosities were measured with a capillary viscometer and compositions were characterized using Fourier Transfer Infrared Spectroscopy with Horizontal Attenuated Total Reflectance and Raman Microspectroscopy. All of the samples proved to be moderately efficient ice nuclei that induced freezing between -23 and -26° C, regardless of whether the INP was solid or liquid. At their ice nucleating temperatures, the viscosity of the liquid samples (squalane and squalene) was 0.6 Poise or greater. Oxidation increased the viscosity of squalene to over 1330 Poise, but decreased viscosity of squalane to 0.07 Poise at room temperature. Most importantly, our results demonstrate that even moderately viscous liquids in contact with water droplets can act to catalyze freezing, plausibly by providing a flexible template which decreases the energy barrier to ice nucleation. The simple soccer ball model of nucleation theory was used to derive the probability of freezing and nucleation rate coefficients as a function of temperature for each type of IN.

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INTRODUCTION In the atmosphere, a perplexing number of pathways between aerosols and water droplets lead to the formation of ice crystals. Aerosol-cloud interactions and specifically ice nucleation are major uncertainties in global climate models to date.1 Homogeneous freezing only occurs at temperatures below ~-36 °C.2 Above this temperature, water droplets freeze when catalyzed by an ice-nucleating particle (INP) either in contact with or immersed within the droplet. Historically, it has been thought the general physical requirement for an aerosol to be an INP is to be a solid possessing a hexagonal crystal lattice structure similar to that of water ice.3-4 This describes many but not all of the substances known to efficiently nucleate ice. The laboratory measurement of Bryant et al. provide a visual demonstration of ice crystal growth initiated on INP with hexagonal crystalline symmetry and a close lattice match to water ice, including silver iodide, lead iodide, copper sulfide, cadmium. However, the authors also observed exceptions to the rule which were good nucleators despite unmatched geometry, such as the rhombohedral calcite and muscovite.5. Additionally, molecular dynamics studies have shown that traits other than lattice match can boost a substances’ propensity to act as an INP, including the formation of a liquid water layer prior to activation, the buckling of the INP material into an ice-like geometry, and nucleation on compact surfaces with high interaction strength.6-7. By definition, contact freezing occurs when an INP collides with a water droplet. Previously, it had been debated whether the root cause of nucleation is the force of impaction or the resulting positioning of the INP at the water-air interface which causes the nucleation.8-9 However, recent studies demonstrate that the presence of an INP at the surface of a water droplet even without a collision, promotes freezing.10-13 Contact freezing has been observed to occur at temperatures several degrees warmer than immersion freezing on identical INP.11

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In the atmosphere, a wide variety of aerosols can act as INP including dust, volcanic ash, diesel emissions, industrial pollutants, organic aerosols, bacteria, pollen, and even cellulose.10-11, 14-17

Volatile organic compounds (VOCs) are emitted in abundance from both biogenic and

anthropogenic sources.18-20 Photochemical oxidation of VOCs produces a variety of lower volatility organic compounds 21-22, which condense to form secondary organic aerosol (SOA) 20, 23-27

. Organic compounds containing a broad range of molecular structures and functional

groups, including those representative SOA, have been observed to nucleate ice.10, 28-36 These results are particularly interesting because SOA have been observed to exist in amorphous semisolid or viscous liquid states, rather than as solids.37-41 Hence, these recent reports of amorphous organics acting as INP force us to reevaluate our underlying understanding of the physical and chemical properties that promote heterogeneous freezing. Amorphous materials are defined according to their viscosities, i.e. liquids (< 103 Poise), semi-solids (~103-1013 Poise), and glassy solids (>1013 Poise).38 In glassy solids, molecular structures lack the high degree of alignment typically associated with a solid, but still maintain characteristics of the solid phase. To account for the ability of non-rigid non-hexagonal surfaces to act as INP, the concept that a flexible substance may self-assemble at an air-water surface to catalyze freezing was introduced.42 When liquid water templates onto a solid surface, strain is inevitably introduced due to an imperfect lattice match. Since the solid is inflexible, all the strain is partitioned to the water. In contrast, if the template itself is flexible, some of the strain could be partitioned to the template. Hence the energy barrier is lowered, making it possible for the water droplet to freeze heterogeneously. This was very recently demonstrated through a series of molecular dynamic runs of heterogeneous ice nucleation on model kaolinite surfaces with varying degrees of flexibility.43 In that case, the flexibility was a characteristic of amphoteric

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kaolinite (001) surfaces in which rotating hydroxyl groups facilitate the nucleation process. Additional experiments with flexible ice nuclei are needed to determine how flexible a substance can be, i.e. how low the viscosity can be, and still support ice crystal nucleation. Viscous templates may be very effective heterogeneous ice catalysts.44 In addition to viscosity, differences in the molecular structures have been proposed as a cause of differences in ice catalysis strength. Amongst the highly effective catalysts of freezing are steroids, such as testosterone and pregnenolene, which catalyze freezing at just 1 °C of supercooling.45 Also, long chain acid alcohols have been observed to act as freezing catalysts.46 Long chain acids undergo a structural rearrangement when cooled from 6 to 1 °C47. It has been suggested that this rearrangement allowed the alcohols such as heptadecanol, to act as effective catalysts of the water freezing. As a catalyst, the heptadecanol outperformed compounds which are not expected to undergo structural rearrangements, i.e. long chain alkanes. It has also been shown that template-directed nucleation can be promoted by organic macromolecules with strong crystalsubstrate-binding free energies, even when the macromolecule structure is unlike that of waterice.48 A survey of recent literature demonstrates that the role of viscosity on heterogeneous ice nucleation processes is complex and depends, in part, on the nucleation mechanism. In the case of immersion freezing, mechanistic differences between nucleation involving viscous liquid vs. glass INP have been observed.34-35 Viscosity may control the repeatability of immersion freezing in subsequent freeze-thaw cycles; it may enhance the freezing ability in subsequent runs occurring in certain cases.49 Finally, viscosity may determine the outcome of competition between water uptake and ice nucleation processes.31, 49 To date, no contact freezing experiments

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on viscous liquids have been conducted. Logically, the impacts on contact mode freezing, a very different process, are expected to differ from impacts on immersion events. The three organic compounds used in this study are present in SOA and vary in phase and viscosity. Octacosane is a straight chain saturated alkane (C28H58) present in solid crystalline form at ambient temperature and pressure. In contrast, squalane and squalene are liquids. Squalane is a highly branched saturated alkane (C30H62), and squalene a branched unsaturated alkene (C30H50). Octacosane and squalane have been used previously in measurements of changes in aerosols as a result of atmospheric oxidation.50-51 Laboratory studies have demonstrated that aging processing including oxidation, multiphase chemistry, and liquid-solid phase transitions lead to improved ice nucleation efficiencies for representative atmospheric INP.32, 34, 37 Variations in repeated others have reported nucleation runs on oxygenated organic INPs.52-53 Oxidation of an INP's surface may add flexibility to a surface structure at the molecular level, through the formation of flexible hydroxyl groups at the surface; it is plausible that the increased flexibility will enhance ice nucleation. Alternatively, in some cases, oxidation may modify the bulk viscosity and thus the larger scale flexibility of the INP. To date, more work is needed to understand how molecular structure and phase modulate contact freezing processes.

1. EXPERIMENTAL METHODS a. Sample Preparation Prior to ice nucleation experiments, solid octacosane samples were size-selected by sifting material with a 3 in. diameter wire mesh sieve (Newark Wire Cloth Co. with a 250-300 µm mesh) and either used directly or exposed to ozone as described below. The compounds used

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as representative INP include commercially available octacosane (Alfa Aesar, 99% purity), squalane (Alfa Aesar, 98% purity), and squalene (Alfa Aesar, 98% purity). To enough solid material for multiple freezing experiments (Figure 1), 10 g of material were placed on a glass fiber filter (Adventec) inside a Nalgene chamber. A HC-30 generator (Ozone Solutions) was used to produce ozone for the oxidation process. A mass flow controller (Model MC-10 SLPM-D, Alicat) provided a 0.01 L/minute flow of oxygen to the generator. Within the generator, the molecular oxygen passes through a corona cell which causes a fraction of it to divide into atomic oxygen. Next, the atomic oxygen reacts with the remaining diatomic oxygen to form high concentrations of ozone (5-14% by weight or 50,000-140,000 ppm). Following this, the ozone is diluted with nitrogen to yield an ozone concentration of ~80 ppm, which enters the Nalgene chamber. A UV absorption analyzer (UV-100, Eco Sensors, Inc., 254 nm) is used to continuously monitor ozone concentration throughout the oxidation process. After a 24-hour exposure period, the sample was removed and stored in an amber jar to prevent further oxidation.

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Figure 1: Schematic of the experimental setup for oxidation. The sample represents the Nalgene chamber or impinger, depending on whether a solid or liquid sample is being oxidized, respectively.

To oxidize sufficient liquid sample for multiple runs, a 10 mL sample (i.e. squalane or squalene) was placed inside a glass impinger instead of the Nalgene chamber. This allows ozone to bubble through the sample and oxidize it. As with the solid sample, the ozone was introduced into the sample at a concentration of ~ 80 ppm for 24 hours. After exposure, the oxidized samples were transferred to amber jars for storage to prevent further oxidation.

Figure 2: Ice microscope apparatus (modified from Fornea et al, 2009).

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b. Ice Nucleation Measurements A custom ice microscope apparatus was used to obtain a series of freezing temperature data points on a single droplet-INP setup following our well-established experimental method.11, 37

The procedure for conducting nucleation experiments is described in detail in Fornea et al.,11

and is thus briefly discussed here. The apparatus includes an Olympus optical microscope (Model BX51M), a digital camera (Q-Imaging Micropublisher 5.0 RTV), and a sealable Linkham cooling stage (Figure 2). With this setup, multiple independent freezing temperature measurements can be performed with a single droplet-INP system. The cooling stage allows temperature control to within ± 0.1 °C.11 In this study, experiments were conducted in contact mode, with the potential INP positioned directly adjacent to a 2.0 µL water droplet. At the start of each experiment, a sample is placed within the stage on a pre-cleaned slide which has been coated with 1.0% AquaSil solution (Pierce Chemical Company) to create a hydrophobic surface. For octacosane experiments, 5 x 10-4 g of solid sample is weighed out using a Quartz Microbalance (Mettler Toledo, PB503-S) and positioned in contact with the water droplet. The micropipette is used to position the solid INP in contact with the droplet. Once the droplet and potential INP are in place, the stage is sealed and cooled at 1.0 °C/min from 5.0 °C down to -40.0 °C. A camera mounted on top of the microscope (Figure 2) captures an image every 6 seconds which corresponds to 1 image every 0.1 °C temperature change. A low humidified nitrogen flow is introduced into the sealed stage to prevent droplet evaporation throughout the experimental runs. This flow is generated through mixing of a flow of dry nitrogen (0.6 lpm) with a second nitrogen flow that has become saturated by passing through a glass bubbler (0.01 lpm). A hygrometer (EdgeTech DewPrime II, Model 2000) is used to monitor the dew point of the moist flow. The dew point is maintained at approximately -39 °C

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to prevent droplet evaporation, but ensure that condensation does not form inside the sealed cooling stage. Once the cooling stage reaches a temperature of -40 °C, it is warmed to 5 °C at 1.0 °C/min. The temperature is maintained at 5 °C for one minute to ensure that the droplet has completely melted before the next cooling cycle begins. Each experiment involves a single droplet-INP setup cycled many times to produce ~ 25 independent freezing data points. The number of cycles varies somewhat due to occasional depletion of the liquid nitrogen, which cools the cold stage, prior to the end of an experiment. Once the experiment is complete, the images are analyzed on a frame-by-frame basis to determine the freezing temperature. A freezing event is distinguished by a change in opacity of the water droplet (transparent to opaque) between frames. For setups involving liquid INP (squalane and squalene), a 2.0 µL sample droplet is micropipetted into position in contact with the water drop. For each compound, at least 4 experiments involving new droplet-INP samples each time were conducted. It should be noted that both the water droplet and the INP are much larger than those found in the atmosphere; the mechanism-specific experimental procedure employed here requires droplets in this size range.

c. Chemical Characterization of Samples Fourier Transform Infrared Spectroscopy (FTIR-HATR) Fourier Transform Infrared Spectroscopy with Horizontal Attenuated Total Reflectance (FTIR-HATR) was used to characterize chemical composition of all samples before and after exposure to ozone. Prior to analysis, samples are placed on a zinc selenium crystal and inserted into a sealable HATR chamber (Pike Technologies). The HATR chamber is then placed into the FTIR spectrometer bench (PerkinElmer Spectrum 100). Employing the method of our previous

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work,7 the solid samples are mixed with reagent grade acetone producing colloidal slurry which adheres well to the crystal. Once the acetone evaporates (~2 hours), a uniform layer of the organic hydrocarbon remains on the crystal. Then, spectra are scanned over the wavenumber range of 700-4000 cm-1 at a resolution of 2 cm-1. Liquid samples are placed directly onto the crystal without acetone. Raman Microspectroscopy In addition to the FTIR-HATR, Raman microspectroscopy was used to characterize changes in the chemical composition of the samples as a result of oxidation. One advantage of Raman microspectroscopy is that it can be used to probe both surface and the bulk samples which could prove to be useful in the case of solid samples in which oxidation may occur only at the surface.54-56 Raman characterization was carried out using a Horiba Jobin Yvon LabRam HR Raman microscope with a 633 nm laser, available at the Texas A&M Materials Facility. Solid samples were analyzed using a 50x SLM objective. A 200 micron pinhole was used to probe the bulk of the solid sample, and a 50 micron pinhole was used in an attempt to penetrate only the surface of the sample. Liquid samples were examined using the 50x SLM objective and a 4x cuvette holder. Due to experimental limitations, differences between the surface and bulk oxidation of liquid samples could not be performed. d. Temperature-Dependent Viscosity Measurements Unexpectedly, we observed that the squalene samples became very thick and highly viscous during oxidation. This phase change poses an interesting question about whether viscosity plays a role in the INP ability of aged aerosol samples. To address this, we conducted a series of experiments to measure the viscosity of the liquid samples before and after ozone

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exposure and as a function of temperature. For comparison, the viscosities of olive oil, water, and ethylene glycol were also determined. To initiate an experiment, a glass capillary viscometer (Ubbelohde Viscometer Cannon Instruments Co., size 3B) (Figure 3) was filled with sample and submerged into a cooling bath (Neslab ULT 80/95) held at a chosen temperature, between 10°C and -40 °C for 20 minutes to equilibrate to a uniform temperature before measurements were taken. After equilibration, the viscometry measurements were conducted three times at each temperature setting. While the timescale for measuring the movement of most samples through the viscometer was on the order 1 to 10 seconds, the movement of oxidized squalene through the viscometer at room temperature (~23.5 °C) required ~9 hours. For oxidized squalene, measurements were not made at lower temperatures.

Following the experiments, viscosity was determined using the following formula: (1) where η is viscosity, t is flow time (s), c is the viscometer constant, and ρ is the density (g/mL). The densities for squalane and squalene are 0.81 g/mL and 0.858 g/mL (determined at 25 °C), according to their Material Safety Data Sheets (Sigma Aldrich). For the Ubbelohde 3B used here, the viscometer constant is 4.858 cSt/s, according to the manufacturer. Note that although the sample was cooled for 20 minutes in the Neslab bath to ensure temperature equilibration, the observation zone of the viscometer was necessarily positioned above the coolant level during readings, which could lead to a slight warming of the sample.

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3. RESULTS AND DISCUSSION a. Sample Characterization Prior to ice nucleation measurements, samples were characterized by FTIR-HATR spectroscopy and Raman microspectroscopy to assess whether or not oxidation has occurred and identify chemical products at the surface and bulk of the samples. The oxidation of octacosane, squalane, and squalene have been used previously as representative SOA compounds in studies of oxidation of SOA.50-51, 57-60 Although alkanes are not as readily oxidized as alkenes, long exposure time and high ozone concentrations, as employed here, are likely to force octacosane and squalane reactions via hydrogen abstraction.50

Figure 3: FTIR-HATR spectra before (fresh) and after exposure to ozone (oxidized) are shown. Spectrum A (light green) and Spectrum B (dark green) represent fresh and oxidized squalane, respectively. Spectrum C (orange) and Spectrum D (brown) represent fresh and oxidized squalene, respectively.

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FTIR-HATR spectra for fresh and oxidized liquid compounds are displayed in Figure 3. Major peaks in the squalane spectra (Spectra 3A and 3B) at 3000-2800 cm-1 and near 1450 cm-1 and 840 cm-1 indicate alkane bonds. Indications of oxidation are noted in several areas on the oxidized squalane spectrum (3B). The most notable indication is at 1390 cm-1, where an O-H bend associated with alcohols appears. A secondary peak near 1180 cm-1 also shows signs of oxidation, with the formation of a C-O stretch, also associated with alcohols. The formation of alcohol functional groups is consistent with the work of Kroll et al.,51 who identified this oxygenated functional group as a product of fragmentation of squalane. In the case of fresh squalene (Spectrum, 3C), the major peaks in FTIR-HATR spectra include an alkene multiplet of CH's at 3000-2850 cm-1, an alkene C=C stretch at 1675 cm-1 and 825 cm-1, and a C-H bend near 1450 cm-1. After ozone exposure (Spectrum 4D), oxidation products are evidenced through the peaks occurring at 1725 cm-1 for an O-H bond of a carboxylic acid, and the peak at 1380 cm-1 that characterizes an O-H bend for alcohols. Additional oxidation peaks occur at 1210 cm-1 and 1100 cm-1, where a C-O stretch associated with alcohols is present. These similar oxidation peaks are also identified in the spectrum of oxidized squalene by Fu et al.58 Compared to squalane, squalene more readily reacts with ozone to produce oxidation products, as is expected for alkene vs. alkane behavior. In the octacosane spectra (not pictured), exposure to ozone caused a small peak to develop near 1480 cm-1 indicative of an O-H bend. This indicates that octacosane underwent minor oxidation, qualitatively comparable to the squalane.

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Figure 4: Raman spectra of liquid samples. Squalane spectra are shown in Spectrum A (fresh, light green line) and Spectrum B (oxidized, dark green line). Squalene spectra are displayed in Spectrum C (fresh, light orange line) and Spectrum D (oxidized, dark orange line).

Raman spectra of the squalane and squalene before and after exposure to ozone are shown in Figure 5. The fresh squalane spectrum (4A) contains major peaks between 2950 cm-1 and 2800 cm-1 due to C-H stretches associated with an alkane, as well as a peak at 1475 cm-1 for a C-H stretch in aliphatic compounds. More C-H stretches are present in the peaks between 800 and 875 cm-1. The peak at 1430 cm-1 shows an indication that fresh sample may have been slightly oxidized prior to oxidation, as there is an O-H stretch representative of a carboxylic acid present. The oxidized spectrum for squalane (4B) provides only a weak indication of further oxidation at 575 cm-1 for the C=O bend found in aldehydes, as well as several C-O peaks at 1300 cm-1, 1150 cm-1, and 1050 cm-1. Identification of peaks in the fresh squalene spectrum (4C) also indicate several bonds associated with alkenes. A multiplet between 2900 cm-1 and 2850 cm-1 and peaks at 1440 cm-1,

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1000 cm-1, and 800 cm-1 are associated with C-H stretches found in alkenes. Similarly, a peak at 1680 cm-1 represents a C=C stretch for an alkene. Several peaks appear in the oxidized squalene spectra (4D) clearly confirming the effects of oxidative aging. A carboxylic acid is identified in the peak at 2950 cm-1 which indicates the presence of an O=H stretch. The peak at 650 cm-1 represents a C-O bend of an alcohol. Like the FTIR-HATR spectra for squalene, the Raman spectra are consistent with the formation of carboxylic acids and alcohols in the oxidized squalene as reported in other studies.58 In the case of octacosane, inspection of the Raman spectra collected using 50 µm pinhole on samples between and after exposure to ozone (not shown) indicated that oxidation has occurred. Comparing the exposed samples collected with the 50 µm pinhole (surface) and 200 µm pinhole (bulk), we found no discernable differences. Thus, any difference in oxidation level between the surface and bulk of the octacosane sample is below the sensitivity of this method. In summary, the Raman and FTIR-HATR spectra depict the effects of oxidation on the sample and confirm that oxidation has occurred to some degree in each of the three sample compositions. b. Ice Nucleation Results Ice Microscope Calibration Following our previous studies,11, 37 a three-point temperature calibration of the ice microscope's sealable cold stage was performed to verify the accuracy of sample temperature (Figure 6). Three compounds, n-dodecane, n-undecane, and n-decane (Sigma Aldrich), were chosen for the calibration based on their well-documented melting temperatures within the temperature range of our experiments.

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The experimental melting point temperatures vs. melting point values reported in the literature were plotted, and the resulting trendline equation fit through the data was y = 1.0x + 2.3, indicating a consistent offset of ~2.3 °C over the experimental temperature range. While the cause of this offset is not known for certain, the distance between the sample location on the cooling stage and the built-in temperature probe is greater than 2 inches which may contribute to the offset. The equation was used to apply a correction to all of the data collected. Fresh and Oxidized Organic Compounds as Ice Nuclei Experimental results for all ice nucleation runs of fresh and oxidized octacosane are indicated in Figure 5 by black circles and open diamonds, respectively. In the figure, each line represents one experimental run. Within the run, each independent freezing temperature determined through the multiple freeze-thaw cycles is shown. As seen in the figure, one oxidized octacosane run stands out as more variable than other runs, with freezing observed at temperatures as high as -12.4 °C in some cycles. The cause of this discrepancy is unclear. Contamination by a more efficient INP may have occurred. Alternatively, had poor INP placement occurred during set up, it would have led to either loss of surface contact or full immersion of the INP into the water, i.e. accidental immersion freezing. In either case, the expected result would most likely be freezing at a colder, not warmer than average freezing temperature. For most, though not necessarily all compounds, immersion freezing is less efficient than surface contact freezing.11, 61

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Figure 5: Freezing temperatures for fresh (black circles) and oxidized (white diamonds) octacosane. Each line represents one experimental run conducted with a single particle as an ice nucleus.

In general, a certain amount of scatter in the data is inherent in ice nucleation measurements in general and in using the ice microscope technique in particular.11, 13, 62-64 In our previous work using soot as potential INP, variations between the freezing temperature and the droplet-INP positioning within the CCD image were closely examined.65 Fluctuations in freezing temperatures were observed to coincide with movement of the soot particle seen freezing and melting. Additional contributing factors may include movement within the droplet itself, preparation of the droplet and soot particle on the slide, breakup of the INP. Particle breakup may cause exposure of surface areas on the particle that was not fully oxidized, thus decreasing the availability of active ice nucleation sites on the particle. One study attributed

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larger changes were due to alterations in INP, including non-random patterns like drifts, jumps, and slow variations.62 Overall, the mean fresh and oxidized octacosane initiated contact freezing temperatures were -26.4 ±0.8 °C and -25.3 ±2.1 °C, respectively. (Uncertainty is reported as the pooled standard deviation in the measurements.) The difference between these temperatures is statistically significant at the 95% confidence level. This improvement in INP ability after exposure to ozone is consistent with previous observations that oxidation of solid organic aerosols leads to improvement in their INP abilities.37 Contact freezing results for squalane INP are shown in Figure 6. Fresh squalane INP initiates freezing of the water droplet at -26.3 ±0.7 °C, on average. Unlike the octacosane case, exposure to ozone causes the squalane to induce freezing at a lower temperature, -28.2 ±1.0 °C on average, statistically significant at the 95% confidence level.

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Figure 6: Freezing temperatures observed for fresh (solid circles) and oxidized (white diamonds) squalane in contact with a water droplet.

Freezing results for squalene before and after exposure to ozone are shown in Figure 7. In this case, oxidation did not cause a significant change in the mean contact freezing temperature which was -27.3 ± 0.6 °C for fresh samples and -27.1 ± 0.8 °C after oxidation. These results are not statistically significant at the 95% confidence interval. We observed that oxidation caused the squalene to become very viscous, making it more challenging to position the INP in contact with the water droplet at the beginning of the run, which may have increased experimental uncertainty. Viscosity is addressed in greater detail in the section below.

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Figure 7: Observed contact freezing temperatures for fresh (black circles) and oxidized (white diamonds) squalene samples.

Contact freezing on many solid INP including organic compounds, bacteria, pollen, peat, soot, volcanic ash, and kaolinite have been reported in the past.11, 15-16, 37, 66 Observations of ice nucleation of water in contact with a liquid INP is unique to this study. Originally, we considered the possibility that the liquid samples might be mixing with the water droplet throughout the experimental run and initiating freezing after becoming a solution. To qualitatively test this possibility, food coloring was added to the water droplet and a contact freezing experiment was conducted. CCD camera images collected at the beginning of the experiment, upon freezing and melting of the droplet, and at the end of the experiment were all closely examined. This qualitative analysis indicated no mixing of the sample and droplet throughout the experiment.

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Table 1: Mean freezing temperatures for fresh and oxidized samples. Uncertainty is reported as a pooled standard deviation.

INP Composition

Diameter (µm)

No. of experiments 5

No. of freezing temperature data points 101

Mean freezing temperature (°C) ± pooled STD -26.4 ± 0.8

Fresh Octacosane Oxidized Octacosane

275 275

4

70

-25.3 ± 2.1

Fresh Squalane

~781

3

49

-26.3 ± 0.7

Oxidized Squalane Fresh Squalene

~781

4

79

-28.2 ± 1.0

~781

4

60

-27.3 ± 0.6

Oxidized Squalene

~781

4

70

-27.1 ± 0.8

Another possibility was that the liquid INP initially froze independently and thus provided the water with a solid INP on which to nucleation after all. To test this, freezing experiments for each organic liquid sample in its pure form (i.e. without a water droplet) were conducted. Pure squalane was observed to freeze at -38.0 ± 1.1 °C, consistent with the literature freezing temperature of -38 °C (Sigma Aldrich). Pure squalene was observed to freeze at -70.3 ± 1.3 °C, slightly above the literature value of -75 °C (Sigma Aldrich). On average, water droplets adjacent to these liquid INP froze 7.7 °C warmer than the homogenous freezing of pure water at ~ -33 °C measured in the identical apparatus.11

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Figure 8: Contact freezing temperatures for fresh and oxidized organic hydrocarbon nuclei are compared to the freezing temperatures of pure liquid samples. Water droplet freezing on fresh and oxidized INP are shown in red and blue, respectively. Pure squalane and pure squalene are show in green and orange, respectively.

Mean value and pooled standard deviations of freezing temperatures for all samples are summarized Table 1 and shown in Figure 8. The droplet diameters in Table 1 are estimated based on the diameter of a sphere. As the figure clearly shown, both pure organic samples remained in the liquid phase until well below the temperatures at which they acted as INP in any of the contact experiments. Our measurements strongly suggest that liquid samples in contact with water droplets can act as a catalyst to freezing. While we cannot say for certain that absolutely no mixing occurs, our observations suggest that any mixing is minimal. The freezing

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temperatures observed here for branched alkane and alkene liquids are colder than freezing temperatures observed of water covered by a thin film heptadecanol 46. In that case, the features of water ice were observed at -17 o C and warmer. However, we note that while our water droplet size of 0.2 µm diameter is larger than droplets in the atmosphere, the thin film method is much larger than ours which likely contributes to the freezing observed at warmer temperatures.

Overall, water droplets in contact with fresh INP froze at a mean temperature of -26.6 + 0.6°C, whereas droplets in contact with oxidized INP froze at a mean temperature of -26.8 + 1.5°C. Given the wide range of compounds included in this range, their freezing properties are remarkably similar. Most notable is the observation that even liquids are moderately effective INP.

c. Viscosity of Ice Nucleating Liquids 0 0 0 0 1 0 0 0 1 1 1 . 0

y t i s o c s i V

0 1

( )

P

0 0 1 1 0 . 0 1 0 0 . 0 0 4 -

0 3 -

(

0 C 2 ° -

e r u t 0 a 1 r e p m

e 0 T

0 1

0 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


)

Figure 9: Measured viscosities as a function of temperature. Red lines indicate squalane data, with closed and open triangles representing the fresh and oxidized samples, respectively. Black lines indicate squalene data, with closed and open triangles representing fresh and oxidized samples, respectively. The cyan line (squares) represents olive

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oil. The green line (diamonds) represents ethylene glycol, and the dark blue line (inverted triangles) represents water. Stars indicate the points at which contact freezing was initiated during ice microscope experiments.

Observed viscosities as a function of temperature are plotted in Figure 9. For oxidized squalene, the room temperature viscosity of oxidized squalene is assumed to be a lower limit of the viscosity at lower temperatures (indicated by the arrows in Figure 9). Note that the olive oil froze at -10 °C, the water froze at 0 °C, and the ethylene glycol froze at -30 °C, and thus could not be tested for viscosity below these points. An estimated experimental uncertainty in temperature of ± 0.8 °C was determined through comparison of the Neslab internal thermometer to an external RTD probe. Accurate viscosity measurements require the use of a viscometer designed for measurements within a limited range of observed flow times. For observed flow times below the recommended range, flow time is corrected for kinetic energy contributions using the Hagenbach-Couette (HC).67

Based on the dimensions of the 3B viscometer, the value

of the HC correction factor (0.2435 s3) is provided by the manufacturer. The corrected flow time can be calculated according to Equation 2:

(2) For the organic compounds included in this study, applying this the correction factor leads to a change in observation time of less than 1% in the majority of cases, with the largest correction, 3.5%, for oxidized squalane at room temperature. Based on the personnel uncertainty, HC correction factors, and uncertainties in density and temperature, the estimated overall uncertainty in viscosity measurements is ± 24%. Unlike the organic samples, the HC correction for pure water (which is much less viscous than the organic compounds studied), is very large and falls outside the range of times for which the ASTM methods (446-07) recommends applying HC corrections appropriate67. However, the observed values for the viscosities of water are in

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excellent agreement with values reported in the CRC Handbook of Chemistry and Physics values over this temperature range.68 In general, observed viscosity increased with decreasing temperature. The viscosity of oxidized squalene was found to be 1330.3 Poise. Aside from the exceptionally viscous oxidized squalene, the organic compounds exhibit low to moderate viscosities, comparable to ethylene glycol. At the temperatures at which the compounds act as INP (stars in the Figure 9), the viscosities range from about 0.6 to over 1330 Poise. While oxidation of squalane resulted in a decrease of viscosity by 76%, the same process causes a dramatic increase in viscosity of more than three orders of magnitude for squalene. This difference can be explained chemically. As a general rule, larger molecules correspond to higher viscosities Fu et al. report that oxidation causes functionalization of squalene resulting in formation of a larger molecule containing carboxylic acids.58 This is consistent with the observed increase in viscosity as well as the carboxylic acids evident in the Raman and FTIR-HATR spectra for squalene. Counter to this, a reduction in viscosity was observed upon oxidation of the squalane. Kroll et al. reported that prolonged aging leads to the formation of carbonyls and alcohols through the fragmentation of squalane.51 These products are consistent with our Raman and FTIR-HATR spectra as well. Taken together, the viscosity and ice nucleation measurements did not reveal a single "critical" minimum viscosity, which must be reached for an organic droplet to act as an INP. However, the results do imply that any hydrocarbons with a viscosity of at least 0.6 Poise is stiff enough to act as an INP and provide flexible templates for the nucleating ice crystals.43 Hence, a broad range of secondary organic aerosols, which are typically overlooked in atmospheric models, must now be considered in the available INP population in the atmosphere.

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d. Application of the Simplified Soccer Ball Model To consider the relative significance of the observed contact freezing behavior on viscous liquids, we calculate ice nucleation rates. Classical nucleation theory has been adapted for heterogeneous freezing to obtain heterogeneous nucleation rate coefficients. A computationally efficient model which combines the key elements of the stochastic (randomized) approach with the singular (temperature-dependent) approach to ice nucleation theory is known as the Simplified Soccer Ball model (SSB).6943 The basis of this model is the concept that each INP contains a specific number of active sites and that nucleation at each of these sites will occur according to a specific nucleation rate coefficient.69-70 The specific nucleation rate coefficient is dependent on the contact angle between the initial critical cluster of ice and the INP. To the best of our knowledge, this is the first application of SSB to the freezing of water droplets in contact with viscous liquids. In SSB, a population of particles contains a number of surface sites, nsite. A Gaussian probability distribution function, p, is used to describe the contact angle, θ, in terms of the mean contact angle of all sites in the population, µθ, and the standard deviation in contact angle, σθ: (3) Using a range of contact angles as opposed to a single value has proven to provide a description of ice nucleation with better agreement to experimental results involving solid INP.37, 71-72

As in our previous work, the model was fit to our experimental data to determine the range

in contact angles and nucleation rates for the samples studied.37 SSB assumes that each particle in the population has a uniform surface, such that nsite = 1. Consequently, every particle in the

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population has the same probability of freezing. For a single particle, the probability of not being frozen is:

(4) where T is temperature, ssite is the surface area of a nucleation site, and t is time. The nucleation rate coefficient, jhet, is defined as: (5) where ∆Fdiff represents the activation energy of water molecules diffusing across a solid-liquid interface, ∆G is the Gibbs free energy of formation for a critical cluster, n is the number density of water molecules at the ice/water interface, f is the wetting parameter, k is the Boltzmann constant, and h is Planck’s constant.73 Following previous studies,74,7b, 7d, 34 the surface area of the liquid INP is estimated as the total to be 4πr2 , the geometric surface area of a sphere. Values for each of these constants are well-known,11, 73 with the exception of wetting parameter, f. The wetting parameter is a function of contact angle, θ, and described below.

(6) It follows that, the fraction frozen for a droplet population interacting with INP of various type at a given temperature is

(7)

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An underlying assumption of the model is that a reduction in contact angle correlates with an increase in hydrophilicity, which yields a more effective INP.74 Taken together, these calculations are commonly used to assess INP.11, 37, 69, 75 Equations 3-7 can be used to determine the theoretical probability of freezing, Pfr, for each sample. Values for all parameters are well known and have been used in our previous work,7 with the exception of the wetting parameter, f, the mean contact angle, µθ, and distribution of contact angles

. To obtain these values the theoretical data is compared to our experimental

results using a construct known as the empirical probability. To determine the empirical probability for one compound, all data points collected during all experimental runs were collected into a single data set to determine the empirical probability of freezing. This probability is linked as a function of temperature to the theoretical probability by:

(8) where N0 is the total number of water droplets, Nf is the number of water droplets that have frozen, T is temperature, and t is time.13, 37 Figure 11 presents the empirical and theoretical probabilities of freezing with for each fresh and oxidized sample. Fits to the empirical probabilities were optimized by altering the mean contact angle, µθ, to match the empirical curve at the 50% frozen (a fraction frozen value of 0.5). Adjusting the mean contact angle transforms the graph by sliding the curve along the x axis. Adjusting the standard deviation in contact angle, σθ, allows for control of the tilt in the probability function. The value of σθ was adjusted to align with the majority of the empirical data points.

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Figure 10: Fraction frozen for 275 micron octacosane and ~781 micron squalane and squalene are shown in part A, B, and C, respectively. The empirical probabilities of freezing fresh and oxidized IN are shown as solid and open circles, respectively, in each panel. The theoretical fraction frozen best fit to the data for fresh and oxidized samples are depicted as solid and open triangles, respectively.

As illustrated by Figure 10, the agreement between empirical and theoretical results is reasonably good in all cases. This is remarkable given that the model doesn’t explicitly take viscosity into account. As reported above, oxidized squalene is especially viscous. Even in this case, the fit to the data is equally good across the full temperature range. Contact angles and wetting parameters derived from the data are summarized in Table 2. The octacosane, which induces freezing at warmer temperatures after oxidation, has reduced values of contact angle and wetting parameter after oxidation. This is consistent with previous observations that oxidation of solid INPs increases hydrophilicity, thereby reducing the contact

29



angle and wetting parameter.7 However, in the case of liquid squalene, the molecular effects of oxidation are counterbalanced by increased viscosity resulting in no significant change in contact angle or wetting parameter.

Table 2: Derived values of mean contact angle, µθ with the standard deviation in contact angle, and the wetting parameter, f, for all IN compositions in this study.

INP Composition

Contact angle, µθ ( o )

Wetting parameter , f

Octacosane, fresh

85.9 ± 5.2

0.45

Octacosane, oxidized

80.2 ± 5.2

0.37

85.9 ± 4.3

0.45

91.7 ± 5.7

0.52

88.8 ± 3.4

0.48

88.8 ± 10.6

0.48

Squalane, fresh Squalane, oxidized Squalene, fresh Squalene, oxidized

J (cm-1, sec-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 11: Heterogeneous nucleation rate coefficients, jhet (cm-1, sec-1), calculated from experimental observations for fresh and oxidized cases are represented as closed and open symbols, respectively. For fresh and oxidized cases, octacosane, squalane, and squalene are shown as circles, triangles, and squares, respectively.

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For all samples in this study, nucleation rate coefficients, jhet, as a function of temperature were calculated using Equation 5 (Figure 11). Nucleation rate coefficients for all fresh samples rapidly increase as the temperature decreases. This implies that on the time scale of our experiments, the increased barrier to molecular rearrangements imposed on the samples by higher viscosities at colder temperatures do not have a significant impact on nucleation rates. In addition, comparing fresh and oxidized samples, there is no clear indication that oxidation increases jhet. Nucleation rate coefficients at -26.5

2.0°C were 0.03 cm-2 s-1 on average for the

liquid samples, compared to 0.52 cm-2 s-1 for the octacosane. In conclusion, SSB can be used to adequately represent ice nucleation even in the case of viscous liquids. Further studies are needed to determine the combined influences of viscosity and oxidation on the kinetics of molecular rearrangement prior to nucleation and on nucleation rates.

4. CONCLUSIONS AND ATMOSPHERIC IMPLICATIONS The results of this study demonstrate that solid and liquid organic compounds in contact with water droplets catalyze freezing events in those droplets. The mean freezing temperature of fresh octacosane, squalane, and squalene was -25.6°C. Exposure to ozone slightly improved the INP ability for the octacosane and squalene, but decreased the INP ability for squalane. These differences are consistent with different reaction pathways due to exposure to ozone. Raman and FTIR spectra confirm all three samples underwent oxidation, with the strongest oxidation occurring for squalene. Further, while the squalane fragments into smaller molecules containing carbonyls and alcohols during oxidation, the squalene becomes functionalized through the addition of carboxylic acids. Concurrent with functionalization, the squalene underwent a large increase in viscosity.

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A significant finding of this study is the freezing of water droplets in contact with liquid aerosols. Several recent studies have examined the importance of amorphous solids and/or viscous liquid aerosols as ice nuclei.29, 33, 36 However, in most measurements on SOA as ice nuclei, the viscosity is inferred, rather than measured. Our study is unique because it is the only study in which the temperature-dependent viscosity was measured on the same samples for which ice nucleation temperatures are determined. Exposure to ozone dramatically increased the viscosity of squalene, which undergoes oxidation and functionalization, whereas ozone exposure decreases the viscosity of squalene, which undergoes fragmentation. While oxidized squalene was highly viscous (1330.3 Poise), the remaining organic samples, all of which were effective INP, remained only mildly viscous with viscosities as low as 0.6 poise at the freezing point. Thus, even moderately viscous organic aerosols are capable of acting as ice nuclei. Application of the simplified soccer ball model to our data reveal that the model can be used to accurately represent the nucleation behavior of liquids. In conclusion, this study expands the range of organic aerosols which can act as INP to include those in the liquid phase. Despite the minor differences in freezing temperatures observed here, the overarching conclusion of this study is that organic compounds found in the atmosphere are all approximately equally effective as contact INP, despite differences in molecular structure, chain length, degree of oxidation, and even phase. Taken together, liquid, glassy, and solid organic aerosols comprise a large fraction of atmospheric INP which is currently underrepresented in most climate models. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: 979-845-5632. Fax: 979-862-4466

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. AKNOWLEDGEMENTS We are grateful for the support of the National Science Foundation, Grant #1309854. Additionally, we thank Dr. Amanda E. Henkes, Materials Characterization Facility, Texas A&M University/Texas A&M Engineering Experiment Station for Raman data collected and Dr. David T. Trowbridge of Cannon Instrument Co for advice on uncertainty in viscometer measurements. ABBREVIATIONS INP, ice nucleating particle; FTIR-HATR, Fourier transform infrared spectroscopy with horizontal attenuated total reflectance; PAH, polycyclic aromatic hydrocarbon; SSB, simplified soccer ball model

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For Table of Contents Use Only Role of Organic Hydrocarbons in Atmospheric Ice Formation via Contact Freezing Kristen N. Collier and Sarah D. Brooks*

Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, United States

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Role of Organic Hydrocarbons in Atmospheric Ice ... - ACS Publications

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