Article pubs.acs.org/JPCA

Effects of Chemical Aging on the Ice Nucleation Activity of Soot and Polycyclic Aromatic Hydrocarbon Aerosols Sarah D. Brooks,* Katie Suter, and Laura Olivarez Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, United States ABSTRACT: The role of soot particles as ice nuclei (IN) in heterogeneous freezing processes in the atmosphere remains uncertain. Determination of the freezing efficiency of soot is complicated by the changing properties of soot particles undergoing atmospheric aging processes. In this study, the heterogeneous freezing temperatures of droplets in contact with fresh and oxidized soot particles were determined using an optical microscope apparatus equipped with a sealed cooling stage and a CCD video camera. Experiments were also conducted using fresh and oxidized polycyclic aromatic hydrocarbons (PAHs), including anthracene, pyrene, and phenanthrene, as potential ice nuclei. Chemical changes at the surface of the aerosols caused by exposure to ozone were characterized using Fourier transform infrared spectroscopy with horizontal attenuated total reflectance (FTIR-HATR). In addition, Brunauer−Emmett−Teller (BET) measurements were used to determine the specific surface areas of the soot particles. Mean freezing temperatures on fresh particles ranged from −19 to −24 °C, depending on the IN composition and size. In all cases, exposure to ozone facilitated ice nucleation at warmer temperatures, by 2−3 °C, on average. In addition, nucleation rate coefficients for a single temperature and IN type increased by as much as 4 orders of magnitude because of oxidation. Furthermore, a fraction of the oxidized soot particles froze at temperatures above −10 °C. A modified version of classical nucleation theory that accounts for a range of contact angles on nucleation sites within an IN population was used to derive the probability of freezing as a function of temperature for each type of IN. In summary, our results suggest that atmospheric oxidation processes may both extend the range over which particles can act as ice nuclei to warmer temperatures and increase heterogeneous nucleation rates on soot and pollutant aerosols.

1. INTRODUCTION At temperatures above the homogeneous freezing threshold (approximately −36 °C), ice nucleation in the atmosphere requires an aerosol to act as an ice nucleus (IN) and to facilitate heterogeneous nucleation. Heterogeneous nucleation may occur through a number of different pathways, including contact, immersion, and condensational freezing, and deposition nucleation.1 The freezing temperature and heterogeneous mechanism depend on chemical and physical properties of the available aerosol.2,3 Ice crystals are effective scatters of sunlight that play a significant role in the Earth’s radiative budget and climate change.4,5 Thus, the temperatures and rates of formation of ice particles in clouds must be accurately known to quantify the role of ice clouds in climate. Assessing the evolving ice-nucleating potential of aging aerosols as ice nuclei is essential to modeling the role of soot and pollution in cloud microphysics and climate. Soot is a primary particulate pollutant emitted to the atmosphere during various combustion processes.6,7 Studies have shown fresh soot to be relatively inefficient at facilitating freezing via any heterogeneous freezing mechanism.2,8−10 However, atmospheric aging processes have been shown to improve the ability of aerosols to act as IN.11−18 For example, it has been shown that coatings of sulfuric acid enhance the IN activity of soot particles.9,19−21 While coatings composed of a limited number of organic compounds on soot particles have not shown enhancement of the IN activity,16 the influence of a wide variety of organic compounds is yet to be determined. For © 2014 American Chemical Society

instance, high concentrations of atmospheric polycyclic aromatic hydrocarbons (PAHs) are present in the atmosphere because of incomplete combustion processes including fossil fuel and biomass burning.22−24 Soot particles may become coated with PAHs concurrently emitted during combustion.25 The surfaces of both soot and PAH particles undergo oxidation when exposed to ozone and become more hydrophilic.26−30 To the best of our knowledge, the IN ability of particles containing purely PAHs and those containing PAHs internally mixed with soot are unknown. At this time, the contribution of contact nucleation on soot particles to ice nucleation is highly uncertain, causing uncertainties in determination of total concentrations of atmospheric IN.31,32 Uncertainties in the contact freezing process stem from the fact that there are a limited number of laboratory measurements of contact freezing measurements, and there is currently no reliable method to measure contact freezing in in situ field measurements.2,9,33−39 Originally, contact freezing was defined as nucleation of a supercooled droplet upon collision with a dry particle.1,40,41 In this definition, immediately after contact, a stable cluster of ice molecules forms and acts as a template, facilitating freezing of the entire droplet. However, more recent findings suggest that contact freezing is initiated by the presence of a lower free Received: August 31, 2014 Revised: October 1, 2014 Published: October 3, 2014 10036

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energy barrier to nucleation at the surface and not by the collision event itself.2,38,42 These studies include experiments in which an ice nucleus is positioned in contact with a droplet surface either outside a droplet or inside the droplet (sometimes labeled “contact freezing inside-out”). Results showed no discernible differences in the observed nucleating behavior freezing temperatures based on the position of the IN.2 In contrast, contact freezing has been observed at temperatures several degrees warmer than immersion freezing on the same type of ice nuclei.2,38,42−49 Since previous results indicate that surface contact between the droplet and IN provides an energetic advantage regardless of IN placement, we define all freezing of droplets in contact with IN as “contact freezing” events in this manuscript. In addition to differences for various mechanisms, ice nucleation depends on the properties of the aerosol acting as IN, which evolve as the aerosol ages.13,16−18,50−52 Insight from molecular modeling suggests that order imposed by the IN surface on adjacent water molecules catalyzes freezing.53,54 However, various atmospheric aging processes may result in either more- or less-ordered surfaces and may reportedly increase, decrease, or not change heterogeneous nucleation depending on conditions and nucleation mechanism involved.11,14,16−18,55 Additional experiments evaluating the influence of aerosol aging on contact freezing temperatures are needed. To derive heterogeneous nucleation rate coefficients, modified versions of classical nucleation theory have been developed for heteorgeneous ice nucleation. Historically, a number of parametrizations of nucleation have been developed based on the concept that heteorogenous nucleation is a stochastic process, which imples that time is an important parameter in constraining nucleation behavior, as summarized in the reviews of Hoose and Möhler and Murray et al.32,56 In contrast, there are a separate set of studies in which a singular approach, which assumes time-independent nucleation occurs at specific temperatures, is employed.56 Fortunately more recent efforts have been developed that combine the key features of both approaches.57−59 The most recent and computionally efficient description of these mixed stochastic− singular models is the simplied Soccer Ball model.59 In that model, it is assumed that each potential IN particle possesses a certain number of active sites and that nucleation on each site will proceed according to a specific nucleation rate coefficient.59 As described below, differences in nucleation rate coefficient arise from a dependence on the contact angle between the ice cluster and the aerosol acting as the IN. Since several laboratory studies have reported that ice nucleation is better described using a distribution of contact angles rather than a single value,34,60−63 this study will rely on the simplified Soccer Ball model to evaluate the probability of freezing and to derive values of the mean and deviation in contact angle as a function of temperature based on the experimental results. To calculate these parameters, we assume a population of particles, each divided into nsite surface sites. The contact angle, θ, for each surface site is determined by a Gaussian probability distribution function described in terms of the mean contact angle, μθ, and the standard deviation in contact angle, σθ: p(θ ) =

⎛ (θ − μ )2 ⎞ 1 θ ⎟ exp⎜⎜ − ⎟ 2 2π σθ 2 σ ⎝ ⎠ θ

One modification introduced in the simplified Soccer Ball model is that particles are assumed to be homogeneous surface, such that nsite = 1, and hence that probabiliy of freezing is identical for all particles. The probability that a single droplet is not frozen is given by Punfr(T , μθ , σθ , t ) =

∫0

π

p(θ ) exp( −jhet (T , θ )ssitet ) dθ

0

+

∫−∞ p(θ) exp(−jhet (T , θ = 0)ssitet ) dθ

+

∫π

∞

p(θ ) exp( −jhet (T , θ = π )ssitet ) dθ

(2)

Here T is temperature, ssite is the surface area of a nucleation site, and t is time. The nucleation rate coefficient, jhet, is defined by jhet (T ) =

⎛ ΔF (T ) ⎞⎡ ⎛ ΔG(T )f ⎞⎤ kT exp⎜ − diff ⎟⎥ ⎟⎢n exp⎜ − ⎝ ⎝ h kT ⎠⎣ kT ⎠⎦

(3)

where ΔFdiff is the activation energy of water molecules diffusing across the solid−liquid interface, ΔG is the Gibbs free energy of formation of a critical ice cluster, n is the number density of water molecules at the ice/water interface, f is the wetting parameter, and k and h are Boltzmann’s and Planck’s constants, respectively. All parametrizations and values needed to calculate jhet are used as described by Zobrist et al. (2007), with one important exception, the wetting parameter, f, which is a function of the contact angle, θ.64,65 f=

1 (2 + cos θ )(1 − cos θ )2 4

(4)

Finally, the fraction frozen at a given temperature for droplet populations interacting with IN of various types is given by Pfr(T , μθ , σθ ,t ) = 1 − Punfr(T , μθ , σθ , t )

(5)

According to the theory above, an increase in hydrophilicity translates to a reduction in the particle’s contact angle, causing the particle becomes a more effective IN nucleus.65 While previous measurements of deposition freezing behavior on fresh and oxidized soot particles did not indicate an improvement in the ability of soot particles to act as IN,17 additional measurements are needed to determine the effects of oxidation on contact freezing. In order to assess the atmospheric lifetimes and role of soot and polycyclic aromatic hydrocarbons in ice nucleation, the IN ability of fresh and aged soot and PAH particles must be known. Here we conducted a series of laboratory experiments on droplets in contact with size-selected particles composed of soot, anthracene, phenanthrene, and pyrene to determine the ability of these to act as ice nuclei. Heterogeneous nucleation rate coefficients are reported for IN of each composition before and after exposure to ozone. In addition, we used the simplified Soccer Ball model to evaluate the fraction frozen as a function of temperature and to determine the wetting parameter and the mean and range of contact angles for IN of each type. To characterize the chemical properties of the particles’ surfaces, Fourier transform infrared spectroscopy with horizontal attenuated total reflectance (FTIR-HATR) was used. To account for the high porosity of soot particles, specific surface areas were determined by the BET method. Our results provide insight on the relationship between the chemical and physical properties of aging aerosols and their ice nucleating ability. This

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Figure 1. Experimental setup for ice nucleation experiments. Adapted from Fornea et al.2

remaining diatomic oxygen to form O3 in high concentrations 5−14% by weight (equivalent to 50 000−140 000 ppm). Next, the gas was diluted with nitrogen gas to produce a steady ozone concentration of 80 ± 5 ppm prior to entering the Nalgene chamber. Each sample was exposed to ozone for 24 h at room temperature and pressure and low relative humidity (less than 5%). Throughout the experiments, ozone concentration was continuously measured by a UV absorption analyzer (UV-100, Eco Sensors, Inc., 254 nm) and was observed to remain stable (±5 ppm) for the duration of the exposure period. At the end of the 24 h exposure period, the sample was removed from the reaction chamber and stored in a dark glass jar to avoid further oxidation prior to use in the FTIR and ice nucleation experiments. The ozone concentration was chosen for practical reasons, in that it was the lowest concentration our apparatus was capable of supplying in a steady concentration for the duration of the oxidation experiments. Also, employing the high O3 concentration and long exposure time improved uniformity of oxidation of all particles in the samples. While 80 ± 5 ppm is much higher than typically encountered in the atmosphere (30−40 ppb),23 particles emitted in an urban environment would also be exposed to other oxidants, including OH and NO2.24 In addition to practical considerations, an advantage of the high concentration is that it may indirectly represent the oxidation effects of longer periods of aging in the atmosphere, where soot particles often remain for several weeks.66 It has been observed that the atmospheric lifetimes of PAHs are shortened upon exposure to ozone. For example, the lifetime of oxidized phenanthrene is between 1 h and 1 day.29,30 Characterization of IN Surface Composition. FTIRHATR was used to identify any changes in the chemical composition of the particles’ surfaces resulting from exposure to ozone. The spectrometer is a PerkinElmer Spectrum 100 equipped with a zinc selenium crystal plate within a sealable

study contributes to our knowledge of the contribution of contact freezing events on soot and pollutant particles to the total freezing events in the atmosphere.

2. EXPERIMENTAL METHODS Heterogeneous freezing temperatures were determined through a series of single droplet−single ice nuclei freezing experiments using a custom ice microscope apparatus. Prior to freezing experiments, samples were size-selected and either used directly or oxidized in the procedure described below. Sample Preparation and Chemical Aging. All samples were prepared by sifting material through 3 in. diameter wire mesh sieves (Newark Wire Cloth Co.) to sort particles into three different size ranges: 250−300, 125−150, and 45−53 μm in diameter, hereafter referred to according to the average diameter in the size range, i.e., 275, 137, and 49 μm, respectively. While larger than atmospheric aerosols, the sizes here are chosen for experimental feasibility; i.e., the particles must be large enough to be maneuvered into position at the droplet’s surface with a micropipette tip prior to each experiment. The soot used in this study was carbon lampblack commercially available from Fisher Scientific. PAH sample materials included anthracene (Alfa Aesar, 99% purity) and pyrene and phenanthrene (Sigma-Aldrich, 98% purity and 99% purity, respectively). After sizing, a portion of each sample was separated to be oxidized, and the remaining particles were stored in airtight amber bottles until used. To oxidize a sample, sieved material was placed on a glass fiber filter (Advantec) and placed inside a Nalgene chamber. Ozone was generated by a HC-30 generator (Ozone Solutions). During generator operation, oxygen entered the generator at a flow rate of 0.01 L/min, controlled by a mass flow controller (model MC-10 SLPM-D, Alicat), and was passed into a corona cell, where a fraction of the molecular oxygen was split into atomic oxygen, and reacts with the 10038

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the stage was sealed (Figure 1). In our previous work, we found freezing results to be insensitive to droplet size in the range of 0.5−2.0 μL droplets.2 The use of sieved samples in this study allowed us to look for any detectable variations in freezing temperature due to differences in particle size. With a droplet−IN system in position, the stage was cooled at 1.0 °C/min down to −40 °C. Another study using a similar experimental apparatus has reported that results depend on the cooling rate and the viewing angle employed in the apparatus.73 However, using our apparatus, we have observed consistent determination of freezing temperatures in experiments when cooling rates of 2.0 °C/min are employed. Digital camera images were recorded at a rate of 1 image every 6 s (1 image every 0.1 °C). To avoid droplet evaporation, samples were maintained under a constant low flow of humidified nitrogen gas. Gas samples of the chosen humidity were generated by passing a flow of nitrogen through a bubbler to produce humid gas and mixing the humid gas with a second dry line of nitrogen (Figure 1). The dewpoint was monitored using an EdgeTech DewPrime II hydrometer, model 2000, and maintained near approximately −45 °C throughout the experiments. This dewpoint was optimal in that it provided enough humidity to avoid evaporation of the droplets, and meanwhile no condensation was observed within the cooling stage. Once a temperature of −40 °C was reached, the stage was warmed at 1.0 °C/min to 5.0 °C and held at 5.0 °C for 1 min to ensure complete melting before the next freezing cycle began. Freezing temperatures, defined here by the onset of visible evidence of freezing, were determined after experiment by frame-by-frame analysis of the recorded digital images. Each experimental setup consisted of a single particle−droplet system that generated a series of ∼33 independent freezing data points (Figure 2).

HATR chamber (Pike Technologies). The HATR method is used to obtain spectra of thin films and solid particles. For the PAHs in this study, samples were dissolved in reagent grade acetone in concentrations of 0.001 mg/L. After dissolution, 30 mL of the PAH solution was pipetted onto the HATR plate. After the acetone evaporated (∼30 min), a thin film of PAH remained on the ZnSe crystal plate which allowed a better spectrum to be obtained.67 Absorbance was measured over the wavenumber range of 4000−700 cm−1 with a resolution of 2 cm−1. Since soot is not soluble in acetone and spectra obtained on solid soot particles did not exhibit adequate signal-to-noise, collection of high quality spectra of the insoluble soot particles required a different procedure. An emulsion of soot particles in olive oil was mixed well and poured onto the HATR crystal to evenly distribute the soot particles on the HATR plate. The spectrum of pure olive oil was subtracted from the spectra of the emulsion. The resulting soot spectrum looked similar to past studies.68 Collected FTIR spectra were used to identify changes in the sample surfaces due to exposure to ozone. Brunauer−Emmett−Teller Surface Area Measurements. Ice nucleation rates depend on particle surface area. For porous materials such as soot, the specific surface area, i.e., the total surface area available to provide nucleation sites, may be much greater than the geometric surface area calculated by assuming the particle is a sphere. For instance, a study by Aubin and Abbatt69 found that the specific surface areas of n-hexane soot samples measured using the BET method were as much as 372 times larger than the calculated geometric surface areas. In this study, BET specific surface areas of sieved soot samples were determined by measuring the adsorption isotherm of kryton on samples of each composition at 77 K.70 To generate an adsorption isotherm, various aliquots of sample material were placed in a sealed chamber and exposed to research purity Krypton (Matheson Tri-Gas Company).71 The difference between the final chamber pressure over the sample compared to the pressure which would have been reached by expansion of the Kr into an empty chamber was used to calculate the number of moles of Kr adsorbed to the surface. Following the IUPAC method, the linearized form of the BET isotherm was used to determine the surface area.72 Specific surface areas for samples sieved into each of the three size ranges and before and after exposure to ozone were determined to probe for any differences in available surface area due to size or chemical aging. Ice Nuclei Sample Preparation. The experimental procedure used here to determine the temperatures of contact freezing events on has been as described in detail in Fornea et al.2 and is described only briefly here. The ice microscope apparatus shown in Figure 1 includes an Olympus optical microscopy (model BX51M), a digital camera (Q-Imaging Micropublisher 5.0 RTV), and a sealable Linkham cooling stage that allows control of samples to ±0.1 °C. This method allows independent measurement of the freezing temperature of a single droplet−IN system multiple times within one experiment. Choice of droplet size is limited by the detection limit of the ice microscope instrument. To initiate an experiment, a 2.0 μL droplet of ultrapure water (HPLC grade) was positioned on a hydrophobic slide which has been pretreated with 1.0% AquaSil solution (Pierce Chemical Company) within the microscope stage. Next a single particle from one of the sieved samples prepared above was placed in direct contact with the surface of the droplet, and

Figure 2. Freezing temperatures for 137 μm diameter fresh and oxidized soot particles are shown as diamonds (connected by solid lines) and circles (connected by dashed lines), respectively. Each line represents a series of experiments conducted using a single particle as the ice nucleus.

There was some variation in the number of data points per run, since in some cases the liquid nitrogen supply was consumed prior the end of the series. A new particle−droplet system was set up for each independent series of experiments. For example, the data shown in Figure 2 were produced from four fresh soot−droplet setups and three oxidized soot−droplet setups 10039

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Figure 3. FTIR spectra of anthracene before (fresh) and after exposure to ozone (oxidized) are shown in spectrum A (black line) and spectrum B (blue line), respectively. The difference spectrum is shown in spectrum C (red line).

rate of reaction between ozone and phenanthrene is slower than that of ozone and anthracene.29 BET Results. The BET method was used to measure the specific surface area of fresh and oxidized samples. According to the results (Table 1), the average surface area for fresh soot was

used to obtain 122 and 97 independent measurements of freezing on fresh and oxidized soot particles, respectively. Overall, a total of 1900 independent measurements of freezing (on 66 independent IN) were obtained in this study.

3. RESULTS FTIR Results. FTIR measurements were used to characterize sample surfaces before and after exposure to ozone. In all cases, exposure to ozone resulted in the formation of oxidized features on the sample surface. For example, Figure 3 illustrates the chemical changes occurring due to oxidation of the surface of an anthracene sample. Spectrum A was obtained for fresh anthracene. Spectrum B was collected after 24 h of exposure to ozone, and spectrum C is the difference spectrum resulting from the subtraction of the fresh spectrum from the exposed one (spectrum B − spectrum A). In the difference spectrum, the peak at 1738 cm−1 represents a CO bond, the peak at 1365 cm−1 represents a bending mode of O−H attached to a carbon molecule, and the peak at 1217 cm−1 peak represents a C−O stretch.28,74 Together these indicate that carboxylic acid and aldehyde groups have formed at the surface of the anthracene. Similarly, the pyrene difference spectra contained evidence of surface oxidation indicated by the presence of C O (peaks at 1735 and 1634 cm−1), O−H (a peak at 1392 cm−1), and C−O (a broad feature 1000−1220 cm−1). The soot spectra also clearly indicated oxidation had occurred. For phenanthrene, exposure to ozone resulted in growth of O−H and C−O peaks at 1303 and 1244 cm−1, respectively. However, in this case, the peaks were smaller than those observed for other the compositions, indicating a weaker and possibly nonuniform modification of the phenanthrene surface. Also, fresh phenanthrene already had a prominent carboxylate peak at 1739 cm−1. Any increase in this peak was below the limit of detection of the FTIR instrument. The observed surface chemical changes are consistent with previous studies that report the formation of carboxylic acids and aldehydes on carbonaceous surfaces after oxidation with ozone.28,30 A possible mechanism for the ozonolysis of PAHs involves the breakage of surface C−C bonds and the formation two aldehydes (one on each of the previously bonded carbon atoms).30 In this study, surface oxidation was observed for all four compositions, though modification of the phenanthrene surface appeared to be less pronounced than in the other cases. This is qualitatively consistent with previous reports that the

Table 1. Surface Area of Fresh Soot and Oxidized Soot Samples diameter (μm) fresh soot average 275 137 50 oxidized soot average 275 137 50

single particle BET surface area (m2/particle)b

single particle geometric surface area (m2/particle)

20.7 21.9 22.9 21.5 ± 2.0

5.1 × 10−4 6.7 × 10−5 3.4 × 10−6

2.4 x10−7 5.9 x10−8 7.9 x10−9

23.0 19.4 22.0

5.7 × 10−4 5.9 × 10−5 3.3 × 10−6

2.4 × 10−7 5.9 × 10−8 7.9 × 10−9

measured BET surface area (m2/g)a 21.8 ± 2.0

a Measurements obtained using 20 mg samples. bBased on an assumed soot density of 2.26 g/cm3 (READE chemical specialties resource).

21.8 ± 2.0 m2/g, in agreement with the reported BET surface area value of carbon lampblack, 22 m2/g.75 Results are also roughly consistent with the results of Aubin and Abbatt69 who observed a range in surface area per gram for n-hexane soot samples depending on the sample mass. Here the measured specific surface areas are much greater than the geometric surface areas. For example, according to our measurements, a single 137 μm diameter soot particle has a measured surface area of 6.7 × 10−5 m2, almost 2 orders of magnitude larger than the geometric surface area calculated by assuming the particle is a sphere of the same diameter. To account for porosity of the soot samples in the calculations of nucleation rate coefficient (cm−2 s−1) below, the BET specific surface areas are used. Note that while the resulting values for nucleation rate coefficient do not depend directly on knowledge of surface area, accurate predictions of nucleation rate coefficients in the atmosphere do require appropriate assumptions of both the number of particles 10040

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available to act as IN and the surface area provided by each particle. An additional motivation for performing the BET measurements was to test for any physical changes occurring in conjunction with the chemical oxidation process. Presumably, if exposure to ozone caused collapse of the soot structure, the specific surface area of the samples would decrease. However, no significant difference was observed between the average BET surface area of fresh (21.8 ± 2.0 m2/g) and oxidized soot samples (21.5 ± 2.0 m2/g). It should be noted, though, that in a subset of the ice nucleation experiments on oxidized soot, particle breakup of the IN particle was observed as described below. While the cause of this behavior is not certain, it is possible that oxidation results in more fragile particles or particles with more soluble components. Finally, BET measurements were conducted on soot particles in each size range in this study to look for possible differences in soot particles with different surface area to volume ratios. The results indicate that there is a slight trend of increasing surface area per gram with increased sieved particle sizes, though this trend is within the uncertainty in the BET measurements. Ice Nucleation Results and Discussion. Freezing experiments were conducted using with 275, 137, and 49 μm diameter soot particles as potential ice nuclei. The majority of experimental runs consisted of ∼33 freezing cycles. Some variation in freezing temperatures was observed even between particles of the same size range and composition. For example in Figure 2, differences in observed freezing temperatures for oxidized vs fresh particles were greater than the variety within individual runs of either type. This type of particle uniqueness has been observed in previous studies.2,57 It suggests that variations in physical or chemical properties may exist even within a single type of IN. The mean freezing temperature for each IN type was calculated by combining all data points collected for each series of experiments on a single IN type (Table 2). Pooled standard deviations were used to equally weight each individual freezing data point in all experimental runs. For fresh soot, values in Table 2 are consistent with our previous measurements2 and with another previous study of

contact-only freezing of kerosene soot that found freezing temperatures in a range from −22 to −28 °C.39 For each IN size, there is a significant difference between fresh and oxidized samples, discernible at the 99% confidence level. For fresh soot particles we see that there is a slight trend toward increased freezing temperature with increased IN diameter. For the oxidized soot samples, it is not clear from our measurements whether such a trend exists, in part because of a greater variation in reported freezing temperatures. In addition, experiments were conducted using fresh and oxidized anthracene, pyrene, and phenanthrene particles with ∼137 μm diameters. Observed freezing temperatures for the PAHs are included in Table 2. To the best of our knowledge, there are no previous reports of contact freezing involving IN with contain polycyclic aromatic hydrocarbons in the literature. For single particles of equal diameter (137 μm), the order of the mean freezing temperatures of droplets in contact with fresh particles from warmest to coldest was pyrene > soot > anthracene > phenanthrene. Considering only chemical properties, this result is somewhat surprising, since the hydrophobicity of soot is much higher than that of any of the PAHs. However, another key parameter in ice-nucleating ability of particles is surface area. The specific surface area of a soot particle is approximately 3 orders of magnitude greater than the geometric surface area for a given diameter (Table 1), whereas the surface area of a nonporous PAH particle is approximately equal to the geometric surface area, giving a soot particle of like diameter a clear advantage in acting as an ice nucleus. In all cases oxidation facilitated freezing at warming temperatures, with an average improvement of 2.9 °C for soot samples and 2.1 °C for polycyclic aromatic hydrocarbons. The greater improvement for soot may be because the original soot is very hydrophobic, and therefore, the surface is most changed by ozonolysis reactions. In addition, in some of the oxidized soot experiments, partial breakup of the IN was observed, resulting in an even greater increase in the surface area. The greater improvement in the soot resulted in an order of mean freezing temperatures of soot > pyrene > anthracene > phenanthrene, for single oxidized particles of equal diameter.

Table 2. Mean Freezing Temperatures for Fresh and Oxidized Samples

4. APPLICATION OF THE SIMPLIFIED SOCCER BALL MODEL To consider the relative significance of the observed contact freezing behavior, we calculate ice nucleation rates. The range in heterogeneous nucleation rate coefficients reported in the literature for IN containing soot is larger than the 6 orders of magnitude.67,76−81 It is known that rates depend on many factors including mechanism, temperature, relative humidity, physical and chemical properties of the IN, and droplet size. To minimize the uncertainty in calculated rate coefficients, we report rates derived directly from the data, following the method used by Koop et al. (2004) and Zobrist et al. (2007).64,82 The ice nucleation rate ω(s−1) can be determined directly from a series of individual freezing point measurements using Poisson statistics, where ω = nnuc/ttot, where ttot is the total observation time and nnuc is the number of nucleation events in ttot.64,82 Since ttot can be divided into a series of incremental changes in temperature, ΔT, the observation time, titot, in a given temperature interval (the ith interval) is the combined times of those droplets that remain liquid and those that freeze during the interval:

IN composition fresh soot

oxidized soot

fresh anthracene fresh phenanthrene fresh pyrene oxidized anthracene oxidized phenanthrene oxidized pyrene a

diameter (μm)

no. of experiments

no. of freezing temperature data points

275 137 49 275 137 49 137

4 4 5 4 3 11 5

117 122 163 99 97 314 121

−23.0 −23.1 −24.2 −20.7 −19.1 −21.9 −23.7

137

4

164

−23.9 ± 1.2

137 137

4 5

111 136

−21.3 ± 1.7 −20.9 ± 0.8

137

13

344

−22.4 ± 2.4

137

4

112

−19.3 ± 2.1

mean freezing temp (°C) ± ± ± ± ± ± ±

1.0a 1.4 1.1 1.3 1.0 4.6 1.1

Uncertainty is reported as the pooled standard deviation. 10041

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ΔT i i (ntot − nnuc )+ = cr

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to −20 °C, the nucleation rate coefficient for oxidized anthracene, 100 cm−2 s−1, is the highest of all compositions and far exceeds that of oxidized phenanthrene, 0.22 cm−2 s−1. While all fresh samples have somewhat similar values of jhet′, i.e., within ∼2 orders of magnitude at a given temperature, once oxidized, the spread in nucleation behavior increases. Some groups have reported nucleation rate coefficients for contact freezing experiments.55,80,83−85 However, in the previous studies, the collision between droplet and IN, not incorporated into our rate coefficients, is considered to be the rate limiting step. Additional efforts to provide contact nucleation rate coefficients are needed. Next, the probability of freezing can be calculated using eqs 1−5 above. Values and temperature dependences for all parameters used in the equations here are summarized in Zobrist et al.,64 with the exception of the wetting parameter, f, and the mean and distribution of contact angles. Wetting parameter and contact angle values are not available in the literature. Hence, we use the empirical data collected in this experiment to derive the best wetting parameter and contact angle values, as described below. It should be emphasized that in this application of classical nucleation theory, a few simplifying assumptions have been made. First, surface sites covering the entire surface area of the IN particle are assumed to interact with the water droplet. In addition, the formulation does not account for differences between contact and immersion freezing mechanisms. Nonetheless, the formulation provides a practical method for estimating the fraction of a droplet population frozen as a function of temperature and for identifying differences in that fraction depending on the composition and aging state of the IN involved. Taking the data points collected for a single composition and size as a collective data set, an empirical probability of freezing is generated.2,38,42 The empirical probability is related to the theoretical probability as a function of time by the following:

i nnuc

∑ Δtnuc,j j=1

(6)

nitot is ninuc is

In eq 6, the number of droplets at the beginning of interval i, the number of droplets that freeze during the interval, and cr is the cooling rate of the experiment. Multiple droplets may freezing during an interval, and Δtnuc,j is the time required for the jth droplet to freeze during interval, defined as 1 i Δtnuc, j = (Tsti − Tnuc, j) cr (7) In eq 7, Tist and Tinuc,j are the starting temperature of interval i and the freezing temperature of the jth sample within interval i, respectively. The mean heterogeneous nucleation rate coefficient of the interval at the mean temperature, Ti, can be calculated according to jhet′ (T i) =

ω het(T i) ni 1 = nuc i AIN AIN t tot

(8)

where AIN is the total available surface area of IN. Nucleation rate coefficients, jhet′, for fresh and oxidized samples as a function of temperature for each IN composition studied are shown in Figure 4. Fresh samples are depicted by

P(time) =

Nf = 1 − exp( − N0

∫0

t

jhet′ dt )

(9)

In eq 9, N0 is the total number of droplets, Nf, is the number of those that have frozen, and t is the time lapsed over an experiment.42,65 Equation 6 can be converted to a freezing probability as a function of temperature, rather than time, by multiplying each side of the equation by the expression dt/dT to yield

Figure 4. Heterogeneous nucleation rate coefficients, Jhet′, cm−2 s−1, calculated from experimental observations for fresh and oxidized cases are shown as solid and open symbols, respectively. For fresh and oxidized cases, soot, anthracene, phenanthrene, and pyrene are shown as diamonds, circles, triangles, and squares, respectively.

P(Temp) =

solid symbols, and oxidized samples are open symbols. Results for soot, anthracene, phenanthrene, and pyrene are shown as diamonds, circles, triangles, and squares, respectively. For all fresh samples, little or no nucleation is predicted above −20 °C. As temperatures decrease, nucleation rates for all fresh samples rapidly increase. Comparing solid symbols to open ones, it is clear that that oxidation increases jhet′ significantly. In the laboratory, individual freezing events at temperatures above −10 °C were observed for a small percentage of the oxidized soot and oxidized phenanthrene particles, though nucleation coefficients were very low. Among the PAHs, oxidized phenanthrene was the only IN to facilitate freezing above −10 °C, and freezing was only observed in less than 1% of the experiments. Recall that the pooled standard deviation in observed freezing temperatures in the case of oxidized phenanthrene was higher than for other PAH compositions. However, when the temperature is reduced

⎛ dt Nf = 1 − exp⎜ − ⎝ dT N0

∫T

T

0

⎞ jhet′ dT ⎟ ⎠

(10)

To compare the data collected in this study to theory, theoretical probability curves for the frozen fraction, Pfr, were generated according to eq 5 above. For example, all recorded freezing temperatures on 137 μm fresh soot particles were combined to create the empirical probability of freezing shown in Figure 5A (solid triangles). A theoretical curve representing the probability that a water droplet in contact with a fresh soot particle will have frozen at a given temperature is shown as the solid circles in Figure 5A. This theoretical curve was adjusted to match the empirical curve at the 50% probability levels by changing the value used for the mean contact angle μθ to match the empircal curve at a fraction frozen of 0.5. The standard deviation in contact angle, σθ, determines the tilt in the probability function, and its value was adjusted to agree with the majority of data points. 10042

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Figure 5. Fraction frozen for 137 μm soot, anthracene, phenanthrene, and pyrene is shown in parts A, B, C, and D, respectively. In each panel, the empirical probabilities of freezing fresh and oxidized IN are shown in solid and open triangles, respectively. The theoretical fraction frozen best fit to the data for fresh samples is depicted by solid circles, and for oxidized samples it is depicted by open circles.

Table 3. Derived Values of the Mean Contact Angle θ, the Standard Deviation of the Contact Angle, and the Wetting Parameter f for IN of All Sizes and Compositions in This Study

The second theoretical curve in Figure 5A (open circles) was fitted to the oxidized soot data (open triangles). As illustrated by the figure, exposure to ozone increases the probability that an IN will induce freezing at a given temperature. The corresponding values of mean contact angle for the fresh and oxidized soot particles (137 μm) are 72 ± 1 °C and 63 ± 2 °C, respectively (Table 3). The corresponding f values are 0.28 and 0.18 for the fresh and oxidized cases, respectively (Table 3). Similar trends were observed for all particle sizes. As a surface becomes more hydrophilic, its contact angle is reduced and its f value is reduced. Thus, the lower values for parameter f observed here for oxidized soot samples are qualitatively consistent with nucleation energy barrier due to the increased hydrophilicity of the oxidized soot surface. The same fitting procedure was followed for anthracene, phenanthrene, and pyrene, as shown in parts B, C, and D of Figure 5, respectively. It is clear that in all cases oxidation increases the probability of freezing at warmer temperatures. Likewise, the derived values of the wetting parameter f were decreased in all cases, consistent with a transition to more hydrophilic surfaces (Table 3). However, the difference between oxidized and nonoxidized freezing temperatures varies with composition. Also, as seen in Figure 5, the goodness of fit between the overall shapes of the probability curves varies with composition. As stated above, the only adjustable parameters in the nucleation equations used here are μθ, which determines f, and standard deviation in contact angle, σθ. An adjustment to the chosen value for μθ does not change the shape of the curve; it merely shifts the curve right or left along the x axis

IN size and composition soot, fresh average (all sizes) 275 μm diameter 137 μm diameter 49 μm diameter soot, oxidized average (all sizes) 275 μm diameter 137 μm diameter 49 μm diameter fresh anthracene fresh phenanthrene fresh pyrene oxidized anthracene oxidized phenanthrene oxidized pyrene a

contact angle θ, deg

wetting parameter f

73 73 72 75

± ± ± ±

4a 2 1 5

0.29 0.28 0.27 0.31

69 69 63 74 73 76 71 67 73 62

± ± ± ± ± ± ± ± ± ±

6 3 2 11 2 3 4 0 3 3

0.24 0.25 0.18 0.30 0.29 0.32 0.26 0.23 0.29 0.17

Uncertainty is reported as 1 standard deviation of θ.

(temperature). An increased spread in σθ results in increased tilt of the probability function. However, it does so in a symmetric fashion. Thus, the theoretical fit best represents the empirical data for the cases in which the data vary over a very narrow range of temperatures, such as the fresh and oxidized anthracene curves (Figure 5B). Anthracene has the narrowest 10043

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range of contact angles, as evidenced in the nearly vertical shape of the curve and in the derived values for σθ of 2 and 0 °C for the fresh and oxidized samples, respectively. In cases where the empirical curves changed over a wider range of temperatures, theoretical fits were still moderately good. However, in some cases, the empirical curves are not symmetric. For example, in the case of oxidized phenanthrene, a number of nucleation events occurred over the ∼5 °C range warmer than mean freezing temperature (Figure 5C). This pattern is not reflected in the modeled results for temperatures colder than the mean. No adjustments to the theoretical probability can account for the observed behavior, and an improved theoretical parametrization would be desirable to improve fit in such cases. It was noted above that the FTIR spectra of the phenanthrene had the weakest indication of oxidation. We speculate that this may have resulted in nonuniform oxidation of the surfaces of particles used in this case. It follows that the contact angles would vary between particles and even between activation sites on an individual particle, perhaps contributing to the greater range of freezing temperatures observed. With the exception of oxidized anthracene, the best fit parametrization for each data set included a nonzero standard deviation in contact angles. This is an indication that the icenucleation abilities of surface sites on the IN are not homogeneous, even for ensembles of aerosol particles of nominally the same composition and size.

ature was much greater than for fresh samples, with single freezing events observed warmer than −10 °C. Additional experiments were conducted to characterize the chemical and physical properties of the aerosols selected as IN this study. FTIR-HATR measurements were conducted to probe the chemical composition of the sample surfaces. After exposure to ozone, the formation of oxidation products was observed on the surfaces of all samples. Based on the relative intensities of the FTIR peaks, we report a possible connection between more strongly oxidized samples and greater changes in mean freezing temperatures relative to fresh samples. BET measurements were used to determine the specific surface area of the soot samples. Based on those measurements, we estimate that a single soot particle’s available surface area may be approximately 3 orders of magnitude greater than that of its geometric surface assuming a solid sphere. Though not proven directly, this high surface area is a likely cause of the relatively high IN ability of the soot particles observed. A recent adaptation of classical nucleation theory, the simplified Soccer Ball model, was used here to determine the probability of freezing, the mean and standard deviation in contact angle, θ, between and IN and the ice cluster forming on it, and the wetting parameter, f, for each IN type. Chemical aging due to exposure to ozone resulted in increased nucleation rate coefficients, reduced mean contact angles, and lower wetting parameters for all cases. Furthermore, oxidation resulted in a population of soot particles in which a small percentage of the particles are predicted to act as IN at temperatures above −10 °C, thus extending the range of conditions of ice cloud formation for a small fraction of the particles. The results of this study indicate that IN ability is an evolving aerosol property that must be included in accurate modeling of the role of atmospheric ice particles in cloud microphysics and climate.

5. CONCLUSIONS Improvements in predictions of heterogeneous freezing processes require a better understanding of those IN characteristics that promote freezing and better constraints on the rate coefficients and conditions under which the freezing occurs. In this study, we have explored the effects of IN composition, size, surface area, and atmospheric aging on the ice-nucleating ability of aerosols composed of soot and PAHs. The average freezing temperatures for all experiments involving soot particles as the representative IN was −23.4 °C. For fresh soot samples with average diameters of 275, 137, and 49 μm, a slight trend toward warmer freezing temperatures with increased IN size was observed. For polycyclic aromatic hydrocarbon samples (137 μm diameter), the average freezing temperature was −23.0 °C for droplets in contact with fresh IN, indicating that even freshly emitted PAH particles in the atmosphere serve as IN under frequently encountered atmospheric conditions. In all cases, oxidation increased the mean freezing temperature by approximately 2−3 °C. Oxidation by exposure to high concentrations of ozone extended the range of nucleation to warmer temperatures and increased the nucleation rate coefficients for a single temperature, and IN type increased by as much as 4 orders of magnitude. Significant differences in freezing temperatures and rate coefficients were found depending on the type of polycyclic aromatic hydrocarbon used. Among the fresh PAH samples, the highest mean freezing temperature was observed for pyrene, followed by anthracene and, last, by phenanthrene. Of all the PAH samples, oxidized pyrene particles were the most efficient ice nuclei of the PAHs, with a mean freezing temperature of −19.3 °C. In comparison, oxidized phenanthrene had a much lower mean freezing temperature, −22 °C. However, in the oxidized phenanthrene case, the variation in freezing temper-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 979-845-5632. Fax: 979862-4466. 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.

■

ACKNOWLEDGMENTS The authors are grateful for support through the National Science Foundation Grants NSF-CAREER -054875 and CHE1309854. Additional thanks go to Alexei Khalizov for providing assistance with the BET measurements.

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ABBREVIATIONS IN, ice nuclei; FTIR-HATR, Fourier transform infrared horizontal attenuated total reflectance; BET, Brunauer− Emmett−Teller; PAH, polycyclic aromatic hydrocarbon

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REFERENCES

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