Deliquescence and Efflorescence Behavior of Ternary Inorganic

(11, 17-24) In fact, it has been suggested that the increase in sulfate aerosols due to the substantial increase of coal burning in China may have cou...
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Deliquescence and Efflorescence Behavior of Ternary Inorganic/Organic/Water Aerosol Particles Andreas Peckhaus, Stefan Grass, Lennart Treuel, and Reinhard Zellner* Institute for Physical Chemistry, University of Duisburg-Essen, Universitaetsstrasse 5-7, 45117 Essen, Germany S Supporting Information *

ABSTRACT: The deliquescence behavior of ternary inorganic (ammonium sulfate and ammonium nitrate)/organic (glutaric acid and malonic acid)/water aerosol particles has been investigated at 293 K using a novel surface aerosol microscopy (SAM) technique. The results obtained for the deliquescence relative humidities (DRH) for particles of variable inorganic/organic contents show a eutectic behavior with the mixed particles showing deliquescence at lower DRH compared to the pure inorganic and organic components, respectively. This behavior has been quantitatively modeled using the extended aerosol inorganics (E-AIM) thermodynamic model of Clegg et al. in combination with the UNIFAC group activity approach to account for organic molecular solutes. In addition, we have investigated the crystallization behavior of supersatured and formerly deliquesced ternary solution droplets using space resolved Raman spectroscopy. It is found that such droplets produce solid particles in which the inorganic and organic phases show some spatial separation with the organic component being predominantly found at the outer part of the particle. Independent measurements of the contact angles of such ternary droplets reveal that their angles are within experimental error identical to those of the purely organic/water solutions.

1. INTRODUCTION The chemical and radiative effects of atmospheric aerosols are size and phase related and hence they are strongly influenced by the ambient relative humidity (RH). Due to water absorbing hygroscopic components, changes in RH will change both particle diameter and wavelength dependent refractive indices.1−8 Therefore, the net effect on chemistry and/or climate for a given atmospheric particle load will also depend on the relative humidity and is hence driven by changes in either temperature or water partial pressure. The hygroscopic properties and phase changes of atmospheric aerosols as well as the surface morphology of solid aerosol particles and chemical surface composition must be understood and represented accurately to improve aerosolclimate models.9 Understanding the influence of individual classes of chemical components on the thermodynamic behavior of complex aerosols is tedious yet inevitable to assess the effects of ambient aerosols.10 Sulfate aerosols are ubiquitous and widely abundant in the atmosphere,11 and due to their scattering behavior12−16 they provide the most significant anthropogenic cooling contribution to the global direct radiative forcing.11,17−24 In fact, it has been suggested that the increase in sulfate aerosols due to the substantial increase of coal burning in China may have © 2012 American Chemical Society

counterbalanced the global temperature increase due to greenhouse gases over the past decade.25 If sulfuric acid aerosols are completely neutralized and an excess of ammonia is present, the formation of ammonium nitrate aerosols becomes feasible. Due to this link to the presence of free ammonia molecules, the effect of nitrate aerosols is sensitive to changes in the concentration of NOx as well as to concentration changes of ammonia. A relevant consequence of this dependency is that the weakening of the radiative forcing of sulfate aerosols due to decreased SO2 emissions is neutralized to a large extent in many regions by the formation of nitrate aerosols.26 The occurrence of nitrate aerosols shows a strong regional dependency with high concentrations mainly found in highly industrialized areas and low concentrations in rural districts.27,28 The radiative forcing of nitrate aerosols over Europe is about 25% that of sulfate aerosols.29 However, the phase behavior as well as the radiative behavior of nitrate aerosols is not well understood and the Special Issue: A. R. Ravishankara Festschrift Received: November 30, 2011 Revised: April 17, 2012 Published: April 23, 2012 6199

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kinetically controlled crystallization from highly supersaturated solutions the resulting crystal structures and, more importantly, the crystal distributions within the effloresced aerosol particle can differ greatly from the predictions referring to thermodynamically controlled conditions. The deliquescence of complex atmospheric aerosol particles will inevitably depend on the crystal structures of the effloresced components present in the aerosol. Moreover, possible differences in the crystal structures and the size of individual crystallites would strongly affect the scattering behavior as well as the surface area and surface composition of such aerosol particles. The elucidation of the crystallization behavior of internally mixed complex atmospheric aerosols is therefore an important task in assessing their influence on atmospheric chemistry and climate. In this work we have investigated the deliquescence behavior of ternary inorganic/organic/water aerosols using surface aerosol microscopy (SAM) to monitor the hygroscopic growth of such particles. The results for DRH measurements for particles with varying relative inorganic/organic composition have been analyzed using the thermodynamic model of Clegg et al.90−92 in combination with the UNIFAC (universal functional group activity coefficient) model to include molecular organic solutes. Moreover, spatially resolved Raman scattering has been used to investigate liquid−solid and solid− solid phase separations in efflorescing supersaturated droplets.

current knowledge remains insufficient for an accurate determination of its influence on the radiative forcing.11 Studies performed by Myhre et al.30 imply that the uncertainty in atmospheric model results is largely due to the uncertainty regarding the hygroscopic growth of atmospheric aerosols. Differences in the model relative humidity could cause differences of up to 60% in the radiative forcing values. These studies spark a new and profound interest in the thorough characterization of the hygroscopic growths of complex aerosols. Depending on factors like location and season, aerosols can contain various ratios of inorganic to organic material31 with the organic material typically accounting for 10−50% of the fine particle mass.32 Other field data indicate that up to 50% or even more organic material can be present in atmospheric aerosols.33 The organic compounds found in atmospheric aerosols frequently include significant amounts of dicarboxylic acids.34,35 Being representative of many polar organic substances, they are predominantly present in condensed phases rather than in the gas phase36−38 due to their low vapor pressures. Owing to these properties, dicarboxylic acids are major constituents of the water-soluble organic compounds (WSOCs).39 Measurements have shown that the organic material is usually internally mixed together with inorganic compounds in tropospheric particles.33,40,41 Although the behavior toward varying relative humidity, as normally described by deliquescence and efflorescence, of pure ammonium sulfate (AS) particles is well established,1,42 information about phase transitions and hygroscopic properties of organic and mixed organic/inorganic particles is still not at the same level.43 Studies of pure organic systems44−51 have shown that their deliquescence behavior strongly depends on the chemical nature of the organic substance. A number of groups have also studied the deliquescence and crystallization as well as thermodynamic properties of mixed organic/ inorganic particles.44,52−76 The results from these studies reveal large deviations from ideal solution behavior and make the properties of the system difficult to predict.77 Thermodynamic models of inorganic/water systems are relatively well developed and widely applied in aerosol science.78−84 Generally, they can be characterized by being based on either mixing rules (e.g., Kusik and Meissner85 and Zdanovski−Stokes−Robinson (ZSR)86,87) or ion interaction equations (e.g., Pitzer molality-based model88 and Pitzer− Simonson−Clegg mole fraction model89). Though the ZSR model can in principle be applied to aerosols containing organic compounds and only values of the molalities are required, it does not deliver accurate results for the activities of neutral (uncharged) species such as organic molecules in mixed solutions containing organics and electrolytes. Clegg et al.90−92 and Friese and Ebel93 proposed a practical approach to overcome these problems by calculating the properties of soluble mixed inorganic/organic/water aerosols. This approach mainly involves the use of pre-existing models and intriguingly combines those models with a self-consisting method of incorporating the mutual influence of ions and organic molecules on the activities of all components. The model has been developed and extended by a collaboration of several scientists, and a range of papers describe its results.41,64,92,94−96 Next to deliquescence, its inverse process, namely the crystallization of supersaturated aerosol solution droplets upon reduction of the relative humidity, called efflorescence, is equally important. Because the efflorescence process leads to a

2. EXPERIMENTAL SECTION Three main experimental techniques were used for the experiments presented in this manuscript: (1) the surface aerosol microscopy (SAM), (2) a newly developed setup to determine the contact angles of deposited aerosol particles on defined surfaces, and (3) a commercial Raman microscope for the acquisition of spatially resolved Raman scans of individual aerosol particles. These setups and the relevant measuring procedures are described in the following paragraphs. 2.1. Surface Aerosol Microscope (SAM) Setup. The surface aerosol microscope (SAM) setup has been described before in detail,97−99 and only a brief summary of the setup will be given here. The central part of the SAM setup is a reflected light optical microscope (KMA 1, Rathenow GmbH) equipped with a 60× or 50× objective (Nikon, M Plan, 60×/0.7 or Nikon, CF Plan, 50×/0.55). Completing the setup, a special measurement cell was designed and built, allowing the deposition of aerosols on carefully prepared surfaces and the precise control of RH and temperature they are subjected to. The SAM system uses essentially the same RH control system described in an earlier publication.61 Relative humidity (RH) and temperature are recorded inside the measuring cell with a combined RH/T sensor (Honeywell, 2C-HIH-4602-C). Throughout the measurements presented in this work, the temperature was kept constant at 293 ± 0.5 K. Throughout the measurements, pictures are taken with the CCD camera every 30 s and stored automatically by the software system together with a time tag, thus allowing a later linkage with the RH and temperature data. Figure 1 shows the outline of the complete SAM setup, including the gas flow system and thermostat units. To measure the full deliquescence/efflorescence cycle of aerosols, aerosols are deposited on the coated surface of the coverslips (see section 2.2) using an air spray nozzle. It has been shown before98 that the results from this technique show no deviations from electrodynamic balance (EDB) measurements 6200

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Figure 1. Schematic setup of the surface aerosol microscope (SAM).

and hence a possible effect of the surface on the measurement results can be ruled out. A LabView software program has been developed allowing the system to change the RH reproducibly in extremely slow and even steps over a period of about 25 h to complete a full efflorescence/deliquescence cycle. This slow procedure ensures that the aerosol particle remains in equilibrium with the surrounding gas phase at all times accounting for possible limitations due to diffusion across the phase boundary or within the liquid aerosol. The recorded data contain the time, temperature, and relative humidity over the complete measuring period, stored in standard ASCII files. These data can be simply imported by standard analysis programs and then be evaluated. A LabView software program has been developed to analyze the droplet pictures, determine the droplet radii, and link the results to the corresponding RH values. 2.2. Preparation of Coverslips. Aerosol particles were deposited on standard microscope coverslips that were surface coated in our laboratory. To minimize the surface influence on the aerosol particles, the surface of these coverslips were carefully coated to ensure hydrophobic behavior. The hydrophobization is achieved by an octadecyltrichlorosiloxane (OTS) coating. Coverslips with such a coating of sufficient quality are not available commercially and had to be prepared in the laboratory via a treatment with octadecyltrichlorosiloxane in a standard dipping procedure. This preparation is necessary to obtain valid results which compare well to EDB and optical tweezers studies with no surfaces present.98 After the reaction, the coverslips were treated with n-hexane to remove any remaining siloxane, thoroughly cleaned with deionized water and dried before use. 2.3. Contact Angle Measurements. For the contact angle measurements a new measuring cell was constructed allowing the simultaneous imaging of single aerosol droplets from two directions. This cell is used with the full RH and temperature control system of the SAM setup described earlier. This setup is schematically shown in Figure 2. The microscope setups are also identical with those used for the SAM experiments but different microscope objectives (Nikon, CF Plan, 50×/0.55 and Carl Zeiss, EC Epi Plan, 10×/ 0.20) are used. Though the droplet imaging from above uses the same reflected light illumination as the SAM setup, the

Figure 2. Schematic representation of contact angle measurement setup.

side-view camera used for the contact angle measurements uses a dimmable LED light source for transmitted light illumination. The whole measuring cell is mounted onto a micrometer precision z-stage so that both the aerosol and the microscope can be adjusted. This setup allows the simultaneous acquisition of efflorescence/deliquescence data (from top-view camera) and the determination of contact angles (from side-view camera). The image data, as well as RH and T data are again recorded and processed using a self-developed LabView program. The contact angles were determined from the recorded images using Image J (1.44p) and the tangent-fitting method, which only leads to negligible deviations from the Young−Laplace method at large contact angles. Using this procedure, we can obtain a precision of ±4° for the contact angles. 2.4. Raman Mapping Experiments. A commercial Raman microscope system (Senterra, Bruker) was used for the measurements presented in this work. The setup is equipped with a fully motorized XY-translation stage that allows the acquisition of scanning probe Raman spectra. A 20× objective (Olympus, M PlanN, 20×/0.40) was used for image and spectra acquisition throughout all experiments presented here. Liquid aerosols are deposited on a silicon wafer using an air pressure nozzle nebulizer. The liquid aerosol droplets are then dried in the RH controlled environment of the surface aerosol 6201

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In the E-AIM approach the activity coefficients γ of an ion and those of an uncharged (organic) solute are expressed by eqs 1 and 2.92

microscope setup until efflorescence occurs. The water-free molar ratio of organic to inorganic is kept constant at 1:1 throughout the different systems under investigation. To exclude effects resulting from a surface influence, a number of control samples have been prepared using the same solutions described below. Instead of drying them on a surface, they have been nebulized using an ultrasonic nebulizer before they were dried in a diffusion dryer and collected after efflorescence in an differential mobility analyzers (DMA) impactor as described in a previous study.100 A single aerosol crystal is then selected and a grid of 50 × 50 data points is placed on it defining the points at which Raman spectra are to be collected. Such spectra are then collected at every single of these points using a laser excitation wavelength of 785 nm, 25 mW power, and an acquisition time of 3 s. The scanning layer is chosen to cut through the middle of the crystal and the thickness of the layer (corresponding to a focal width of 2.39 μm) is small compared to the size of the overall crystal. Yet, the focal volume is large enough to allow probing a sufficient spatial average at any point. For the evaluation of the spectra a characteristic Raman band is chosen and integrated in all spectra. The areas are then color coded and used to generate space resolved color coded images of the crystal, revealing information about the spatial distribution of individual substances. The Raman bands used for the evaluation of the individual substances and their spectral assignments are listed in Table 1.

ln(γi) = Δln(γi[ion−water]) + Δln(γj([ion−organic]) (1)

and ln(γn) = Δln(γn[organic−water]) + Δln(γj[ion−organic]) (2)

Each Δ element in these equations is an individually determinable contribution to the activity coefficient resulting from ion−water, organic-water, and ion-organic interactions respectively. The advantage of this approach consists to a large extent in the fact that different models can be used to calculate the individual contributions to the overall activity coefficient. Once the individual contributions are determined, the above equations also allow the determination of the overall activity coefficient of water and hence the resulting DRH. The organic− water interactions can be calculated using the UNIFAC model based on group contributions. This is of special importance where the behavior of multicomponent system is unknown. The UNIFAC model was developed by Fredenslund et al.104 and has since been further developed and modified. The UNIFAC parameters utilized in the Clegg et al. model are updated from the Fredenslund et al.104 paper to include the changes made by Hansen et al.,105 Wittig et al.,106 as well as those of Balsev and Abildskov.107 The UNIFAC model has been frequently improved by including new or corrected interaction parameters. For the description of the mixed aerosol droplets under investigation in this work, the modified UNIFAC parameters derived by Peng et al.48 proved to be useful. This is due to the fact that the standard UNIFAC model estimations frequently fail to describe highly concentrated solutions with sufficient accuracy. Peng et al.48 used electrodynamic balance (EDB) measurements to optimize the UNIFAC parameters for the description of such situations. Essentially, their modifications emphasize the importance and impact of H-bonds between polar organic compounds and they modify the interactions between hydroxyl/carboxyl groups and water to accurately describe their experimental results. For the organic contributions three different approaches have been used: The Redlich−Kister equations (fitted activity equation, FAE), the standard UNIFAC method and the modified UNIFAC method using the parameters derived by Peng et al.48 The Redlich−Kister equations are based on a polynomial approach. From the coefficients of the polynomial expansion the excess enthalpy and hence the activity coefficients for a thermodynamic system can be deduced.

Table 1. Raman Bands Used for the Evaluation of Raman Scanning Results and Their Spectral Assignments101,102 substance

range of integration (cm−1)

assignments101−103

ammonium sulfate (AS) ammonium nitrate (AN) malonic acid (MOA) glutaric acid (GAA)

1015−960 1100−970 950−895 958−915

ν(S−O) ν(N−O) ν(C−C) ν(C−C)

2.5. Chemicals. Throughout the studies ultrahigh purity Millipore water (resistivity = 18.2 MΩ·cm) was used to minimize the influence of possible impurities. The following inorganic and organic substances were used: ammonium sulfate, AS (Arcos Organics, 99.5%), ammonium nitrate, AN (Arcos Organics, 99%), glutaric acid, GAA (Sigma Aldrich, 99%), malonic acid, MOA (Arcos Organics, 99%).

3. MODELING METHODS In the E-AIM model it is assumed that the activity coefficient of an anion or cation in solution may be expressed by the logarithm of its activity coefficient in a solution containing ions only (at their molalities in the mixture), plus additional terms containing ions and molecular solutes (again at their molalities in the mixture). This assumption is consistent with the Pitzer equations. Clegg et al.90−92 assume further that the purely ionic contributions to a molality based activity coefficient and those arising from molecular solutes can be expressed independently from each other. This makes it possible to use different models for the determination of these coefficients and combine them in a self-consistent way to estimate the properties of the mixture, which is the key element of their approach.

4. RESULTS AND DISCUSSION We first present deliquescence measurements of ammonium sulfate and ammonium nitrate aerosols containing different amounts of glutaric acid (GAA) and malonic acid (MOA), respectively. The deliquescence behavior determined in this work is compared to literature data and also to model data from the extended aerosol inorganics model (E-AIM) as developed by Clegg et al.90−92 and Friese and Ebel.93 The results obtained from contact angle measurements on the same systems will be correlated with the deliquescence and the RH dependent 6202

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growth of the mixed organic/inorganic aerosols. Finally, Raman scans revealing the spatial distribution in the effloresced crystals of the aerosols used for the deliquescence and contact angle measurements will be presented and discussed. 4.1. Deliquescence Behavior of the Glutaric Acid/ Ammonium Sulfate (GAA-AS) System. The first system under investigation was an aqueous aerosol containing different ratios of mole fractions of glutaric acid (GAA) and ammonium sulfate (AS), respectively. The water-free organic mole fraction is varied from 0 (pure AS present in the solution) to 1 (pure GAA). This situation exceeds the organic fraction that is frequently found in atmospheric aerosols. However, the authors believe that it is useful to investigate the whole range of relative concentrations to increase the basic understanding of the underlying thermodynamic and kinetic behavior. The components in an ideal mixed ternary system (inorganic/organic/water) have a larger solubility (and hence a lower DRH) than these components in binary mixtures as described by the Gibbs−Duhem equation. Whereas for this system its enthalpy of dissolution will be approximately the sum of the (mole fraction weighted) molar enthalpies of the individual components, this is not the case for the corresponding entropy of dissolution. In this case, the entropy change for the mixed system is larger and more positive than the sum of the entropy changes of the components due to the entropy of mixing. This results in a less positive change of ΔG and hence in a eutectic behavior of the phase diagram (see following paragraphs). The phase diagram that can be constructed by plotting the deliquescence relative humidity (DRH) against the organic mole fraction is displayed in Figure 3, where the upper panel shows a comparison with the literature data available for the same system and the lower panel illustrates the comparison with the model data obtained from the E-AIM model using different approaches to describe the water-organic interactions. The phase diagram shows a slight decrease of the DRH as the organic mole fraction is increased from 0 to about 0.5 where the DRH has dropped to 77% RH from 81% determined for pure ammonium sulfate. In analogy to a melting diagram, the eutectic point represents the minimal DRH for the system. Increasing the organic mole fraction beyond this point leads to an increase in the DRH for the system. The results from these measurements show very little deviations from other experimental data52,54,59,98,108 originating from a variety of different methods including bulk measurements (Brooks et al.54), optical microscope techniques (Pant et al.52), conventional EDB measurements (Treuel et al.61) and scanning EDB measurements (Choi and Chan108). We therefore consider this system to be quantitatively well described. As can be seen from the lower panel of Figure 3, the experimentally derived phase diagram is reasonably well reproduced using the modified UNIFAC approach and also by using the FAE method. Both methods also predict the eutectic point with a reasonable precision at a mole fraction of 0.6, which lies between 0.5 and 0.6 in the experimental data. The standard UNIFAC method gives good results at low organic concentrations but deviates strongly from the other approaches and especially from the experimental data at high organic concentrations. This underlines that the parametrization of the interactions between the organic components is insufficient in this model. Also the concentrations of the eutectic point are determined rather poorly by the standard UNIFAC model for this system.

Figure 3. Dependence of the DRH of glutaric acid (GAA)/ammonium sulfate (AS) mixtures on the (GAA) mole fraction. Upper panel: comparison with literature data. Lower panel: comparison of our experimental data with E-AIM model results (lines drawn to guide the eye).

Overall, the experimental situation demonstrates that the system is quantitatively well described. This is not true for the model simulations. Although the experimental data show a very good agreement and little scatter between the literature data, the model data are still lacking precision, especially in certain concentration ranges. The Peng-UNIFAC version presents a much improved parametrization of the UNIFAC model which results in acceptable accuracy modeling the GAA/AS system. However, further modifications could help to assess the remaining differences. 4.2. Deliquescence Behavior of the Glutaric Acid/ Ammonium Nitrate (GAA-AN) System. To elucidate the influence of the inorganic component, we substituted the ammonium sulfate (AS) by the more soluble ammonium nitrate (AN). The phase diagram of the binary system (GAA/ AN) system again shows a eutectic point, but much less GAA mole fraction is needed to reach this point at an organic mole fraction of 0.2 and a DRH of 59% RH (cf. Figure 4). After this DRH minimum, the DRH increases with an increasing organic mole fraction until the DRH of pure GAA is reached at 88% RH. Pant and co-workers have previously described the phase diagram of GAA and sodium chloride,52 but no literature data 6203

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Figure 4. Dependence of the DRH of glutaric acid (GAA)/ammonium nitrate (AN) mixtures on the (GAA) mole fraction. Comparison of our experimental data with E-AIM model results (lines drawn to guide the eye only).

exists on the phase behavior of the GAA/AN system to the best of our knowledge. We can therefore only compare our results to those obtained from the E-AIM model. The model results show an excellent agreement with the experimental data, when the Peng-UNIFAC parameters are used. This holds true for both the eutectic point and for the overall DRH trend. The reason that the eutectic DRH value for GAA/AN is shifted to lower GAA mole fractions compared to the GAA/AS system can simply be explained by the lower DRH of AN relative to AS. The FAE method also results in a reasonable description of the experimental situation. Again, the standard UNIFAC method describes the system with an acceptable accuracy at low organic concentrations but completely fails to describe the experimental situation at higher organic concentrations, with deviations of more than 20%RH between the experimental and model data. 4.3. Deliquescence Behavior of the Malonic Acid/ Ammonium Sulfate (MOA-AS) System. The impact resulting from a change of the organic component on the overall behavior of the mixed aerosol has also to be considered. To this end, we have used malonic acid (MOA) as the organic component and measured its impact on the behavior of ammonium nitrate and ammonium sulfate containing aerosols, respectively. Figure 5 depicts the resulting dependence of the DRH on the MOA concentration. The DRH of pure AS decreases upon an increasing MOA content until a minimum DRH of 68% is reached at a mole fraction of around 0.8. After this value, the DRH increases upon addition of further MOA until the DRH of pure MOA is reached at 72%. The data are well supported by most of the earlier literature reports.44,54,58,108 However, the values reported by Salcedo71 seem to contradict our own experimental data as well as the remaining literature reports. As can be seen from the lower panel of Figure 5, the thermodynamic modeling of the DRH values results in very similar data for the Peng-UNIFAC and for the FAE parametrization. However, both approaches underestimate the DRH values and differences between the model results and the experimental data on this system deviate by more than 10% RH

Figure 5. Dependence of the DRH of malonic acid (MOA)/ ammonium sulfate (AS) mixtures on the MOA mole fraction. Upper panel: comparison of the data from literature and from the present work. Lower panel: comparison of our experimental data with E-AIM model results (lines drawn to guide the eye only).

at some mole fractions, with the predicted DRH values being systematically lower than experimentally observed. In addition, the position of the eutectic point is shifted somewhat toward lower MOA mole fractions. Again, the standard UNIFAC parametrization leads to results that deviate strongly from the experimental data as the organic mole fraction is increased. 4.4. Deliquescence Behavior of the Malonic Acid/ Ammonium Nitrate (MOA-AN) System. The phase diagram of the MOA-AN system shows a strong decrease of the DRH even for small mole fractions of MOA present in the mixture, as illustrated by Figure 6. The eutectic point is reached at a mole fraction of around 0.5 and a DRH of 41% RH. Hence the substitution of AS by the better soluble AN shifts the eutectic point toward lower organic mole fractions, which is well in line with the results reported for the GAA-AN system in this work. No literature data exists on experimental descriptions of the phase behavior of the MOA-AN system to the best of our knowledge. The model results using the FAE and PengUNIFAC methods show a very good agreement with our experimental data for MOA mole fractions between 0 and 0.4 as well as for MOA mole fractions of 0.7 and above. Again, the 6204

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Figure 6. Dependence of the DRH of malonic acid (MOA)/ ammonium nitrate (AN) mixtures on the MOA mole fraction. Comparison of our experimental data with E-AIM model results (lines drawn to guide the eye only).

standard UNIFAC method gives a reasonably good reproduction of the experimental data at low MOA mole fractions but fails to describe the system at higher organic mole fractions. For the values between 0.4 and 0.7 and hence around the eutectic point, the agreement is poor. This underlines the importance for an enhanced understanding of the efflorescence process of this system including the problems associated with this behavior. In this context it may be worthy of note that for the mole fractions around the eutectic point no efflorescence could be observed for a large portion of the aerosol particles. Even when the system was kept at an RH below 1% for up to 24 h, the majority of the aerosol droplets showed no efflorescence. The DRH values reported for these droplets therefore are limited to those individual droplets that showed efflorescence in the experiments. Drying the aerosol at higher temperatures (353 K) also results in efflorescence of the system. However, deviating from our studies at 295 K, DRH values larger than 80% with poor reproducibility are observed for these crystals indicating the possible formation of solid hydrates. The detailed efflorescence behavior and its crystallization mechanisms therefore remain challenging and need a more detailed evaluation. 4.5. Contact Angle Measurements. The deliquescence curve of a 1:1 molar mixture of glutaric acid (GAA) and ammonium sulfate shows the onset of volume increase due to the water uptake at a DRH of 78%RH (cf. Figure 7, upper panel). Upon a further increase of the ambient RH, the droplet continues to take up water essentially following the Köhler curve. The observed DRH is below that of the individual components as discussed earlier in this manuscript. With the new setup introduced above contact angles of the solution droplets on an OTS covered hydrophobic substrate have also been measured. Their variations as a function of RH for the GAA/AS system are compared with the growth data in Figure 7 (upper panel). It is apparent that the contact angle increases slightly during and after the deliquescence until an end value is reached with a contact angle around 90° approaching that of pure water. This relatively straightforward result is contrasted by the behavior of the MOA/AS system. Here, the RH dependent

Figure 7. Comparison of deliquescence curves and contact angles for 1:1 molar mixtures of organic acid and ammonium sulfate (AS). Upper panel: glutaric acid GAA/AS. Lower panel: malonic acid MOA/AS. The contact angle data are represented by the open circles; the growth factors, by the continuous series of black dots.

growth factor confirm the two-step deliquescence reported before for this system.98 A similar behavior involving more than one clear deliquescence step has also been described for systems containing multiple salts109,110 and indications for an earlier onset of the deliquescence have been stated for other AS/organic mixtures.76,111 A first step of water uptake for the MOA/AS system can be seen at a RH around 50% and a second step around 70%. Interestingly, the measured contact angles follow this two-step water uptake, showing contact angles between 60° and 65° for the partly liquid system between 50% and ∼65% RH. During the second water uptake, the contact angle increases until again a contact angle of ∼90°, approaching that of pure water at 92° on this surface, is reached after deliquescence. The contact angle of a pure water droplet does not change when the droplet grows (or shrinks) due to humidification (or drying) and all other conditions (in particular the nature of its supporting surface) remain constant and none of the interfacial energies involved change. The same is not true for a deliquescing system of solutes because of dilution and/or change of relative composition. The above results clearly suggest differences in the liquid phase composition of the aerosol depending on the ambient RH. These differences must arise from different organic/inorganic mole fractions in the liquid phase depending on the RH. 6205

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Figure 8 shows the Raman scan of an effloresced glutaric acid/ammonium sulfate (GAA/AS) aerosol (1:1 molar concentration ratio) and reveals the spatial distributions of both substances within the crystal. Though AS has a high abundance in the center of the crystal, GAA has its highest concentrations toward the edges of the crystalline particle. The Raman micrograph of the GAA/AN (Figure 9) mixture reveals a similar picture. However, even though the substances are clearly not distributed evenly throughout the crystal, the separation does not occur symmetrically with one substance in the center and the other surrounding it. The Raman micrographs of the MOA-AS mixture (Figure 10) again reveal a spatial separation of MOA and AS in the effloresced crystal. This spatial distribution is in line with the expectations from earlier studies100 and with the behavior of the two systems described above. The temporal resolution of our experimental approach prevents the detection of a possible sequential efflorescence for these systems and a higher time resolution would be desirable to further elucidate the basis for this distribution. Nevertheless, in trying to elucidate the causes of this behavior we were able to scan an aerosol droplet in the state of partial efflorescence. Figure 11 shows exemplary Raman spectra taken in the two different regions of a partially effloresced aerosol particle. The Raman scans clarify that the inner, crystalline part consists of effloresced AS whereas the outer liquid part consists mainly of MOA. In contrast to the efflorescence, the sequential deliquescence of a MOA-AS mixture can be better resolved in time, as the components of the mixture have a different solubility and thus more widely separated DRH values. First malonic acid takes up water, so that an aqueous solution of malonic acid is formed around a crystalline core of ammonium sulfate. At a RH of 76%

Table 2. Measured Contact Angles of Ammonium Sulfate (AS), Glutaric Acid (GAA), and Malonic Acid (MOA) as Well as of Their 1:1 Mixtures substance

contact angle prior to efflorescence (deg)

AS GAA MOA AS/GAA AS/MOA

90 55 61 56 64

Our measurements on drying aerosols reveal that their contact angle on the OTS covered surface is determined by that component which has the strongest influence on the contact angle. Table 2 summarizes this situation for aerosols containing the pure inorganic (AS) or organic substances (GAA and MOA) and for those containing both of them internally mixed. Though the pure AS aerosols have contact angles around 90° and hence close to that of pure water even just prior to their efflorescence, the organic aerosols produce contact angles of only 55° (GAA) and 61° (MOA). Mixtures of AS with the organic acids produce contact angles approximately equal to those of the organic component. 4.6. Raman Spectroscopy of Ternary Aerosols. Using Raman scanning microscopy, the spatial distribution of individual substances within the effloresced aerosol can be elucidated. For identification, the characteristic Raman bands of Table 1 have been chosen. As has been shown previously by our group using X-ray spectroscopy,97 the solids that form from effloresced ternary aerosols are composed of pure crystals of either organic or inorganic component exclusively. There was no indication of mixed crystals being formed; the spatial distribution of these two solid phases, however, was not uniform.

Figure 8. Spatial distribution of glutaric acid (GAA) and ammonium sulfate (AS) in an effloresced aerosol of 1:1 composition. Left: reflected light intensity. Center: glutaric acid Raman signal. Right: ammonium sulfate Raman signal.

Figure 9. Spatial distribution of glutaric acid (GAA) and ammonium nitrate (AN) in an effloresced aerosol of 1:1 composition. Left: reflected light intensity. Center: glutaric acid Raman signal. Right: ammonium nitrate Raman signal. 6206

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Figure 10. Spatial distribution of malonic acid (MOA) and ammonium sulfate (AS) in an effloresced aerosol of 1:1 composition. Left: reflected light intensity. Center: malonic acid Raman signal. Right: ammonium sulfate Raman signal.

Figure 11. Spatial distribution of Raman spectra of malonic acid (MOA) and ammonium sulfate (AS) in a partially effloresced aerosol at 30% RH. Left: Raman spectrum of ammonium sulfate. Center: reflected light intensity. Right: malonic acid Raman signal.

Figure 12. Spatial distribution of malonic acid (MOA) and ammonium sulfate (AS) in a partially deliquesced aerosol of 1:1 composition at 65% RH. Left: reflected light intensity. Center: MOA Raman signal. Right: AS Raman signal.

signals observed for partially effloresced malonic acid/ ammonium sulfate aerosols (cf. Figure 11). When under such conditions one component nucleates faster than the other, it is likely that this faster component crystallizes in the center of the droplet, while the solution of the other component surrounds it. When the system loses more water, the second component nucleates, thus forming a shell around the previously crystallized component in the center as discussed above. Looking down onto a droplet as done in the reflected light intensity setup (Figure 11, center panel) used in our experiments, the largest column volume is found in the center of the droplet with less volume toward the edges. Because the homogeneous nucleation rate is volume dependent, this leads to the droplet crystallizing with higher probability in its center. The provision for this to happen is (i) an exclusively homogeneous nucleation mechanism and (ii) the absence of concentration gradients. The presence of concentration gradients would otherwise cause crystallization to commence at or near the surface, where heterogeneous mechanisms might be operative and where droplets would become more

complete deliquescence occurs. In Figure 12, the liquid−solid phase separation in a partially deliquesced aerosol of 1:1 composition is shown. The Raman micrographs of the malonic acid/ammonium nitrate (MOA-AN) mixture are displayed in Figure 13. These scans show a somewhat different picture compared to the other mixtures studied in this work. In this case a spatial separation of the two substances cannot be seen, which again is a reproducible result. Spatial separation of efflorescing ternary (organic/inorganic/ water) aerosols including dicarboxylic acids (glutaric, malonic, maleic, and succinic acid) and ammonium sulfate has previously observed in our laboratory.101 The current work presents yet additional examples. Nevertheless, a clear picture of the mechanistic details of these observations has as yet not emerged. Because efflorescence is a nonequilibrium process that is kinetically controlled, a likely explanation for phase separation is sequential efflorescence of the two components due to differences in their individual nucleation rates and transport processes.101 This is exemplified, for instance, by the Raman 6207

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Figure 13. Spatial distribution of malonic acid (MOA) and ammonium nitrate (AN) in an effloresced aerosol of 1:1 composition. Left: reflected light intensity. Center: malonic acid Raman signal. Right: ammonium nitrate Raman signal.

Moreover, the chemical composition of the liquid phase changes dramatically when one substance effloresces. The dynamic measurements of the contact angles indicate that the contact angle and hence the surface tension of an internally mixed ternary aerosol are influenced by the organic component. This could possibly indicate an enrichment of the organic component at the surface with the following consequences: On one hand, an organic-rich phase at the surface could effect the hygroscopic activation of particles to convert into cloud droplets112 and, on the other hand, heterogeneous reactions such as the hydrolysis of N2O569,113 may change.

concentrated earlier on upon drying. We believe that this is avoided in our experiments because of sufficiently low drying rates. The rates of homogeneous nucleation depend on the extent of supersaturation and hence on solubility and concentration. Of all the systems studied in this work, the glutaric acid/ ammonium sulfate mixture has the lowest solubility. As a consequence, the nucleation rates during efflorescence are highest. Moreover, because the rate of spatial separation by diffusion is larger for sulfate than for glutaric acid, we can qualitatively explain the sequential nucleation of AS and GAA, with the inorganic component in the center of the aerosol. The malonic acid/ammonium nitrate system, on the other hand, has the highest solubility. Therefore, the nucleation rates and hence the tendency for spatial separation will be low during efflorescence. It is clear that the mechanistic details of our findings remain to be further explored. Interestingly though, the results from the contact angle studies lend independent support to the above arguments. As shown above, these measurements indicate that the contact angles of the ternary mixtures just prior to efflorescence are identical to those of the binary organic/water mixtures.

6. CONCLUSIONS In the present work we have further quantified the deliquescence behavior of a number of ternary organic/ inorganic/water aerosol particles by studying such systems over the whole range of relative mole fractions. It is found that all of the systems studied show a eutectic behavior with the DRH values of the mixtures being lower than those of the individual components. The exact concentration dependences of the DRH values have been reproduced using theoretical thermodynamic modeling. The agreement between experiment and theory is very satisfactory on the inorganic rich side of the phase diagram but is generally poorer on the organic side. This is due to the remaining difficulties in describing theoretically the activity coefficients of highly concentrated solutions, in particular of organic solutes in otherwise ionic solutions. However, the modifications by Peng et al. are a much better data set for modeling these aerosols and a satisfactory description of the experimental situation can be achieved in some cases. In this work we have also performed contact angle measurements of concentrated ternary aerosol droplets prior to their crystallization by efflorescence. Interestingly, such particles show contact angles that very much resemble those of the binary organic/water system. Hence it may be concluded that the surface energy of the ternary aerosols is mainly being determined by the organic component. Whether or not this interpretation requires an accumulation of organic species at or near the surface remains to be solved. Nevertheless, the observations are in agreement with the observation of a phase separation in the effloresced crystals. A final and important finding of this work comes from our Raman studies of the effloresced particles. It could be shown that crystallization is not spatially homogeneous but rather produces a separation of the crystallizing components. The exact mechanisms responsible for this spatial separation remains elusive and needs to be further investigated. However,

5. ATMOSPHERIC IMPLICATIONS The observations from this work may have wide reaching practical implications for ambient tropospheric aerosols with regards to the following aspects: The observed deliquescence/efflorescence behavior of ternary aerosols will impact on the radiative properties of such aerosols. First, the addition of organic acids to ammonium sulfate or nitrate will decrease their DRH below the deliquescence of the binary systems and hence enhance the hygroscopic growth and the scattering intensity of such particles. Second, sequential efflorescence and the spatial inhomogeneous distribution of individual components in the resulting crystalline particle will impact on its complex refractive index and its scattering behavior. Third, a clear influence on the scattering behavior is also expected from the formation of solid inclusions in liquid particles and, moreover, from the influence of a core−shell morphology in the finally completely effloresced particles. Concerning the atmospheric chemical relevance of these findings, it has to be noted that the surface chemistry and the surface area (due to morphological differences in the crystal structures resulting from the efflorescence of chemically different components) in the crystalline particles will show an inhomogeneous behavior with a dependence of the total surface on the individual nucleation rates of the comprised substances. 6208

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a preliminary interpretation shows that sequential processes based on different nucleation and transport rates may be operative.

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ASSOCIATED CONTENT

S Supporting Information *

Raman spectra of the chemical substances and mixtures used in this work. This material is available free of charge via the Internet at http://pubs.acs.org

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.T. acknowledges support of this work from the German Israeli foundation (GIF).

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REFERENCES

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