review deliquescence and e†orescence processes of aerosol

CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 21, NUMBER 1 FEBRUARY 27, 2008

REVIEW

Deliquescence and Efflorescence Processes of Aerosol Particles Studied byin situ FTIR and Raman Spectroscopy

Li-jun Zhaoa,b, Feng Wanga,Kun Zhanga, Qing-xuan Zengb, Yun-hong Zhanga∗

a. The Institute of Chemical Physics, School of Science, Beijing Institute of Technology, Beijing 100081,China;b. School of Aerospace Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

(Dated: Received on May 6, 2007; Accepted on July 9, 2007)

Deliquescence and efflorescence are the two most important physicochemical processes of aerosol particles.In deliquescence and efflorescence cycles of aerosol particles, many fundamental problems need to be inves-tigated in detail on the molecular level, including ion and molecule interactions in supersaturated aerosols,metastable solid phases that may be formed, and microscopic structures and deliquescence mechanisms ofaerosol particles. This paper presents a summary of the progress made in recent investigations of deli-quescence and efflorescence processes of aerosol particles by four common spectral techniques, which areknown as Raman/electrodynamic balance, Fourier transform infrared/aerosol flow tube, Fourier transforminfrared/attenuated total reflection, and confocal Raman on a quartz substrate.

Key words: Aerosol, Deliquescence, Efflorescence, Raman/electrodynamic balance, Fourier transform in-frared/aerosol flow tube, Fourier transform infrared/attenuated total reflection, Confocal Raman

I. INTRODUCTION TO ATMOSPHERIC AEROSOLS

Although generally understood as colloidal systemsof particles suspended in a gas, aerosols refer mostly toliquid or solid particles in atmospheric research [1,2].As an important component of the atmosphere, aerosolparticles span four orders of magnitude from 0.01 µmto 100 µm in size, and can take part in many impor-tant physicochemical processes. Aerosol particles canabsorb sunlight to cause direct radiative forcing, con-tributing significantly to regional and global climaticchanges. Acting as cloud condensation nuclei (CCN),aerosol particles also play an important role in hydro-logical cycles in the atmosphere. Aerosols can furtheradsorb and concentrate air pollutants, provide the vehi-cles for gas/liquid and/or gas/solid interface reactions,and directly partake in many important chemical reac-tions. The chemical species in aerosol particles and/oron their surfaces may act as catalysts, determining therates of some chemical and photochemical reactions. Bythe way of multiphase or heterogeneous chemical reac-tions relating to aerosols, human activities have led tothe modifications of the chemical compositions of theatmosphere, which subsequently deteriorate the visibil-ity and threaten public health.

II. DELIQUESCENCE AND EFFLORESCENCE CYCLESOF AEROSOLS AND RELATED PROBLEMS

The macroscopic properties of aerosols can be relatedto many microscopic physicochemical processes occur-

∗Author to whom correspondence should be addressed. E-mail:[email protected], Tel: +86-10-86668406

ring between aerosols and the milieu, among which, del-iquescence and efflorescence are the two most importantones [1].

A. Phase transitions in deliquescence and efflorescencecycles of aerosols

Deliquescence is, by definition, the process in whichsolid particles take up water from the air with the in-crease of relative humidity (RH) and suddenly dissolveinto aerosol droplets at RHs specific to various systemsand conditions. By contrast, efflorescence is the processin which aerosol droplets lose water gradually with thedecreases of RH and abruptly change into solid particlesat specific RHs [1]. The uptake and loss of water withchanging RH directly reflects the hygroscopic proper-ties of aerosols. In deliquescence and efflorescence cy-cles, aerosol hygroscopic properties can largely fashionthe size distributions, the radiation and precipitationproperties, and the chemical reactivity of aerosol par-ticles, affecting relevant physicochemical processes andultimately changing the evolution of climate [1,2].

One typical hygroscopic curve of aerosols is shownin Fig.1, which describes the hygroscopic properties ofNaNO3 aerosols in deliquescence and efflorescence cy-cles that were previously obtained by Tang et al. intheir electrodynamic balance (EDB) experiments [3].The relative water content in aerosol particles is ex-pressed as water-to-solute molar ratio (WSR). In theefflorescence process, with decreasing the RH, NaNO3

aerosols pass in turn through the saturation point (7 inFig.1) and the efflorescence point (8 in Fig.1). Prior tothe saturation point, aerosols are unsaturated dropletsas indicated by 1, between the saturation and efflores-cence points, aerosols appear as supersaturated droplets

DOI:10.1088/1674-0068/21/01/1-11 1 c©2008 Chinese Physical Society

2 Chin. J. Chem. Phys., Vol. 21, No. 1 Li-jun Zhao et al.

1

7 6

2

83

4 5

9

0 20 40 60 80 100

0

5

10

15

WSR

RH / %

FIG. 1 Hygroscopic curve of NaNO3 aerosol particles in del-iquescence and efflorescence cycles. WSR: water to solutemolar ratio, RH: relative humidity. 1, 6: Unsaturation re-gion, 2: Supersaturation region, 3: Solid region in efflores-cence process, 4, 5: Solid region in deliquescence process, 7:Saturation point, 8: Efflorescence point, 9: Deliquescencepoint.

as indicated by 2, and below the efflorescence point,aerosols exist as solid particles as indicated by 3. In thedeliquescence process, with the increase of RH, NaNO3

aerosols remain as solid particles at low RHs prior tothe deliquescence point (9 in Fig.1), when the uptakeof water suddenly turns solid particles into saturateddroplets. The deliquescence RH is, by theory, equal(but not necessarily) to the RH of saturation point [1,4].Below the deliquescence point, aerosols are solid parti-cles (4 and 5 in Fig.1), and above the point, solid par-ticles dissolve into unsaturated droplets (6 in Fig.1).The hygroscopic traces overlap with each other in theunsaturation region covered by 1 and 6, as well as thesolid region covered by 3 and 4, in efflorescence anddeliquescence processes, respectively.

B. Important problems in deliquescence and efflorescencecycles of aerosols

As shown in Fig.1 and discussed previously, stepsfrom 1 to 6 correspond respectively to distinct physico-chemical states and properties of aerosol particles. Thescientific problems derived therein have aroused greatinterest of researchers in such diverse fields as atmo-spheric science, biology, and crystallography. Becauseunsaturated solutions can be prepared with ease, muchhas already been learned with respect to steps 1 and 6.However, in other regions, especially the supersaturatedregion and regions in the vicinity of deliquescence point,many important problems have yet to be investigated.These problems can be briefly summarized as follows.

1. Ion and molecule interactions in supersaturated aerosoldroplets

Supersaturated states are ubiquitous in aerosol sys-tems. In supersaturated aerosol droplets, complicatedinteractions usually occur between ions, molecules, andion and molecules, giving rise to such problems as thestructures and properties of ion pairs, and hydrogenbond structures of water molecules in extremely super-saturated solutions as well as the structures of aerosolgels. These complicated interactions are closely relatedto the hygroscopic properties of aerosol particles, af-fecting accordingly their physicochemical and opticalproperties. For example, the study of MgSO4 aerosolsin water uptake process disclosed the existence of seri-ous mass transfer limitations at low RHs, which werebelieved to be caused by the formation of ion pairs[5,6]. By using the Raman/electrodynamic balance(Raman/EDB) technology, Chan’s group has probedthe structures of ion pairs in supersaturated MgSO4

droplets, and obtained Raman spectral evidence of com-plicated interactions like monodentate and bidentatecontact ion pairs [6]. In addition, water probably doesnot hold bulk-like structures for aerosols in supersatu-rated states. For example, in supersaturated perchlo-rate droplets, water molecules have been found to occurmainly as monomers [7].

2. Metastable solid phases that may be formed

In extremely supersaturated states, the solidificationof aerosol droplets is mainly controlled by kinetic ratherthan thermodynamic factors. As a result, new crys-tals with novel structures are usually expected to form,and considerable defects may be found on the surfacesof solid particles. These metastable solid phases haveunique microscopic structures and chemical properties,and their hygroscopic properties are also different fromstable ones. For example, in the Raman investigationsof Na2SO4 aerosols, Tang et al. have observed the for-mation of Na2SO4 crystals at low RHs instead of ther-modynamic stable Na2SO4·10H2O crystals [8]. In theimaginary deliquescence process conforming to thermo-dynamic models, Na2SO4 crystals should be convertedinto Na2SO4·10H2O crystals once the RH increase upto 81%, and then the aerosols deliquesce to form satu-rated droplets once the RH increase up to 93%, whichis the deliquescence RH for Na2SO4·10H2O crystals.However, experiments found that aerosol particles ofNa2SO4 crystals completely deliquesce at 84% RH toform saturated Na2SO4 droplets. Similar observationshave also been made for LiClO4, Sr(NO3)2, and KHSO4

aerosol particles [8]. Due to the lack of the knowledgefor new species formation from supersaturated aerosols,deviations usually occur to varying extent in predictionsmade by many theoretical models of aerosol properties[1].

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Chin. J. Chem. Phys., Vol. 21, No. 1 Deliquescence and Efflorescence of Aerosol Particles 3

3. Microscopic structures of aerosol particles

When efflorescence or solidification occurs in aerosolparticles, the stoichiometry of the formed solid (i.e.water-to-solute molar ratio) may not be equal to anycrystals. Therefore, residual water can generally befound either in the inner parts or on the concave sur-faces of effloresced particles. Atmospheric aerosols aremostly a mixture of solids and residual liquids ratherthan being totally dried, displaying complicated anddiversified morphologies [9,10]. Colberg et al. haveproposed several possible structures for atmosphericaerosols as shown in Fig.2 [10], which are a single crys-tal (I), an agglomerate of single crystals or a polycrys-talline solid (II), a polycrystalline material with sev-eral open cavities filled with liquid (III), a solid poly-crystalline shell with embedded liquid (IV), a solid sin-gle crystalline shell with embedded liquid (V), a singlecrystal with surrounding liquid (VI), a polycrystallinesolid with surrounding liquid (VII), and a totally liq-uid aerosol particle (VIII), respectively. Theoretically,the surfaces are in higher concentrations with respectto the inner parts of aerosol droplets because of wa-ter evaporation, so that nucleation and crystallizationprobably start on the surfaces to enclose residual waterin the particles. Therefore, structure (IV) should bepreferred in atmospheric aerosols [11]. However, com-plicated morphologies can be found even for simple two-component aerosol particles. For example, both aerosolflow tube (AFT) [9] and electrodynamic balance (EDB)[12] investigations have confirmed the presence of resid-ual water (probably in the concave parts) of crystal-lized NaCl aerosol particles. The solvent properties ofthe surfaces of NaCl aerosol particles as a function ofRH have recently been characterized by the excited-state absorption spectroscopy and ionization thresholdmeasurements of coumarin 314 adsorbed on the particle

I II III IV

V VI VII VIII

FIG. 2 Possible structures of aerosol particles: (I) singlecrystal, (II) agglomerate of single crystals or a polycrys-talline solid, (III) polycrystalline material with several openbut liquid-filled cavities, (IV) solid polycrystalline shell withembedded liquid, (V) solid single-crystalline shell with em-bedded liquid, (VI) single crystal with surrounding liquid,(VII) polycrystalline solid with surrounding liquid, (VIII)liquid aerosol particle. (Reproduced with permission fromRef.[10]. Copyright 2004 American Chemical Society.)

surfaces [13].

4. Deliquescence mechanisms of aerosol particles

In view of classic theories, with the increase of RH,aerosol particles usually deliquesce at the equilibriumRH of corresponding saturated salt solutions, whentheir mass abruptly increases, i.e. prompt deliques-cence of aerosols [1]. However, nonprompt deliques-cence has been observed by Hameri et al. in theirpioneering work on the hygroscopic growth behaviorsof NaCl and (NH4)2SO4 ultrafine aerosols [14,15], andseveral models of aerosol deliquescence were accord-ingly developed, predicting either prompt or nonpromptaerosol deliquescence [16-19]. In recent studies [20-22],both deliquescence and efflorescence processes were in-vestigated by Martin et al. for the aerosol nanoparticlesof NaCl and (NH4)2SO4. In contrast, prompt deliques-cence was always observed. Due to the enormous diffi-culties involved, only tandem differential mobility ana-lyzer (TDMA) has been used in their investigations.

The liquids contained in the fissures or concave partsof aerosol particles with complicated structures tend totake up water from the milieu prior to deliquescenceRH, and aerosol particles grow accordingly in mass withincreasing the RH. Pre-deliquescence of aerosol parti-cles can thus occur [10]. Relevant investigations areof great importance, because heterogeneous reactionsof aerosol particles can be seriously affected by sur-face liquid films [23]. More recent theoretical investi-gations further found that substrate can decrease sig-nificantly the deliquescence RH of deposited small NaClaerosol particles, posing a potential corrosion problemfor metallic substrates [24]. In contrast, the “annealing”accomplished by repeated exposure of a slab of NaClcrystal to RHs slightly lower than that of saturated so-lutions could reduce surface defects, and an increaseof 10% in deliquescence RH was accordingly observed.This observation of the increase in deliquescence RHfor NaCl crystal has been called “nucleated deliques-cence” by Cantrell et al. to imply the requirement ofsurmounting the energy barrier for deliquescence [25].Recently, molecular dynamics has been used to sim-ulate the interface with two equal-sized slabs of solidNaCl and liquid water in contact, and it was concludedthat the void fraction (including both bulk and surfacedefects) required to trigger dissolution varies between15%-20% at 300 K and 101.3 MPa [26]. According tothe above investigations, it seems that minimal surfacedefect fractions are required to initiate aerosol deliques-cence.

III. APPLICATIONS OF IN SITU FTIR AND RAMANSPECTROSCOPY TO THE STUDY OF AEROSOLPARTICLES

Since the early days of atmospheric aerosol research,the changes of aerosol particles in electric mobility, light



scattering and mass have been observed and quantifiedin an effort to investigate the aerosol phenomena occur-ring in atmospheric processes. Orr et al. have madethe first observations of metastable states (i.e. super-saturated droplets) by measuring the electric mobilityof aerosol particles [4]. Then, by using a gravimetricmethod, Winkler and Junge have made further obser-vations on some supersaturated aerosol droplets such asNaCl and KCl [27]. The surface structures and phasetransitions at various RHs of aerosol particles could alsobe observed and investigated by AFM (atomic forcespectroscopy) [28]. In addition, other methods suchas optical microscope, differential scanning calorimetry(DSC) have also been used for the observation of aerosolphase transitions [1].

The spectral techniques of molecular vibrations,namely infrared and Raman, are well known as thetwo eyes for the observation of microscopic world, andcan provide molecular information for aerosol particleson micron scale. These two techniques have played animportant role in studying advanced topics, and foundalso wide applications in fields like atmospheric researchand the study of dynamic processes. Infrared and Ra-man are complementary in the observation of molecu-lar vibrations. Therefore, infrared and Raman investi-gations of deliquescence and efflorescence processes ofaerosols can provide detailed information on molecu-lar structures, deepening our knowledge on molecularlevel of the ion interactions in aerosols of various com-positions and concentrations, and disclosing the molec-ular basis for hygroscopic properties of aerosols. Inparticular, a number of techniques such as electrody-namic balance (EDB), aerosol flow tube (AFT), atten-uated total reflection (ATR), and optical levitation havebeen combined with infrared and Raman respectively togreatly expand the applications to aerosol research ofthe spectral techniques of molecular vibrations. Greatachievements have already been made in understand-ing the hydrogen bond structures of water, the processof ion hydration, and the formation and interchange ofvarious contact ion pairs, elucidating the structures ofsolid phases formed in efflorescence process, and gettinga deep insight into the adsorption behaviors of watermolecules onto the surfaces of solid particles and so on.These techniques will be introduced and discussed indetail in the following part of this paper, as well as theresults and progress of their applications to the studyof deliquescence and efflorescence processes of aerosolparticles.

A. Raman in situ study of single levitated aerosolparticles in deliquescence and efflorescence cycles

It has been generally agreed that levitated aerosoldroplets can depress both homogeneous and hetero-geneous nucleation to perfectly simulate aerosols in areal atmospheric environment, so that metastable solids

and/or supersaturated states occur frequently. This isvery helpful for the study of physicochemical processesbetween aerosols and the milieu. For a long time, re-searchers have endeavored to produce and capture smallparticles through ultrasonic [29-31], electric and opticallevitations.

Eletrodynamic balance is a technique capable of cap-turing and levitating by electric field a single dropletor solid particle on micron order in a given environ-ment, allowing accordingly for the investigations ofmetastable solids and supersaturated states in aerosols.The schematic diagram of an EDB is shown in Fig.3.EDB was firstly used to study atomic electrificationsby Paul and Steinwedel [32], and was later extensivelyadopted for capturing single electriferous particles [33-35].

Single particle levitation techniques integrated within situ spectral techniques of molecular vibrations canprovide molecular information on many aerosol physic-ochemical processes, conducive to elucidating on molec-ular level the hygroscopic properties and chemical reac-tions of aerosols, and have been widely used in atmo-spheric research and the study of dynamic processes.As early as 1987, Grader et al. had successfully fo-cused an infrared ray on a single levitated (NH4)2SO4

aerosol particle captured and levitated by an EDB,and made some spectroscopic investigations [36]. Incontrast to infrared, Raman combined with an EDB,i.e. Raman/EDB, has been amply reported to studyaerosols. Shown in Fig.4 is a schematic diagram ofa Raman/EDB [37]. For an electriferous particle lev-itated by an EDB in a given environment (N2 at givenRHs or other gases), the balancing voltage is propor-tional to its gravity. Therefore, for a small droplet withknown chemical compositions at prefixed RHs, changes

Gas inlet

AC:0-600 V30-10 kHz

AC

Gas outlet

Window

He-Ne laser beam

DC:0-60 V

Insulator

FIG. 3 Schematic diagram of an eletrodynamic balance(EDB). (Reproduced with permission from Ref.[6]. Copy-right 2000 American Chemical Society.)



Saturated water vapor

Dry gas

Microscope

EDB

M3

L1 L2

Mono-chromator

Computer

M1 M2

He-N

e las

er

Argo

n las

er

514.5

nm lin

e notc

h filt

er

CCD63

2.8 nm

514.5

nm

FIG. 4 Schematic diagram of a Raman/EDB system for themeasurement of Raman spectra of single levitated particlesat given RHs. (Reproduced with permission from Ref.[37].Copyright 2002 American Chemical Society.)

in RH in the sample cell of an EDB should be accompa-nied by corresponding changes in the balancing voltageagainst its gravity, and the composition of the dropletat certain RH can be accordingly determined. TheRaman spectra of aerosol droplets with known soluteconcentrations have greatly advanced the study of ioninteractions, metastable solid phases, and microscopicstructures in deliquescence and efflorescence processesof aerosol particles. The applications of EDB as wellas optical levitation technique combined with Ramanspectroscopy to the study of important problems in del-iquescence and efflorescence cycles of aerosol particleshave made several contributions, as follows.

1. Spectroscopic investigations of the structures of contaction pairs

The structures of contact ion pairs have always beenone of the most fundamental topics, which are sharedby many fields like solution chemistry, crystallography,biochemistry, electrochemistry and atmospheric aerosolchemistry, for which our knowledge is still presentlyvery limited. In addition, spectral analysis of ion pairshas been hotly disputed for a long time. For exam-ple, even though it is possible for SO4

2− to form sol-vent separated or contact ion pairs with metal ionsin bulk solutions, the concentrations are too low andrelevant spectral analyses are very controversial. Afaint shoulder was reported to occur on the high fre-quency side of the SO4

2− symmetric stretching band(981 cm−1), but no agreement has been reached on itsassignments. Though Rull et al. agreed the faint shoul-der was caused by cations, they also argued the asym-metric shape (caused by the shoulder) of SO4

2− sym-metric stretching band is due to the changes induced by

cations of the rotational correlation times of the watermolecules attached directly to SO4

2−, instead of theproposed formation of contact ion pairs [38]. On theother hand, Rudolph et al. have directly estimated thepercentage of contact ion pairs by using the area of theshoulder [39].

The integration of single droplet levitation with insitu spectral techniques has ushered in a new chapter inthe study of the structures of contact ion pairs. Hetero-geneous nucleation is not favored for single droplets levi-tated in the air and with diameters ranging from severalto tens of microns, which become extremely supersatu-rated with the decrease of RH. Cations and anions havedifficulty forming full hydration layers in supersatu-rated droplets, allowing for the formation of large quan-tities of contact ion pairs with diversified structures.Favorable conditions can thus be created for spectro-scopic investigations. By taking advantage of Ra-man/EDB technique, Tang and Fung have studied thedeliquescence and efflorescence processes of Ba(NO3)2,Sr(NO3)2, and Ca(NO3)2 aerosol particles, respectively,and obtained the spectral evidences for the existence ofcontact ion pairs in supersaturated solutions [3]. Chan’sgroup has used a Raman/EDB to systematically inves-tigate the deliquescence and efflorescence processes ofsingle MgSO4 droplets [6]. According to the evolutionsof SO4

2− stretching vibrations in Raman spectra, somechain-like structures formed by contact ion pairs (biden-tate) of MgSO4 were proposed to account for the gelformation and mass transfer limitations in supersatu-rated MgSO4 droplets. In particular, the formationof contact ion pairs has been correlated on molecularlevel with the turning point of the hygroscopic curveof MgSO4 aerosols [6]. Chan’s group has made furtherRaman investigations by changing RH of single sulfatedroplets of alkali and alkaline earth metals, disclosingthe formation conditions of diversified contact ion pairsand elucidating the novel metastable structures formedfrom supersaturated solutions [37]. Similar observationswith the formation of contact ion pairs have also beenmade for some carboxylates with relatively simple struc-tures. For example, Wang et al. have investigated theefflorescence processes of some acetates aerosols by us-ing a Raman/EDB. Through the analysis of the Ra-man spectral evolutions of C−O and C−C stretchingvibrations, the research confirmed the formation of con-tact ion pairs with monodentate, bidentate, and bridgebidentate structures by Mg2+ and CH3COO− in super-saturated Mg(CH3COO)2 aerosol droplets [40].

2. Observations of water monomers in supersaturated droplets

The integration of single-droplet levitation with insitu spectral techniques has provided new opportuni-ties for the study of water structures in supersatu-rated solutions. Chan’s group has made in situ spec-troscopic investigations of the deliquescence and efflo-



rescence processes of levitated Mg(ClO4)2, NaClO4,LiClO4 droplets, and obtained their Raman spectra inextremely supersaturated states with WSRs as low asabout 2 [7]. Water monomers, which were previouslyobserved only in the glassy states of perchlorates at lowtemperatures [41], could be identified by the Raman fea-tures in supersaturated droplets at room temperatures.In addition, they have also elaborated on the incompletehydration structures of three cations and the structuraldifferences of hydrogen bonds in the first and secondhydration layers.

3. Characterizations of supersaturated droplets with core andshell structures

Enrichment of anions on the surfaces of aerosols canhave important implications for many atmospheric re-actions [42-44]. The spatially resolved Raman spec-tra of single Mg(NO3)2 droplets were firstly obtainedwith using CCD (charge-coupled device) imaging tech-nique. Heterogeneity and homogeneity between coresand shells could be found for supersaturated and di-luted droplets, respectively [45]. Contact ion pairs withdiversified structures were believed to characteristicallydistribute on the shells and in the cores, as well asin the medium of supersaturated Mg(NO3)2 droplets.Since fluorescent probe pyranine can differentiate be-tween the states of water in solutions, i.e., free waterand bound water [46], Choi et al. have used it to studysome aerosol droplets of inorganic salts by fluorescencespectroscopy combined with EDB [47,48]. They foundthat the surfaces of NaCl droplets are enriched withbound water at low RHs, suggesting a core and shellstructure. However, the relative amount of free waterand bound water was further observed to oscillate spa-tially within NaCl droplets, therefore, even more com-plicated aerosol structures, which are difficult to de-scribe by a normal core and shell structure, probablyexist for NaCl droplets [47].

4. Observations of the coagulation process of opticallylevitated particles

Reid’s group has developed a novel method to con-trol aerosol particles via light [49-53]. Figure 5 shows aschematic diagram of a 3-dimensional optical trap levi-tating a single particle by a beam of tightly focused lightfrom a laser [51]. The forces exerted on a particle by thelight restrict the motion of particles in all three axes,allowing the particle to be effectively immobilized. Theposition of a trapped particle can be moved by manipu-lating the light beam. By virtue of forming multiple op-tical traps from multiple light beams, multiple particlescan be trapped and manipulated simultaneously. Com-bined with Raman spectroscopy, they have succeededin studying mass and heat transfer dynamics of evapo-

Blue LED illusmination: 455 nm

RH probe

Nebuliser flow

CoverslipImmersion oil Microscope objective

Dichroic mirror

Long pass filter>600 nm

Raman light

Trapping beam

Beam expansion optics

Humidified

flownitrogen gas

FIG. 5 Experimental designs for optical levitation of aerosolparticles. A single aerosol droplet is trapped from a neb-ulized flow. Illumination of the particle for imaging isachieved with a blue LED. A narrow band mirror (99% re-flective at 532 nm) is used to steer the trapping beam ontothe microscope objective while allowing the blue light andRaman-scattered light around 650 nm to pass through. Animage of a 5.609 µm radius water droplet is shown. (Re-produced with permission from Ref.[51]. Copyright 2006American Chemical Society.)

rating aerosol droplets [49,50], examining the temporalevolution of droplet size, composition and temperature[51,52], and observing the coagulation process betweenaerosol particles [53].

Single particle levitation techniques combined withRaman spectroscopy can be used to study the compli-cated interactions of ions and molecules in supersatu-rated droplets. However, strong morphology-dependentresonances (MDRs) can occur for single droplets, whichhave an impact especially on the wide O−H stretchingband and make it very difficult for the study of wa-ter structures. Moreover, only the symmetric stretch-ing bands with strong Raman signals of SO4

2−, NO3−,

and ClO4− and so on can be studied, and little can be

known of the antisymmetric stretching vibrations capa-ble of yielding further structural information of contaction pairs. Therefore, infrared technique is needed toprovide the complementary information on molecularlevel on atmospheric aerosols, especially the ion inter-actions in supersaturated droplets.

B. FTIR/AFT in situ study of deliquescence andefflorescence cycles of aerosols

In the current study of aerosols, an FTIR spectrom-eter and an aerosol flow tube (AFT) can be assembled



Humidifier

Buffering Multi-step desiccation cells

Humidity temperature meter

To computer

ExhaustAir pump

IR observation cellMirror

Detector

IR source

cell

FIG. 6 Schematic diagram of a FTIR/AFT system for themeasurement of FTIR spectra of aerosol flow changing withRH. (Reproduced with permission from Ref.[54]. Copyright2005 Royal Society of Chemistry.)

into a widely used FTIR/AFT system [1,54-56]. Be-cause infrared and Raman respond complementarily tomolecular vibrations, FTIR/AFT can make up for thegap uncovered by Raman/EDB and becomes an effec-tive method.

FTIR/AFT is generally composed of an aerosolgenerator, a RH-regulating and recording system, apipeline system, and an infrared (IR) observation celland so on [54-56], as shown in Fig.6. Aerosol gen-erator can produce aerosol droplets from diluted so-lutions at given concentrations, which are then flownalong with carrier gas with controlled RHs into theIR observation cell. With changing the RH, FTIRin situ study of aerosols can bring about new under-standings on molecular level about the phase transi-tions (deliquescence and efflorescence) of particles aswell as the supersaturated structures [54-59]. By con-trast to single particle levitation techniques, the re-sults provided by FTIR/AFT are of statistic signifi-cance. With a temperature-regulating system beingadded, FTIR/AFT can be used to study the phase tran-sitions of aerosols at different temperatures [60-63]. Inaddition, FTIR/AFT has also been successfully used inin situ study of aerosol heterogeneous chemical reac-tions, such as the reactions between NO2 and sea saltaerosol particles at various RHs [64] and the acid cat-alyzed heterogeneous reactions giving rise to the growthof organic aerosol particles [65].

The study of deliquescence and efflorescence pro-cesses of aerosols by FTIR/AFT and Raman/EDB canyield complete information on molecular structures, ad-vancing our understandings on molecular level of ion in-teractions in aerosol droplets and disclosing the hygro-scopic properties of aerosols. By using FTIR/AFT inthe study of deliquescence and efflorescence processes,the infrared spectra can be recorded of aerosol particlesat various RHs. Meanwhile, the compositions of aerosoldroplets at given RHs can be obtained by EDB experi-ments. Relating the infrared spectra of aerosol particlesto their compositions can facilitate the spectral anal-ysis and more quantitative results have been accord-

ingly obtained. FTIR/AFT investigations of NaClO4

aerosols have proved the formation of metastable an-hydrous NaClO4 at low RHs instead of thermody-namic stable NaClO4·H2O, and further confirmed wa-ter monomers as major component in supersaturatedNaClO4 droplets [54], consistent with Raman/EDB in-vestigations [7]. According to the FTIR/AFT spectraof NaClO4 aerosols at different RHs, dominant waterstructures can be observed to transform from monomersto incomplete hydrogen bonds and then to strong hy-drogen bonds with increasing the RH. FTIR/AFT hasalso been used by Zhao et al. to study deliques-cence and efflorescence processes of MgSO4 aerosols[55]. Through the analysis of the infrared spectra andobservations of the evolution of the hygroscopic curveof MgSO4 aerosols, contact ion pairs with diversifiedstructures were found to occur abundantly in supersat-urated MgSO4 droplets, and MgSO4 gels were believedto form in sea water aerosol particles at low RHs. Thesefindings are relevant to seawater aerosol characteriza-tion and maritime environment monitoring [55]. Thistechnique can be also applied to other meaningful sys-tems, and is suitable for the study of the hygroscopicprocess of organic aerosols and inorganic/organic mixedaerosols.

C. FTIR/ATR in situ study of deliquescence andefflorescence cycles of aerosols

In the early days when Harrick [66,67] and Fahren-fort [68] independently designed their attenuated totalreflection (ATR), some potentials of its applications hadalready been proposed, including the measurements ofoptical constants by a plotting method or through theapplication of Kramers-Kronig relation, which laid thefoundation for many ensuing important explorations.With the steady improvement of internal reflection ac-cessories, and the strengthening capability and growingpopularity in routine analysis of FTIR spectrometers,ATR is also evolving rapidly and becoming increasinglymature. Therefore, though ATR historically expandedthe applications of infrared spectroscopy, the improve-ments and popularity of FTIR spectrometers have re-ciprocally been providing a sound platform for ATR toplay its full role [69].

In the past two decades, ATR has been widely usedto probe the molecular adsorption behaviors on thesolid/liquid interfaces between salt solutions and cleanATR crystal surfaces, coated ATR crystal surfaces, andfinely dispersed particulate matter in contact with ATRcrystal surfaces, respectively [70]. In addition, ATRcould also be used to study solid/solid interfaces, suchas the buried interfaces in molecular junctions [71]. Andin recent years, Max et al. [72-74] and Zhang et al. [75-77] have investigated many bulk solutions of electrolytesby FTIR/ATR, and extracted the hydration informa-tion of many ions like Na+, Mg2+, Zn2+, and ClO4

−,



making a unique contribution to the development of so-lution chemistry.

In the late 1970s and early 1980s, ATR had al-ready been used in the study of atmosphere aerosols[78,79]. Specially designed ATR impactors were thenused to collect aerosol samples in the atmosphere foroff line analysis of chemical compositions. Some re-searchers have recently used ATR to identify the chem-ical compositions of aerosols collected on Teflon filters[80,81]. More recently, the optical constants of mixedSO4

2−-NO3−-NH4

+ aerosols under tropospheric condi-tions have been obtained by using ATR spectral tech-niques [82]. In addition, ATR has also played an im-portant role in other aerosol relevant researches, suchas the formation and structures of sulfuric acid and sul-furic acid monohydrate films as potential substrates forspectroscopic probes of the surface chemistry of atmo-spheric sulfate particles [83], the surface compositionsduring the well-known reactions of nitrogen oxide gasesNO2 and N2O5 with NaCl powders [84], and the interac-tions of ozone at 296 K with unsaturated self-assembledmonolayers [85].

Study of deliquescence and efflorescence processes ofaerosols by FTIR/ATR has rarely been reported inthe literature, especially the physicochemical events oc-curring in aerosols as either supersaturated dropletsor solid particles. Zhang’s group has recently modi-fied a standard ATR sample cell as the key unit of aFTIR/ATR aerosol experimental setup, which is shownschematically in Fig.7, for the study of the deliques-cence and efflorescence processes of atmospheric aerosolparticles [86-88]. An ultrasonic atomizer was used togenerate aerosol particles with diameters ranging from1 µm to 5 µm. Guided by a vacuum pump, aerosol par-ticles were then deposited on the surfaces of an ATRcrystal and the amount could be controlled by moni-toring the ATR spectra of aerosols. By changing theRH in the sample cell, the deliquescence and efflores-cence processes could be investigated for aerosol parti-cles. This method was used to collect the infrared spec-tra of NaClO4 aerosols at various RHs, which were ana-lyzed for the information of residual interfacial water inNaClO4 particles [86]. In the investigations of efflores-cence and deliquescence cycles, complicated morpholo-gies in phase transitions of Mg(NO3)2 aerosol particleshave been observed [87]. Optical distortions on ATRspectra of aerosols were further carefully examined anda method for measuring the optical constants of super-saturated aerosols have been proposed based on theo-retical calculations and experimental designs [88].

With a similar method, Grassian’s group has alsoinvestigated the hygroscopic properties of several in-organic aerosols [89,90]. In addition, they have fur-ther studied the reactions of CO2, HNO3, and NO2

with mineral particles and soots by FTIR/ATR [91]and Knudsen Cell combined with FTIR [92-94], respec-tively. In our unpublished results with the study of themolecular events occurring in the deliquescence and ef-

Dry N2 N2 saturated vapor Exhaust

Pump

RH meter

IR sourceMCT detector

Ultrasonic humidifier

Aerosol chamber

ZnSe

FIG. 7 Schematic diagram of a FTIR/ATR system for themeasurement of FTIR spectra of aerosol particles depositedon a ZnSe substrate with changing RH. (Reproduced withpermission from Ref.[86]. Copyright 2005 Science in ChinaPress.)

florescence processes of NaNO3 aerosol particles, it hasbeen found that water molecules should be adsorbed tothe surfaces of NaNO3 crystals at low RHs, mainly inthe forms of monomers and dimers, as shown in Fig.8.

D. Confocal Raman in situ study of deliquescence andefflorescence cycles of aerosol particles deposited on aquartz substrate

Morphology-dependent resonances (MDRs) are in-herent in Raman/EDB study of single levitated dropletsof spherical shape, which randomly overlap with thespectral features resulting from the interactions rele-vant to ion pairs and make it difficult for detailed spec-tral analysis. As for the vibrations of SO4

2−, only thesymmetric stretching vibration υ1 with strong intensityin Raman spectra can be analyzed on the informationof various ion pairs, while the other three modes se-riously affected by MDRs can not yield useful struc-tural information [6,37]. Recently, a novel method, i.e.confocal Raman in situ study of single aerosol dropletson a substrate, whose schematic diagram is shown inFig.9, has been reported by us to circumvent this prob-lem [95]. According to this method, sample solutionswere sprayed by a syringe and deposited on a quartzsubstrate situated at the bottom of a sample cham-ber. Due to greatly depressed homogenous nucleation,small droplets can attain some extent of supersaturationeven when deposited on substrates [88]. In addition,aerosol droplets deposited on substrates are usually ofhalf ellipsoid shape causing no MDRs. The sample cellconnected directly with the RH-regulating pipeline wassealed with a piece of PE film and fixed on the man-ual platform of a microscope. The environmental RHs



1 2 3

4 5 6

7 8 9

(b)

FIG. 8 (a) Hygroscopic growth of NaNO3 aerosol particlesobtained by FTIR/ATR. (b) Possible adsorption modes ofwater monomers and dimers. The blue balls, red balls, pur-ple balls, and grey balls denote N atoms, O atoms, Na atoms,and H atoms, respectively. For interpretation of the color inthis Figure legend, the reader can refer to the web versionof this article.

in the sample cell could be precisely controlled over abroad range by mixing and adjusting two flows of dryand water saturated nitrogen gases, and were monitoredby a humidity temperature meter. Apparatus specifica-tions and experimental procedures of this method havebeen detailed elsewhere [95,96].

With decreasing the RH, monodentate contact ionpairs in supersaturated MgSO4 droplets were observedto evolve and merge in sequence as bidentate and chain-like bidentate contact ion pairs, adding to our struc-tural understanding of mass transfer limitations [95].The species interactions in NaNO3 aerosol droplets havealso been investigated, especially the complicated con-

GratingMicroscopy

Sample

PE film

Quartz

Dry N2 Water saturated N2

Hygrometer Exhaust

Pinhole

Laser

CCD detector

FIG. 9 Schematic diagram of a confocal Raman system forthe in situ study of deliquescence and efflorescence cycles ofsingle aerosol particles on a quartz substrate. (Reproducedwith permission from Ref.[95]. Copyright 2005 AmericanChemical Society.)

tact ion pairs formed by Na+ and NO3− as well as the

structures of hydrogen bonds between water moleculesin extremely supersaturated droplets [96]. The effects ofdifferent morphologies of solid NaNO3 aerosol particleson distinct deliquescence processes have been furtherinvestigated to disclose the involved molecular mecha-nisms [96].

IV. CONCLUSION

This paper has briefly reviewed the studies of deli-quescence and efflorescence processes of aerosol parti-cles by in situ FTIR and Raman spectroscopy. In par-ticular, the four spectral techniques, i.e. Raman/EDB,FTIR/AFT, FTIR/ATR, and confocal Raman on aquartz substrate were highlighted with regard to theirapplications in the investigations of many importantproblems involved in deliquescence and efflorescence cy-cles of aerosol particles. With the rapid development ofdetection techniques and introduction of new spectralmethods, it can be expected that related research willmake even greater progress in the future.

V. ACKNOWLEDGMENTS

This work was supported by the National Natural Sci-ence Foundation of China (No.20073004, No.20473012,No.20673010, and No.20640420450), the 111 ProjectB07012, and the China Postdoctoral Science Founda-tion (No.20070410466). The Trans-Century TrainingProgram Foundation for the Talents by the Ministry ofEducation of China was also gratefully acknowledged.



VI. NOTE

Several new advances have come to our attentionsince the acceptance of this paper. Experimental ev-idences of small angle neutron scattering strongly sug-gested that aqueous organic nanodroplets display com-plicated core/shell structures [97], resulting probablyin increased water condensation due to the presenceof “effective” negative surface tension [98]. The efflo-rescence RHs of (NH4)2SO4 aerosols [99], as well asmixed NaCl/Na2SO4 droplets at various ratios [100],have been successfully predicted using classical homo-geneous nucleation theory.

[1] S. T. Martin, Chem. Rev. 100, 3403 (2000).[2] S. Fuzzi, M. O. Andreae, B. J. Huebert, M. Kulmala,

T. C. Bond, M. Boy, S. J. Doherty, A. Guenther,M. Kanakidou, K. Kawamura, V. M. Kerminen, U.Lohmann, L. M. Russell, and U. Poschl, Atmos. Chem.Phys. Discuss. 5, 11729 (2005).

[3] I. N. Tang and K H. Fung, J. Chem. Phys. 106, 1653(1997).

[4] C. Orr, F. K. Hurd, and W. J. Corbett, J. Colloid Sci.13, 472 (1958).

[5] C. K. Chan, R. C. Flagan, and J. H. Seinfeld, J. Am.Ceram. Soc. 81, 646 (1998).

[6] Y. H. Zhang and C. K. Chan, J. Phys. Chem. A 104,9191 (2000).

[7] Y. H. Zhang and C. K. Chan, J. Phys. Chem. A 107,5956 (2003).

[8] I. N. Tang, K. H. Fung, D. G. Imre, and H. R. Munkel-witz, Aerosol. Sci. Technol. 23, 443 (1995).

[9] D. D. Weis and G. E. Ewing, J. Geophys. Res. 104(D17), 21275 (1999).

[10] C. A. Colberg, U. K. Krieger, and T. Peter, J. Phys.Chem. A 108, 2700 (2004).

[11] K. H. Leong, J. Aerosol Sci. 18, 511 (1987).[12] C. Braun and U. K. Krieger, Opt. Express 8, 314

(2001).[13] E. Woods III, S. F. Morris, C. N. Wivagg, and L. E.

Healy, J. Phys. Chem. A 109, 10702 (2005).[14] K. Hameri, M. Vakeva, H. C. Hansson, and A. Laak-

sonen, J. Geophys. Res. 105(D17), 22231 (2000).[15] K. Hameri, A. Laaksonen, M. Vakeva, and T. Suni, J.

Geophys. Res. 106(D18), 20749 (2001).[16] P. Mirabel, H. Reiss, and R. K. Bowles, J. Chem. Phys.

113, 8200 (2000).[17] Y. S. Djikaev, R. Bowles, H. Reiss, K. Hameri, A.

Laaksonen, and M. Vakeva, J. Phys. Chem. B 105,7708 (2001).

[18] L. M. Russell and Y. Ming, J. Chem. Phys. 116, 311(2002).

[19] D. O. Topping, G. B. McFiggans, and H. Coe, Atmos.Chem. Phys. 5, 1205 (2005).

[20] G. Biskos, A. Malinowski, L. M. Russell, P. R. Buseck,and S. T. Martin, Aerosol Sci. Technol. 40, 97 (2006).

[21] G. Biskos, L. M. Russell, P. R. Buseck, and S. T. Mar-tin, Geophys. Res. Lett. 33, L07801 (2006).

[22] G. Biskos, D. Paulsen, L. M. Russell, P. R. Buseck, andS. T. Martin, Atmos. Chem. Phys. 6, 4633 (2006).

[23] B. J. Finlayson-Pitts and J. C. Hemminger, J. Phys.Chem. A 104, 11463 (2000).

[24] Y. Gao, L. E. Yu, and S. B. Chen, J. Phys. Chem. A111, 633 (2007).

[25] W. Cantrell, C. McCrory, and G. E. Ewing, J. Chem.Phys. 116, 2116 (2002).

[26] R. Bahadur, L. M. Russell, S. Alavi, S. T. Martin, andP. R. Buseck, J. Chem. Phys. 124, 154713 (2006).

[27] P. Winkler and C. Junge, J. Rech. Atmos. 6, 617(1972).

[28] G. Friedbacher, M. Grasserbauer, Y. Meslmani, N.Klaus, and M. J. Higatsberger, Anal. Chem. 67, 1749(1995).

[29] N. Leopold, M. Haberkorn, T. Laurell, J. Nilsson, J.R. Baena, J. Frank, and B. Lendl, Anal. Chem. 75,2166 (2003).

[30] S. Santesson, J. Johansson, L. S. Taylor, L. Levander,S. Fox, M. Sepaniak, and S. Nilsson, Anal. Chem. 75,2177 (2003).

[31] B. R. Wood, P. Heraud, S. Stojkovic, D. Morrison, J.Beardall, and D. McNaughton, Anal. Chem. 77, 4955(2005).

[32] W. Paul and H. Steinwedel, Z. Naturforsch. A 8, 448(1953).

[33] H. Straubel, Die Naturwissenschaften 42, 506 (1955).[34] H. Straubel, Zeit. Electrochem. 60, 1033 (1956).[35] R. F. Wuerker, H. Shelton, and R. V. Langmuir, J.

Appl. Phys. 30, 342 (1959).[36] G. S. Grader, S. Arnold, R. C. Flagan, and J. H. Se-

infeld, J. Chem. Phys. 86, 5897 (1987).[37] Y. H. Zhang and C. K. Chan, J. Phys. Chem. A 106,

285 (2002).[38] F. Rull, C. Balarew, J. L. Alvarez, F. Sobron, and A.

Rodriguez, J. Raman Spectrosc. 25, 933 (1994).[39] W. W. Rudolph, G. Irmer, and G. T. Hefter, Phys.

Chem. Chem. Phys. 5, 5253 (2003).[40] L. Y. Wang, Y. H. Zhang, and L. J. Zhao, J. Phys.

Chem. A 109, 609 (2005).[41] H. Kanno and J. Hiraishi, Chem. Phys. Lett. 83, 452

(1981).[42] J. H. Seinfeld, Science 288, 285 (2000).[43] P. J. Jungwirth, J. Phys. Chem. A 104, 145 (2000).[44] E. M. Knipping, M. J. Lakin, K. L. Foster, P. Jung-

wirth, D. J. Tobias, R. B. Gerber, D. Dabdub, and B.J. Finlayson-Pitts, Science 288, 301 (2000).

[45] Y. H. Zhang, M. Y. Choi, and C. K. Chan, J. Phys.Chem. A 108, 1712 (2004).

[46] R. Chakraborty and K. A. Berglund, J. Cryst. Growth125, 81 (1992).

[47] M. Y. Choi, C. K. Chan, and Y. H. Zhang, J. Phys.Chem. A 108, 1133 (2004).

[48] M. Y. Choi and C. K. Chan, J. Phys. Chem. A 109,1042 (2005).

[49] J. P. Reid and L. Mitchem, Annu. Rev. Phys. Chem.57, 245 (2006).

[50] R. J. Hopkins and J. P. Reid, J. Phys. Chem. B 110,3239 (2006).

[51] L. Mitchem, J. Buajarern, R. J. Hopkins, A. D. Ward,R. J. J. Gilham, R. L. Johnston, and J. P. Reid, J.Phys. Chem. A 110, 8116 (2006).

[52] R. J. Hopkins, C. R. Howle, and J. P. Reid, Phys.



Chem. Chem. Phys. 8, 2879 (2006).[53] L. Mitchem, J. Buajarern, A. D. Ward, and J. P. Reid,

J. Phys. Chem. B 110, 13700 (2006).[54] L. J. Zhao, Y. H. Zhang, L. Y. Wang, Y. A. Hu, and

F. Ding, Phys. Chem. Chem. Phys. 7, 2723 (2005).[55] L. J. Zhao, Y. H. Zhang, Z. F. Wei, H. Cheng, and X.

H. Li, J. Phys. Chem. A 110, 951 (2006).[56] D. J. Cziczo, J. B. Nowak, J. H. Hu, and J. P. D.

Abbatt, J. Geophys. Res. 102(D15), 18843 (1997).[57] D. D. Davis and G. E. Ewing, J. Geophys. Res.

101(D13), 18709 (1996).[58] C. E. Braban, M. F. Carroll, S. A. Styler, and J. P. D.

Abbatt, J. Phys. Chem. A 107, 6594 (2003).[59] Q. Xu, M. Dewitte, and J. J. Sloan, Atmos. Environ

37, 911 (2003).[60] D. J. Cziczo and J. P. D. Abbatt, J. Phys. Chem. A

104, 2038 (2000).[61] D. J. Cziczo and J. P. D. Abbatt, J. Geophys. Res.

104(D11), 13781 (1999).[62] H. M. Hung, A. Malinowski, and S. T. Martin, J. Phys.

Chem. A 106, 293 (2002).[63] H. M. Hung, A. Malinowski, and S. T. Martin, J. Phys.

Chem. A 107, 1296 (2003).[64] D. D. Weis and G. E. Ewing, J. Phys. Chem. A 103,

4865 (1999).[65] M. Jang, S. Lee, and R. M. Kamens, Atmos. Environ.

37, 2125 (2003).[66] N. J. Harrick, J. Phys. Chem. 64, 1110 (1960).[67] N. J. Harrick, Phys. Rev. Lett. 4, 224 (1960).[68] J. Fahrenfort, Spectrochim. Acta 17, 698 (1961).[69] J. P. Coates, The Industrial Applications of Infrared

Internal Reflection spectroscopy, In: Internal Reflec-tion Spectroscopy: Theory and Applications, F. M. Jr.Mirabella, Ed. New York-Basel-Hong Kong: MarcelDekker, Inc. 53 (1992).

[70] A. R. Hind, S. K. Bhargava, and A. McKinnon, Adv.Colloid Interface Sci. 93, 91 (2001).

[71] Y. Jun and X. Y. Zhu, J. Am. Chem. Soc. 126, 13224(2004).

[72] J. J. Max and C. Chapados, Appl. Spectrosc. 53, 1601(1999).

[73] J. J. Max, S. D. Blois, A. Veilleux, and C. Chapados,Can. J. Chem. 79, 13 (2001).

[74] J. J. Max and C. Chapados, J. Chem. Phys. 115, 2664(2001).

[75] Y. Chen, Y. H. Zhang, and L. J. Zhao, Phys. Chem.Chem. Phys. 6, 537 (2004).

[76] J. H. Liu, Y. H. Zhang, L. Y. Wang, and Z. F. Wei,Spectrochim. Acta A 61, 893 (2005).

[77] Z. F. Wei, Y. H. Zhang, L. J. Zhao, J. H. Liu, and X.H. Li, J. Phys. Chem. A 109, 1337 (2005).

[78] S. A. Johnson, R. Kumar, and P. T. Cunningham,Aerosol Sci. Technol. 2, 401 (1983).

[79] R. L. Tanner, R. Kumar, and S. Johnson, J. Geophys.Res. 89(D5), 7149 (1984).

[80] H. Shaka and N. A. Saliba, Atmos. Environ. 38, 523(2004).

[81] A. Ghaucha, P. A. Deveau, V. Jacob, and P. Baussand,Talanta 68, 1294 (2006).

[82] G. J. Boer, I. N. Sokolik, and S. T. Martin, J. Quant.Spectrosc. Radiat. Transfer 108, 17 (2007).

[83] A. B. Horn and K. J. Sully, Phys. Chem. Chem. Phys.1, 3801 (1999).

[84] R. M. Sayer and A. B. Horn, Phys. Chem. Chem. Phys.5, 5229 (2003).

[85] Y. Dubowski, J. Vieceli, D. J. Tobias, A. Gomez, A.Lin, S. A. Nizkorodov, T. M. McIntire, and B. J.Finlayson-Pitts, J. Phys. Chem. A 108, 10473 (2004).

[86] Y. H. Zhang, Y. A. Hu, F. Ding, and L. J. Zhao, Chin.Sci. Bull. 50, 2149 (2005).

[87] X. H. Li, J. L. Dong, H. S. Xiao, P. D. Lu, Y. A. Hu,and Y. H. Zhang, Sci. China Ser. B, In press (2008).

[88] L. J. Zhao, Y. H. Zhang, Y. A. Hu, and F. Ding, At-mos. Res. 83, 10 (2007).

[89] J. Schuttlefield, H. Al-Hosney, A. Zachariah, and V.H. Grassian, Appl. Spectrosc. 61, 283 (2007).

[90] P. K. Hudson, J. Schwarz, J. Baltrusaitis, E. R. Gib-son, and V. H. Grassian, J. Phys. Chem. A 111, 544(2007).

[91] J. Baltrusaitis and V. H. Grassian, J. Phys. Chem. B109, 12227 (2005).

[92] H. A. Al-Abadleh and V. H. Grassian, J. Phys. Chem.A 104, 11926 (2000).

[93] E. R. Johnson, J. Sciegienka, S. Carlos-Cuellar, andV. H. Grassian, J. Phys. Chem. A 109, 6901 (2005).

[94] G. M. Underwood, T. M. Miller, and V. H. Grassian,J. Phys. Chem. A 103, 6184 (1999).

[95] F. Wang, Y. H. Zhang, S. H. Li, L. Y. Wang, and L.J. Zhao, Anal. Chem. 77, 7148 (2005).

[96] X. H. Li, F. Wang, P. D. Lu, J. L. Dong, L. Y. Wang,and Y. H. Zhang, J. Phys. Chem. B 110, 24993 (2006).

[97] B. E. Wyslouzil, G. Wilemski, R. Strey, C. H. Heath,and U. Dieregsweiler, Phys. Chem. Chem. Phys. 8, 54(2006).

[98] P. Chakraborty and M. R. Zachariah, J. Phys. Chem.A 111, 5459 (2007).

[99] Y. Gao, S. B. Chen, and L. E. Yu, J. Phys. Chem. A110, 7602 (2006).

[100] Y. Gao, L. E. Yu, and S. B. Chen, J. Phys. Chem. A111, 10660 (2007).


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