Aerosol Hygroscopicity on A Single Particle Level Using Microscopic and Spectroscopic Techniques: A Review

Studies on the hygroscopic properties of aerosol particles can provide critical insights into important aerosol properties such as (1) alteration of aerodynamic properties; (2) cloud-droplet nucleation efficiency; (3) optical properties; and (4) physicochemical changes through complex heterogeneous chemical reactions with atmospheric gas-phase species. Investigations of the hygroscopic properties have been carried out using various analytical techniques for individual or bulk aerosol particles with deviating chemical compositions. Solid information on the hygroscopic properties, morphology, and chemical composition of airborne particles on a micrometer scale can be achieved by using a single particle approach. In this short review, recent microscopic (such as optical microscope, atomic force microscopy, environmental SEM and TEM, and scanning transmission X-ray microscopy) and spectroscopic (including Raman microspectrometry, FTIR spectroscopy, and their combination with levitation techniques) studies of hygroscopicity of aerosol particles on a single particle level are provided.


INTRODUCTION
Atmospheric aerosol particles composed of inorganic and organic species in various phase states play significant roles in global climate change directly by scattering or absorbing incoming solar radiation and indirectly by serving as cloud condensation nuclei (CCN) and/or ice nuclei (IN) (Seinfeld and Pandis, 2016).The radiative effects, optical properties, and chemical reactivity of atmospheric aerosol particles depend on their chemical compositions, sizes, aerosol phases, and mixing states (Martin, 2000).Studies on the heterogeneous chemistry of aerosol particles in the air is of vital importance, since airborne particles can react with gaseous pollutants, such as SO x or NO x , and their physicochemical properties can be modified in turn, through heterogeneous chemical reactions.In addition, the chemistry of the Earth's atmosphere is influenced by reducing photolysis rates of important atmospheric gas-phase species through heterogeneous chemical reactions with the atmospheric aerosol particles.Therefore, increasing attention has been devoted to the study of physicochemical characteristic changes of aerosol particles (Poschl and Shiraiwa, 2015;Usher et al., 2003).It has been well recognized that the ability of aerosol particles to contain water as a function of relative humidity (RH), referred to as the hygroscopicity, plays important roles in some heterogeneous reactions (Freedman, 2017;Tang et al., 2016a, b).For example, gaseous HNO 3 can be dissolved in the surface water and gradually react with CaCO 3 particles which originally do not absorb a significant amount of water on the surface, resulting in the total consumption of CaCO 3 species, the completion of irreversible heterogeneous reaction, and the production of hygroscopic Ca(NO 3 ) 2 species (Kelly, 2005).When the chemical compositions of particles are modified by aging processes including heterogeneous reactions, their hygroscopic properties are altered accordingly, which in turn alters their phase states.Thus, Ca(NO 3 ) 2 particles were observed as liquid droplets when they were formed in the air (Laskin et al., 2005;Krueger et al., 2004;Krueger, 2003).Furthermore, hygroscopicity of particles determines their size and refractive index, affecting their optical properties and consequently their impacts on visibility and direct radiative forcing (Tang et al., 2019;Krieger et al., 2012).
Since the understanding of the hygroscopic properties of airborne particles is important to analyze their physicochemical changes (Krieger et al., 2012), there have been many studies on the deliquescence and efflorescence behaviors of atmospheric-related aerosol particles (Freney et al., 2009;Mikhailov et al., 2009;Wise et al., 2007;Hoffman et al., 2004;Ebert et al., 2002;Martin, 2000;Ge et al., 1998Ge et al., , 1996;;Tang and Fung, 1997;Tang and Munkelwitz, 1994;Cohen et al., 1987a, b, c).The sizes of hygroscopic particles in the atmosphere can vary depending on the ambient RH.In other words, particles can grow by absorbing water with increasing RH (humidification process) or shrink when water evaporates with decreasing RH (dehydration process).For aerosol particles which can deliquesce and effloresce during humidification and dehydration processes, respectively, the size of dry particles remains unchanged with increasing RH until the deliquescence RH (DRH) is reached, after which the solid particles become aqueous droplets, and they experience hygroscopic growth above the DRH.When RH is decreased, the concentration of salts in the aqueous droplets becomes dense and can finally crystallize at their efflorescence RH (ERH).
For atmospheric aerosol particles, deliquescence is a thermodynamic process and a range of thermodynamic models, such as the Extended Atmospheric Inorganics Model (E-AIM) (http://www.aim.env.uea.ac.uk/aim/ aim.php) and the Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficient (AIOMFAC) model (http://www.aiomfac.caltech.edu),have been developed to predict the deliquescence behavior or the ionic activity coefficients of single or two-component aerosol particles (Zuend et al., 2012(Zuend et al., , 2011;;Wexler and Clegg, 2002;Ansari and Pandis, 1999;Clegg et al., 1998;Tang, 1976).Efflorescence is a kinetic or rate-driven process that occurs by overcoming a kinetic barrier, which in turn depends on a range of factors, such as the solubility and concentration of the chemical components, vapor pressure, interfacial tension, inter-ionic forces, solutewater and solute-solute interactions, etc. (Martin, 2000).Therefore, the best way to understand efflorescence behavior of aerosol particles is through experimental measurements (Seinfeld and Pandis, 2016).It is also known that the ERH of aerosols is sometimes significantly lower than their DRH, which is called hysteresis.
Regarding the deliquescence of a two-component inorganic solid mixture, it has been predicted theoretically that two-stage phase transitions should occur, i.e. an aqueous phase with a eutonic composition is formed first at their mutual DRH (MDRH), and the remnant solids in the core dissolve later at their second DRH which depends on the composition of the particle (Wexler and Seinfeld, 1991).The MDRH should be independent of the mixing ratios of the two-component salt mixtures and the MDRH in multi-component systems should always be lower than the DRHs of the pure individual salts (Wexler and Seinfeld, 1991).Multi-component aqueous droplets should show step-wise efflorescence transitions: a component in the aqueous droplets precipitates first at their ERH and the aqueous phase of the eutonic composition effloresces at their mutual ERH (MERH), therefore, forming a heterogeneous, core-shell crystal structure (Ge et al., 1996).Full phase diagrams, covering the entire range of mixing ratios, are generally helpful to fully understand the hygroscopic behavior of multi-component aerosol particles (Martin, 2000).
Numerous studies on the hygroscopic properties of particles with various chemical compositions have been carried out using diverse analytical techniques (Tang et al., 2019;Krieger et al., 2012).The particles can be either collected by a cascade impactor aerosol sampler or generated through an atomizer from aqueous solutions.All the analytical techniques have proved to be very useful for studies on the hygroscopic properties of different types of particles under controlled RH.For example, rapid single particle mass spectrometry (Ge et al., 1998(Ge et al., , 1996) ) and online Fourier transform infrared (FTIR) spectroscopy (Braban et al., 2001;Cziczo and Abbatt, 2000;Cziczo et al., 1997) were used to examine the hygroscopic properties of aerosol particles, depending on their chemical compositions or mixing states.Hygroscopicity-tandem differential mobility analyzer (H-TDMA) has been widely used for the hygroscopic studies of size-segregated mono-disperse aerosol particles (typically in sub-micro meter range) (Mikhailov et al., 2009;Gysel et al., 2002;Weingartner et al., 2002;Rader and McMurry, 1986).Because H-TDMA cannot provide chemical compositional information on aerosol particles, the H-TDMA system has sometimes been combined with other techniques, such as scanning or transmission electron microscopy (SEM/TEM), single-particle laser-ablation time-offlight mass spectrometry (SPLAT-MS), or aerosol timeof-flight mass spectrometry (ATOFMS) (Park et al., 2009;Herich et al., 2009Herich et al., , 2008;;Zelenyuk et al., 2008;McMurry et al., 1996).In addition, the hygroscopic properties of single particles can be examined using levitation techniques (Krieger et al., 2012;Lee et al., 2008;Tang et al., 2007;Parsons et al., 2006;Tang et al., 1997;Tang and Fung, 1997;Tang and Munkelwitz, 1994;Cohen et al., 1987a, b, c).To obtain the chemical compositions along with the hygroscopic properties of single particles, the levitation techniques have been used in combination with nonintrusive analytical techniques such as electro-dynamic balance (EDB) with Raman microspectrometry (RMS), laser induced fluorescence, or Mie-scattering (Treuel et al., 2010;Lee et al., 2008;Choi and Chan, 2005;Choi et al., 2004;Tang and Fung, 1997) and optical levitation with RMS ( Jordanov and Zellner, 2006).On the other hand, many hygroscopic studies have been performed for individual or bulk aerosol particles collected on a range of substrates such as TEM grids, silicon wafer.ZnSe window, and Si 3 N 4 window by micro-FTIR in a flow-cell or static mode chamber (SMC) (Ghorai et al., 2011;Liu et al., 2008a;Lu et al., 2008;Braban et al., 2001).The hygroscopicity of submicrometer salt particles were studied by scanning transmission X-ray microscopy/X-ray absorption spectroscopy (STXM/XAS) and noncontact environmental atomic force microscopy (e-AFM), using a Si 3 N 4 X-ray transmission window (Ghorai et al., 2011;Zelenay et al., 2011) and oxidized silane-treated silicon wafer (Bruzewicz et al., 2011) as the substrate, respectively.The liquid water content of aerosol particles deposited on Fluoropore and Teflon filters was determined by gravimetric analysis and ion chromatography (McInnes et al., 1996) and by gas chromatography with a thermal conductivity detector (Lee and Chang, 2002;Lee and Hsu, 2000;Hsu, 1998), respectively.Some excellent review papers focusing on the techniques for investigating aerosol hygroscopicity of aerosol particles have been published in the previous studies (Tang et al., 2019(Tang et al., , 2016a;;Kreidenweis and Asa-Awuku, 2014;Krieger et al., 2012;Swietlicki et al., 2008).Latest advances for characterizing complex chemical and physical properties of atmospheric aerosols using key spectroscopic and microscopic techniques were also summarized and discussed (Ault and Axson, 2017).For further details, interested readers are directed to those literatures.
Atmospheric particles are chemically and morphologically heterogeneous so that the average composition and aerodynamic diameter, which are closely related to their physicochemical properties, obtained from bulk analyses do not describe well the chemical modification of the particles.Nevertheless, single particle analysis can provide the hygroscopic properties, morphology, chemical composition, mixing state, and heterogeneous reactions of airborne particles on a micrometer scale (Li et al., 2016;Ro et al., 2005).In addition, microscopic and spectroscopic techniques could simultaneously investigate the hygroscopic growth, phase transformation, chemical compositional evolution, and morphological changes of individual aerosol particles according to relative humidity change (Gupta et al., 2015a).
In this review, hygroscopic studies on a single particle level using microscopic and spectroscopic techniques will be provided in two major sections.The microscopic section includes hygroscopic system coupled with optical microscope, atomic force microscopy, environmental SEM and TEM, and scanning transmission X-ray microscopy.The spectroscopic section covers in-situ Raman and FTIR and the levitation techniques.For each technique, some typical results are introduced for better elucidating how these techniques contribute to investigation of hygroscopicity of aerosol particles.

MICROSCOPIC STUDIES
Hygroscopicity on a single-particle level can be stud-ied using various microscopic techniques.Visual observation on changes in particle size at different RHs can be used to determine hygroscopic growth factors and phase transitions (Tang et al., 2019).

1 Optical Microscopy (OM)
Optical microscopy (OM) with an apparatus which produces an air stream of variable humidity was first utilized to perform humidification measurement of single aerosol particles collected in Sydney, which were sea-salts based on their solid-to-liquid phase transition occurring in the range of ~71-75% (Twomey, 1954(Twomey, , 1953)).
A flow cell coupled with OM was employed to study the deliquescence behavior of malonic, succinic, glutaric, and adipic acid particles of 2-40 μm size, for which the results were in agreement with those from the literature and model calculations (Parsons et al., 2004b).The similar apparatus was then employed to investigate the hygroscopic behavior of ammonium sulfate (AS) and NaCl mixed with glutaric acid particles and AS particles mixed with water-soluble organic compounds, showing that the organic compounds only slightly influence the DRHs of the inorganic moiety, while they may drastically decrease the ERHs of the inorganic components depending on their types and concentrations (Pant et al., 2004;Parsons et al., 2004a).Soot and kaolinite particles were reported to have no impact on the ERH of the AS droplets and to increase it, respectively, using the above apparatus (Pant et al., 2006).
An OM technique was used to perform the visual observation of the phase transformation and hygroscopic growth of aerosol particles on a single particle level (Ahn et al., 2010).Fig. 1 shows the schematic diagram of the setup, which is composed of three parts: (A) the seethrough impactor, (B) an optical microscope, and (C) a humidity controlling system (Ahn et al., 2010).Collecting substrates, such as TEM grids, Al, and Ag foils on which either wet or dry deposited particles were seated, were placed on the impaction plate in the see-through impactor.The RH inside the impactor was controlled by mixing dry and wet (saturated with water vapor) N 2 gases.The wet N 2 gas was obtained by bubbling through deionized water reservoirs.The flow rates of the dry and wet N 2 gases were controlled by mass flow controllers (MFC) to obtain the desired RH in the range of ~3-95%, which was monitored using a digital hygrometer (Te s to 645).Optical images that contained information on the phase transformation and hygroscopic growth of individual particles according to RH change were recorded by using a digital camera through an optical microscope as shown in Fig. 2 (Eom et al., 2014).The change in size of the individual particles on optical images was used to generate growth factors (GFs) by dividing two-dimensional (2-D) areas of the particle at different RHs by that of the dry particle before starting the humidification process.From the 2-D area measurement of all the particles in the image field at each RH, the humidification and dehydration curves for all the particles were obtained as shown in Fig. 3, so that DRHs and ERHs of the particles can be determined (Ahn et al., 2010).Particles larger than ~0.5 μm could be analyzed using this system.The practical applicability of this analytical methodology was validated by investigating the hygroscopic properties of artificially generated NaCl, KCl, (NH 4 ) 2 SO 4 , and Na 2 SO 4 aerosol particles of micrometer size collected on TEM grid, which were well agreed with those reported in the literature.The technique was then employed to characterize the hygroscopic properties on individual ambient aerosol particles containing two or three components and energy-dispersive electron probe X-ray microanalysis (ED-EPMA) was used to perform a quantitative chemical speciation of the same individual particles after the measurement of the hygroscopic property (Ahn et al., 2010), which clearly identified the hygroscopic properties and morphologies of pure, reacted, and mixed seasalt, mineral, and carbonaceous particles.Hygroscopic studies using the analytical system on a single-particle basis were performed for laboratory-generated single or mixed inorganic particles.The hygroscopic behavior of wet dispersed and dry deposited NaNO 3 particles of 2.5-4 μm was examined (Kim et al., 2012).Most of wet dispersed NaNO 3 particles continuously grew and shrank during humidification and dehydration processes, respectively, and yet all the dry deposited particles had reproducible DRHs and ERHs.The different behavior of the NaNO 3 particles is attributed to different nucleation mechanisms, i.e. the homogeneous and heterogeneous nucleations, for pure and impure (seed-containing) NaNO 3 particles, respectively (Kim et al., 2012).Similar observation was reported for ammonium nitrate (AN) (Sun et al., 2018;Cziczo and Abbatt, 2000;Lee and Hsu, 2000;Lightstone et al., 2000;Dougle et al., 1998;Neubauer et al., 1998;Tang, 1980) and NH 4 HSO 4 (Cziczo and Abbatt, 2000;Lee and Hsu, 2000;Tang and Munkelwitz, 1994;Mozurkewich and Calvert, 1988).
A study systematically examined full hygroscopic properties of NaCl and KCl mixture aerosol particles in nine mixing ratios (mole fractions of KCl (X KCl ) = 0.1-0.9),obtained experimental phase diagrams for their deliquescence and efflorescence, and elucidated the efflorescence mechanism (Li et al., 2014c).K and Na salts are major components of sea salt (Seinfeld and Pandis, 2016) and KCl is also a major constituent of young smoke produced from biomass burning along with soot particles (Pósfai et al., 2003).Stepwise deliquescence and single-stage efflorescence transitions were observed.Elemental X-ray mappings of the effloresced NaCl-KCl mixture particles at all mixing ratios were performed using SEM/EDX for better understanding the efflorescence process and it was suggested that a more supersaturated salt homogeneously nucleated to be crystallized in the center and the other salt almost simultaneously underwent the heterogeneous crystallization on the former (Li et al., 2014c).
The influence of six collecting substrates with different physical properties (see Table 1) on the hygroscopicity measurements of inorganic aerosol surrogates and the pot ential applications of these substrates by optical microscopy were examined (Eom et al., 2014).The TEM grids were found to be most suitable for the hygroscopic measurements of individual inorganic aerosol particles by optical microscopy, especially when multiple analytical Table 1.General characteristics of six different substrate materials used for hygroscopic measurements.Reprinted with permission from Eom, H.J., Gupta, D., Li, X., Jung, H.J., Kim, H., Ro, C.U., Anal. Chem. 2014, 86, 2648-2656 techniques, such as scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM/EDX), TEM/EDX, and/or RMS, are applied for the characterization of the same individual particles (Eom et al., 2014).
When "genuine" (or nascent) NaCl sea-salt aerosols react with nitrogen oxides in the atmosphere, NaNO 3 species can be formed, resulting in Cl loss (Gibson et al., 2006).Laboratory-generated, micrometer-sized NaCl and NaNO 3 mixture particles at nine mixing ratios (mole fractions of NaCl (X NaCl ) = 0.1 to 0.9) were examined systematically to observe their hygroscopic behavior (Gupta et al., 2015b).Stepwise phase transitions were observed during the hygroscopic processes, and thus this observation can have important atmospheric implications (Wang and Laskin, 2014;Ault et al., 2013b).As the NaCl-NaNO 3 mixture aerosol particles can maintain an aqueous phase over a wider RH range than pure NaCl particles as the SSA surrogate, it makes their heterogeneous chemistry more probable (Gupta et al., 2015b).The aqueous surface region is crucial for atmospheric heterogeneous chemistry as heterogeneous reactions with gas phase species, such as N 2 O 5 (Ryder et al., 2014;Ault et al., 2013a) or organics (Wang and Laskin, 2014), can be promoted due to the facile gas-particle partitioning (Woods et al., 2012).X-ray elemental mappings using SEM/EDX indicated that the effloresced NaCl-NaNO 3 particles at all mixing ratios were composed of a homogeneously crystallized NaCl moiety in the center as shown in Fig. 4 (Gupta et al., 2015b).Aqueous moieties of particles were reported to effloresce more easily by the heterogeneous nucleation in the presence of seeds.The formation of the core-shell type were reported for a range of binary mixed aerosol particles, such as NaCl-KCl, KCl-KI, and (NH 4 ) 2 SO 4 -NH 4 NO 3 system (Ge et al., 1996).
An individual particle hygroscopicity (IPH) system employing an optical microscope was used for a hygroscopic behavior study of field-collected aerosol particles such as Asian dust and haze particles to better understand their physicochemical properties (Li et al., 2014b).Most of haze particles dissolved at RH = 68-70%, due to the presence of dominant sulfates, nitrates, and organics.Some particles collected during an Asian dust storm  showed DRH = 73-75% due to the presence of external sea salt mixture.The transformation of CaCO 3 to Ca (NO 3 ) 2 was also observed, changing the hydrophobic dust particles to the hydrophilic ones in the continental region (Li et al., 2014b).The hygroscopicity of aerosol particles collected at Mt. Lu, an acid precipitation area, measured by the IPH system showed that individual particles started to deliquesce at RH = 73-76% and finished the deliquescence at RH = 80% due to the presence of dominant NH 4 + and SO 4 2-ions (Li et al., 2014a).In addition, soluble organic coatings on secondary particles lowered initial DRH of particles to 63-73%, and yet with the complete dissolution at RH = 80%.Given the ambient RH of 65-85%, the secondary particles were supposed to be in the liquid phase or liquid-solid multiphase in the air (Li et al., 2014a).The hygroscopic measurements on the standard ammonium sulfate (AS) and ammonium nitrate (AN) mixture particles and the individual particles collected during haze events were also conducted using the IPH system (Sun et al., 2018).AS and AN particles comprise the major components of urban fine aerosols and their heterogeneous reactions in aqueous phase can accelerate the haze formation in China (Sun et al., 2018).AS-AN particles showed twostage deliquescence and single-stage efflorescence during humidification and dehydration processes, respectively.
As for urban haze particles, they displayed a solid core and aqueous shell at RH = 60-80% and aqueous phase at RH>80% during humidification, and it was concluded that most haze particles exist as the core-shell structure.The liquid water around the particles can provide an important surface for further heterogeneous reactions, forming secondary aerosols more easily in the polluted air (Li et al., 2019).As particles emitted from residential coal burning are an important contributor to air pollution, the IPH system was also performed for hygroscopic measurement of these fine primary particles (Zhang et al., 2018).Organic matter (OM), OM-S (sulfur-containing OM), soot-OM, S-rich, metal, and mineral particles are the major species for the coal-burning emitted particles.OM and soot particles from residential coal burning exhibited extremely low hygroscopicity, while inorganic sulfate salts or a mixture of sulfates and chlorides within individual particles can completely change the hygroscopicity of soot and OM particles, inducing a dramatic growth in size at RH = 68-85% as shown in Fig. 5 (Zhang et al., 2018).The IPH system was also used for observing the hygroscopic growth of fresh primary biological aerosol particles (PBAPs), showing that the fresh PBAPs displayed low hygroscopicity (Li et al., 2020).TEM measurements for these aerosol samples confirmed the chemical compositions of the particles (Li et al.,Fig. 5. Hygroscopic growth curves of particles emitted from residential coal burning as a function of relative humidity (RH).The left panel shows the growth factor (GF) of organic matter (OM), soot-OM, OM-S (1), and OM-S (2) according to RH change with the temperature of 20°C.The right panel shows TEM images of typical particles in four samples and optical images of the corresponding samples with increasing RH from 5% to 94%.Reprinted with permission from Zhang, Y., Yuan, Q., Huang, D., Kong, S., Zhang, J., Wang, X., Lu, C., Shi, Z., Zhang, X., Sun, Y., Wang, Z., Shao, L., Zhu, J., Li, W., J. Geophys.Res.Atmos. 123, 12,964-12,979, 2018.Copyright 2018 The Authors.The Creative Commons Attribution-Non-Commercial-NoDerivs License.2020, 2014a, b;Sun et al., 2018;Zhang et al., 2018) and the exemplar particle images are shown in Fig. 5.

2 Atomic Force Microscopy (AFM)
Atomic force microscopy (AFM) is a popular technique in surface science, with a very high spatial resolution down to a nanometer level, providing 3-D imaging including size, phase, and height information by scanning the specimen surface with a sharp tip (probe) at the end of a cantilever, which can be operated under ambient conditions (Tang et al., 2019(Tang et al., , 2016a;;Dazzi and Prater, 2017;Li et al., 2016;Laskina et al., 2015a, b;Morris et al., 2015;Ghorai et al., 2014;Lehmpuhl et al., 1999).The technique, however, has been traditionally limited by its incapacity to offer chemical information of single particles (Li et al., 2016).The coupling of chemical information with AFM, such as IR and Raman spectrometry, is expected to expand the utility of the technique (Ault and Axson, 2017;Dazzi and Prater, 2017;Wang et al., 2017a;Dazzi et al., 2012;Freedman et al., 2010).
The hygroscopicity of both laboratory-generated and field-collected particles were examined by AFM.The structures formed on the (100) cleavage surface of NaCl in respect to RH change were studied using AFM (Dai et al., 1997).A uniform layer of water was observed to be formed on the surface of the NaCl crystal when RH> 35%, and the surface with the uniform water layer evolved slowly until RH = 73%, where the water layer structure disappeared due to the deliquescence.The AFM technique, which can provide information on the surface of the particles, successfully observed that the salt surfaces can be covered with water even though RH is far below the DRH, which may not be captured by other techniques (Dai et al., 1997).Similar observations were reported in a study for NaCl nanoparticles using non-contact environmental AFM (e-AFM), suggesting that the deliquescence of NaCl nanoparticles is more complex than an abrupt first-order phase transition (Bruzewicz et al., 2011).AS particles collected at the North Atlantic Ocean were investigated by the combined use of AFM and TEM (Pósfai et al., 1998).Accurate airborne volume of hydrated AS particles can be obtained under ambient conditions through 3-D morphological information using AFM.Supplementary TEM data on the same particles showed that the particle volume is four times smaller than that from AFM due to the loss of water when they were exp osed to TEM vacuum chamber.AFM was used to qua n tify the shape of fine and ultrafine ambient aerosol particles by height-to-diameter ratios, based on that the phase of ambient aerosol particles could be determined (Wittmaack and Strigl, 2005).The growth factor (GF) of particles deposited on substrates was traditionally monitored by 2-D analysis of area changes responding to RH changes, assuming the droplets grow equally in all directions, which may not be true when the hygroscopic growth is not isotropic in height and diameter (Morris et al., 2016).AFM was established as a reliable and accurate method to determine GFs of substrate-deposited, individual submicrometer particles through a 3-D view including both height and area, so that the volume of each particle can be obtained in a wide range of RH at ambient conditions to derive humidification and dehydration curves (Morris et al., 2016).The 3-D analysis results using AFM for the atmospheric relevant particles such as NaCl, malonic acid, and their binary mixture system were compared with those obtained by 2-D analysis and the substrate-free HTDMA.As shown in Fig. 6, the AFMdetermined 3-D volume-equivalent GF of the mixed  NaCl and MA particle agreed well with that from the substrate-free HTDMA method, while that determined by 2-D area analysis was biased (Morris et al., 2016).

3 Environmental SEM and TEM (ESEM and ETEM)
Simultaneous measurements of hygroscopic behavior and chemical compositions, mixing states, and morphology of individual particles can be performed using environmental SEM and TEM (ESEM and ETEM) which are powerful for the investigation of hygroscopic behavior of ambient aerosol particles with diameters down to approximately 100 nm (Adachi et al., 2011;Freney et al., 2010Freney et al., , 2009;;Shi et al., 2008;Semeniuk et al., 2007a;Wise et al., 2007;Hoffman et al., 2004;Ebert et al., 2002).The hygroscopic behavior of individual particles can be obtained based on high resolution secondary/transmission electron images showing morphological change in res pect to RH change.The elemental compositions of individual particles can be obtained from EDX spectral analysis.

3. 1 ESEM
SEM can provide detailed information on the surface of individual aerosol particles (Li et al., 2016).ESEM with a spatial resolution of 8-15 nm was used to analyze the hygroscopicity of inorganic salt and soot particles (Ebert et al., 2002).The hygroscopic behavior of the inorganic salts was generally consistent with the previous literature results and the activation of soot particles was also observed using the high-resolution ESEM.Gaseous HNO 3 can react with CaCO 3 particles, resulting in the production of hygroscopic Ca (NO 3 ) 2 species, which was observed as liquid droplets using ESEM (Shi et al., 2008;Laskin et al., 2005;Krueger et al., 2004).The ESEM measurement for NaNO 3 particles showed that they exist as metastable, amorphous solids with dropletlike structure at low RH and undergo continuous growth with increasing RH as shown in Fig. 7 (Hoffman et al., 2004).(NH 4 ) 2 SO 4 particles of 1-2 μm size were examined using ESEM by filling the sample chamber with water vapor at a pressure of 600 Pa and with a cooling stage controlling the RH inside the chamber (Matsumura and Hayashi, 2007).The (NH 4 ) 2 SO 4 droplets were observed to experience hygroscopic growth when RH was increased from 80% to 98% (Matsumura and Hayashi, 2007).The GFs of the (NH 4 ) 2 SO 4 droplets were obtained by calculating the diameters, which were consistent with the theoretically estimated values, indicating the validity of the ESEM technique.The hygroscopic behavior of individual aerosol particles in Ni refineries was investigated by ESEM.Thin surface coatings of sulfates on insoluble Ni compounds were observed for some particles, which might have some implications for health assessments (Inerle-Hof et al., 2007).Individual agriculture aerosol particles were characterized for their morphological, hygroscopic, and chemical properties using ESEM (Hiranuma et al., 2008).Most of the particles exhibited low water uptake when exposed to up to 96% RH and a small fraction of particles in the coarse mode showed DRH = ~75-80% and reached twice their original sizes at RH = 96% due to the presence of K-salts (Hiranuma et al., 2008).ESEM was utilized to examine the water uptake by individual pollen particles, a kind of primary biological aerosols (PBAs), as a function of RH, and it was observed that the surface of pollen is wettable at high sub-saturated humidity, suggesting that the pollen grains can readily act as cloud condensation nuclei even though they are only slightly hygroscopic (Griffiths et al., 2012;Pope, 2010).

3. 2 ETEM
TEM can analyze nano-sized particles and observe their internal mixing state due to its excellent spatial resolution down to 1 nm (Tang et al., 2019;Li et al., 2016).ETEM was used for investigating the hygroscopic behavior of ambient particles after being validated for standard salt particles such as NaBr, CsCl, NaCl, (NH 4 ) 2 SO 4 , and KBr (Adachi et al., 2011;Freney et al., 2010Freney et al., , 2009;;Wise et al., 2009Wise et al., , 2007Wise et al., , 2005;;Semeniuk et al., 2007a, b).The hygroscopic behavior of carbonaceous particles (soot, tar balls, and particles internally mixed with S-, K-, Mg-or Na-rich inorganic species) from biomass burning showed that soot and tar balls did not take up water, whereas the mixed organic-inorganic particles took up water at RH = 55-100%, depending on the compositions of their water-soluble phase (Semeniuk et al., 2007b).The deliquescence behavior of atmospheric particles such as sulfate-coated NaCl/silicate aggregates, sulfate-coated seasalt particles, and Mg-rich, chloride-coated sea-salt particles was observed by ETEM (Semeniuk et al., 2007a).The particles started to uptake water at RH = 50-60% and deliquescent spheres appeared RH = 70-76%, indicating that the water uptakes below and above RH = 76% were due to the coating components and NaCl, respectively, when particles underwent a multi-step deliquescence process.The deliquescence and efflorescence of K-containing particles, such as KCl, KNO 3 , and K 2 SO 4 , and their mixtures related to biomass burning emissions were investigated using ETEM (Freney et al., 2009).Some internally mixed aerosol particles with various hygroscopic properties were examined by ETEM to observe solid inclusions present inside aqueous droplets at high RH (e.g.,>65%) as shown in Fig. 8 (Freney et al., 2010).Such core-shell structures have strong ability to scatter light (Adachi et al., 2011;Freney et al., 2010).

4 Scanning Transmission X-ray Microscopy
(STAM) Scanning transmission X-ray microscopy coupled with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) is a promising technique due to its capability for chemical imaging on a spatial resolution of 40 nm and it can determine local chemical environments within single submicron particles and heterogeneous materials (Tang et al., 2019;Ault and Axson, 2017;Li et al., 2016;Kelly et al., 2013;Krieger et al., 2012).In addition, less beam damage is induced compared to SEM and TEM as soft X-rays are probed to the sample, making it uniquely capable of quantitative analysis of light elements (such as C, N, O) (Moffet et al., 2011;de Smit et al., 2008;Maria et al., 2004).Hygroscopic properties of individual submicrometer AS particles were measured using STXM/NEXAFS and the results were reported to be consistent with previous literature results (Zelenay et al., 2011).The STXM/NEXAFS technique was further used to study the morphology of AS/adipic acid mixture particles by examining spectral features at both carbon and oxygen edges as a function of RH and the heterogeneous distributions of the mixtures under both dry and wet conditions were reported (Zelenay et al., 2011).STXM/ NEXAFS was used to observe hygroscopic growth and chemical change of fine malonic acid particles according to RH change (Ghorai et al., 2011).Hygroscopic GFs, which were obtained from water-to-solute ratios (WSRs) based on STXM/NEXAFS and micro-FTIR data, agreed well with previous data reported in the liter- ( ature.The efficient keto-enol tautomerization of malonic acid was observed with the enol form dominating at high RH.A compact gas-phase reactor for performing in situ STXM was assembled for the measurement under water vapor, which provided a more stable condition for the hygroscopic study (Kelly et al., 2013).The setup was calibrated by examining NaBr, NaCl, (NH 4 ) 2 SO 4 , and KCl with different DRH values.A following study using the reactor coupled to STXM was performed to measure the mass-based hygroscopicity of atmospheric particles and characterize their elemental and carbon functional group compositions simultaneously (Piens et al., 2016).The setup was firstly used to measure hygroscopic behavior of laboratory generated NaCl, NaBr, KCl, (NH 4 ) 2 SO 4 , levoglucosan, and fructose particles and the results agreed well with those calculated from AIOMFAC.Field-collected atmospheric particles with unknown compositions were then determined by in-situ STXM/ NEXAFS and SEM/EDX and particles with Na and Cl contents were observed to have high hygroscopicity (Piens et al., 2016).The dependence of shikimic acid ozonolysis on the humidity and thereby viscosity was investigated by in-situ STXM/NEXAFS, showing that the degradation kinetics of shikimic acid depend on the relative humidity, which in turn are influenced by the increased viscosity of the shikimic acid-water mixture (Steimer et al., 2014).STXM/NEXAFAS was also applied to investigate field-collected particles under controlled water vapor environment.For example, the hygroscopic behavior of aerosol particles collected during an anthropogenic polluted period at the Amazonian rain forest which were internally mixed with carbonaceous (such as secondary organic aerosols and soot) and inorganic (such as ammonium sulfate) components was analyzed using STXM/NEXAFAS (Pöhlker et al., 2014).Efflorescence during the humidification was claimed to occur as the impacted ambient organic-inorganic mixture aerosols initially had amorphous or poly-crystalline structures and underwent restructuring through kinetic water and ion mobilization, resulting in the crystallization of inorganic salts during the humidification (Pöhlker et al., 2014).As shown in Fig. 9, cubic structures in Fig. 9B and F suggest that sulfate salts crystallized upon the humidification.Highly absorbing spots in carbon maps represent soot particles, which were incorporated in amorphous inorganic material (i.e., ammonium sulfate) and localized on the surface of inorganic crystals (white arrows in Fig. 9H) at 0% and 80% RH, respec-tively (Pöhlker et al., 2014).

SPECTROSCOPIC STUDIES
Vibrational spectroscopic techniques such as Raman and FTIR spectroscopies can be quite powerful for the hygroscopicity study of individual aerosol particles when an environmental cell, named in-situ system, is installed because the spectroscopic techniques can provide realtime chemical compositional evolution based on functional groups of individual particles during hygroscopic measurements (Wu et al., 2019;Li et al., 2017;Wang et al., 2017b;Yeung et al., 2009;Lee et al., 2008;Liu et al., 2008b;Reid et al., 2007;Li et al., 2006).A built-in or coupled optical microscope can also be used for observing morphological change of individual particles during hygroscopic measurements (Li et al., 2017;Gupta et al., 2015a).Both spectral and optical image information can be used to investigate the hygroscopic behavior of individual particles including the determination of DRH and ERH values.In addition, the vibrational spectra obtained during hygroscopic measurements can provide detailed information on chemical functional groups, molecular interactions, and particle phase to allow better understanding of the hygroscopic behavior of complex aerosol particles (Li et al., 2017;Wang et al., 2017b;Lee et al., 2008).

1 In-situ Raman Microspectrometry (RMS)
Individual single salt, mixed, or ambient aerosol particles collected on a variety of substrates have been examined by in-situ Raman micro-spectrometry (RMS) (Baustian et al., 2012;Baustian et al., 2010;Ciobanu et al., 2009;Treuel et al., 2009;Yeung et al., 2009;Liu et al., 2008b;Li et al., 2006).Hygroscopic properties of individual MgSO 4 and NaNO 3 droplets deposited on a quartz substrate were investigated based on Raman spectral evolution in respect to RH change (Wang et al., 2005;Li et al., 2006).The hygroscopic behavior of individual Ca(NO 3 ) 2 and internally mixed Ca(NO 3 ) 2 / CaCO 3 particles deposited on Teflon film was investigated using RMS (Liu et al., 2008b).The RH dependence of the water-to-solute ratio (WSR) was quantified for Ca(NO 3 ) 2 and the phase transitions were successfully determined by Raman spectral analysis related to chemical structure evolution.The different hygroscopic behavior and phase transitions of AS-adipic acid particles with various mixing ratios deposited on a hydrophobic substrate were studied using RMS, and the hygroscopic behavior of the AS fraction in the mixed particles was found not to be much influenced by the organic moiety (Yeung et al., 2009).The similar setup was later used to determine the hygroscopicity and phase transitions of AS, AN, malonic acid (MA), glutaric acid (GA), glyoxylic acid (GlyA), as well as two mixed particle systems AS-MA and AS-GA, and the results very well corre-sponded to the previous literature and model values (Yeung et al., 2010;Yeung and Chan, 2010).In-situ RMS was used to observe the water uptake of the mixture aerosols composed of various inorganic and organic species, such as Ca(NO 3 ) 2 and CaCO 3 mixed with H 2 C 2 O 4 and dicarboxylic acids mixed with NaCl or AS (Wang et al., 2017b;Ma et al., 2013;Ma and He, 2012).The chemical reactions between the organic and inorganic compositions, which drastically change the chemical compositions and hygroscopic behavior of the mixtures, were found to occur based on Raman spectral analysis.
Since Cl -is the most abundant anion in nascent sea spray aerosols (SSAs) and Na + and Mg 2+ are the first and second most abundant cations, respectively, (Seinfeld and Pandis, 2016), NaCl-MgCl 2 binary mixture particles can be the better surrogate for the nascent SSAs than pure NaCl aerosols.Due to the major role of the MgCl 2 moiety in the hygroscopic behavior of the nascent SSAs, the study on the NaCl-MgCl 2 mixture particles may have important implications for nascent SSA heterogeneous chemistry (Ault et al., 2013a;Woods et al., 2010;Wise et al., 2009;Liu et al., 2007).An in situ RMS setup, which was composed of three parts: (A) see-through impactor, (B) Raman microscope/spectrometer, and (C) humidity controlling system, was employed for hygroscopic studies of pure MgCl 2 and NaCl-MgCl 2 mixture aerosol particles (Gupta et al., 2015a).The exp eri mental setup was similar to that used for optical micro scopy, with the Raman spectrometer replacing the optical microscope (Gupta et al., 2015a;Ahn et al., 2010).As for the hygroscopic behavior for pure MgCl 2 particles, characteristic OH-stretching Raman signatures indicated the crystallization of MgCl 2 •4H 2 O at low RH as shown in Fig. 10, suggesting that the kinetic barrier to the more stable MgCl 2 •6H 2 O crystallization was not overcome in the timescale of the dehydration measurements (Gupta et al., 2015a).The kinetic barrier to MgCl 2 •6H 2 O could be overcome when there was sufficient condensed water for optimally sized crystalline NaCl acting as heterogeneous nucleation seeds.
The hygroscopicity of AS and AN mixture particles was studied theoretically and experimentally (Ling and Chan, 2007;Ge et al., 1998) although their hygroscopic behavior has been under no comprehensive understanding.In-situ RMS measurements on pure AS, AN, and AS-AN mixture aerosol particles with various mixing ratios were performed (Wu et al., 2019).Two types of hygroscopic behavior of pure AN particles are shown in Fig. 11.The AS and AN mixture droplets can crystallize as the mixtures of pure AS crystal and stable and/or meta stable double salts (2AN•AS and 3AN•AS, respectively) and the degree of metastability might differ under different conditions (Ling and Chan, 2007).Hygroscop-ic growth, corresponding optical images, and Raman spectra at specific RHs of a particle of mole fraction of AS (X AS ) = 0.6 is shown in Fig. 12 as an example (Wu et al., 2019).During the humidification process, the size and shape of the particles remained constant until a slight decrease in size was observed at RH = 60-68.5%,which

(A) (B) (C) (D) (E)
is also seen in the optical image of the particle at RH = 66.6%.Three Raman spectra at RH = 66.6% were obtained at different positions on the particle.As shown in the middle green-colored Raman spectrum which is for the upper part of the particle, the NO 3 -peak at RH = 66.6% showed double peaks at 1043 and 1051 cm -1 , indicating the transformation of a metastable 3AN•AS to a stable 2AN•AS double salt (Ling and Chan, 2007).The  center and bottom parts of the particle (the orange-and blue-colored Raman spectra, respectively) seemed to be the mixture of pure AS and double salt crystals as the pure AS crystal peak at 973 cm -1 and the double peaks at 1043 and 1051 cm -1 were observed.It seems that heterogenous chemical distributions occurred during the structural rearrangement at RH = 60-68.5%.The particle showed a first partial deliquescence transition at RH = 68.9%,which is the DRH of the eutonic composition.The particle had a liquid eutonic shell with a solid core structure as shown in the optical image of the particle at RH = 77.3%.Raman spectra of the center and edge parts of the particle showing the SO 4 2-peak at 973 and 977 cm -1 , respectively, indicating the solid core is a pure AS crystal.Upon the further increase in RH, the particle absorbed water and grew in size until a second deliquescence transition occurred at RH = 77.8%,i.e., a second DRH, where the AS solid core dissolved completely, after which the particle grew continuously with the further increase in RH.The AS-AN mixture particles with different mixing ratios were observed to crystallize into different forms, leading to different MDRHs and DRHs with bigger gap than those from theoretical models, which can promote their capability of probable heterogeneous chem istry on aqueous aerosol surface (Wu et al., 2019).
Nascent SSAs were reported to undergo the chloride depletion mainly due to the reactions of NaCl/MgCl 2 with inorganic NO x /HNO 3 and SO x species (Ault et al., 2013a;Beardsley et al., 2013;Prather et al., 2013;Saul et al., 2006;ten Brink, 1998).Ambient SSAs were also claimed to react with water-soluble organic acids, which was driven by the liberation of gaseous HCl (Laskin et al., 2012).In situ RMS analysis for the hygroscopic behavior of NaCl-malonic acid (MA) mixture aerosols with different mixing ratios clearly showed that the chemical reaction between NaCl and MA occurred rapidly and considerably, producing mono-sodium malonate.The chemical reaction was facilitated when MA is more enriched in the aerosols because the controlling factor for the reactivity of the aerosols is the availability of H + ions dissociated from MA (Li et al., 2017).

2 In-situ FTIR Spectroscopy
Similar to Raman analysis, FTIR spectroscopy can be used to monitor chemical compositional changes of particles when exposed to water vapor as FTIR and Raman spectroscopies are sensitive to functional groups such as OH band (Tang et al., 2019).The hygroscopicity of inor-ganic particles such as (NH 4 ) 2 SO 4 , NH 4 HSO 4 , NaCl, MgCl 2 and NH 4 NO 3 , and (NH 4 ) 2 SO 4 mixed with organics, were investigated by transmission FTIR spectroscopy (Zawadowicz et al., 2015;Braban et al., 2001;Cziczo and Abbatt, 2000;Cziczo et al., 1997).The transmission FTIR analysis on heterogeneous reactions of nitric acid with oxide particles of crustal elements were reported to be significantly enhanced in the presence of water vapor (Goodman et al., 2001).Attenuated total reflection FTIR (ATR-FTIR) and diffusion reflectance infrared Fourier transform spectroscopy (DRIFTS) can also be used for hygroscopicity study.The hygroscopicity of MgSO 4 particles were studied by ATR-FTIR, in which the evolution of the SO 4 2-and the water O-H peaks was directly related to their hygroscopic behavior, which agreed well with the previous reports (Zhao et al., 2006).ATR-FTIR was also used to monitor the water uptake and phase transitions for atmospherically relevant particles such as NaCl, NaNO 3 , NH 4 NO 3 , (NH 4 ) 2 SO 4 , Ca(NO 3 ) 2 , SiO 2 , levoglucosan, oxalic acid, malonic acid, succinic acid, phthalic acid, Na-carboxylate, and fly ash particles as a function of RH (Gao et al., 2018;Navea et al., 2017;Jing et al., 2016;Zhang et al., 2014;Schuttlefield et al., 2007).The spectral features including absorption band frequencies, full-width half-maximum (FWHM) of the vibrational modes, and integrated area of water stretching and bending regions can distinguish various processes such as water adsorption onto the surface, water absorption to become hydrate forms, and deliquescence to become droplets when particles uptake water vapor (Schuttlefield et al., 2007).ATR-FTIR was used to investigate the supersaturation and crystallization process of NaNO 3 -Na 2 SO 4 mixture droplets deposited on ZnSe with mixing ratios of 1 : 1, 3 : 1, and 10 : 1, in which the crystals and supersaturated droplets formed during dehydration when the substrate effects were present and absent, respectively (Tong et al., 2010).Temperature-dependent hygroscopic behavior of sodium methanesulfonate (CH 3 SO 3 Na) salt was examined using ATR-FTIR, suggesting that sea-salt particles would have quite different DRHs and ERHs depending on temperature once they reacted with methanesulfonic acid, which would have important atmospheric implications (Zeng et al., 2014).IR spectra of MSA-Na particles undergoing hygroscopic growth at 296 K is shown in Fig. 13 as an example (Zeng et al., 2014).As RH increases, an absorbed water IR peak is observed in the region of 3660 to 2750 cm -1 and gradually becomes significant at the higher RH region.A substantial amount of water is absorbed by the solid MSA-Na particles when RH approaches 70%, after which the water band increases rapidly with increasing RH.During the dehydration process, the deliquesced MSA-Na droplets gradually lose the water with the water band decreasing accordingly until a rapid transition occurs at RH = ~52%, where particles undergo efflorescence and become crystal form again.An FTIR spectrometer coupled with optical microscope (micro-FTIR) was used to investigate the hygroscopic behavior of atmospheric relevant particles (Liu and Laskin, 2009;Liu et al., 2008a;Wu et al., 2007), which was claimed to agree well with that reported in previous studies.Water-to-solute ratios (WSRs) in particles as a function of RH, which were obtained from the absorbance ratio between water IR peak at 3400 cm -1 and the solute peak, were used to derive the hygroscopic curves.Chemical compositions and hygroscopic properties of laboratory generated NaCl particles mixed with malonic acid (MA) and glutaric acid (GA) at different molar ratios were studied using the micro-FTIR, and the formation of sodium malonate and sodium glutarate salts resulting from HCl evaporation were observed (Ghorai et al., 2014).In-situ DRIFTS was demonstrated to be a reliable technique for studying the water adsorption on NaCl and mineral particles (Ibrahim et al., 2018;Joshi et al., 2017;Ma et al., 2010).The hygroscopic behavior for MgO, α-Al 2 O 3 , and CaCO 3 particles mixed with acetic acid was analyzed with this technique (Ma et al., 2012).A later study investigated heterogeneous reactions of acetic acid with γ-Al 2 O 3 , SiO 2 , and CaO in a range of RHs using transmission FTIR.The reaction rate of acetic acid with γ-Al 2 O 3 increased significantly with increasing RH, while acetic acid and water were found to compete for surface adsorption sites on SiO 2 particles (Tang et al., 2016b).

3 Levitation Studies of Single Particles
When individual particles are levitated for the study of their hygroscopic behavior, they are free of probable heterogeneous nucleation due to no contact with solid surfaces, which makes it an effective way to explore highly supersaturated solutions and solution thermodynamics (Davis, 1997).The levitation techniques are generally categorized into electrodynamic balance, optical levitation, and acoustic levitation.

1 Electrodynamic Balance (EDB)
Electrodynamic balance (EDB) was described in detail in previous studies (Tong et al., 2015;Krieger et al., 2012;Lee et al., 2008;Reid and Sayer, 2003;Davis, 1997).Briefly, a particle sized in 1-100 μm can be levitated inside an EDB chamber with properly adjusted AC and DC electric fields (Tang et al., 2019).Fig. 14(A) shows a design of EDB with a pair of DC endcap electrodes and an AC ring electrode.A particle will be charged by induction before entering the EDB chamber, after which the electrostatic force imposed on the particle by the DC voltage balances the weight of the particle, leading to the confinement of the particle at the center of the EDB (Zhang and Chan, 2000).The mass of a particle is proportional to the DC balancing voltage (Pope, 2010).The EDB technique has been widely used for hygroscopicity studies for atmospheric relevant particles, in which mass or size changes of the levitated particles as a function of RH were used to derive the humidification and dehydration curves (Griffiths et al., 2012;Haddrell et al., 2012; (A) (B) Hargreaves et al., 2010;Pope, 2010;Pope et al., 2010;Peng and Chan, 2001;Peng et al., 2001;Tang and Fung, 1997;Tang and Munkelwitz, 1994;Cohen et al., 1987a, b, c).As shown in Fig. 14(B), EDB can be combined with spectroscopic techniques such as RMS to obtain chemical evolution information according to RH change in hygroscopic studies (Zhang and Chan, 2002).Molecular structures of droplets including equal molar Na 2 SO 4 / MgSO 4 mixture, Na 2 SO 4 , (NH 4 ) 2 SO 4 , MgSO 4 , ZnSO 4 , and CdSO 4 single salts were examined using molar waterto-solute ratios (WSRs) and Raman spectra obtained at different RHs in an EDB (Zhang andChan, 2002, 2000).
The formation of contact ion pairs at high concentrations was reported based on the spectral characteristics.The combination of EDB and RMS was also used to inv e s tigate the phase transformation in AN/AS mixed particles and the hyg ro scopicity of organic and inorganic mixtures particles (Lee et al., 2008;Ling andChan, 2008, 2007).Changes in Raman peak positions and full width at half height (FWHH) were used to examine the molecular interactions and phase transformation (Lee et al., 2008).AN/AS mixtures were reported to form the metastable 3AN•AS double salt when crystallization occurred, which gradually transformed into the stable 2AN•AS  double salt depending on RH (Ling and Chan, 2007).AS/MA (malonic acid) mixture particles remained partially crystallized at RH down to 16%, whereas AS/GA (glutaric acid) and AS/SA (succinic acid) mixture particles completely became dried at RH = ~30-36% and below, respectively (Ling and Chan, 2008).Partial deliquescen ce was observed at RHs of <10% to 79%, 70% to 80%, and 80% to >90% for the AS/MA, AS/ GA, and AS/SA particles, respectively (Ling and Chan, 2008).Fig. 15 shows the Raman spectra evolution of the AS/MA particle during dehydration and humidification processes.At RH = 16%, a small portion of MA crystallized onto the AS seed as shown in Fig. 15(B) (Ling and Chan, 2008).EDB was coupled with fluorescence spectroscopy for hygroscopic measurements, where the solvated-to-free water ratios in the droplets were determined (Choi and Chan, 2005;Choi et al., 2004).The solvated and free water were distributed heterogeneously in the droplets and the crystallization or supersaturation of the droplets were found to occur when their amounts become equal (Choi et al., 2004).The distributions of solvated and free water in NaCl, Na 2 SO 4 , (NH 4 ) 2 SO 4 , MgSO 4 , Mg(NO 3 ) 2 , and a mixture of NaCl and Na 2 SO 4 droplets were studied using EDB coupled with fluorescence spectroscopy system (Choi and Chan, 2005;Choi et al., 2004).

2 Optical Levitation
The optical trapping system for particles was described well in previous studies (Tang et al., 2019;Gong et al., 2018;Krieger et al., 2012;Wills et al., 2009;Mitchem and Reid, 2008;Ashkin, 2000).Briefly, the interaction between the incident laser beam and a particle induces a scattering and a gradient force, which results in optical levitation and optical tweezer techniques, respectively, for trapping particles optically (Tang et al., 2019;Krieger et al., 2012).The optical levitation was achieved by overcoming the gravitational force of particles (20-100 μm) with backwards light scattering using a vertically propagating laser beam, while the optical tweezer was realized by applying a strong gradient restoring force many orders of magnitude larger than the scattering and gravitational forces of particles (1-10 μm), leading to 3-dimensional stabilization in position with a single laser beam (Krieger et al., 2012).Fig. 16 shows a schematic diagram of an optical tweezer, where a trapping beam passes through two sets of beam expansion optics and it is split into two separate beams, after which the beams are focused into the sample cell through a coverslip by an oil immersion objective, allowing the two droplets to be trapped (Mitchem et al., 2006c).Hygroscopicity of opticallytrapped particles can be investigated when imaging and/ or spectroscopic systems are combined (Tang et al., 2019;Krieger et al., 2012;Mitchem and Reid, 2008).Optical trapping techniques were used to examine the deliquescence and efflorescence of atmospherically relevant particles in the presence and absence of a surface-contacted seed solid, in which the contact efflorescence of the particles was reported (Davis et al., 2015a, b).For optically levitated multicomponent microdroplets incl uding glycerin, water, and ammonium sulfate, evaporation and phase transition from the liquid to the solid state of ammonium sulfate were observed by morphology-dep end ent resonances observed in Raman spectra (Trunk et al., 1997).The hygroscopic behavior of NaCl droplet trapped in an optical tweezer was investigated by Raman spectroscopy through the variations of OH stretching band subjected to RH change (Treuel et al., 2010;Mitchem et al., 2006a).Stimulated Raman scattering can be used to determine the size of the trapped droplet with nanometer precision and with a time resolution of 1 s (Mitchem et al., 2006a).The capability of the combination of optical tweezers and Raman spectroscopy to study the coagulation of decane and water droplets was reported (Mitchem et al., 2006b).By the combined use of Raman spectroscopy and optical tweezers, mixed decane/NaCl aqueous droplets were probed for investigating their internal structures, where the decane formed a layer on the surface of the core region in aqueous droplets (Buajarern et al., 2007).Hygroscopic behavior of oleic acid and NaCl mixture particles before and after ozonolysis was examined using the optical tweezer and the phase and morphology were investigated as a function of RH using a cavity enhanced Raman spectroscopy (CERS) (Dennis-Smither et al., 2012).The hygroscopic behavior of NaCl was claimed not to be influ en ced by the organic moiety.The efflorescence promptly occurred, whereas the deliquescence was a gradual process lasting around 20 s, during which the dissolution of the inorganic component and the adoption of an equilibrium morpho logy occurred as shown in Fig. 17 (Dennis-Smither et al., 2012).The water adsorption onto optically trapped mineral oxides under controlled RH was observed by Raman spectroscopy (Rkiouak et al., 2014).The optically trap ped SiO 2 particles were monitored by Raman spectroscopy for chemical composition changes when exposed to N 2 O 5 flow at different RHs and the heterogeneous reaction of N 2 O 5 on SiO 2 surface were found to be enhanced by increasing RH (Tang et al., 2014).

3 Acoustic Levitation
Particles, larger than 20 μm, can be trapped in the acoustic levitator due to the standing sound wave generated and reflected by properly positioned piezoelectric transducer and concave reflector, respectively, as shown in Fig. 18 (Brotton and Kaiser, 2013).Acoustic levitation technique coupled with optical microscope or spectroscopy has been employed to monitor the hygroscopic behavior (Seng et al., 2018;Brotton and Kaiser, 2013;Schenk et al., 2012).Imidazolium-based ionic liquids with two types of anions such as fluorinated (BF 4 -and PF 6 -) and alkylsulfate anions were investigated using acoustically levitated droplets and Raman spectral analysis indicated that the ionic droplets with sulfate anions took up much more water than those with fluorinated   anions (Schenk et al., 2012).The hygroscopic properties of single, ultraviolet-irradiated NaNO 3 droplets in an acoustic levitator coupled with RMS were examined and the irradiation significantly affected their hygroscopic behavior of NaNO 3 particles (Seng et al., 2018).As shown in Fig. 19, the photochemical aging processes modified the hygroscopic behavior of the NaNO 3 particles (Seng et al., 2018).The production of NO 2 -decreased the DRH values.For long irradiation times (>5 h), these values were even more affected, which is due to the additional production of peroxynitrite and car bon ate ions in individual droplets as distinguished by Raman spectra shown in Fig. 20 (Seng et al., 2018).The NaNO 3 -NaNO 2 deliquescence phase diagram cannot explain the hygroscopic behavior of long-term irradiated particles and the influence of CO 2 on the photo-transformation process in NaNO 3 droplets was claimed (Seng et al., 2018).

SUMMARY
The ability of aerosol particles to uptake water in the air depends on their hygroscopicity, which is one of the most important physicochemical properties (Tang et al., 2019;Freedman, 2017).Therefore, studies on the hygroscopicity of aerosol particles are important to comprehensively understand and predict their behavior when interacting with water vapor and further their impacts on the heterogeneous chemical reactions, atmospheric environment, and human health.The inhalable aerosol particles can typically transport and deposit in the respiratory tract of human bodies, during which the hygroscopicity of particles can influence the deposition fraction due to either growth or shrinkage of the particles as they experience the RH change when passing through the warm humid respiratory tract (Xi et al., 2013).Atmospheric particles are intrinsically internal or external mixtures with inorganic and organic species and their chemical mixing state is of vital importance to understand their complex physicochemical properties, which motivates the necessity to illustrate the aerosol particles individually (Ault and Axson, 2017).In addition, microscopic and spectroscopic techniques enable the studies of physical properties and chemical compositions within homogeneous and heterogeneous individual particles.The results and data presented in this review show that the  hygroscopic measurements of atmospherically relevant aerosol particles are essential and can be achieved by a range of microscopic and spectroscopic techniques on a single particle level.
Visual observation on changes in particle size at different RHs by microscopic techniques can be used to determine hygroscopic growth factors and phase transitions.OM provides important information on the intuitive 2-D morphological evolution of pure and mixed aerosol particles on an image field as a function of RH typically for the particles with size of larger than 1 μm.AFM expands the analysis of the hygroscopicity of particles to a 3-D imaging including area and height information and even to a submicron, ultrafine level with its high spatial resolution although only one particle can be analyzed at a time.Both OM and AFM techniques are limited because they cannot provide chemical information, so that the off-line measurements by other techniques are generally used to investigate the chemical species under certain RH conditions.ESEM and ETEM equipped with EDX are powerful analytical tools as they can provide information on hygroscopic behavior, elemental compositions, mixing states, and morphology of individual particles.However, the high energy electrons used to probe the particles tend to destroy beam sensitive particles such as organics and ammonium nitrate and sulfate.STXM/NEXAFS technique is promising as it can provide chemical imaging on a spatial resolution of 40 nm and determine local chemical environments within single submicron particles, and its elemental maps allow the heterogeneous distribution of elements to be illustrated during hygroscopic measurement.In addition, the soft X-rays used in the STXM/ NEXAFS measurements assure the minimum damage of the particles having light elements.
Vibrational spectroscopic techniques such as Raman and FTIR spectroscopies have been widely used because their spectra of organic and inorganic compounds are quite specific, depending on their chemical functional groups, phase states, crystallinity, and neighboring environments, which guarantee a better understanding of the hygroscopic behavior of complex aerosol particles during the hygroscopic measurement.The advantage is that the chemical functional groups, water contents, molecular interactions, and physicochemical mixing state of the aerosol particles can be directly associated with the morphological changes during the hygroscopic processes at ambient pressure once the spectroscopic methods are coupled with the RH-controlled environmental cell and optical microscopic techniques, particularly for the analysis of single particles.Raman and FTIR spectroscopies are complementary to each other as it is necessary for the molecule to have a dipole moment and a polarizability changes during the vibrations for IR and Raman signals, respectively.The spectroscopic methods can distinguish species that are challenging for the X-ray analysis such as NO 3 -.The florescence emitted from some particles after being probed by a Raman laser often makes the Raman spectral analysis challenging, and many efforts have been made to minimize the influences (Ault and Axson, 2017).The size of the particles analyzed by Raman spectroscopy is generally large (>5 μm), while the recently developed SERS (surface enhanced Raman spectroscopy) and TERS (tip-enhanced Raman spectroscopy) have extended the analysis to particles in the more atmospherically relevant size fraction (around 1 μm) (Tirella et al., 2018;Fu et al., 2017;Gen and Chan, 2017;Ofner et al., 2016;Craig et al., 2015).
Most of the techniques mentioned above are based on the investigation of substrate-deposited particles, which may have some substrate influences such as a facilitated heterogeneous nucleation, which can be eliminated in the levitation systems due to the substrate-free and contactless situation, even though the individual trapping particles are generally supermicron ones and only one par ticle can be measured in each experiment.One stren gth of the levitation systems is their ability to measure highly supersaturated droplets.The evolution of size, mass, chemical functional groups, and phase states during hygroscopic measurements of the levitated particles can be captured when the levitation systems are coupled with imaging and spectroscopic tools.
The future directions and expansions of the techniques for better elucidating hygroscopicity of aerosol particles have been well summarized and discussed in previous reviews (Tang et al., 2019;Ault and Axson, 2017), and the readers are directed to those literatures.Briefly, the availability of on-line, field-portable methods, the accurate employments of RH, temperature, and pressure range, and the commerciality of the techniques would help better predict the aerosol effects on climate change and human health.

Fig. 11 .
Fig. 11.Hygroscopic curve, corresponding optical images, and Raman spectra at specific RHs of two types of AN particle.The recorded transition RHs in both dehydration and humidification processes are marked with arrows in the hygroscopic curve.Reprinted with permission from Wu, L., Li, X., Ro, C.-U., Asian J. Atmos.Environ, 13, 3, 196-211, 2019.Copyright 2019 by Asian Journal of Atmospheric Environment.The Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/).