Surface Disordering and Film Formation on Ice Induced by

Jul 7, 2014 - The coldfinger and the connecting Schlenk line were under a low vacuum. At 77 K, HCHO forms a white solid. The coldfinger was then place...
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Surface Disordering and Film Formation on Ice Induced by Formaldehyde and Acetaldehyde Min H. Kuo, Samar G. Moussa,† and V. Faye McNeill* Chemical Engineering Department, Columbia University in the City of New York, 500 W 120th Street, New York, New York 10027, United States S Supporting Information *

ABSTRACT: Small aldehydes, such as formaldehyde (HCHO) and acetaldehyde (CH3CHO), are known to contribute significantly to the OH and O3 budgets in polar atmospheres via chemical interaction with snow and ice surfaces. However, our understanding of how these small aldehydes interact with ice surfaces is rather limited. In this work, ellipsometry was used to study the interaction of gas-phase HCHO and CH3CHO with ice under partial pressure and temperature conditions akin to polar snowpack interstitial space. Both HCHO and CH3CHO were found to induce surface change consistent with the formation of a disordered interfacial layer (DIL) at temperatures below which no intrinsic DIL is expected to exist (T < 238 K). For HCHO, induced surface disorder showed both temperature and partial pressure dependence. For CH3CHO, temperature seemed to be the dominant factor; a DIL surface transition was observed in the presence of CH3CHO at 223 ± 2 K, in agreement with earlier findings that partitioning behavior of CH3CHO is different above and below this transition temperature. Exposure to HCHO or CH3CHO was also observed to cause the formation of opaque domains on the ice surface, which may correspond to hydrate formation.

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INTRODUCTION Small aldehydes, such as formaldehyde (HCHO) and acetaldehyde (CH3CHO), are the most abundant carbonylcontaining volatile organic compounds (VOCs) in the atmosphere. They can be emitted directly to the atmosphere from combustion sources or form from secondary sources through the photo-oxidation of VOCs of both biogenic and anthropogenic origins.1−3 As a result of their prevalence in the atmosphere and their reactive nature, small aldehydes play a central role in atmospheric chemistry.1,4,5 Particularly, aldehydes and other carbonyl compounds are known to undergo photolysis to produce hydroxyl radicals (OH), a primary atmospheric oxidant.1 While the main daytime sink for formaldehyde is photolysis, removal of HCHO through dry and/or wet deposition is prevalent at nighttime.6 For acetaldehyde, reaction with atmospheric NO3 radicals is the main removal process.6 In Polar Regions, aldehydes have been found to contribute significantly to the OH budget, which determines the oxidizing capacity of the polar boundary layer.7,8 In addition, aldehydes also play a role in modulating the amount of ozone (O3) in the polar atmosphere during the spring. Aldehydes react with bromine atoms and thereby terminate chain reactions, which deplete O3.9,10 In addition to their prevalence in the atmosphere, HCHO and CH3CHO have also been found to be present in both Arctic and Antarctic snowpacks in concentrations higher than those in the overlying air.9,11−23 It has been proposed that HCHO and CH3CHO incorporation into snow involves both physical and chemical processes. Physical incorporation © 2014 American Chemical Society

mechanisms include dissolution into snow crystal lattice during their growth in the atmosphere, adsorption onto snow crystal surfaces, or integration as organic aerosol components.16,19,24,25 Organic aerosols can be incorporated into snow via several pathways. For example, aerosols may be scavenged by snow crystals during their growth or fall, or simply dry-deposited onto the snow surface after precipitation.16,19 In addition, the presence of aldehydes in snow can also be due to in situ photochemical production from photolysis or other reactions of precursor species within snowpacks.9,13,16,20,23 This hypothesis is consistent with the observation of higher aldehyde fluxes from the snowpack to air during polar springs as compared to wintertime measurements.16,20 Since they are both small carbonyl-containing VOCs, the interaction of acetaldehyde with ice might be expected to be similar to that of formaldehyde in some respects, but the presence of the extra methyl group can lead to very different chemistry and partitioning behavior as compared to formaldehyde. For example, the effective Henry’s law coefficient for HCHO uptake on water surface has been estimated to be 3 orders of magnitude higher than that of CH3CHO at 298 K.26 Furthermore, its larger size may prevent acetaldehyde from being incorporated into the ice matrix to form solid solutions.23 However, CH3CHO can undergo aldol condensation in the Special Issue: John C. Hemminger Festschrift Received: May 1, 2014 Revised: July 3, 2014 Published: July 7, 2014 29108

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presence of water and acid or base, whereas HCHO cannot.27 Like HCHO, formation of hydrates and heavier secondary organic material can also occur in CH3CHO−ice systems.28 The HCHO−ice and CH3CHO−ice systems have been studied using computer simulations based on molecular dynamics and grand canonical Monte Carlo methods.29−31 Both HCHO and CH3CHO were predicted to undergo Langmuir-type adsorption via a single hydrogen bond with water and remain intact on the surface. This has been supported experimentally in the case of acetaldehyde adsorption onto pure and nitric acid trihydrate (NAT) ice by Lasne et al. (2012) using infrared spectroscopy.32 In the case of formaldehyde, lateral dipole−dipole type interactions were also found to stabilize adsorbed molecules.29 Only a small number of laboratory studies have been undertaken to elucidate the nature of the interaction of HCHO or CH3CHO with ice. Taken together, these studies suggest that HCHO−ice and CH3CHO−ice interactions can vary widely when temperature and partial pressure conditions change. HCHO has been found to adsorb to the ice surface in ways consistent with the Langmuir mechanism,33,34 dissolve into the ice matrix,35 polymerize to form solid poly oxomethylene glycols,35 form hydrogen-bonded networks with water,36 or even exist as clathrates.37 Petitjean et al. (2009) observed reversible, Langmuir-type adsorption of CH3CHO to ice and NAT between 203 and 253 K,38 whereas Hudson et al. observed no uptake at lower temperatures.39 Characterizing trace gas−ice interactions in general is complicated by the dynamic nature of the ice surface.40 Under typical environmentally relevant conditions, trace gas uptake onto ice can also be affected by the presence of interfacial layers such as impurity-induced brine layers (BL)41−43 and, under more pristine conditions, disordered interfacial layers (DILs), known variously as quasiliquid layer, premelting layer, surface roughening, etc. in the literature. DILs are of two general types: an intrinsic DIL existing on pure ice near but below the melting temperature, or an induced DIL arising from ice surface modification by solutes. Previously, our work with hydrochloric44,45 and nitric46 acids on ice showed that the DIL can be induced when pure ice is exposed to trace gases, and this surface modification has significant implications for trace gas uptake and heterogeneous chemistry. McNeill et al. (2006, 2007) and Moussa et al. (2013) showed that HCl- or HNO3-induced disorder leads to different trace gas uptake profiles as compared to conditions where no disorder was observed using ellipsometry. Unlike the strongly acidic trace gases, HCl and HNO3, studied previously, HCHO and CH3CHO are much weaker acids (pKa 13.3 and 13.6, respectively), and therefore their interactions with the ice surface likely differ from those of HCl and HNO3. For example, HCl and HNO3 tend to disassociate on ice surfaces upon adsorption. Aldehydes are unlikely to do so, although they may participate in self-reactions on the surface such as aldol condensation or hydration and polymerization (e.g., paraformaldehyde formation).27,28 Studies of HCHO− or CH3CHO− ice interactions have never been performed using a technique such as ellipsometry to monitor the state of the ice surface. Thus, the potential impacts of these species on the ice surface are not known. The goal of this study was to understand the interaction of HCHO and CH3CHO with ice surfaces under conditions relevant to the polar boundary layer. Ellipsometry was used to directly probe the ice surface to detect changes in the surface

state induced by exposure to HCHO or CH3CHO. Results of this study have implications for snowpack processing and boundary layer concentrations of these trace gases in the Polar Regions, as well as interpretations of snow pit and ice core chemical measurements.

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EXPERIMENTAL METHODS Ice sample preparation, experimental setup, and general procedures used here for the ellipsometry experiments have been described previously in McNeill et al. 2006 and Moussa et al. 2013.44,46 Briefly, zone-refined (ZR) ice samples were prepared following a modified version of the Bridgman technique.47 The ice sample processed using this technique is optically transparent and contains large crystal grains on the order of a few centimeters. A schematic of the experimental setup can be found in the Supporting Information. During the experiments, ice samples were placed in double-jacketed flow tubes cooled by a recirculating chiller (ThermoFisher, NESLAB ULT80). The double-jacketed flow tube was designed with a break in the coolant layer so that the paths of the incident and reflected beams were unobstructed. The entire flow tube was position inside the ellipsometer (Beaglehole Instruments, Picometer Ellipsometer), which was equipped with a Ne/He laser with a wavelength of 632.8 nm. Experiments were conducted at flow tube temperatures between 217 and 250 K and pressures of 40 to 50 Torr in order to reduce gas-phase diffusion limitations.46 The gas phase concentrations of probe molecules were monitored using chemical ionization mass spectrometry (CIMS) with H3O+·(H2O)n as the chemical ionization reagent ion (H3O+·H2O, m/z 37, was the dominant peak). The parent ions were generated by passing a N2 stream saturated in water through a 210Po radioactive source (NRD Nuclecel, P-2031). HCHO and CH3CHO were monitored as the sum of their ion clusters with H3O+ and H2O. A small piece of ice placed upstream of the main sample ensured that the ice surface was in equilibrium with water vapor in the gas phase. The trace gases, carried by N2, were delivered through a stainless steel injector mounted axially at the centerline of the flow tube. The injector tip, initially positioned downstream of the sample, is displaced to expose the ice surface to the trace gas once a signal baseline (from ellipsometer or CIMS) had been established. Changes in signals were monitored and analyzed. There existed a small radial temperature gradient within the flowtube (the center of the flowtube was ≤2 °C warmer than the cooled flowtube walls). All temperatures reported here are internal flowtube temperatures, and are representative of the ice sample temperature. While the temperature inside the sample cell was not monitored continuously, the relationship between chamber temperature and the coolant temperature was carefully calibrated. The associated uncertainty in reported temperatures is ±2 K. Formaldehyde Preparation and Delivery. A Pyrex vessel containing paraformaldehyde powder (Sigma-Aldrich, 95%) was submerged in an oil bath, which was heated to 413 K (140 °C), at which temperature paraformaldehyde thermally decomposed into gaseous HCHO. The HCHO gas was captured in a coldfinger precooled to 77 K using liquid nitrogen. The coldfinger and the connecting Schlenk line were under a low vacuum. At 77 K, HCHO forms a white solid. The coldfinger was then placed inside a dewar containing a silicone oil bath (Dow Chemicals, Syltherm XLT), the temperature of which was controlled by adding liquid nitrogen. At typical bath temperatures (128 to 188 K), HCHO was a viscous liquid. A 29109

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Figure 1. Predicted x and y signals for DIL with varying refractive indices. For a three-layer system composed of air, DIL, and ice, the multilayer evaluation tool in the Beaglehole Picometer software was used to predict x and y signals for a growing DIL layer having a range of assumed refractive indices bracketing that of ice at n = 1.308. n = 1.33 corresponds to the refractive index of liquid water. Signal amplitudes and phases of x, y signals are both important in determining the refractive index of observed surface transitions.

flow controller (MKS, 0−10 sccm) directed 2 to 8 sccm of N2 into the dip tube of the coldfinger/bubbler, and the outgoing stream saturated in HCHO entered the flow tube through the movable injector. The concentration of HCHO in the flow tube was calculated by first using Antoine’s coefficients48 to determine the vapor pressure of HCHO at the given bath temperature and then taking into account known dilution ratios. Bath temperature was recorded frequently throughout the experiment to track variations in HCHO partial pressure inside the flow tube. Typical HCHO partial pressures were in the range of 2 × 10−5 Torr to 8 × 10−4 Torr. Acetaldehyde Preparation and Delivery. Acetaldehyde (Sigma-Aldrich, >99.5%) stored inside a coldfinger was subject to three cycles of freeze−pump−thaw. After the last thawing step, CH3CHO was discharged via a Schlenk line into a 5 L glass bulb at low vacuum. The pressure in the Schlenk line and the bulb was monitored using an analog pressure gauge (Varian, 801). Excess CH3CHO was pumped away until the desired partial pressure was achieved, typically in the range of 100 to 300 mTorr. The bulb was then filled with N2 to 760 Torr. Dilute CH3CHO from the bulb was delivered into the injector via a flow controller (MKS, 0−10 sccm). Ellipsometry Data Analysis. The detection of surface transition by ellipsometry is accomplished via changes in the phase and amplitude of the x and y signals, which reflect how much the refractive index of the surface deviates from that of ice. Figure 1 illustrates how x and y signals change when the assumed refractive index of a DIL at the air-ice interface is varied. If the refractive index of the DIL (nDIL) is very close to that of ice (nice = 1.308), x and y signal amplitudes will be very small, as depicted in the panel labeled nDIL = 1.31 in Figure 1. At a constant nDIL, a DIL growing in thickness can be detected as oscillating x and y signals. A change in surface state, on the other hand, results in a change in nDIL, detectable by a shift in the amplitude and phase of the x and y signals. Under real experimental conditions, however, both the refractive index and the thickness of the DIL may change simultaneously and

possibly also continuously. Thus, observed x and y signals may not closely resemble the theoretical predictions shown in Figure 1. Nevertheless, observed amplitude and phase changes in signal can still be reliably used to estimate the refractive index and thickness of the DIL when carefully analyzed.44,49−51 The y signal is typically used when determining the change in signal amplitude in our data analysis because all experiments were conducted at the Brewster angle of ice (52.6 degrees), at which y signals are more sensitive to changes in the refractive index of the surface. The predictions from the multilayer model can be compared to experimental data of x and y signals plotted against time to estimate the refractive index of the DIL. Where an abrupt change in x and y signals occurred in the experimental data, the phases of the two signals relative to each other and the magnitude of change in the y-signal were noted. These two properties were then compared to predictions of the multilayer evaluation tool generated for various nDIL values. The nDIL value that yields an x−y phase relationship and signal amplitude that most closely approximates those found in the experimental data was identified as the estimated refractive index of the DIL formed.

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RESULTS Formaldehyde. Ellipsometry was used to detect changes in the reflectivity of the ice surface as pure ice was exposed to a known partial pressure of HCHO. We conducted experiments in the HCHO−H2O solid solution stability regime on the binary phase diagram of Barret et al. (2011).35 As can be seen from Figure 2, Barret et al. (2011) identified that a solid poly oxomethylene glycols phase could form via polymerization reactions between water and HCHO at high HCHO concentrations. This solid polymer region was not explored in this study since the HCHO partial pressures needed significantly exceed typical concentrations found in the atmosphere or firn air. The HCHO−H2O solid solution regime on the phase diagram can be subdivided into two regions, based on our ellipsometry measurements on pure ice, at the 238 K 29110

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Figure 3 shows two variable-pressure experiments as an example of raw data from which the points in Figure 2 were

Figure 2. HCHO variable pressure ellipsometry experiments. Variable pressure ellipsometry experiments plotted on a HCHO−H2O phase diagram (Barret et al. 2011). Data markers differentiate between different types of surface states as explained in the text. A red open circle (○) marks experiments in which no significant signal changes were observed. Orange arrows represent the variation in HCHO partial pressure during the experiment. Bold numbers next to data points are elapsed time (seconds) from the start of the experiment.

mark, above which intrinsic DIL from pure water in the absence of trace gases exists and below which it does not. This distinction is important since the degree of disorder in ice structure near the surface may affect the availability and flexibility of water molecules to interact with gas-phase species. Two types of ellipsometry experiments were performed: (a) constant temperature experiments in which the ice sample is exposed to increased HCHO partial pressure, and (b) variable temperature experiments. In both types of experiments, significant changes in the ellipsometry signal (>5 times the noise level) were noted and subsequently analyzed. Observed surface states were classified into four distinct categories Stable, Growing DIL, Induced DIL, and Opaqueas explained below. Figure 2 summarizes the results of the variable pressure ellipsometry experiments. Each pair of symbol and arrow represents one experiment. Earlier ellipsometry experiments on pure ice done in our laboratory suggested that an intrinsic DIL exists at temperatures above ∼238 K.46 Thus, at T ≥ 238 K, the intrinsic DIL on the surface of the ice sample either remains stable or grows (blue ◆) when exposed to HCHO. At T < 238 K, since there is no intrinsic DIL, surface transitions indicated by large changes (>5 times noise level) in x and y ellipsometry signals were interpreted as DIL formation (green ◆) induced by HCHO. In the variable partial pressure experiments performed at temperatures below 238 K, where no intrinsic DIL was present in the beginning of the experiment, induced DILs were only observed at high HCHO partial pressures and/or low temperatures. In addition, it appeared that DIL induction occurred more rapidly at lower temperature, which is consistent with greater partitioning of HCHO to the condensed phase expected as temperature decreases. For the two experiments performed at T > 238 K (to the left of dash line), it appeared that temperature, rather than partial pressure, dominated in determining whether the intrinsic DIL grows.

Figure 3. HCHO variable pressure ellipsometry data at 231 and 235 K. The gray trace in each of the three panels corresponds to an experiment conducted at 231 K, and the black trace corresponds to one at 235 K. The top panel maps the variation in HCHO partial pressure inside the flow tube throughout the experiment. Although pHCHO in the two cases were nearly identical, x and y signals showed significant changes at 231 K but remained relatively constant at 235 K.

derived. As shown in the top panel of Figure 3, the ice samples in these two experiments were exposed to very similar HCHO partial pressure profiles. It is therefore worth noting that, while x and y signals changed substantially at 231 K (gray trace), they remained fairly constant for the experiment at 235 K (black trace). Since both experiments were performed at temperatures below 238 K, no intrinsic DIL from pure ice premelting was present in either of the two cases. An interesting feature in the y signal from the experiment at 231 K is worth pointing out. Note that between ∼5500 and ∼6700 s, the y signal in gray exhibits a cyclic pattern. Using the multilayer evaluation tool in the Picometer software, this signal change is consistent with a growing DIL having a refractive index of ∼1.311 (only slightly larger than the 1.308 of ice) and a layer thickness between 85 and 112 nm. Thus, this data point (231 K) was marked as “Induced” in Figure 2. In contrast, since x and y signals showed negligible change at 235 K, this data point was designated by a “no change” symbol. Nevertheless, closer inspection of both data sets shown in Figure 3 reveals that the signals had an overall upward trend, albeit very slight for the T = 235 K case. This feature was observed in all variable pressure experiments, and suggests that the refractive index might be changing continuously at a much slower rate than what was observed in variable temperature experiments (Figure 3). This slow change 29111

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in refractive index is likely linked to the extent of HCHO coverage on the ice surface. Figure 4 summarizes the results of four sets of variable temperature experiments. Arrows track the progress of the

Figure 5 shows a summary of all the CH3CHO−ice ellipsometry experiments performed. While most experiments

Figure 5. Summary of CH3CHO ellipsometry experiments. Experiments in which x and y signals showed significant change are marked by green diamond symbols. Open red circles mark experimental conditions under which x and y signals remained relatively constant. A black dot in the center of a symbol indicates that the sample developed a nearly opaque white layer during the experiment. The blue arrow represents the path of a cooling experiment, and the orange arrow a heating experiment. The gray vertical line at T = 238 K demarcates the temperature above which intrinsic DIL from pure ice is present. The green vertical line T = 225 K separates regions in which induced DIL was and was not observed.

Figure 4. HCHO variable temperature ellipsometry data. Variable temperature ellipsometry experiments plotted on HCHO−H2O phase diagram (Barret et al. 2011). Legends and other notations are the same as in Figure 2 unless otherwise noted. Blue arrows track the progress of cooling experiments, and red arrows heating experiments.

experiment through surface transitions represented by the symbols in the legend. There was some change in the partial pressure of HCHO during each experiment due to small variations in sample bath temperature. Signal change consistent with DIL growth was observed in each experiment summarized in Figure 4. In addition, we observed that the initially clear and reflective ice surface can become opaque (•) during the course of the variable temperature experiments under some conditions. A rapid drop in the intensity of light arriving at the detector and an increase in noise in the x and y signals accompanied the formation of an opaque layer. For the three cooling experiments shown in Figure 4, as the ice surface was exposed to HCHO, the intrinsic DIL grew. As the temperature decreased and solute loading increased, the sample eventually became opaque. Comparing the three cooling experiments, one can see that the opaque layer formed more rapidly at high partial pressures of HCHO. For the heating experiment shown in Figure 4 (marked by red arrow), although no intrinsic DIL was present in the beginning of the experiment due to the low starting temperature, the combination of high HCHO partial pressure and low temperature were conducive to HCHO partitioning to the condensed phase. This led to an induced DIL followed fairly rapidly by transition to opaque film as sufficient HCHO loading/saturation was reached. Acetaldehyde. We conducted a series of ellipsometry experiments in order to better characterize CH3CHO−ice interactions under conditions found in the polar boundary layer. The range of temperatures and CH3CHO partial pressures used were [218 < T < 242 K] and [1.1 × 10−5 < pCH3CHO < 3.4 × 10−5 Torr] (equivalent to [4.6 × 1011 < CCH3CHO < 1.4 × 1012 molecules/cm3]), respectively. These conditions are within the submonolayer coverage regime on the adsorption isotherms produced by Petitjean et al. (2009).38

were performed at constant temperature, two temperatureramping experiments, one heating and one cooling, were also conducted. Surface changes due to exposure to CH3CHO occurred only at temperatures below ∼225 K. Opaque domains were formed in some of the CH3CHO−ice exposure experiments, however, unlike in the HCHO experiment, the x and y traces merely exhibited greater noise near the end of the experiment, but did not undergo large changes in amplitude. Formation of opaque domains on ice exposed to CH3CHO did not correlate with temperature or partial pressure. Figure 6 shows examples of raw data from two experiments at the same CH3CHO partial pressure, but different temperatures. At 239 K (black trace), there were no significant changes in the signals. On the other hand, at 224 K (gray trace), sizable signal variations indicating changes on the surface were observed. The bottom panel of Figure 6 shows x and y signals at 224 K between [2800 < t < 3300 s]. In this view, the oscillatory pattern in both x and y became apparent. As mentioned in the discussion of HCHO ellipsometry results above, signal oscillations in sinusoidal form could be a manifestation of growing DIL atop the ice substrate. In this case, analysis of the data using the multilayer evaluation tool yielded an estimated refractive index of 1.30 and a DIL thickness of ∼240 nm. Figure 7 shows data from two variable temperature experiments, one heating (panel A) and the other cooling (panel B). In both cases, the temperature of the flow tube was ramped gradually between 221 and 250 K. In panel A of Figure 7, two points of major transition were apparent from rapid changes in x and y signals. The transition at T = ∼238 K was expected to arise due to the onset of pure ice DIL. The 29112

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Figure 7. CH3CHO variable temperature data. Data from experiments in which the sample temperature was ramped between 221 and 250 K. The top panel shows a heating experiment, and the bottom panel shows a cooling one. The partial pressure of CH3CHO in both cases was held constant at ∼2.4 × 10−5 Torr. Dashed lines serve to aid the eye in identifying temperatures at which major transitions were observed.

Figure 6. CH3CHO ellipsometry data at 224 and 239 K. Top two panels show x and y signals from two sets of experiments, one in which surface change was observed (gray) and another where it was not (black). The bottom panel is a close up of a portion of the x and y signals from the experiment at 224 K, corresponding to the gray trace in the top two panels. Both experiments were conducted at a CH3CHO partial pressure of 1.11 × 10−5 Torr.

of an intrinsic DIL at T > 238 K. This is perhaps consistent with the significantly lower solubility of CH3CHO in water as compared to HCHO.26 This trend is also consistent with previous observations of physisorption of CH3CHO at temperatures above ∼225 K.31,32 Our observation of DIL induction by CH3CHO at T < 225 K is consistent with Petitjean et al.’s finding of greater partitioning to the surface below 223 K. Figure 8 shows a contrast-enhanced photograph of a typical ice sample which developed opaque white domains during the course of a CH3CHO exposure experiment, with opaque domains outlined in white. One can see that the film was not uniform over the surface but existed in domains roughly 5 mm in size. Opaque domains formed via exposure to HCHO were visually similar to this. Although a study by Winkler et al. suggested very weak interactions between HCHO and ice, since formaldehyde is highly soluble in water, in the presence of disordered interfacial layers, there is potential for HCHO to dissolve into the DIL, form hydrates or solid solution with H2O, or undergo polymerization reactions to produce secondary organic material.23 The HCHO partial pressures used in our experiments are not high enough for poly oxomethylene glycols to form on the ice surface.35 Therefore, we hypothesize that the opaque domains observed are likely solid hydrates. Chazallon et al. (2005, 2008) performed low-temperature micro-Raman spectroscopic studies of frozen bulk solutions of HCHO and H2O and observed a bulk-phase hydrogen-bonded network between HCHO oligomers and water molecules for T > 213 K, with a second phase at 163 < T < 213 K consisting of oligomers interacting with surrounding water molecules through weak

transition at 223 K, however, was most likely linked to ice surface modification by acetaldehyde. A transition point at 223 K again echoes with the observation of Petitjean et al. of different partitioning behaviors demarcated by T = 223 K, as discussed earlier.38 One may notice that the magnitude of signal changes at T = 238 ± 2 K in panel A was ∼5 times greater than those in panel B. Recall that the amplitude of signal change is a function of the refractive index at the air−ice interface. Thus, a larger x, y signal change means greater deviation from the refractive index of pure ice. In the heating experiment shown in panel A, the signal change at T = 223 K corresponded to a refractive index of 1.294. In panel B, although a small dip can be seen in the y signal at T = 221 K, its complementary x signal showed negligible change, indicating that no surface transition took place. The fact that a transition at T = 223 K was only observed during the heating experiment (A) but not in the cooling experiment (B) could indicate that (1) the transition is path dependent or (2) longer equilibration time at low temperatures is required to achieve the loading necessary to cause surface change.

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DISCUSSION We have observed three main types of changes in the ice surface due to exposure to HCHO and CH3CHO in the gas phase: growth of an intrinsic DIL at temperatures greater than 238 K, induction of a DIL, and formation of an opaque layer on the surface. While the behavior of these two species was similar, unlike HCHO, CH3CHO was not observed to induce growth 29113

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the ellipsometry signal was different for the CH3CHO−ice system. Additionally, whereas opaque film formation was favored at high HCHO partial pressures and low temperatures, formation of opaque domains on ice exposed to CH3CHO did not correlate with temperature or partial pressure. Further investigation is warranted to conclusively identify the nature of the opaque films.

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CONCLUSIONS AND IMPLICATIONS To our knowledge, this is the first report of aldehyde-induced surface disorder on ice surfaces in the literature. We found that when ice with an intrinsic DIL (T > 238 K) was exposed to formaldehyde, the DIL grew in thickness, possibly as a result of HCHO dissolution into the DIL. This was not observed in the case of acetaldehyde. At temperatures below 238 K, both HCHO and CH3CHO were observed to induce DIL formation. Opaque domains on the ice surface were formed in some HCHO−ice and CH3CHO−ice experiments. These observations of ice surface modification by HCHO and CH3CHO suggest that the interaction between these small aldehydes and ice are more complex than simple Langmuir-type surface adsorption, and may help explain discrepancies between field measurements and model predictions of these key atmospheric components. Our work is consistent with the proposal by Chazallon and co-workers that hydrates of HCHO may play a role in modulating air−ice partitioning of HCHO in polar environments.36,37 Since polar snow regularly undergoes temperature cycling, gas-phase aldehydes in and above snow can cause the type of surface transitions (DIL, opaque layers) observed on our laboratory ice samples. In sunlit months, photolysis of organics in snow can lead to the production of aldehydes within the snowpack. If the temperature is warm enough for an intrinsic DIL to exist, HCHO can dissolve into the DIL, and as temperature decreases, HCHO may be trapped in the opaque hydrate form we observed. Even if the temperature is too cold for an intrinsic DIL to exist, we have shown that both HCHO and CH3CHO can induce disorder on the ice surface, potentially affecting exchange of trace gases between snow and air. An organic hydrate phase at the firn air− snow interface would serve as a reservoir of gas phase organic species, potentially leading to higher concentrations of these species in firn as compared to ambient air, as has been observed in many field studies.9,11−23 The presence of the DIL on snow may also enhance the uptake of HCHO, CH3CHO, and other trace gases, as we observed previously in the case of the HCl− ice system.44,45 Again, enhanced uptake can lead to a reservoir of organic material in the condensed phase. However, further studies are necessary before we can quantitatively connect our observations to the observed discontinuities between ambient air and firn air concentrations of HCHO and CH3CHO.

Figure 8. Ice sample with opaque domains. Ice sample previously exposed to CH3CHO is backlit, and the contrast has been enhanced in postprocessing. Opaque domains are outlined in white. Light and dark patches show different crystal grains in the sample.

forces.36 Furthermore, under conditions where ice film porosity was low, formation of nearly pure HCHO-clathrate was observed.37 The interpretation of hydrate formation is also consistent with computer simulations showing HCHO−H2O hydrogen bonds on the surface of ice and lateral stabilization of adsorbed HCHO.29,52 The formation of opaque domains by HCHO was dependent on HCHO partial pressure, temperature, and exposure time, consistent with hydrate formation by other trace gases such as HCl and HNO3 on ice surfaces.45,53 Comparing the three cooling experiments shown in Figure 4, one can see that the opaque layer formed much more rapidly at high partial pressures of HCHO. No opaque layers were formed in any of the variable pressure experiments performed, even at long times (9000 s). This suggests that a temperature-induced transition is required for its formation. When the signal change corresponding to formation of the opaque layer during the HCHO cooling experiments was analyzed using the multilayer evaluation tool, the refractive index and thickness of the hydrate layer were inferred to be ∼1.29−1.30 and ∼80 nm, respectively. Unfortunately, no literature values of HCHO hydrate or oligomer refractive indices at low temperatures are available for comparison. Most published refractive indices of organic materials are measured to be greater than that of ice (or greater than water’s 1.33 in most cases), but such measurements were typically taken at room temperature when the substances are in the liquid phase. A refractive index lower than that of ice is uncommon, but not impossible. For example, amorphous Teflon (Dupont, Teflon AF) is reported to have refractive indices ranging from 1.29 to 1.31. Furthermore, theoretical estimates by Groh and Zimmermann (1991) place the lower limit of organic polymer refractive index at around 1.29.54 Little is known about the phase behavior of the bulk CH3CHO−ice system, making diagnosing the opaque films in these experiments more difficult. Extrapolation of CH3CHO vapor pressure to experimental temperatures suggests that condensation on the surface was unlikely.55 Although the opaque domains formed by acetaldehyde on ice were visually similar to the hydrates found in HCHO−ice experiments, they may be different in nature. The signature of the opaque films in

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ASSOCIATED CONTENT

S Supporting Information *

A schematic diagram of the experimental setup has been included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Phone (212) 854-2869; Fax (212) 854-3054. 29114

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Present Address

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Atmospheric Science and Technology Directorate, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, Canada. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by a NSF CAREER award for V.F.M. (ATM-0845043).

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REFERENCES

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