Efflorescence Relative Humidity Research Articles

Optical and Physicochemical Properties of Brown Carbon Aerosol: Light Scattering, FTIR Extinction Spectroscopy, and Hygroscopic Growth.

A great deal of attention has been paid to brown carbon aerosol in the troposphere because it can both scatter and absorb solar radiation, thus affecting the Earth's climate. However, knowledge of the optical and chemical properties of brown carbon aerosol is still limited. In this study, we have investigated different aspects of the optical properties of brown carbon aerosol that have not been previously explored. These properties include extinction spectroscopy in the mid-infrared region and light scattering at two different visible wavelengths, 532 and 402 nm. A proxy for atmospheric brown carbon aerosol was formed from the aqueous reaction of ammonium sulfate with methylglyoxal. The different optical properties were measured as a function of reaction time for a period of up to 19 days. UV/vis absorption experiments of bulk solutions showed that the optical absorption of aqueous brown carbon solution significantly increases as a function of reaction time in the spectral range from 200 to 700 nm. The analysis of the light scattering data, however, showed no significant differences between ammonium sulfate and brown carbon aerosol particles in the measured scattering phase functions, linear polarization profiles, or the derived real parts of the refractive indices at either 532 or 402 nm, even for the longest reaction times with greatest visible extinction. The light scattering experiments are relatively insensitive to the imaginary part of the refractive index, and it was only possible to place an upper limit of k ≤ 0.01 on the imaginary index values. These results suggest that after the reaction with methylglyoxal the single scattering albedo of ammonium sulfate aerosol is significantly reduced but that the light scattering properties including the scattering asymmetry parameter, which is a measure of the relative amount of forward-to-backward scattering, remain essentially unchanged from that of unprocessed ammonium sulfate. The optical extinction properties in the mid-IR range (800 to 7000 cm(-1)) also showed no significant changes in either the real or the imaginary parts of the refractive indices for brown carbon aerosol particles when compared to ammonium sulfate. Therefore, changes in the optical properties of ammonium sulfate in the mid-IR spectral range due to reaction with methylglyoxal appear to be insignificant. In addition to these measurements, we have characterized additional physicochemical properties of the brown carbon aerosol particles including hygroscopic growth using a tandem-differential mobility analyzer. Compared to ammonium sulfate, brown carbon aerosol particles are found to have lower deliquescence relative humidity (DRH), efflorescence relative humidity (ERH), and hygroscopic growth at the same relative humidities. Overall, our study provides new details of the optical and physicochemical properties of a class of secondary organic aerosol which may have important implications for atmospheric chemistry and climate.

Contact efflorescence as a pathway for crystallization of atmospherically relevant particles

Inadequate knowledge of the phase state of atmospheric particles represents a source of uncertainty in global climate and air quality models. Hygroscopic aqueous inorganic particles are often assumed to remain liquid throughout their atmospheric lifetime or only (re)crystallize at low relative humidity (RH) due to the kinetic limitations of efflorescence (salt crystal nucleation and growth from an aqueous solution). Here we present experimental observations of a previously unexplored heterogeneous nucleation pathway that we have termed "contact efflorescence," which describes efflorescence initiated by an externally located solid particle coming into contact with the surface of a metastable aqueous microdroplet. This study demonstrates that upon a single collision, contact efflorescence is a pathway for crystallization of atmospherically relevant aqueous particles at high ambient RH (≤80%). Soluble inorganic crystalline particles were used as contact nuclei to induce efflorescence of aqueous ammonium sulfate [(NH4)2SO4], sodium chloride (NaCl), and ammonium nitrate (NH4NO3), with efflorescence being observed in several cases close to their deliquescence RH values (80%, 75%, and 62%, respectively). To our knowledge, these observations represent the highest reported efflorescence RH values for microdroplets of these salts. These results are particularly important for considering the phase state of NH4NO3, where the contact efflorescence RH (∼20-60%) is in stark contrast to the observation that NH4NO3 microdroplets do not homogeneously effloresce, even when exposed to extremely arid conditions (<1% RH). Considering the occurrence of particle collisions in the atmosphere (i.e., coagulation), these observations of contact efflorescence challenge many assumptions made about the phase state of inorganic aerosol.

Hygroscopic behavior of NaCl–MgCl&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; mixture particles as nascent sea-spray aerosol surrogates and observation of efflorescence during humidification

Abstract. As Na+, Mg2+, and Cl− are major ionic constituents of seawater, NaCl–MgCl2 mixture particles might represent sea-spray aerosols (SSAs) better than pure NaCl. However, there have been very few hygroscopic studies of pure MgCl2 and NaCl–MgCl2 mixture aerosol particles despite the MgCl2 moiety playing a major role in the hygroscopic behavior of nascent SSAs. Laboratory-generated pure MgCl2 and NaCl–MgCl2 mixture aerosol particles with 12 mixing ratios (0.01 ≤ mole fraction of NaCl (XNaCl) ≤ 0.9) were examined systematically by optical microscopy (OM), in situ Raman micro-spectrometry (RMS), and scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDX) elemental X-ray mapping to observe their hygroscopic behavior, derive the experimental phase diagrams, and obtain the chemical micro-structures. Dry-deposited MgCl2 ⋅ 6H2O particles exhibited a deliquescence relative humidity (DRH) of ~ 33.0 % and an efflorescence RH (ERH) of 10.8–9.1 %, whereas the nebulized pure MgCl2 and MgCl2-dominant particles of XNaCl = 0.026 (eutonic) and 0.01 showed single-stage transitions at DRH of ~ 15.9 % and ERH of 10.1–3.2 %. The characteristic OH-stretching Raman signatures indicated the crystallization of MgCl2 ⋅ 4H2O at low relative humidities (RHs), suggesting that the kinetic barrier to MgCl2 ⋅ 6H2O crystallization is not overcome in the timescale of the dehydration measurements. The NaCl–MgCl2 mixture particles of 0.05 ≤ XNaCl ≤ 0.9 generally showed two-stage deliquescence: first at the mutual DRH (MDRH) of ~ 15.9 %; and second with the complete dissolution of NaCl at the second DRHs depending on the mixing ratios, resulting in a phase diagram composed of three distinct phases. During dehydration, most particles of 0.05 ≤ XNaCl ≤ 0.9 exhibited two-stage efflorescence: first, by the homogeneous nucleation of NaCl; and second, at mutual ERH (MERH) of ~ 10.4–2.9 %, by the crystallization of the MgCl2 ⋅ 4H2O moiety, also resulting in three distinct phases. Interestingly, particles of XNaCl = 0.1 and 0.2 frequently showed three different types of mutual deliquescence behaviors. The first type exhibited an MDRH at ~ 15.9 %. The second exhibited the first MDRH at ~ 15.9 %, efflorescence to MgCl2 ⋅ 6H2O (confirmed by in situ RMS) at RH of ~ 16.1–25.0 %, and a second MDRH at ~ 33.0 %. The third showed an MDRH at ~ 33.0 %. Some particles of XNaCl = 0.1 and 0.2 also exhibited higher MERHs = 15.2–11.9 % and 23.7–15.3 %, respectively, forming MgCl2 ⋅ 6H2O. These observations suggest that the presence of sufficient condensed water and optimally sized crystalline NaCl (XNaCl = 0.1 and 0.2) acting as heterogeneous nucleation seeds helps overcome the kinetic barrier, leading to the structural growth and crystallization of MgCl2 ⋅ 6H2O. SEM/EDX elemental X-ray mapping showed that the effloresced NaCl-rich particles contain homogeneously crystallized NaCl in the center, surrounded by MgCl2 ⋅ 4H2O. The observation of an aqueous phase over a wider RH range for NaCl–MgCl2 mixture particles indicates their more probable heterogeneous chemistry compared to pure NaCl particles as a nascent SSA surrogate.

Ozone and secondary organic aerosol formation from Ethylene-NO x -NaCl irradiations under different relative humidity conditions

The formation of ozone and secondary organic aerosol (SOA) from ethylene-NO x -NaCl(aerosol) irradiations was studied under various relative humidity (RH) conditions in an indoor smog chamber. In the absence of NaCl seed aerosols, SOA was hardly formed and peak O3 concentrations decreased linearly with increasing RH in ethylene irradiations. For the irradiations with NaCl seed aerosols, when RH <48 % (efflorescence relative humidity of NaCl), NaCl existed as solid phase and had little effect on peak O3 concentrations. The infrared spectra from sampled particles showed that SOA was rarely formed on solid NaCl particles. However, when NaCl was in the aqueous phase as RH ≥ 48 %, the peak O3 concentration was sharply reduced by over 20 % as compared to experiments without NaCl aerosol, and the absorption of NaNO3 in aerosols was coincidently increased with RH. Model results indicated that the heterogeneous reaction of N2O5 with aqueous NaCl aerosols was the main cause for the sharp decrease of O3. Besides, the absorptions from C-H, C = O, ONO2 and COO groups all greatly increased with RH. Our results show that SOA from ethylene-NOx irradiations was mainly formed through aqueous reactions. The yields of SOA from ethylene were measured to be 1.5 and 2.3 % at RH of 65 and 84 %, respectively.

Determination of interfacial parameters of a soluble particle in a nonideal solution from measured deliquescence and efflorescence humidities

Abstract. In order to study the growth/shrinking of a hygroscopic nanoparticle during hydration/dehydration in an atmosphere of water vapour, we have employed a thermodynamic approach proposed by Shchekin et al. (2008). This approach uses the mechanic and thermodynamic concept of disjoining pressure of thin films and allows, among others, the prediction of the humidity growth factor of both (i) a homogeneous solution droplet with completely dissolved residual core and (ii) a heterogeneous solution droplet with partially dissolved residual core as a function of the ambient relative humidity. For application to a nanometric sodium chloride particle we have extended the original approach by (i) considering the nonideality of the solution through the dependence of molecular volumes of the solvent and solute molecules and the solute and solvent activities on the solution concentration, (ii) deriving an equation for the estimation of the efflorescence properties of a homogeneous solution droplet, and (iii) combining the empirical power law fittings for the size dependence of the deliquescence and efflorescence relative humidity values by Biskos et al. (2006a). It was demonstrated how the solution/solute interface energy and the correlation length of a thin solution film can be determined from a combination of experimentally determinable efflorescence and deliquescence humidities with the present calculus. The solution/solute interface energy was found to be in close agreement with some previous values reported in the literature, while it strongly differs from data of some other sources. The calculated deliquescence humidity shows a low sensitivity to the choice of the numerical value for the film correlation length. The estimated film correlation length of 1 nm for a nanometric sodium chloride particle with dry particle radius of 5 nm was found to be reconcilable with available a priori estimates of the correlation length from the literature when the measurement uncertainty of the deliquescence humidity is considered. Considering the combination of an extensive calculus, a comprehensive set of thermophysical constraints, and independent measurements of the deliquescence and efflorescence humidities as functions of dry particle radius, the obtained values of the solution/solute interface energy and the correlation length are in close agreement with previous estimations. The humidification of sodium chloride particles in the initial hydration stages was found to be very sensitive to the specification of the disjoining pressure. The enhancement of the wettability of the particle surface leads to an earlier onset of hygroscopic growth.

In Situ Observation on the Dynamic Process of Evaporation and Crystallization of Sodium Nitrate Droplets on a ZnSe Substrate by FTIR-ATR

Sodium nitrate is a main component of aging sea salt aerosol, and its phase behavior has been studied repeatedly with wide ranges observed in the efflorescence relative humidity (RH) in particular. Studies of the efflorescence dynamics of NaNO3 droplets deposited on a ZnSe substrate are reported, using an in situ Fourier transform infrared attenuated total reflection (FTIR-ATR) technique. The time-dependence of the infrared spectra of NaNO3 aerosols accompanying step changes in RH have been measured with high signal-to-noise ratio. From the IR difference spectra recorded, changes of the time-dependent absorption peak area of the O-H stretching band (ν-OH, ∼3400 cm(-1)) and the nitrate out-of-plane bending band (ν2-NO3(-), ∼836 cm(-1)) are obtained. From these measurements, changes in the IR signatures can be attributed to crystalline and solution phase nitrate ions, allowing the volume fraction of the solution droplets that have crystallized to be determined. Then, using these clear signatures of the volume fraction of droplets that have yet to crystallize, the homogeneous and heterogeneous nucleation kinetics can be studied from conventional measurements using a steady decline in RH. The nucleation rate measurements confirm that the rate of crystallization in sodium nitrate droplets is considerably less than in ammonium sulfate droplets at any particular degree of solute supersaturation, explaining the wide range of efflorescence RHs observed for sodium nitrate in previous studies. We demonstrate that studying nucleation kinetics using the FTIR-ATR approach has many advantages over brightfield imaging studies on smaller numbers of larger droplets or measurements made on single levitated particles.

Formation of aqueous solutions on Mars via deliquescence of chloride–perchlorate binary mixtures

Perchlorate salts, known to exist on Mars, can readily absorb water vapor and deliquesce into aqueous solutions even at low temperatures. The multiple soluble ionic species, such as chloride salts, present in the Martian subsurface may affect this deliquescence. Here we study the deliquescence (solid to aqueous transition) and efflorescence (aqueous to solid transition) of three perchlorate/chloride mixtures: KClO4/KCl at 253 K, NaClO4/NaCl at 243 and 253 K, and Mg(ClO4)2/MgCl2 at 243 and 253 K. A Raman microscope with an environmental cell was used to monitor the phase transitions of internally mixed ClO−4/Cl− particles as a function of the perchlorate mole fraction. The eutonic relative humidity (where deliquescence begins to occur regardless of ClO−4 mole fraction), deliquescence relative humidity (DRH, where complete deliquescence occurs), and efflorescence relative humidity (ERH) were measured for several perchlorate mole ratios for each cation system. At the temperatures studied, the eutonic relative humidity was measured to be 28% RH for Mg(ClO4)2/MgCl2 mixtures, 38% RH for NaClO4/NaCl mixtures, and 82% RH for KClO4/KCl mixtures. The DRH depends on the perchlorate mole ratio, but is below the DRH of the least deliquescent (highest DRH) pure salt. When humidity is lowered around an aqueous salt mixture, we find that efflorescence occurs at an RH below the DRH due to the kinetic inhibition of crystallization. The ERH values of the salt solutions were as low as 5% RH for Mg(ClO4)2/MgCl2 mixtures, as low as 13% RH for NaClO4/NaCl mixtures, and as low as 66% RH for KClO4/KCl mixtures. The low eutonic RH values for the Na+ and Mg2+ perchlorate/chloride mixtures are important: wherever Mg(ClO4)2 and MgCl2 or NaClO4 and NaCl coexist at the temperatures studied, mixtures will contain a stable aqueous phase above 28 or 38% RH, respectively, regardless of the perchlorate mole fraction. This liquid water may persist until 5 or 13% RH, respectively.

In-situ FTIR-ATR spectroscopic observation on the dynamic efflorescence/deliquescence processes of Na2SO4 and mixed Na2SO4/glycerol droplets

The FTIR-ATR spectra of Na2SO4 and mixed Na2SO4/glycerol (1:1) aerosols are monitored in the dehumidifying and humidifying process. It shows that glycerol suppresses both heterogeneous and homogeneous nucleation of Na2SO4/glycerol aerosols on the ZnSe substrate, while the water uptake of surrounding glycerol leads to lower deliquescence relative humidity of mixed Na2SO4/glycerol aerosols. In mixed Na2SO4/glycerol aerosols, glycerol restrains the formation of contact ion pairs between Na+ and SO42−, resulting in the delay of the efflorescence relative humidity. And glycerol molecular conformations transform from γγ to αα type in the dehumidifying process.

Impact of Organic Coating on Optical Growth of Ammonium Sulfate Particles

Light extinction by particles in Earth's atmosphere is strongly dependent on particle size, chemical composition, hygroscopic growth properties, and particle mixing state. Here, the influence of an organic coating on particle optical growth was studied. The particle optical growth factor, fRHext, was measured using cavity ring-down aerosol extinction spectroscopy at 532 nm. The particles were composed of ammonium sulfate (AS), 1,2,6-hexanetriol, and mixed particles containing a wet or dry ammonium sulfate core and a 1,2,6-hexanetriol coating. Dry, coated particles were generated by atomization followed by drying. Wet, coated particles were formed via liquid-liquid phase separation (LLPS). LLPS was achieved by deliquescing and then drying the particles to a relative humidity (RH) between the phase separation RH and the efflorescence RH. For the LLPS particles, the fRHext at each RH was between the fRHext of ammonium sulfate and that of 1,2,6-hexanetriol. In contrast, for the mixed dry, coated particles, the fRHext was the same as 1,2,6-hexanetriol particles. At room temperature, the water uptake properties of AS coated with 1,2,6-hexanetriol are largely dictated by the phase of the AS. Thus, the total water uptake depends on the RH history of the particle and the resulting phase of AS.

Effects of NOy Aging on the Dehydration Dynamics of Model Sea Spray Aerosol

The reactions of NO(y) species in the atmosphere with sea spray aerosol replace halogen anions with nitrate. These experiments show the effect of increasing the nitrate content of model sea spray aerosol particles on the morphology changes and the phase transitions driven by changes in relative humidity (RH). The components of the model particles include H2O, Na+, Mg2+, Cl-, NO3-, and SO4(2-). Tandem differential mobility analyzer (TDMA) measurements yield the water content and efflorescence relative humidity (ERH) of these particles, and probe molecule spectroscopic measurements reveal subsequent phase transitions and partially characterize the salt composition on the surface of dry particles. The results show three effects of increasing the nitrate composition: decreasing the EFH (46 to 29%), production of a metastable aqueous layer on the surface of effloresced particles, and decreasing the sulfate content near the surface of dry particles. For the mixtures studied here, the initial crystallization event forms a core of NaCl. For particles that contain a substantial metastable aqueous layer following efflorescence, probe molecule spectroscopy shows a second crystallization at a lower RH. This subsequent phase transition is likely the formation of Na2SO4. Homogeneous nucleation theory (HNT) using a semiempirical formulation predicts the ERH of all mixtures within 2.0% RH, with a mean absolute deviation of 1.0%. The calculations suggest that structures associated with highly concentrated or supersaturated magnesium ions strongly affect the interfacial tension between the NaCl crystal nucleus and the droplet from which it forms.

Morphologies of mixed organic/inorganic/aqueous aerosol droplets

Despite major progress in the understanding of properties of tropospheric aerosol particles, it remains challenging to understand their physical state and morphology. To obtain more detailed knowledge of the phases, phase transitions and morphologies of internally mixed organic/inorganic aerosol particles, we evaluated liquid-liquid phase separation (LLPS), deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of 33 organic/ammonium sulfate (AS)/H2O systems from our own and literature data. The organic fraction consists of single compounds or mixtures with up to ten aliphatic and/or aromatic components with carboxylic acid, hydroxyl, carbonyl, ether, and ester functionalities, covering O : C ratios between 0.29 and 1.33. Thirteen out of these 33 systems did not show LLPS for any of the studied organic-to-inorganic mixing ratios, sixteen underwent LLPS showing core-shell morphology, and four showed both core-shell and partially engulfed configurations depending on the organic-to-inorganic ratio and RH. In all cases the organic fractions of the systems with partially engulfed configurations consisted of dicarboxylic acids. AS in mixed organic/AS/H2O particles deliquesced between 70 and 84% RH. AS effloresced below 58% RH or remained in a one-liquid-phase state. AS in droplets with LLPS always showed efflorescence with ERH between 30 and 50% RH, providing clear evidence that the presence of LLPS facilitates AS efflorescence. Spreading coefficients of the organic-rich phase on the AS-rich phase for systems containing polyethylene glycol 400 (PEG-400) and a mixture of dicarboxylic acids are in agreement with the optically observed morphologies of droplets deposited on the hydrophobic substrate. Analysis of high resolution elastic Mie resonance spectra allowed the detection of LLPS for single levitated droplets consisting of PEG-400/AS/ H2O, whereas LLPS was difficult to detect in (2-methylglutaric acid + 3-methylglutaric acid + 2,2-dimethylsuccinic acid)/AS/H2O. Measured Mie spectra of PEG-400/AS/H2O at 93.5% and at 80.9% RH agreed with computed Mie spectra for a homogeneous and a core-shell configuration, respectively, confirming the results obtained from droplets deposited on a hydrophobic substrate. Based on the presented evidence, we therefore consider the core-shell morphology to be the prevalent configuration of liquid-liquid-phase-separated tropospheric organic/AS/H2O particles.

Deliquescence, efflorescence, and phase miscibility of mixed particles of ammonium sulfate and isoprene-derived secondary organic material

Abstract. The hygroscopic phase transitions of ammonium sulfate mixed with isoprene-derived secondary organic material were investigated in aerosol experiments. The organic material was produced by isoprene photo-oxidation at 40% and 60% relative humidity. The low volatility fraction of the photo-oxidation products condensed onto ammonium sulfate particles. The particle-phase organic material had oxygen-to-carbon ratios of 0.67 to 0.74 (±0.2) for mass concentrations of 20 to 30 μg m−3. The deliquescence, efflorescence, and phase miscibility of the mixed particles were investigated using a dual arm tandem differential mobility analyzer. The isoprene photo-oxidation products induced deviations in behavior relative to pure ammonium sulfate. Compared to an efflorescence relative humidity (ERH) of 30 to 35% for pure ammonium sulfate, efflorescence was eliminated for aqueous particles having organic volume fractions &amp;amp;varepsilon; of 0.6 and greater. Compared to a deliquescence relative humidity (DRH) of 80% for pure ammonium sulfate, the DRH steadily decreased with increasing &amp;amp;varepsilon;, approaching a DRH of 40% for &amp;amp;varepsilon; of 0.9. Parameterizations of the DRH(&amp;amp;varepsilon;) and ERH(&amp;amp;varepsilon;) curves were as follows: DRH(&amp;amp;varepsilon;)= ∑i ci,d &amp;amp;varepsilon;i valid for 0 ≤ &amp;amp;varepsilon; ≤0.86 and ERH(&amp;amp;varepsilon;)= ∑ i ci,e &amp;amp;varepsilon;i valid for 0 ≤ &amp;amp;varepsilon; ≤ 0.55 for the coefficients c0,d= 80.67, c0,e = 28.35, c1,d = −11.45, c1,e = −13.66, c2,d = 0, c2,e = 0, c3,d = 57.99, c3,e = -83.80, c4,d = −106.80, and c4,e = 0. The molecular description that is thermodynamically implied by these strongly sloped DRH(&amp;amp;varepsilon;) and ERH(&amp;amp;varepsilon;) curves is that the organic isoprene photo-oxidation products, the inorganic ammonium sulfate, and water form a miscible liquid phase even at low relative humidity. This phase miscibility is in contrast to the liquid-liquid separation that occurs for some other types of secondary organic material. These differences in liquid-liquid separation are consistent with a prediction recently presented in the literature that the bifurcation between liquid-liquid phase separation versus mixing depends on the oxygen-to-carbon ratio of the organic material. The conclusions are that the influence of secondary organic material on the hygroscopic properties of ammonium sulfate varies with organic composition and that the degree of oxygenation of the organic material, which is a measurable characteristic of complex organic materials, is an important variable influencing the hygroscopic properties of mixed organic-inorganic particles.

Hygroscopic behavior of wet dispersed and dry deposited NaNO3 particles

Previous controversial studies on the hygroscopic behavior of NaNO3 aerosol particles and our frequent observation of crystalline NaNO3-containing ambient particles prompted this hygroscopic study on NaNO3 particles. In this work, the hygroscopic behavior of individual NaNO3 particles of 2.5–4.0 μm in diameter is investigated on a single-particle basis using an optical microscopic technique. Quite different hygroscopic behavior between particles wet dispersed by the nebulization of aqueous NaNO3 solutions and dry deposited powdery particles was observed; i.e., most of wet dispersed particles continuously grew and shrank during humidifying and dehydration processes, respectively, and yet all the dry deposited particles had reproducible deliquescence and efflorescence relative humidities (DRHs and ERHs). The different behavior of the two NaNO3 systems is due to the different nucleation mechanisms. Our hygroscopic study of wet deposited NaNO3 particles indicates that they nucleate via homogeneous nucleation, but the time scale for the nucleation to occur is too long to be atmospherically relevant. And thus no efflorescence of the particles has been observed in the laboratory measurements. However, when chemical species acting as heterogeneous nuclei are present, then efflorescence occurs which can explain the observation of ambient crystalline NaNO3 particles. It is imperative to work with heterogeneous nucleation systems which are more relevant to the real world.

Caking of sodium chloride: Role of ambient relative humidity in dissolution and recrystallization process

Both, producers and users of divided solids regularly face the problem of caking after periods of storage and/or transport. Particle agglomeration depends not only on powder water content, temperature and applied pressure, but also on the interactions between the solid substance and water molecules present in the atmosphere, i.e., on relative humidity (RH) at which the product is stored. Ambient humidity plays an important role in most of events leading to caking: capillary condensation of water at contact points between particles, subsequent dissolution of a solid and formation of saturated solution eventually followed by precipitation of solid during evaporation of water. Here we focus on the kinetics of dissolution followed by evapo-recrystallization of hygroscopic powder sodium chloride under controlled temperature and RH, with the aim of anticipating caking by predicting rates of water uptake and loss under industrial conditions. Precise measurements of water uptake show that the rate of dissolution is proportional to the difference between the imposed RH and deliquescence RH and follows a model based on the kinetic theory of gases. Evaporation seems to be governed by more complex phenomena related to the mechanism of efflorescence and crystal growth from supersaturated salt solution. The proposed methods provide values of Deliquescence Relative Humidity (DRH) and Efflorescence Relative Humidity (ERH) for sodium chloride in good agreement with literature data.

Suppression of NaNO3 Crystal Nucleation by Glycerol: Micro-Raman Observation on the Efflorescence Process of Mixed Glycerol/NaNO3/Water Droplets

Although the hygroscopicity of a NaNO(3)/water microdroplet and a polyalcohol/water microdroplet, two of the most important aerosols in atmosphere, has been widely studied, little is known about the relationship between the hygroscopic behavior of mixed NaNO(3)/polyalcohol/water droplets and their structures on the molecular level. In this study, the hygroscopicity of mixed glycerol/NaNO(3)/water droplets deposited on a hydrophobic substrate was studied by micro-Raman spectroscopy with organic-to-inorganic molar ratios (OIRs) of 0.5, 1, and 2. In the mixed glycerol/NaNO(3)/water droplets, glycerol molecules tended to combine with Na(+) and NO(3)(-) ions by electrostatic interaction and hydrogen bonding, respectively. On the basis of the analyses of the changes of symmetric stretching (v(s)-CH(2)), asymmetric stretching (v(a)-CH(2)), their area ratio (Av(a)-CH(2)/Av(s)-CH(2)) of glycerol, and symmetric stretching band of NO(3)(-) (ν(1)-NO(3)(-)) with relative humidity (RH), it was found that the conformation of glycerol was transformed from αα mainly to γγ and partly to αγ with a decreasing RH in the mixed droplets, contrary to the case in the glycerol/water droplet. In addition, the glycerol with γγ and αγ conformation had strong interaction with Na(+) and NO(3)(-) respectively, which suppressed the formation of contact of ions and delayed the efflorescence relative humidity (ERH) for the mixed droplets compared to the NaNO(3)/water droplet.

Depositional ice nucleation onto crystalline hydrated NaCl particles: a new mechanism for ice formation in the troposphere

Abstract. Sea-salt aerosol (SSA) particles are ubiquitous in the marine boundary layer and over coastal areas. Therefore SSA have ability to directly and indirectly affect the Earth's radiation balance. The influence SSA have on climate is related to their water uptake and ice nucleation characteristics. In this study, optical microscopy coupled with Raman spectroscopy was used to detect the formation of a crystalline NaCl hydrate that could form under atmospheric conditions. NaCl(s) particles (~1 to 10 μm in diameter) deliquesced at 75.7 ± 2.5% RH which agrees well with values previously established in the literature. NaCl(aq) particles effloresced to a mixture of hydrated and non-hydrated particles at temperatures between 236 and 252 K. The aqueous particles effloresced into the non-hydrated form at temperatures warmer than 252 K. At temperatures colder than 236 K all particles effloresced into the hydrated form. The deliquescence relative humidities (DRH) of hydrated NaCl(s) particles ranged from 76.6 to 93.2% RH. Based on the measured DRH and efflorescence relative humidities (ERH), we estimate crystalline NaCl particles could be in the hydrated form 40–80% of the time in the troposphere. Additionally, the ice nucleating abilities of NaCl(s) and hydrated NaCl(s) were determined at temperatures ranging from 221 to 238 K. Here, depositional ice nucleation is defined as the onset of ice nucleation and represents the conditions at which the first particle on the substrate nucleated ice. Thus the values reported here represent the lower limit of depositional ice nucleation. NaCl(s) particles depositionally nucleated ice at an average Sice value of 1.11 ± 0.07. Hydrated NaCl(s) particles depositionally nucleated ice at an average Sice value of 1.02 ± 0.04. When a mixture of hydrated and anhydrous NaCl(s) particles was present in the same sample, ice preferentially nucleated on the hydrated particles 100% of the time. While both types of particles are efficient ice nuclei, hydrated NaCl(s) particles are better ice nuclei than NaCl(s) particles.

Laboratory studies of perchlorate phase transitions: Support for metastable aqueous perchlorate solutions on Mars

Perchlorate salts, recently discovered on Mars, are known to readily absorb water vapor from the atmosphere and deliquesce into the aqueous phase at room temperature. Here we study the deliquescence (crystalline solid to liquid transition) and efflorescence (liquid to crystalline solid transition) of perchlorate salts at low temperatures relevant to Mars. A Raman microscope and environmental cell were used to determine the deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of NaClO 4 and Mg(ClO 4) 2 as a function of temperature and hydration state. We find that the deliquescence of anhydrous NaClO 4 is only slightly dependent on temperature and occurs at ~ 38% RH. The DRH of NaClO 4·H 2O increases with decreasing temperature from 51% at 273 K to 64% at 228 K. The DRH of Mg(ClO 4) 2·6H 2O also increases with decreasing temperature from 42% at 273 K to 64% at 223 K. The efflorescence of both NaClO 4 and Mg(ClO 4) 2 salt solutions occurs at a lower RH than deliquescence due to the kinetic inhibition of crystallization. For all temperatures studied, the ERH values of NaClO 4 and Mg(ClO 4) 2 are 13% and 19%, respectively. These results indicate perchlorate salts can exist as metastable, supersaturated solutions over a wide range of RH and temperature conditions. Summer diurnal temperature and relative humidity cycles at low latitudes on Mars could allow the surface salts to be aqueous for several hours per day.

The Dynamic Shape Factor of Sodium Chloride Nanoparticles as Regulated by Drying Rate

The influence of drying rate on the dynamic shape factor χ of NaCl particles was investigated. The drying rate at the efflorescence relative humidity (ERH) of 45% was controlled in a laminar flow tube and varied from 5.5 ± 0.9 to 101 ± 3 RH s–1 at ERH, where RH represents one percent unit of relative humidity. Dry particles having mobility diameters of 23–84 nm were studied, corresponding to aqueous particles of 37–129 nm at the RH (57%) prior to drying. At each mobility diameter and drying rate, the critical supersaturation of cloud-condensation activation was also measured. The mobility diameter and the critical supersaturation were combined in an analysis to determine the value of χ. The measured values varied from 1.02 to 1.26. For fixed particle diameter the χ value decreased with increasing drying rate. For fixed drying rate, a maximum occurred in χ between 35- and 40-nm dry mobility diameter, with a lower χ for both smaller and larger particles. The results of this study, in conjunction with the in...

Efflorescence of Ammonium Sulfate and Coated Ammonium Sulfate Particles: Evidence for Surface Nucleation

Using optical microscopy, we investigated the efflorescence of ammonium sulfate (AS) in aqueous AS and in aqueous 1:1 and 8:1 (by dry weight) poly(ethylene glycol)-400 (PEG-400)/AS particles deposited on a hydrophobically coated slide. Aqueous PEG-400/AS particles exposed to decreasing relative humidity (RH) exhibit a liquid-liquid phase separation below approximately 90% RH with the PEG-400-rich phase surrounding the aqueous AS inner phase. Pure aqueous AS particles effloresced in the RH range from 36.3% to 43.7%, in agreement with literature data (31-48% RH). In contrast, aqueous 1:1 (by dry weight) PEG-400/AS particles with diameters of the AS phase from 7.2 to 19.2 mum effloresced between 26.8% and 33.9% RH and aqueous 8:1 (by dry weight) PEG-400/AS particles with diameters of the AS phase from 1.8 to 7.3 mum between 24.3% and 29.3% RH. Such low efflorescence relative humidity (ERH) values have never been reached before for AS particles of this size range. We show that these unprecedented low ERHs of AS in PEG-400/AS particles could not possibly be explained by the presence of low amounts of PEG-400 in the aqueous AS phase, by a potential inhibition of water evaporation via anomalously slow diffusion through the PEG coating, or by different time scales between various experimental techniques. High-speed photography of the efflorescence process allowed the development of the AS crystallization fronts within the particles to be monitored with millisecond time resolution. The nucleation sites were inferred from the initial crystal growth sites. Analysis of the probability distribution of initial sites of 31 and 19 efflorescence events for pure AS and 1:1 (by dry weight) PEG-400/AS particles, respectively, showed that the particle volume can be excluded as the preferred nucleation site in the case of pure AS particles. For aqueous 1:1 (by dry weight) PEG-400/AS particles preferential AS nucleation in the PEG phase and at the PEG/AS/substrate contact line can be excluded. On the basis of this probability analysis of efflorescence events together with the AS ERH values of pure aqueous AS and aqueous PEG-400/AS particles aforementioned, we suggest that in pure aqueous AS particles nucleation starts at the surface of the particles and attribute the lower ERH values observed for aqueous PEG-400/AS particles to the suppression of the surface-induced nucleation process. Our results suggest that surface-induced nucleation is likely to also occur during the efflorescence of atmospheric AS aerosol particles, possibly constituting the dominating nucleation pathway.

Deliquescence and Efflorescence of Potassium Salts Relevant to Biomass-Burning Aerosol Particles

Particles containing potassium salts are common in biomass burning plumes and include compounds such as KCl, KNO 3 , and K 2 SO 4 . We determined the deliquescence and efflorescence relative humidities of single- and multi-component potassium salts using an environmental transmission electron microscope and an environmental scanning electron microscope. KCl particles deliquesced at a relative humidity (RH) of 85% and effloresced at 56%. K 2 SO 4 particles deliquesced at 96% and effloresced at 60%. KNO 3 particles grew continuously with increasing humidity. If KNO 3 particles were heat-treated at 104°C overnight, they deliquesced at an RH above 90%. Atomized solutions of multi-component potassium compounds (KCl & KNO 3 and KCl & K 2 SO 4 ) formed particles that contained distinct grains of each compound upon drying. Particles formed from the KCl & KNO 3 mixtures grew continuously with increasing humidity until deliquescence occurred at an RH of 82% and efflorescence occurred at 58%. Particles from the KCl & K 2 SO 4 mixture began to take up water at an RH of 84%. They deliquesced at >90% and effloresced at 52%. This work provides new data for mixed aqueous electrolytes that are important for atmospheric science. Efflorescence values for K 2 SO 4 , KNO 3 , and internally mixed particles of K 2 SO 4 and KCl are reported here for the first time.