Deliquescence Phase Transition Measurements by Quartz Crystal

Jun 28, 2012 - Deliquescence Phase Transition Measurements by Quartz Crystal Microbalance Frequency Shifts. Kathleen Jane L. Arenas, Steven R. Schill,...
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Deliquescence Phase Transition Measurements by Quartz Crystal Microbalance Frequency Shifts Kathleen Jane L. Arenas, Steven R. Schill, Ammaji Malla, and Paula K. Hudson* California State University, Fullerton, 800 N. State College, Fullerton, California 92834, United States ABSTRACT: Measurements of the hygroscopic properties of aerosols are needed to better understand the role of aerosols as cloud condensation nuclei. Several techniques have been used to measure deliquescence (solid to liquid) phase transitions in particular. In this study, we have measured the deliquescence relative humidity (DRH) of organic and inorganic salts, organic acids (glutaric and succinic acid), and mixtures of organic acids with ammonium sulfate using a quartz crystal microbalance (QCM). The QCM allows for measurement of the deliquescence phase transition due to inherent measurement differences between solids and liquids in the oscillation frequency of a quartz crystal. The relative humidity dependent frequency measurements can be used to identify compounds that adsorb monolayer amounts of water or form hydrates prior to deliquescence (e.g., lithium chloride, potassium and sodium acetate). Although the amount of water uptake by a deliquescing material cannot be quantified with this technique, deliquescence measurements of mixtures of hygroscopic and nonhygroscopic components (e.g., ammonium sulfate and succinic acid (DRH > 95%)) show that the mass fraction of the deliquescing portion of the sample can be quantitatively determined from the relative change in oscillation frequency at deliquescence. The results demonstrate the use of this technique as an alternative method for phase transition measurements and as a direct measurement of the mass fraction of a sample that undergoes deliquescence. Further, deliquescence measurements by the QCM may provide improved understanding of discrepancies in atmospheric particle mass measurements between filter samples and the tapered element oscillating microbalance given the similar measurement principle employed by the QCM.

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INTRODUCTION It is well established that tropospheric aerosols play a role in our climate system directly, by absorbing and scattering radiation, and indirectly, by acting as cloud condensation nuclei (CCN), thus modifying cloud microphysical properties, cloud formation properties, and cloud lifetime.1−7 Although it has been established that aerosols have a cooling effect on the Earth’s surface, the magnitude of radiative forcing due to aerosols is poorly understood, particularly in the case of the indirect effect.8 The degree to which aerosols affect climate depends, in part, on the aerosol size, phase, and water content, which is known to vary with changing relative humidity (RH).6,9,10 The hygroscopic properties of aerosols have been studied in great detail using a variety of techniques to examine deliquescence (solid to liquid) and efflorescence (liquid to solid) phase transitions. Experimental variables have included particle size (nanometer to bulk sample measurements), temperature, and physical state (e.g., whether the particles are suspended in an air stream or electric field or impacted on a surface or grid substrate). Although studies have shown that particle size affects the deliquescence relative humidity (DRH),11−14 aerosol particles with a diameter greater than 50 nm yield similar results to those of a bulk sample due to the minimization of the Kelvin effect.15 Whether the particle is deposited on a substrate or suspended in air as the aerosol appears to have a nearly negligible effect on deliquescence © 2012 American Chemical Society

properties. For example, Fourier transform infrared (FTIR) spectroscopy measurements for water uptake and deliquescence measurements of ammonium sulfate and sodium chloride particles deposited on an attenuated total reflectance crystal (ATR-FTIR)16 or suspended in an air stream in an aerosol flow tube (AFT-FTIR)17−20 are in good agreement. Similarly, optical measurements using atomic force microscopy (AFM)21 or scanning or transmission electron microscopy (SEM or TEM),22−24 where particles are deposited on substrates and grids, are in good agreement with tandem differential mobility analyzer (TDMA)25−30 and electrodynamic balance (EDB) measurements,31−37 where particles are suspended in an air stream or in an electric field, respectively. Another factor involved in the measurement of the deliquescence phase transition of aerosol depends on the ability to dry the aerosol prior to relative humidity exposure and knowledge of the degree to which the aerosol is dried. In situ measurements, like the TDMA or AFT-FTIR, where aerosols are generated, dried, and exposed to relative humidity in a continuous air stream, can have difficulty completely drying aerosols prior to relative humidity exposure. EDB instruments are typically capable of drying particles more completely and, in Received: November 1, 2011 Revised: May 17, 2012 Published: June 28, 2012 7658

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oscillating microbalance (TEOM), employed in atmospheric field measurements of particle mass, uses a similar technology to the QCM, laboratory studies using the QCM under changing relative humidity conditions may provide insight into some of the conflicting values and data reported between TEOM field and filter based measurements.53,54

the case of particles that form hydrates, result in compositions other than the most thermodynamically stable form.32,38 For example, Peng and Chan (2001) measured a stoichiometry of a half-hydrate when drying a sodium acetate particle to an RH < 10%, rather than the thermodynamically favored trihydrate.32 Differences between DRH measurements or theoretical values have been attributed to the possibility that the particle was dried to a less stable crystalline phase than expected, may have contained several different crystalline phases or trapped water, or had a glassy structure.38 A recent study by Yeung et al. (2010) suggests that discrepancies between DRH measurements of glutaric acid could similarly result from different polymorphic states (stable β- and metastable α-forms) of glutaric acid. Differences in measured DRH values of glutaric acid determined by bulk39−42 EDB41,43−46 and TDMA measurements25 are therefore attributed to experimental differences in the phase of the measured particles. Additionally, only one TDMA measurement of glutaric acid DRH and growth has been reported due to the inability to fully effloresce the aerosol or resulting experimental difficulties from structural rearrangements and/or particle shrinking.29,41,46 Although a number of factors (e.g., particle size, surface deposition) do not affect DRH measurements enabling the use of multiple techniques, particle drying and knowledge of the composition of the “dry” particle is necessary for reporting accurate DRH values. The quartz crystal microbalance (QCM) determines particle mass deposited on a piezoelectric crystal based on the frequency of oscillation of the crystal. The QCM has been used previously for a wide range of mass related studies including biological, physical, and analytical applications.47−49 The QCM may be considered a viable technique to measure solid to liquid phase transitions because the change in mass between the solid particles deposited on the quartz crystal and conversion to liquid droplets can be directly measured. Previous studies have shown that particle size and deposition on a surface, as is the case with the QCM, have negligible effects on the measurement of the deliquescence relative humidity. Unlike in situ drying measurements (e.g., TDMA, AFT-FTIR), particles deposited on the QCM crystal can be exposed to dry air for as long as needed for particle efflorescence to occur. Not only does this allow for complete drying of a given sample but also extends the deliquescence relative humidity measurements to compounds with low efflorescence relative humidities. Previous studies using the QCM indicate that it has the capability to quantify water uptake by atmospherically relevant particles including organic films,50,51 SiO2,16 and clay samples.52 Previous QCM measurements by Schuttlefield et al. (2007) have shown that the QCM can be used to quantify monolayer water uptake, or several monolayers of water uptake, by SiO2 and clay samples.16,52 However, the measured water uptake was not a result of compounds undergoing phase transitions. Deliquescence-like behavior was reported by Demou et al. (2003) for certain organic films; however, quantitative deliquescence measurements of known salts using the QCM have not been reported. The purpose of this study was to examine the efficacy of the QCM to measure the DRH of salts with a variety of compositions including organic and inorganic components. The technique has further been applied to hygroscopic and nonhygroscopic dicarboxylic acids, glutaric acid, and succinic acid, respectively, and mixtures of the two with ammonium sulfate. Because the tapered element

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EXPERIMENTAL METHODS The deliquescence relative humidity of inorganic (ammonium nitrate and sulfate; lithium, magnesium, and sodium chloride; and potassium iodide) and organic salts (potassium and sodium acetate), organic acids (glutaric and succinic acid), and mixtures of the noted organic acids with ammonium sulfate were measured using a quartz crystal microbalance (SRS Instruments, QCM200). The experimental apparatus is shown in Figure 1. ACS grade salts and organic acids, used as received

Figure 1. Schematic of the experimental apparatus for the measurement of deliquescence relative humidity values of inorganic and organic salts deposited on a quartz crystal microbalance crystal. Relative humidity is generated as a stepwise RH increase (A) or constant RH ramp (B) by mixing ratios of wet and dry air. The salt is exposed to the RH through a flow cell attachment.

with no additional purification, were deposited on the gold plated electrode of the quartz crystal from 1% solutions by mass using an airbrush (Badger, 250). The solution was sprayed on to the crystal using a plastic mask with a 1/4” diameter hole resulting in small solution drops deposited in the middle of the electrode surface. Visual inspection of the quartz crystal after drying particles showed this method generated small clusters of particles with an average dry particle mass of 20 ± 10 μg cm−2. A variety of deposition surface areas (1/8”−1/2”) as well as range of particle masses (9−45 μg cm−2) resulted in similar QCM response behavior and DRH results. It was necessary to have good contact between the deposited particles and the electrode for the measurement of deliquescence phase transitions. After particle deposition, the crystal was placed in the QCM with flow cell attachment and exposed to dry air. Because the relative humidity over the sample can be controlled for an extended period of time, the amount of drying time is controllable, and compounds with low efflorescence relative humidity values were able to be dried effectively prior to starting a deliquescence experiment. The particles deposited on the quartz crystal were exposed to relative humidity via one of two possible methods using either a stepwise increase in relative humidity (Figure 1A) or a constant relative humidity ramp (Figure 1B). With either method, the 7659

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Figure 2. Measured relative humidity (top) and change in frequency data (bottom) as a function of time for typical ammonium sulfate deliquescence experiments given a stepwise increase in RH (A) or constant RH ramp (B). The light gray dots (A) show all frequency and relative humidity measurements, whereas the dark squares show frequency changes under stable RH conditions (RH changing less than 0.11% RH min−1, the rate of the constant RH ramp (B)).

relative humidity was controlled by combining ratios of wet and dry air with a total flow of dry or humidified air reaching the QCM flow cell attachment of 0.10 L/min (lpm). Dry air was generated from a purge gas generator (Parker Balston, 75-62 NA) which removes carbon dioxide and dries the air to a dew point of less than −73 °C. Wet air was generated from dry air flow through a bubbler (Chemglass, AF-0513-A) or Nafion tubing (Perma Pure, LLC, MH-110-48P-4). The Nafion tubing consists of two concentric tubes, an inner Nafion tube and an outer polypropylene tube. Water is circulated through the inner tube using a variable-flow peristaltic pump (Fisher Scientific, 13-876-2) while dry air flows between the inner and outer tubes. The water vapor equilibrates across the Nafion material humidifying the circulated dry air. When the wet and dry air were combined in different ratios, a range of relative humidity was generated. The stepwise RH increase (Figure 1A) was produced from a single dry air line separated into wet and dry lines with a constant flow of 3.0 lpm total. An adjustable needle valve (Swagelok, B-1RS4) on the dry line was manually closed forcing air through the wet line to become humidified. The two lines are then recombined yielding an increasingly humidified air flow. A constant RH ramp (Figure 1B) was produced using two mass flow controllers (Alicat Scientific, Inc., MC-5 SLPM-D) with a total combined flow of 3.0 lpm. The flows of the wet and dry mass flow controllers were incrementally changed in small steps (increasing the wet flow and decreasing the dry flow) resulting in a constant RH ramp with a steady total flow. The rate of increase of the RH ramp was varied by changing the magnitude of the incremental steps. For both the stepwise RH increase and constant RH ramp, the excess humidified air was exhausted through an adjustable valve such that the total flow reaching the QCM flow cell attachment was maintained at 0.10 lpm. The increased total flow of the wet and dry air allows for greater precision in changing the RH with both methods.

The generated relative humidity was measured using relative humidity sensors (Vernier, RH-BTA) connected to a LabPro data acquisition module (Vernier, LABPRO) and computer. The crystal oscillation frequency and resistance was measured with a stand alone Labview application provided by SRS Instruments. Relative humidity and crystal oscillation frequency were measured independently with 1 s time resolution.

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RESULTS AND DISCUSSION Two typical experiments for the deliquescence of ammonium sulfate are shown in Figures 2A and 2B. The relative humidity over the sample is increased either stepwise (Figure 2A top) or as a constant ramp of 0.11% RH min−1 (Figure 2B top), respectively. The light gray dots in Figure 2A show all relative humidity measurements where the dark squares indicate stable RH conditions (i.e., under the same conditions as the constant ramp rate or where the RH was changing by less than 0.11% RH min−1). The bottom panel of Figures 2A and 2B show the corresponding change in the crystal oscillation frequency relative to the blank quartz crystal as the relative humidity over the ammonium sulfate salt is increased. Again, the dark squares in Figure 2A (bottom) correspond to frequency changes with stable RH conditions. The initial change in frequency value of −1560 and −1440 Hz (Figures 2A and 2B bottom) is directly related to the mass of ammonium sulfate deposited on the crystal relative to the blank crystal through the Sauerbrey equation: Δf = −Cf Δm, where Δf is the change in oscillation frequency, Δm is the mass change, and Cf is a constant that includes the resonant frequency, density, and shear modulus of the quartz crystal.55 The measured frequency remains fairly constant as the RH is increased until a large increase in the frequency is observed at 8.9 × 103 and 4.7 × 104 s, respectively, for Figures 2A and 2B, as the ammonium sulfate particles deliquesce and change from solid particles to liquid droplets. Although the Sauerbrey equation dictates a linear response between frequency and mass loading for the QCM, 7660

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the observed increase in frequency due to conversion from solid particle to liquid droplet is not unexpected due to inherent measurement differences between mass loading and liquid damping.56 The Sauerbrey equation has been used previously to quantify monolayer water uptake on atmospherically relevant particles using the QCM.16,50−52 In these studies, a decrease in oscillation frequency is observed with increasing relative humidity as water is adsorbed on or incorporated into the solid particles.16,50−52 In this study, a deliquescence phase transition is occurring. The solid particles have become liquid droplets when the deliquescence relative humidity is reached resulting in an increase in the liquid damping of the crystal and the observed increase in oscillation frequency. Note that this observed response is not due to a saturation of the QCM oscillation frequency due to overloading the mass deposited on the crystal which is seen as a decrease in frequency to approximately −6 × 106 Hz rather than the increase in frequency observed in this study. Attempts to decrease the magnitude of the liquid damping effect by decreasing the initial amount of particle mass deposited on the crystal by a factor of 5 still yielded similar results in the observed frequency change. As a result, water uptake that involves a deliquescence phase transition cannot be quantified because the oscillation frequency change is a response to the liquid damping of the crystal and therefore should not be considered analogous to EDB mass fraction solute or TDMA growth factor curves (e.g., see Cohen et al. (1987) or Prenni et al. (2001)). Although the amount of water uptake during deliquescence may not be quantifiable, the difference in oscillation frequency between the solid particles and liquid droplets, the magnitude of the frequency shift at deliquescence, may contain information regarding the deliquescing mass fraction of the sample, vide infra. Further, this observation has implications for atmospheric particle mass measurements by the TEOM which uses a similar methodology to the QCM. Discrepancies between TEOM and filter based measurements may in fact be attributed to particle phase due to the inherent measurement differences between solid and liquid particles.53,54 The RH and frequency data in Figure 2 are each measured independently as a function of time. The two measurements can then be matched in time in order to link the occurrence of the frequency change due to deliquescence to a relative humidity. Figure 3A shows the corresponding results for ammonium sulfate particles exposed to relative humidity using the constant RH ramp increase of 0.11% RH min−1 (darker line) or the stepwise RH increase (light dots with dark squares). The light gray dots show all frequency and relative humidity measurements, whereas the dark squares show frequency changes under stable RH conditions. The sharp increases in frequency observed in Figures 2A and 2B are now directly correlated to a particular RH and occur near 81 and 86% RH, respectively. For comparison, the DRH of ammonium sulfate has been previously measured by a number of different techniques with an average value of 79.9 ± 1.0%.16−20,25,26,28,29,35−39,57−59 Figure 3B (bottom) shows an enlargement of the dashed boxed region in Figure 3A and the derivative of the frequency data as a function of relative humidity (top). Closer examination of the deliquescence region shows a sharp transition near 86% RH for the constant RH ramp (dark dots). No stable RH data from the stepwise RH increase are present during the actual deliquescence transition as was often the case with the stepwise deliquescence measurements. It can

Figure 3. (A) Frequency and RH data from Figure 2 matched in time for the deliquescence of ammonium sulfate particles using the stepwise RH increase (light gray dots with dark squares) or the constant RH ramp (black dots). The dark squares represent regions of stable RH (RH changing less than 0.11% RH min−1, the rate of the constant RH ramp). (B) Boxed region in (A) is enlarged in the bottom panel showing a response delay of the RH sensor with large RH changes. The top shows smoothed derivatives of the constant RH ramp (black line) and the stepwise RH data multiplied by a factor of 10 (gray line). The maximum value of the derivative yields the measured DRH value.

only be stated that deliquescence occurs between 76.8 and 83.7% RH, the stable RH data for the stepwise increase that bracket the deliquescence transition. Smaller steps in RH result in improved measurement of the DRH. The complete stepwise RH increase data (light dots) shows the deliquescence transition occurring from 80 to 82% RH with far fewer data points (one data point per second) than the constant RH ramp. The slightly broader deliquescence transition observed for the stepwise RH increase is likely due to the response time of the RH sensor relative to that of the QCM. Large stepwise increases in RH during an experiment result in an observed delay between the RH measurements and corresponding QCM response (i.e., the RH measurement is lower than the observed QCM response). For example, when the RH was increased from 77.0 to 83.7% (Figure 2A top), it took nearly 5 min for the RH signal to reach stable conditions (where the RH was changing by less than 0.11% RH min−1). Further, because this 7661

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manual relative humidity change occurred near the ammonium sulfate deliquescence phase transition, a broadened deliquescence transition from 80 to 82% RH is observed. Similar results are observed when using a faster constant RH increase (0.60% RH min−1, not shown). When using a faster constant RH ramp, the measured DRH of ammonium sulfate decreases to 69.8 ± 1.9% RH, again a lower RH measurement compared to literature values accompanying the observed QCM response. In general, smaller increases in relative humidity result in a faster equilibration time yielding more accurate results. Although a broader observed deliquescence phase transition is observed with the stepwise increase, the determined value of the DRH is not greatly affected. The derivative of the frequency data as a function of relative humidity is used to determine the relative humidity that corresponds to the sharpest increase in frequency. The top panel of Figure 3B shows a 30 point binomially (Gaussian) smoothed derivative of the frequency data for the complete stepwise RH increase data (gray line) and the constant RH ramp (black line). The derivative corresponding to the stepwise data has been multiplied by a factor of 10 for ease in data comparison. A comparison of the width of the derivative shows the broader deliquescence transition with the stepwise RH increase. The maximum value of the derivative corresponding to the DRH for ammonium sulfate is determined as 81.5% and 86.1% RH for the complete stepwise increase data and the constant RH ramp, respectively, for these two experiments. Multiple experiments using the two different techniques result in an average of 82.6 ± 2.2% RH for the stepwise RH increase and 85.0 ± 1.0% RH for the constant RH ramp. Similar deliquescence experiments were completed for several inorganic (ammonium nitrate and sulfate; lithium, magnesium, and sodium chloride; and potassium iodide) and organic salts (potassium and sodium acetate) to examine the versatility of the QCM technique to measure deliquescence phase transitions for a variety of compounds with a range of established DRH values using multiple experimental techniques. Two types of frequency behavior were observed for the salts studied when using the constant RH ramp. Ammonium nitrate, sodium chloride, and potassium iodide behave similarly to ammonium sulfate (shown in Figure 3A) where the frequency of oscillation is fairly constant prior to and after deliquescence. Additionally, the maximum frequency reached is 95.5 ± 1.2% of the initial starting frequency (e.g., on a scale normalized to the initial starting frequency, the frequency increases from −1.0 to −0.045). Figure 4 shows typical deliquescence experiments for potassium and sodium acetate compared to ammonium nitrate and sulfate. The measured change in frequency has been normalized relative to the magnitude of the starting frequency for each compound shown. For example, the ammonium sulfate constant ramp data shown in Figure 3A is divided by the magnitude of the starting frequency (1440 Hz) such that the normalized change in frequency starts at −1.0 and does not increase above 0, the frequency of the blank crystal. Dashed lines are present at −1.0, the normalized starting frequency, and −0.045, the maximum normalized frequency for ammonium nitrate, sulfate, sodium chloride, and potassium iodide. Potassium and sodium acetate both show decreases in frequency with increasing RH prior to deliquescence compared to ammonium nitrate and sulfate. Further, the maximum frequency value during the deliquescence transition is lower for potassium and sodium acetate than that of ammonium nitrate and sulfate. The decrease in

Figure 4. Normalized frequency data (relative to initial starting frequency) as a function of relative humidity for typical constant RH ramp deliquescence experiments of hydrate forming (potassium and sodium acetate) and nonhydrate forming (ammonium nitrate and sulfate) salts. Dashed lines at −1.0 and −0.045 indicate normalized starting frequency and standard maximum normalized frequency values observed for nonhydrate forming salts, respectively.

frequency prior to deliquescence and lower maximum frequency at deliquescence were similarly observed for lithium chloride to a much larger degree (e.g., minimum normalized frequency = −1.8). Measured DRH values for potassium and sodium acetate and ammonium nitrate using the constant RH ramp are 20.1 ± 0.1%, 43.0 ± 0.6%, and 65.5 ± 1.0%, respectively, in good agreement with previous measurements.16,31−33,58 Previous studies have shown that monolayer water uptake on atmospherically relevant particles can be quantified when phase changes do not occur and is indicated by a decrease in the frequency with increasing relative humidity.16,50−52 It therefore follows that an observed decrease in frequency results from uptake of water not resulting in a phase change (e.g., monolayer water uptake or hydrate formation) while an increase in frequency is due to the solid to liquid phase transition. The decrease in frequency prior to deliquescence was primarily observed for three of the seven salts studied: potassium and sodium acetate and lithium chloride. Of these salts, hydrate formation has been measured for lithium chloride and sodium acetate using infrared (IR) spectroscopy and EDB, respectively.32,60 Using observed structural changes in the OH stretching region of concentrated salt solutions when cooled from 280 to 200 K, Pandelov et al. (2010) established an empirical rule that a salt may form a hydrate upon cooling of its aqueous solution if the enthalpy of solution for the salt is lower than the standard enthalpy of fusion of ice (6 kJ mol−1).60 Given this rule, it then follows that potassium acetate could similarly form a hydrate upon cooling (ΔHsoln = −15.3 kJ mol−1).61 This study shows that potassium acetate, similar to the hydrate forming lithium chloride and sodium acetate, also likely forms a hydrate under room temperature conditions. Although the relative humidity at which the large increase in frequency was observed (the DRH) was consistent across multiple constant RH ramp experiments for hydrate forming salts, the amount of decrease prior to deliquescence and, in particular, the maximum frequency reached at deliquescence varied by as much as 20% for sodium acetate. Fluctuations in the maximum frequency reached during deliquescence for lithium chloride and potassium acetate were much smaller, near 6%, compared to the maximum 2% deviation for nonhydrate 7662

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Table 1. Experimental DRH Values for Different Experimental Conditions Compared to Literature LiCl KOOCCH3 MgCl2 NaOOCCH3 NH4NO3 KI NaCl (NH4)2SO4

stepwise

constant ramp

fast constant ramp

10.3 ± 1.1

11.2 ± 0.3 20.1 ± 0.1

10.7 ± 0.4 19.0 ± 0.8

30.7 ± 0.9

69.2 ± 1.2 77.1 ± 2.1 82.6 ± 2.2

43.0 65.5 72.5 81.5 85.0

± ± ± ± ±

0.6 1.0 2.6 1.6 1.0

35.9 53.1 58.0 65.7 69.8

± ± ± ± ±

0.7 1.2 0.7 1.6 1.9

literature 11.3 22.5 32.8 43.5 62.7 68.0 75.0 79.9

± ± ± ± ± ± ± ±

0.3a 0.3a 0.2a 0.8b 1.2c−e 1.3a,f 0.7a,e−o 1.0a,e,g−l,o−y

a

Greenspan (1977). bPeng and Chan (2001). cLightstone et al. (2000). dRichardson and Hightower (1987). eSchuttlefield et al. (2007). fWoods et al. (2007). gCohen et al. (1987). hCruz and Pandis (2000). iCziczo et al. (1997). jDai et al. (1997). kGysel et al. (2002). lJoutsensaari et al. (2001). m Krueger et al. (2003). nRichardson and Snyder (1994). oTang and Munkelwitz (1993). pBrooks et al. (2002). qDougle et al. (1998). rGibson et al. (2006). sHan and Martin (1999). tJohnson et al. (2008). uMinambres et al. (2010). vOnasch et al. (1999). wPrenni et al. (2001). xTang and Munkelwitz (1991). yTang and Munkelwitz (1994).

and size and mass measurements.12,25−28,30−38,59 This further supports the use of a QCM to measure deliquescence relative humidity values that are independent of particle size (for samples greater than 50 nm) and substrate deposition. The salts measured here with DRH values 95% RH) and is beyond our experimental range.32,39,40,42,62,69 Figure 7A shows typical deliquescence experiments for ammonium sulfate, succinic acid, and a variety of molar mixtures of succinic acid:ammonium sulfate (2:1, 1:1, and 1:2) corresponding to 36, 52, and 70 wt % (wt %) ammonium sulfate, respectively. Data is shown from 60 to 100% RH although experiments were started below 5% RH. No deliquescence is observed for pure succinic acid due to the experimental limitations of our RH system. DRH values below that of ammonium sulfate are observed for the 2:1, 1:1, and 1:2

Figure 6. Calibrated normalized frequency data (relative to initial starting frequency) as a function of relative humidity for typical constant RH ramp deliquescence experiments of ammonium sulfate, glutaric acid, and a 1:1 molar mixture of glutaric acid and ammonium sulfate. Dashed lines at −1.0 and −0.045 indicate normalized starting frequency and standard maximum normalized frequency observed for nonhydrate forming salts, respectively.

Figure 7. (A) Calibrated normalized frequency data (relative to initial starting frequency) as a function of relative humidity for typical constant RH ramp deliquescence experiments of ammonium sulfate, succinic acid (0 wt % AS), and 36, 52, and 70 wt % succinic acid:ammonium sulfate mixtures. The dotted lines represent the theoretical frequency reached at deliquescence given the deliquescing (ammonium sulfate) mass percent. (B) Correlation of the measured mass percent deliquesced (percent increase in normalized frequency) and ammonium sulfate mass percent of deposited ammonium sulfate, succinic acid (0 wt % AS), and 36, 52, and 70 wt % succinic acid:ammonium sulfate mixtures. The dashed line is the best-fit linear regression through the data corresponding to the given equation and correlation coefficient.

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molar succinic acid:ammonium sulfate solutions at 79.6 ± 0.6%, 78.6 ± 1.1%, and 79.4 ± 1.8%, respectively. An average value of 1:1 molar mixtures (mass mixtures) of 78.0 ± 0.6% RH has been reported from EDB and TDMA measurements.65,66,68 Although specific 2:1 and 1:2 molar succinic acid:ammonium sulfate mixtures have not been measured, all measured values for multiple mole ratios fall in the range of 76−83% RH, in good agreement with the results presented here.39,42,65,66,68,70 Unlike the glutaric acid:ammonium sulfate mixtures shown in Figure 6, the succinic acid:ammonium sulfate mixtures do not reach the standard maximum normalized frequency at deliquescence (−0.045). This behavior is somewhat similar to hydrate forming compounds where the maximum normalized frequency is less than the standard maximum normalized frequency. However, in these cases, the maximum frequency reached at deliquescence only slightly increases after deliquescence where the frequency of the hydrate forming compounds continued to increase after deliquescence (Figure 4). Because the deliquescence transition of succinic acid occurs at such a high relative humidity and because the eutonic composition of succinic acid and ammonium sulfate mainly consists of ammonium sulfate (0.004:0.094 mol mol−1 succinic acid:ammonium sulfate),39 it can be assumed that the ammonium sulfate within the sample is the only component deliquescing in this particular mixture. This assumption is further supported by TDMA and EDB measurements of succinic acid:ammonium sulfate mixtures that show decreased hygroscopic growth (decreased water uptake) that can be modeled using the Zdanovskii−Stokes−Robinson (ZSR)65,68 theory given the assumption that only the ammonium sulfate fraction is hygroscopic. As stated earlier, the QCM measures a change in frequency that is proportional to the change in mass on the crystal. However, because solids and liquids are inherently measured differently, an observed increase in the frequency corresponds to the conversion from a solid particle to a liquid droplet. Given these assumptions, the maximum normalized frequency reached for the succinic acid:ammonium sulfate mixture at deliquescence should correspond to the mass fraction of the deliquescing compound (ammonium sulfate) in the mixture. The dotted lines in Figure 7A represent the value the frequency should reach at deliquescence given the mass fraction of the ammonium sulfate in the sample. For example, a mixture that contains 70 wt % ammonium sulfate could reach a normalized value of −0.3. Figure 7B shows the linearity in the response of the QCM of the maximum frequency reached during deliquescence as a function of the mass fraction of the deliquescing component (mass % ammonium sulfate) in the sample for pure succinic acid, pure ammonium sulfate, and 36, 52, and 70 wt % ammonium sulfate (2:1, 1:1, and 1:2 molar ratios of succinic acid:ammonium sulfate). The dashed line shows the best-fit linear regression where y = 0.93 x + 3.50 with a corresponding R2 = 1.00. Although the amount of liquid water absorbed by the particles cannot be quantified, the mass fraction of solid particle remaining on the crystal can be determined for this particular internal mixture of a hygroscopic inorganic salt (ammonium sulfate) and a nonhygroscopic organic acid (succinic acid). These results differ from other hygroscopic growth measurement techniques, in that the QCM is able to directly measure the mass fraction of the compound that has deliquesced (or the mass fraction of the remaining nondeliquesced material) due to the inherent measurement differences of solids and liquids.56 As

only one specific case has been presented in the present study, the full extent of the ability of the QCM to measure the deliquescing mass fraction will be explored with other inorganic salt and dicarboxylic acid mixtures as well as mixtures of dicarboxylic acids in future studies.

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CONCLUSIONS In this study, the quartz crystal microbalance has been shown to be an effective method for the measurement of deliquescence relative humidity values ranging from 10% to 90% RH for a number of organic and inorganic salts (both hydrate and nonhydrate forming), organic acids (glutaric and succinic acid), and mixtures of organic acids with ammonium sulfate. As with previous literature results, we find that the deliquescence relative humidity is independent of particle size (for particles greater than 50 nm) and particle deposition on a substrate. DRH values can be determined via a stepwise or constant RH ramp under fast or slow rates, but calibration conditions must be considered in all cases. Similar to EDB measurements, when the deliquescence of hydrate forming salts is measured, the particle deliquesces at the lowest DRH of its hydrates. The amount of water uptake during deliquescence cannot be quantified due to inherent measurement differences between solid and liquid particle phases with the QCM due to liquid damping. However, for a specific mixture of ammonium sulfate and succinic acid, the mass fraction of the particle that deliquesces can be quantified. This measurement principle differs from other commonly used techniques that measure changes in particle size (TDMA) or changes in particle mass (EDB) yet are unable to differentiate between particle phases. Spectroscopic measurements have been able to gain similar particle phase information where studies have focused on mixtures of similar hygroscopic/nonhygroscopic components (e.g., succinic acid with ammonium sulfate or ammonium nitrate)19,31,66 as well as soluble components (e.g., malonic or glutaric acid with ammonium sulfate).64,66 Future studies will explore other inorganic salt and organic acid mixtures as well as mixtures of dicarboxylic acids given differences between observed and predicted growth.69 Because of the similar technology between the QCM and the TEOM, these observations could provide insight into conflicting TEOM measurements of atmospheric particle mass. The physical state of particles sampled must be considered as liquid (deliquesced) particles will be measured with a smaller mass than the solid counterparts. Further, the mass fraction of hygroscopic particles present can be readily determined by increasing the relative humidity over atmospherically sampled particles.

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AUTHOR INFORMATION

Corresponding Author

*Phone 657-278-2641; fax 657-278-5316; e-mail phudson@ fullerton.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by California State University, Fullerton startup funds and state funded mini grants. P.K.H. thanks Tram Le, Karen Duong, Jenny Lam, Jennifer Taing, and Tiffany Lopez for critical foundation experiments resulting in the culmination of work presented here. 7665

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