求助 b702651j

Deliquescence behaviour of single levitated ternary salt/carboxylic acid/water microdropletsw L. Treuel,a S. Schulze,b Th. Leisnerb and R. Zellner*a Received 21st February 2007, Accepted 16th April 2007 First published as an Advance Article on the web 3rd August 2007 DOI: 10.1039/b702651j The deliquescence relative humidities (DRH) of ammonium sulfate as well as ammonium sulfate/dicarboxylic acid (glutaric, maleic and tartaric) mixtures as a function of temperature and relative composition have been studied using an electrodynamic balance (EDB) in connection with optical microscopy and Mie scattering. The absolute DRH values for pure ammonium sulfate as well as their temperature dependence are consistent with literature data and with the AIM model of Clegg et al. The addition of either glutaric or maleic acid to ammonium sulfate leads to a decrease of the DRH value, with the temperature dependence either remaining constant (glutaric acid) or increasing (maleic acid) with increasing acid concentration. This difference is attributed to the higher acidity of maleic acid, which generates stronger ionic interactions with the ammonium sulfate system. In the case of tartaric acid, the deliquescence behaviour of ammonium sulfate is substantially influenced by the formation of insoluble ammonium tartrate. 1. Introduction Atmospheric aerosols have significant effects on climate, atmospheric turbidity and air quality.1,2 In addition to these effects, they also play an important role in many chemical processes occurring in the atmosphere.3,4 The heterogeneous conversion of N2O5 to HNO3 on aerosol surfaces presents an example of the importance of aerosol composition and phase on the rate of reaction. As a result, this reaction is now recognized as playing a crucial role in controlling the fate of nitrogen oxides in both the stratosphere and troposphere. The reactivity of the N2O5 on the aerosol surface strongly depends on the phase and water content of the aerosol particles.5 Furthermore, the protuberant role of atmospheric aerosol particles in the radia- tive transfer within the atmosphere by scattering and absorbing electromagnetic radiation6 makes them an important parameter in modelling the Earth’s climate. Since chemical and radiative effects of atmospheric aerosols are size and phase related,7,8 they are strongly influenced by the ambient relative humidity (RH) due to water absorbing hygroscopic components, changing both particle diameter and a Institute for Physical and Theoretical Chemistry, University of Duisburg-Essen, Germany. E-mail: reinhard.zellner@uni-due.de b Institute for Environmental Physics, University of Heidelberg, Germany and Institute for Meteorology and Climate Research, Forschungszentrum Karlsruhe Germany { The HTML version of this article has been enhanced with colour images. PAPER www.rsc.org/faraday_d | Faraday Discussions Faraday Discuss., 2008, 137, 265–278 | 265This journal is �c The Royal Society of Chemistry 2007 D ow nl oa de d by H ar bi n In sti tu te o f T ec hn ol og y on 2 6 M ar ch 2 01 2 Pu bl ish ed o n 03 A ug us t 2 00 7 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /B7 026 51J View Online / Journal Homepage / Table of Contents for this issue wavelength dependent refractive indices.9–13 As a result, for a given atmospheric particle load the net effect on chemistry and/or climate will depend on the relative humidity, and will hence be modified by either temperature or water partial pressure. Phase and water content of particles govern their mass and hence their surface area and reactivity. The hygroscopic properties and phase changes of atmospheric aerosols as well as the surface morphology (of solid particles) and chemical surface composition must be understood and represented accurately in order to improve aerosol-climate models. Sulfate aerosols are widely abundant in the atmosphere and represent the largest anthropogenic mass source for the accumulation mode of atmospheric particles.14 Being non-absorbing in the visible region of the electromagnetic spectrum, they provide the most significant anthropogenic cooling contribution to the global direct radiative forcing.6,14–20 Depending on factors like location, aerosols can contain various ratios of inorganic-to-organic material.21 Results from field measurements indicate that organic material typically accounts for 10–50% of the fine particle mass,1 with the organic material originating from both anthropogenic and natural sources.3 Recent field data confirm these findings, indicating that indeed up to 50% or more organic material may be present in atmospheric aerosols.22 Dicarboxylic acids are amongst the chemical compounds found in atmospheric aerosol particles.23,24 Like many other polar organic substances, they are predomi- nantly present in condensed phases rather than in the gas phase,25–27 resulting from their low vapour pressures. In aqueous aerosol particles, dicarboxylic acids are found to be major constituents of the water soluble organic compounds (WSOCs),28 and composition measurements have shown that the organic material is internally mixed together with inorganic compounds in tropospheric particles.22,29 The physical behaviour of dicarboxylic acids is likely to be typical of many polar WSOCs in the atmosphere,30 which puts them in the focus of many current laboratory studies. Dicarboxylic acids are found in many different environments.23 They have been abundant in aerosols from the urban,31–34 marine,35 polar36 and tropical atmo- sphere.37,38 The sources of organic aerosols include fossil-fuel combustion and biomass burning, whose global emission rates are estimated to be 28.5 and 44.6 Tg yr�1, respectively.39 From their field data, Mochida et al.23 suggest that deposition is more important than chemical decomposition as a sink of diacids and that they are relatively stable end products in the atmosphere. Whilst the behaviour towards varying relative humidity, as normally described by deliquescence and efflorescence, of pure ammonium sulfate (AS) particles is well established,9,40 information about phase transitions and hygroscopic properties of organic and mixed organic/inorganic particles is not at a level comparable to inorganic particles.41 Several groups have studied the deliquescence of pure organic systems42–49 and these works have shown that the deliquescence strongly depends on the chemical nature of the organic substance. Moreover, most of the studies have been performed at room temperature only, although there is substantial interest in the temperature dependence of this property as well. A number of groups have also studied the deliquescence and crystallisation of mixed organic/inorganic particles.50–66 Studies with such internally mixed organic/ AS particles54 have shown that the organic component changes the deliquescence relative humidity (DRH) relative to pure ammonium sulfate. The water content of atmospheric aerosols is governed by the ambient relative humidity. Hence, high aqueous phase concentrations can be attained at low relative humidities. These result in large deviations from ideal solution behaviour and make the properties of the system difficult to predict.67 In the current work we present results from deliquescence measurements of AS and mixed dicarboxylic acid/ammonium sulfate crystals using an electrodynamic balance (EDB). The temperature dependence of the DRH of pure AS solutions, 266 | Faraday Discuss., 2008, 137, 265–278 This journal is �c The Royal Society of Chemistry 2007 D ow nl oa de d by H ar bi n In sti tu te o f T ec hn ol og y on 2 6 M ar ch 2 01 2 Pu bl ish ed o n 03 A ug us t 2 00 7 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /B7 026 51J View Online solutions of dicarboxylic acids and water as well as of ternary mixtures containing AS/dicarboxylic acid/ water has not previously been studied in EDB experiments. In the following sections of the paper, a short introduction of the experimental setup and the techniques involved will be given. The experimental results will be presented and discussed in the subsequent sections. A final chapter serves to summarise the results, give a conclusion of their significance and suggest further experimental and theoretical efforts in order to enhance the current level of comprehension of the processes involved. 2. Experimental 2.1 Electrodynamic balance All deliquescence experiments described in this paper have been carried out with single levitated particles using an EDB.68 The experimental setup used for this work was already described elsewhere in great detail69–73 and only a brief introduction will be given here. However, specific aspects of the present setup, such as the height control and peripheral devices needed for temperature and humidity control, will be described more extensively. The EDB is of the classical hyperbolical geometry as suggested by Paul.74,75 It consists of a toroidal centre electrode connected to a liquid nitrogen cryostate, which is machined from gold-plated copper and serves as a climate chamber. Six optical ports allow access to the trap, two of which accommodate the temperature and humidity sensors. Two hyperboloidal endcap electrodes seal the top and the bottom of the climate chamber. Here, an AC voltage (approximately 2 kV, 200 Hz) is applied to trap charged particles. A superimposed DC field compensates the gravitational force on the particle. Fig. 1 shows the EDB in its opened aluminium climate chamber together with the relevant sensing devices. 2.2 Particle microscopy The trap is integrated into a long working-distance microscope (Mitutoyo), which is equipped with a cooled CCD camera (PCO sensicam) and illuminated by an ultrafast spark flash lamp (HSP nanolight) in order to take microscopic still images of the particles at various stages of the efflorescence/deliquescence cycles (cf. Fig. 2). 2.3 Temperature control In order to achieve a precise and reliable temperature control within the EDB, a copper cooling finger (CryoVac) is directly attached to the trap via two cooling rings to ensure a good thermal conductivity. To avoid condensation of ambient water on the trap, it is placed in an evacuated aluminium chamber. Cooling is achieved by evaporating liquid nitrogen in a heat exchanger at the tip of the cooling finger. A membrane pump and a needle valve control the flow of gaseous nitrogen and allow adjusting the cooling power. A regulated heating system is applied to stabilize the trap at the desired temperature. The interplay of heating system and cooling setup allows it to achieve and retain constant temperatures over a wide temperature region with a precision of o0.5 K. The EDB temperature is determined by a Pt-100 thermometer, which is integrated in the setup and has direct contact with the EDB body. A combined RH/Tempera- ture sensor (Honeywell 3602, accuracy: �2% RH, �0.5 K) is also used to monitor the temperature within the trap. Faraday Discuss., 2008, 137, 265–278 | 267This journal is �c The Royal Society of Chemistry 2007 D ow nl oa de d by H ar bi n In sti tu te o f T ec hn ol og y on 2 6 M ar ch 2 01 2 Pu bl ish ed o n 03 A ug us t 2 00 7 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /B7 026 51J View Online 2.4 Humidity control An Ansyco (SycosHS) moisturiser has been used to regulate the RH inside the EDB. It relies on vapour diffusion through a semi-permeable membrane and allows to keep the gas flow through the EDB constant at all RH values. The RH is measured directly inside the EDB with the combined RH/T probe described in Section 2.3. An Fig. 1 Photograph of the experimental setup used. (1)—injector port, (2)—laser diode for droplet illumination, (3)—CCD camera for droplet imaging, (4)—CCD camera for Mie scattering detection, (5)—CCD column for automated height control, (6)—cooling finger, (7)—N2 suction pipe of the cryostat, (8)—connection to vacuum pump, (9)—coupling for U-lifter. Fig. 2 A series of photographs taken before (A), during (B-E) and after (F) the deliquescence of a pure ammonium sulfate particle. 268 | Faraday Discuss., 2008, 137, 265–278 This journal is �c The Royal Society of Chemistry 2007 D ow nl oa de d by H ar bi n In sti tu te o f T ec hn ol og y on 2 6 M ar ch 2 01 2 Pu bl ish ed o n 03 A ug us t 2 00 7 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /B7 026 51J View Online additional RH value is determined internally by the Asyco (SycosHS) in the outgoing moisturised gas flow. The setup has an automated height control that keeps the trapped droplet at the EDB centre by changing the DC voltage accordingly. This allows following any changes in the mass/charge ratio of the droplet. Assuming that no loss of charge takes place during an experiment,76 the height control data can be used to determine changes in the droplet mass. This allows the mass fraction of the solute (mfs = mass of solute on a dry basis/mass of solution) of a levitated particle as a function of ambient RH to be determined. 2.5 Experimental procedure The droplet is inserted into the preconditioned EDB using a piezoelectric injector (Gesim mbH) and the equilibration process of the droplet can be observed. Subsequently, the RH can be changed in small steps until efflorescence of the droplet is observed. After a sufficient extent of drying, the RH is slowly raised until a sudden increase of the droplet mass indicates the deliquescence. Typical images of a droplet undergoing deliquescence are shown in Fig. 2. The dry and crystalline particle (A) takes up water and loses all fine edges (B). Further water uptake (C–E) leads to the slow dissolution of the crystal and results ultimately in a completely liquid solution droplet (F). Since water is a dielectric fluid, it seems plausible that electric fields might alter the structure of aqueous solutions and hence alter their phase transition behaviour. The influence of the EDB on phase transition behaviour of trapped particles has been probed in freezing experiments. Good agreement exists in literature, that electric fields will only be influential if field strengths of a couple of hundred kV m�1 are reached. Hobbs77 gives a good overview of this problem. In the centre of the EDB used in the experiments, only a field strength in the order of a few V mm�1 is realised.69 Kra¨mer et al.73,76 have probed the influence of the droplet charge on the nucleation rate and have not found any effect. An influence of particle motion in the EDB on the phase transition can also be ruled out since work on similar setups69,76 have found a good agreement of their data with literature data from other, non-EDB experiments. The data presented in this work also shows good agreement with literature values from different experimental techniques, thus underlining the reliability of this technique. 2.6 Chemicals The purities and sources of the chemicals used are as follows: Ammonium Sulfate (AS), Fluka Z 99%, Glutaric Acid (GA), Acros organics, 99%, Maleic Acid (MA), Fluka, Z 99%, L-(+)-Tartaric Acid (TA), puriss. p.a. Z 99,5%. These chemicals were used without further purification. Water used for the preparation of solutions was purified by a Milipore apparatus. 3. Results and discussion The deliquescence behaviour of mixed organic/inorganic solutions is a complex process. This paper primarily tries to contribute to a better understanding of the influence that dicarboxylic acids have on the deliquescence of AS. Two dicarboxylic acids, GA and MA have been chosen for their atmospheric relevance and chemical structure (Fig. 3). A third—TA—has solely been chosen for its chemical structure. It has no reported atmospheric relevance. GA is a dicarboxylic acid containing a saturated carbon chain, whereas MA has a slightly shorter chain length and a double bond, introducing additional polarity into the molecule. Two hydroxyl-groups in the TA molecule add even more polarity and allow investigating the effect of higher polarity without a change in the carbon chain Faraday Discuss., 2008, 137, 265–278 | 269This journal is �c The Royal Society of Chemistry 2007 D ow nl oa de d by H ar bi n In sti tu te o f T ec hn ol og y on 2 6 M ar ch 2 01 2 Pu bl ish ed o n 03 A ug us t 2 00 7 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /B7 026 51J View Online length. Before discussing the behaviour of mixed solutions, we will first present the results for the pure substances, starting with AS. 3.1 Pure Ammonium Sulfate (AS) A typical result of one single deliquescence measurement of pure AS is shown in Fig. 4. For these results, a solution droplet has been injected into the trap and, after the complete efflorescence, the droplet was dried further down to ensure the complete loss of water from the crystalline particle. The relative humidity was increased again until deliquescence could be observed. The data in Fig. 4 shows the sudden rise in particle mass between 79 and 80% RH. Different experiments with AS showed a high degree of reproducibility. The data points shown in Fig. 4 are taken directly from the height control signal and the RH sensor signal. A calibration correction for the RH signal has been applied and it has been normalised relative to a concentration of 1 mole SO4 2�. The deliquescence behaviour of the pure inorganics/H2O system, as observed in the current work, has been compared with predictions based on the Aerosol Inorganics Model II (AIM) model. The AIM78,79 is a multi-component mole-fraction-based model to represent aqueous phase activities, equilibrium partial pressures and saturation with respect to solid phases. Carslaw et al.80 have developed a method using the mole-fraction-based equations of Pitzer, Simonson and Clegg,81–84 which allows calculating activity coefficients for the system HCl–HNO3–H2SO4–H2O. Clegg et al. 78 applied this model to the system H+–NH4 +–SO4 2�–NO3 �–H2O. Comparisons suggest that the model satisfactorily represents salt solubilities and water activities.78 Mole fractions are calculated on the basis of the individual ionic and molecular species present. Equations for the activity Fig. 3 Structures of glutaric-, maleic- and tartaric acid. Fig. 4 Deliquescence of an AS droplet; comparison of experiment and AIM model.78 270 | Faraday Discuss., 2008, 137, 265–278 This journal is �c The Royal Society of Chemistry 2007 D ow nl oa de d by H ar bi n In sti tu te o f T ec hn ol og y on 2 6 M ar ch 2 01 2 Pu bl ish ed o n 03 A ug us t 2 00 7 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /B7 026 51J View Online coefficients of neutral species, cations and anions in arbitrarily complex mixtures are derived from an equation for the excess Gibbs energy of a liquid mixture.81–83 This equation accounts for three major contributions. First, an extended Debye– Hu¨ckel term accounting for long range forces between ions, a contribution most important in dilute solutions. Second, a higher order electrostatic term essentially modifying the Debye–Hu¨ckel contribution and arising from the unsymmetrical mixing of ions of the same sign but different charge. The third contribution is an expression for the short range forces between components that dominate in concentrated solutions. This term is based upon a Margules expansion85 of terms in the mole fraction. The equations generally contain parameters describing interactions in binary solutions and ternary mixtures whose values are determined by fitting to empirical data. The AIM model has been made available online at http://mae.ucdavis.edu/ Bsclegg/aim.html. The comparison of the experimental results with respect to the DRH value with the predictions from the AIMmodel calculation shows that the experimental result is