Secondary organic aerosols (SOA) can exist in a glassy or semisolid state under low relative humidity (RH) conditions, in which the particles show nonequilibrium kinetic characteristics with changing ambient RH. Here, we selected internally mixed sucrose/NaNO3 droplets with organic to inorganic molar ratios (OIRs) of 1:8, 1:4, 1:2, and 1:1 as a proxy for multicomponent ambient aerosols to study crystal nucleation and growth processes and water transport under a highly viscous state with the combination of an RH-controlling system and a vacuum Fourier transform infrared (FTIR) spectrometer. The initial efflorescence RH (ERH) of NaNO3 decreased from ∼45% for pure NaNO3 droplets to ∼38.6 and ∼37.9% for the 1:8 and 1:4 sucrose/NaNO3 droplets, respectively, while no crystallization of NaNO3 occurred for the 1:2 and 1:1 droplets in the whole RH range. Thus, the addition of sucrose delayed the ERH and even completely inhibited nucleation of NaNO3 in the mixed droplets. In addition, the crystal growth of NaNO3 was suppressed in the 1:4 and 1:8 droplets most likely due to the slow diffusion of Na+ and NO3- ions at low RH. Water uptake/release of sucrose/NaNO3 particles quickly arrived at equilibrium at high RH, while the hygroscopic process was kinetically controlled under low RH. The half-time ratio between the liquid water content and the RH was used to describe the mass transfer behavior. For the 1:1 droplets, no mass limitation was observed with the ratio approaching to 1 when the RH was higher than 53%. The ratio increased 1 order of magnitude under an ultraviscous state with RH ranging from 53 to 15% and increased a further 1 order of magnitude at RH < 15% under a glassy state.

•The RH pulse method can measure hygroscopicity of aerosols in time scale of seconds.•Water diffusion coefficient in citric acid aerosols was obtained.•The addition of citric acid suppressed crystallization of ammonium sulfate.•Water transfer limitation was enhanced for the 1:3 mixture.In this research, we applied a pulsed RH controlling system and a rapid scan vacuum FTIR spectrometer (PRHCS-RSVFTIR) to investigate hygroscopicity of internally mixed (NH4)2SO4(AS)/citric acid (CA) particles. The water content and efflorescence ratio of AS in the particles and ambient relative humidity (RH) as a function of time were obtained with a subsecond time resolution. The hygroscopic behavior of AS aerosols in two different RH control processes (equilibrium and RH pulsed processes) showed that AS droplets crystallize with RH ranging from 42% to 26.5%. It was found that the half-life time ratio between the water content in the CA particles and the gas phase under RH pulsed change was greater than one under low RH conditions (<40% RH), indicating the significant water transfer limitation due to the high viscosity of CA aerosols at low RH, especially at RH<20%. In addition, water diffusion constants between 10−12 m2 s−1 and 10−13 m2 s−1 in micron size CA aerosols were obtained in a sub-second and second timescale. The addition of AS enhanced the water transfer limitation in the mixed aerosols. The efflorescence relative humidity (ERH) of the mixed particles with AS/CA by molar ratio 3:1 was found between 22.7% and 5.9%, which was much lower than AS particles. No efflorescence process was observed for the 1:1 mixed particles, indicating that CA greatly suppressed nucleation of AS. Our results have shown that the PRHCS-RSVFTIR is effective to simulate hygroscopicity and water transport of aerosols under fast variations in RH in atmosphere.Download high-res image (280KB)Download full-size image

•The extinction efficiency of aerosols controls the cooling effect.•The scattering effect in IR spectra can be found.•The extinction efficiency (Qext) of microsphere has been calculated based on the scattering spectral signals.•By micro-FTIR technology, the extinction efficiency of sphere with the diameter of 2.5 μm–25 μm can be gained.The IR spectra of a single, isolated polystyrene sphere with diameter of 4.46 μm under different aperture sizes have been measured by Micro-FTIR spectrometer and the scattering signal can be seen obviously. Based on Mie scattering theory, a feasible method has been proposed to estimate the extinction efficiency (Qext) of microsphere. Qext from Mid-IR spectroscopy is consistent well with that derived from MiePlot software. It shows that the extinction efficiency of microsphere with the size of the Mid-IR range (2.5 μm–25 μm), which exhibits weak IR absorption, can be obtained by using the present method based on recorded Micro-FTIR spectra.Download high-res image (195KB)Download full-size image

We report a new method to investigate water transport kinetics in aerosol particles by using rapid scan FTIR spectroscopy combined with a custom-built pulse relative humidity (RH) control system. From real time in situ measurements of RH and composition using high time resolution infrared spectroscopy (0.12 s for one spectrum), and through achieving a high rate of RH change (as fast as 60% per second), we are able to investigate the competition between the gas and condensed phase diffusive transport limits of water for particles with mean diameter ∼3 μm and varying phase and viscosity. The characteristic time (τ) for equilibration in particle composition following a step change in RH is measured to quantify dissolution timescales for crystalline particles and to probe the kinetics of water evaporation and condensation in amorphous particles. We show that the dissolution kinetics are prompt for crystalline inorganic salt particles following an increase in RH from below to above the deliquescence RH, occurring on a timescale comparable to the timescale of the RH change (<1 s). For aqueous sucrose particles, we show that the timescales for both the drying and condensation processes can be delayed by many orders of magnitude, depending on the viscosity of the particles in the range 101 to 109 Pa s considered here. For amorphous particles, these kinetics are shown to be consistent with previous measurements of mass transfer rates in larger single particles. More specifically, the consistency suggests that fully understanding and modelling the complex microphysical processes and heterogeneities that form in viscous particles may not be necessary for estimating timescales for particle equilibration. A comparison of the kinetics for crystalline and amorphous particles illustrates the interplay of the rates of gas and condensed phase diffusion in determining the mass transport rates of water in aerosols.

Ozone initiated heterogeneous oxidation of micron-sized oleic acid (OA), linoleic acid (LA), and linolenic acid (LOA) single droplets was investigated using a gas-flow system combined with microscopic Fourier transform infrared (micro-FTIR) spectrometer. The pseudo-first-order rate constant (kapp) and the overall uptake coefficient (γ) are obtained by quantitatively estimating the changes in absorbance area of the CO stretching band at 1710 cm−1, which is assigned to the carboxyl group of the reactant. The overall kinetics is dominated by surface reaction. And the effect of surface adsorption, which is derived from the ozone concentration and particle size effects on reaction kinetics, plays an important role during the reaction. Comparison of the kapp values corresponding to OA, LA and LOA shows the positive correlation between double bonds and reaction rate. In the view of RH effect, both kapp and γ are strongly enhanced by over a factor of three for the LOA/O3 reaction system as the relative humidity (RH) increases from ∼0% to 83%. The LA/O3 reaction system exhibits a weaker RH dependence. In contrast, the kapp and γ of the OA/O3 reaction system are independent of the RH changes. Moreover, the various hygroscopicities of the three acids and corresponding products lead to different reactivities.

•The reactions kinetics of soot particles with O3 and O3/SO2 are studied as a function of RHs.•Water vapor significantly promotes the proceeding of the heterogeneous reactions.•Reaction rates and uptake coefficients at different RHs are the first reported.•The possible reaction mechanism is proposed for the first time.The heterogeneous reactions of soot particles with O3 and the mixture of O3 and SO2 were studied as a function of relative humidities (RHs). The reactions were followed in real time using microscopic Fourier transform infrared (micro-FTIR) spectrometer to obtain kinetic data. The results show that the ketone (CO) group is the main product of the O3/soot reaction, and the sulfate is identified on the surface of soot particles in the presence of O3/SO2. Both reactions are sensitive to RHs and surrounding water significantly promotes the proceeding of the heterogeneous reactions. For the O3/soot reaction, the pseudo-first-order rate constant increases from 3.2 × 10−4 s−1 to 7.1 × 10−4 s−1 with increasing RH in the range of 1%–82%. When O3 and SO2 exist simultaneously during the reaction, the reaction rate and uptake coefficient are all enhanced by about an order of magnitude as the RH increases from 1% to 83%. The high productions of the ketone and sulfate on soot surface are of highly hydrophilic, which play a key role in environmental effect under humid environment. The possible reaction mechanism speculates that products of aromatic carbonyls and dihydrofuran species on soot particles will be more harmful to human health.Download high-res image (251KB)Download full-size image

•O3 or NO2 leads to rapid conversion of SO2 to sulfate on α-Al2O3 particles.•Compared to NO2, O3 is easier to oxidize SO2 to sulfate.•Reactions of SO2/O3, SO2/NO2 on α-Al2O3 particles exhibit RH dependence.•The reaction rates and uptake coefficients increase with increasing RH.Heterogeneous reactions of SO2/O3 and SO2/NO2 with α-Al2O3 particles at different RHs were investigated using a gas-flow system combined with microscopic Fourier transform infrared (micro-FTIR) spectrometer. The results show that the trace gas O3 or NO2 leads to rapid conversion of SO2 to sulfate on the surface of α-Al2O3 particles in initial stage and then conversion rate decreases in the following stages. The rate of sulfate formation and uptake coefficient (γ) for SO2 in the two systems as a function of relative humidity (RH) are determined for the first time, which are all strongly enhanced more than seven-fold as the RH increase from 15% to 95% in initial stage for SO2/O3 and SO2/NO2. Moreover, the γ in the system of SO2/O3 on α-Al2O3 particles is more than 3-fold than that of SO2/NO2 for the similar RH condition. Our results may be broadly applicable to understand the effects of RH and trace gases (e.g., O3, NO2) for the converting SO2 into sulfate on the mineral dust, which supply basic data for atmospheric chemistry modeling studies.Download high-res image (210KB)Download full-size image

The in situ infrared spectra of sodium nitrate (NaNO3) and mixed NaNO3/glycerol droplets with organic to inorganic molar ratio (OIR) of 1:8, 1:4, 1:2, 1:1, and 2:1 on the ZnSe substrate were collected using the Fourier transform infrared attenuated total reflection (FTIR–ATR) technique in the RH linearly decreasing process. When the efflorescence process occurred in the RH decreasing process, the stochastric transformation from NaNO3 droplets to NaNO3 solid particles resulted in gradually increasing of a new band at 836 cm–1 and contineously decreasing of an initial band at 829 cm–1, which were assigned to the v2-NO3– mode in crystal phase state and in liquid state, respectively. There were excellent isobesic points between the two bands in the transformation processes, indicating the synchronization between the disappearence of NO3– in solutions and the production of NaNO3 crystal. The nucleation ratio, i.e., the amount of the droplets crystallized at a given RH upon the total amount droplets, was obtained by using the absorbance of ν2-NO3– band at 836 cm–1, which was used to calculate the nucleation rates of NaNO3 either for heterogeneous or for homogeneous nucleation process. While the glycerol molecules delayed the efflorescence RHs (ERH) of NaNO3 in the mixed NaNO3/glycerol droplets (OIR = 2:1) to 15%, greatly lower than the ERH for pure NaNO3 droplets at 62.5%, they also greatly suppressed the heterogeneous nucleation rate with increase of the OIR ratio. Two different kinetic mechanisms were suggested in the mixed droplets with OIR = 1:8, 1:4, 1:2, and 1:1, i.e., homogeneous nucleation at higher supersaturation and heterogeneous nucleation at lower supersaturation. For the mixed droplets with 2:1 OIR, they fell into the homogeneous nucleation region completely.

Atmospheric aerosols are usually complex mixtures of inorganic and organic compounds. The hygroscopicity of mixed particles is closely related to their chemical composition and interactions between components, which is still poorly understood. In this study, the hygroscopic properties of submicron particles composed of NaCl and dicarboxylic acids including oxalic acid (OA), malonic acid (MA), and succinic acid (SA) with various mass ratios are investigated with a hygroscopicity tandem differential mobility analyzer (HTDMA) system. Both the Zdanovskii–Stokes–Robinson (ZSR) method and extended aerosol inorganics model (E-AIM) are applied to predict the water uptake behaviors of sodium chloride/dicarboxylic acid mixtures. For NaCl/OA mixed particles, the measured growth factors were significantly lower than predictions from the model methods, indicating a change in particle composition caused by chloride depletion. The hygroscopic growth of NaCl/MA particles was well described by E-AIM, and that of NaCl/SA particles was dependent upon mixing ratio. Compared with model predictions, it was determined that water uptake of the NaCl/OA mixture could be enhanced and could be closer to the predictions by addition of levoglucosan or malonic acid, which retained water even at low relative humidity (RH), leading to inhibition of HCl evaporation during dehydration. These results demonstrate that the coexisting hygroscopic species have a strong influence on the phase state of particles, thus affecting chemical interactions between inorganic and organic compounds as well as the overall hygroscopicity of mixed particles.

A novel approach based on a combination of a pulse RH controlling system and a rapid scan vacuum FTIR spectrometer (PRHCS-RSVFTIR) was utilized to investigate dynamic hygroscopicity of two atmospheric aerosols: ammonium sulfate ((NH4)2SO4) and magnesium sulfate (MgSO4). In this approach, rapid-scan infrared spectra of water vapor and aerosols were obtained to determine relative humidity (RH) in sample cell and hygroscopic property of aerosols with a subsecond time resolution. Heterogeneous nucleation rates of (NH4)2SO4 were, for the first time, measured under low RH conditions (<35% RH). In addition, studies of MgSO4 aerosols revealed that water mass transport may be limited by different processes depending on RH values (surface limited at 40% < RH < 52% and bulk phase limited at RH < 40%). Furthermore, we are also the first to report water diffusion constants in micron size MgSO4 aerosols at very low RH values. Our results have shown that the PRHCS-RSVFTIR is well-suited for determination of hygroscopicity of atmospheric aerosols and water transport and nucleation kinetics of liquid aerosols.

The effect of gel formation on the mass transfer of water during evaporation or condensation from MgSO4 droplets is studied using aerosol optical tweezers coupled with Raman spectroscopy. In particular, the kinetics of water transport during hydration and dehydration are followed for variable step changes in relative humidity and compared with previous measurements using different methodologies. Slow diffusion of water in the particle bulk is shown to limit water evaporation and condensation from the aerosol. Desorption of water continues over a long time at the very low RH region and this is validated with complementary studies made by FTIR-ATR and measurements of water adsorption isotherms. The observations can be rationalized when considering the possible phase transformation of the gel structure at very low RHs. Finally, the influence of the duration of the drying time (RH ≤ 10%) on the kinetics of condensation during hydration is investigated. Apparent diffusion coefficients of water molecules in the gel are obtained, showing little dependence on the water activity and droplet composition, and are consistent with the slow removal of water during drying from pores formed at the gel transition RH.

Tropospheric aerosols are usually complex mixtures of inorganic and organic components, which can influence the hygroscopicities of each other. In this research, we applied confocal Raman technology combined with optical microscopy to investigate the relationship between the hygroscopic behavior and the molecular interactions of mixed glycerol/Mg(NO3)2/water droplets. Raman spectra provide detailed structural information about the interactions between glycerol molecules and Mg2+ ions, as well as information about the interactions between glycerol and NO3– ions through electrostatic interaction and hydrogen bonding. The change of the CH2 stretching band of glycerol molecules in mixed droplets suggests that the backbone structures of glycerol mainly transform from αα to γγ in the dehumidifying process, and the additional Mg2+ ions strongly influence the structure of glycerol molecules. Because the existence of glycerol suppresses the crystallization of Mg(NO3)2·6H2O in the dehumidifying process, Mg(NO3)2 molecules in mixed droplets form an amorphous state rather than forming crystals of Mg(NO3)2·6H2O when the relative humidity is lower than 17.8%. Moreover, in mixed droplets, the molar ratio of NO3– to glycerol is higher in the center than in the outer region.

•FTIR technique was used to study the trans/gauche translation.•The structural information was gained by analyzing the differential spectra.•As the increase of temperature, PVA film took the conformational transition from trans to gauche.The conformational change of poly(vinyl alcohol) has been studied by Fourier transform infrared spectroscopy at various temperatures in the 4000–400 cm−1 region. The molecular motion and the trans/gauche content are sensitive to the CH, CC stretching modes. FTIR spectra show that the I2920/I2849 decreases from 1.84 to 1.0 with increasing temperature, companying the decrease in I1047/I1095 from 0.78 to 0.58, implying the conformational transition from trans to gauche in alkyl chain. Based on the van’t Hoff relation, the enthalpies and entropies have been calculated in different temperatures, which are 4.61 kJ mol−1 and 15.23 J mol−1 K−1, respectively, in the region of 80–140 °C. From the CO stretching mode and OH band, it can be concluded that the intermolecular hydrogen bonds decrease owing to elevating temperature, which leads to more gauche conformers.

The confocal Raman spectra of mixed phthalic acid/ammonium sulfate (AS) droplets deposited on a polytetrafluoroethylene (PTFE) substrate and a hydrophilic glass substrate are collected. The evolution of the physical state of the mixed droplet deposited on the PTFE substrate at various relative humidities consists of three states: well-mixed liquid state, liquid–liquid phase-separated state, and crystalline state. When the mixed droplets exist in liquid–liquid phase-separated state, the morphologies of the droplets on a PTFE substrate and on a glass slide are totally reverse, that is, an aqueous AS inner phase surrounded by an organic outer phase and an organic inner phase surrounded by an AS outer phase, respectively. We propose that the salting-out effect may induce the diffusion and formation of the organic phase, thus leading to the generation of liquid–liquid phase separation. The surface tension and hydrophobicity/hydrophilicity of substrates influence the spatial distribution of mixed aerosols. The understanding about the evolution of the physical state of mixed droplets and the effect of the substrates are important for probing the formation of atmospheric aerosols’ morphology in the dehumidifying process.

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.

Density functional theory (DFT) calculations were applied to investigate the adsorption of water monomer, water clusters on NaNO3(0 0 1) surface. Single water molecule is more likely to locate on the bridge site with its H atom attracted by the O atom of nitrate ion and its O atom adjacent to Na+. Mulliken population analysis shows that fewer electrons transfer from the Na atom of substrate to water molecule. A systematic study of water clusters adsorption at high coverages ranging from 0.5 monolayer (ML), 0.75 ML, 1 ML, 1.25 ML, and 1.5 ML on NaNO3(0 0 1) surface was also investigated, and the results indicate that for 1 ML water adsorption on NaNO3(0 0 1) surface, a water chain is formed among four water molecules through hydrogen bonds. Interestingly, the water molecules are linked through hydrogen bonds to form a 14-membered macrocyclic water ring for 1.5ML adsorption on NaNO3(0 0 1) surface. Our estimated O–H symmetric stretching frequency (νO–H) will have blueshift with decrease of water coverage, which is consistent with the tendency given by experiments.Graphical abstractHighlights► We study the adsorption of water monomer, water clusters on NaNO3(0 0 1) surface. ► Single water molecule is more likely to locate on the bridge site. ► A water chain is formed for 1 ML water adsorption on NaNO3(0 0 1) surface. ► A 14-membered water ring is formed for 1.5 ML adsorption on NaNO3(0 0 1) surface. ► Our estimated νO–H will have blueshift with decreased water coverage.

•A novel anatase crystal was studied by Raman mapping technique for the first time.•The Raman spectra of the crystal were observed to vary with measurement position.•The spectral differences among different crystal facets result from symmetry rules.•The spectral differences observed on the same crystal facet indicate lattice defects.After it has been successfully synthesized in 2008, so far, no Raman investigations on the micro-sized anatase TiO2 single crystal which has a large percentage of the reactive (0 0 1) facets have been conducted to the best of our knowledge. In the present work, this unique anatase TiO2 single crystal was investigated by noninvasive and nondestructive Raman mapping technique. Raman images of both non-polarized and polarized measurements showed that the Raman features of the crystal varied with measurement position. The differences among the Raman spectra measured on different crystal facets were believed to result from the orientations and the symmetry rules. Whereas the differences among those measured at different points of the same crystal facet under the same measurement condition were supposed to indicate the defects of the crystal structure, such as oxygen vacancies, local lattice disorder etc. Furthermore, the appearance of the two second order Raman peaks of 803 and 918 cm−1 as well as the blue-shift of 395 cm−1 peak implies the anharmonicity of the crystal structure, which is also probably caused by the crystal defects. Our results provide useful information about the structure of this unique anatase TiO2 material, and could be complementary to those that acquired by other characterization techniques.

The Fourier transform infrared-attenuated total reflectance (FTIR-ATR) difference spectra of aqueous MgSO4, Na2SO4, NaCl and MgCl2 solutions against pure water were obtained at various concentrations. The difference spectra of the solutions showed distinct positive bands and negative bands in the O–H stretching region, indicating the influences of salts on structures of hydrogen-bonds between water molecules. Furthermore the difference spectra of MgCl2 solutions against NaCl solutions and those of MgSO4 solutions against Na2SO4 solutions with the same concentrations of anions (Cl− or SO42−, respectively) allowed extracting the structural difference of the first hydration layer between Mg2+ and Na+. Using SO42− as a reference ion, structural information of the first hydration layer of the Cl− anion was obtained according to the difference spectra of MgCl2 solutions against MgSO4 solutions and those of NaCl solutions against Na2SO4 solutions containing the same concentrations of cations (Mg2+ or Na+, respectively). The positive peak at ~3,407 cm−1 and negative peak at ~3,168 cm−1 in these spectra indicated that adding Cl− decreased the strongest hydrogen-bond component and increased the relatively weaker one.

High quality micro-Raman spectra of the LiClO4 droplet with mass of nanogram scale were obtained at various concentrations from dilute to supersaturated state. From component band analysis of the v1-ClO4− band, four peaks at 933.3, 936.8, 942.1 and 950.7 cm−1 were identified and assigned to free solvated perchlorate anion, solvent-shared ion pair, contact ion pair and complex ion aggregates, respectively. As expected, the signature of free solvated ClO4− ion was observed to decrease in intensity with the increase in concentration. The intensity of the signature from solvent-shared ion pair was observed to rise with increase in concentration from 1.8 mol/kg to 5.0 mol/kg before decreasing as the concentration was further increased to 5.6 mol/kg. Signatures of contact ion pair and of complex ion aggregates were shown to increase as the concentration was enhanced. Based upon the Eigen mechanism, we show that three association equilibria can be used to describe the transformations between free solvated perchlorate anion, solvent-shared ion pair, contact ion pair and complex ion aggregates. The overall association constant, K, and the stepwise association constants Ki (i = 1 to 3) in the Eigen mechanism were determined separately with values of 0.025 ± 0.003, 0.023 ± 0.002, 0.068 ± 0.033 and 0.686 ± 0.174. Based on these constants, the electronic performance can be reasonably predicted by the optimum choice of electrolyte concentrations.

Heterogeneous reaction between ozone and linoleic acid (LA) thin film was investigated by a flow reactor coupled to attenuated total reflection infrared spectroscopy (FR-ATR-IR) over wide ranges of temperature, relative humidity (RH), and ozone concentration under atmospheric pressure condition. Pseudo-first-order rate constants kapp and overall reactive uptake coefficients γ were acquired on the basis of changes in absorbance from peaks located near 1743, 1710, 1172, and 1110 cm–1, which can be assigned to C═O in ester, C═O in acid, and C—C and C—O stretching modes, respectively. Results showed that the kapp and γ increased nearly by a factor of 6 with increasing temperatures from 258 to 314 K. It was noted the temperature effect on the reaction kinetics was much more pronounced at lower temperatures. Such behavior can be explained by a change in the physical state of LA at lower temperatures. In addition, kapp and γ were enhanced by 2-fold as the RH increased from 0 to 80%. Moreover, the effect of ozone concentration on the reaction kinetics was reported for the first time. kapp was found to display a Langmuir–Hinshelwood dependence on ozone concentration with KO3 = (1.146 ± 0.017) × 10–15 molecules cm–3 and k[S] = 0.0522 ± 0.0004 s–1, where KO3 is a parameter that describes the partitioning of ozone to the thin film surface, and k[S] is the maximum pseudo-first-order coefficient at high ozone concentration. Furthermore, yields and hygroscopic properties of reaction products were also investigated by FTIR spectroscopy. The intensity ratio of two C═O stretching bands, A1743/A1710, which was utilized as an indicator of the product yields, increased sharply with increasing temperatures in the lower temperature region (258–284 K), and then remained nearly constant in the higher temperature region (284–314 K). The product yields showed no significant variation with RH, for the intensity ratio of A1743/A1710 barely changed in the wide RH range 0–80%. Water uptake studies showed that the LA thin film absorbed water with an increasing RH, and the hygroscopicity of the thin film was enhanced after ozone exposure.

A single K2HPO4 droplet with size of ∼50 μm on a Teflon substrate was forced to enter into the supersaturated state by decreasing the relative humidity (RH), allowing accurate control over the concentration of the solute within a droplet of a nanogram. The K2HPO4 solutions from dilute (0.1–1.0 mol·L–1 bulk) to concentrated state (a droplet from RH 98.2% to 25.1%) were studied through micro-Raman spectroscopy in the spectral region of about 200–4000 cm–1. The area ratio between the water stretching band to the sum of the ν1-PO3, ν2-POH, and ν4-PO3 bands of the HPO42– at various RHs was used to describe the dehydration behavior of a microsized single K2HPO4 droplet in dehumidifying process. The peak position of the v1-PO3 band for the 1 mol·L–1 bulk solution appeared at 991 cm–1 and moved to 986 cm–1 at 98.2% RH, to 978 cm–1 at 70.2% RH, and then to 964 cm–1 at 30.0% RH for a droplet, accompanying an increase of the full width at half-height (fwhh) of this peak from 16.3 to 17.2, 22.2, and then to 24.2 cm–1, indicating transition of the HPO42– anions from monomers to dimers/trimers/oligomers and then to polyanions with chain structures in the K2HPO4 solutions. After 25.1% RH, the solid was proved to be K2HPO4·3H2O according to the Raman spectral features. Furthermore, the O–H stretching envelope of a K2HPO4 droplet showed that the intensity ratios of the strong hydrogen bonding component (3255 cm–1) to the weak one (3417 cm–1) and the cage-like water (2925 cm–1) to the weak one (3417 cm–1) were sensitive to the HPO42– association structures, which can be used to understand the effects of dimers/trimers/oligomers and chain structures of the HPO42– associations on the hydrogen bonding of water molecules.

Although the hygroscopicity of a NaNO3/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 NaNO3/polyalcohol/water droplets and their structures on the molecular level. In this study, the hygroscopicity of mixed glycerol/NaNO3/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/NaNO3/water droplets, glycerol molecules tended to combine with Na+ and NO3– ions by electrostatic interaction and hydrogen bonding, respectively. On the basis of the analyses of the changes of symmetric stretching (vs-CH2), asymmetric stretching (va-CH2), their area ratio (Ava-CH2/Avs-CH2) of glycerol, and symmetric stretching band of NO3– (ν1-NO3–) 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 NO3– respectively, which suppressed the formation of contact of ions and delayed the efflorescence relative humidity (ERH) for the mixed droplets compared to the NaNO3/water droplet.

Morphological changes with decreasing relative humidity (RH) of supersaturated sea-salt aerosol droplets on a quartz substrate were observed using a high-speed video-camera. Stable gypsum (CaSO4·2H2O) or the metastable hemihydrate (CaSO4·0.5H2O) were precipitated as the RH decreased. The dynamic process of crystal growth under steady-state humidity was studied by controlling the RH; the metastable hemihydrate was precipitated at 70.5%–77.1% RH, and the apparent crystal growth rate was between 1.42 and 2.33 μm3/s. Stable gypsum was formed at 80.7%–82.2% RH, and the apparent crystal growth rate was between 0.70 and 0.81 μm3/s.

Ion association ratios as a function of concentration were estimated in single NaNO3 droplets (5–60 μm) on a polytetrafluoroethylene (PTFE) substrate with molar water-to-solute ratios (WSRs) of 0.8–28 and bulk NaNO3 solutions with WSRs of 35–200 by combining micro-Raman spectroscopy and component band analysis. Concentrations of the NaNO3 droplets were accurately controlled by adjusting relative humidity (RH) in a sample chamber. As the WSRs decreased from 200 to 0.8, symmetric stretching band (ν1-NO3–) was observed to shift from 1047 to 1058 cm–1 along with a change in full width at half-maximum (fwhm) from ∼10 to ∼16 cm–1, indicative of formation of ion pairs with different structures. Through the component band analysis of the ν1-NO3– band, five bands centered at 1040.0, 1042.9, 1048.5, 1053.5, and 1057.0 cm–1 were identified and assigned to coupled wagging modes of water molecules hydrated with nitrate ions, free hydrated nitrate anions, solvent-shared ion pairs (SIPs), contact ion pairs (CIPs), and the complex ion aggregates (CIAs), respectively. There were large amounts of SIPs and CIPs in dilute NaNO3 solution even at an extremely low concentration (WSR ∼ 200), and each accounted for 50% and 20% of total nitrate species, respectively. This finding is in good agreement with earlier reported observations. In the dilute solutions (45 < WSR < 200), there is the same amount of free hydrated ions transformed into SIPs as that of SIPs transformed into CIPs. As a result, the overall amount of SIPs remained unchanged over the concentration range. With a decrease in WSR from 45 to 0.8, the amounts of SIPs and free solvated NO3– ions kept decreasing, whereas the amount of CIPs rose to a maximum at WSR = 7 and then fell with a further decreasing WSR. Formation of CIAs started at WSR ∼ 45, and its amount continuously increased as the WSR is further reduced to 0.8. The effect of temperature on ion association structure in the NaNO3 droplets was also studied in the present work. An increase in temperature promoted formation of both CIPs and CIAs, and the latter was more pronounced. At 80 °C, the most concentrated NaNO3 droplets had a WSR approximately equal to 0.12 and were in amorphous state with cations and anions aggregated in a complicated manner, highly similar to ionic liquid.

The hydrated bisulfate ion clusters (HSO4–(H2O)n, n = 1–10) were optimized at the M06/6-311++G(d,p) level. The factors affecting ν2-SOH of the clusters involved vibration coupling between ν2-SOH and the water wagging libration mode (W-H2O) and hydrogen bonding effect. In order to understand the vibration coupling between W-H2O and ν2-SOH for the bisulfate clusters, D2O instead of H2O and Se instead of S were used to estimate the uncoupling frequency of ν2-SOH and W-H2O, respectively. For HSO4–·H2O-I, the uncoupling frequencies of ν2-SOH and W-H2O were obtained at 752.0 and 753.4 cm–1. After coupling, the frequencies appeared at 782.2 and 732.6 cm–1. H2S and NH4+ instead of D2O in HSO4–·D2O-II were compared to analyze the effect of hydrogen bond. The sequence of hydrogen bond strength was found to be HSO4–·H2S-II < HSO4–·D2O-II < HSO4–·NH4+-II with the respective ν2-SOH at 736.7, 740.5, and 802.2 cm–1 increasing in the same order. In HSO4–·(H2O)n, coupling appeared when n was from 1 to 8. For HSO4–·(D2O)n, no coupling between ν2-SOH and D2O librations made it possible to understand the hydrogen bonding effect on the ν2-SOH. The frequencies of ν2-SOH for clusters HSO4–(D2O)n almost linearly decreased from 752.0 to 854.6 cm–1 with n from 1 to 10.

The conformational behaviors of monosodium glutamate (MSG) in a dehydration process were studied by Micro-Raman spectroscopy in combination with Hartree–Fock calculations using 6-31+G* method. The dehydration process of the MSG droplet was performed by decreasing the ambient relative humidity (RH). The intensity ratio of the 935 cm−1 band to 884 cm−1 band (I935/I884) kept decreasing when RH decreased. By optimizing the geometries with different fixed dihedral angles, the downtrend of (I935/I884) is found to be due to the reduction of MSG molecular volume.

Temporally and spatially resolved investigations of the efflorescence of equimolar mixtures of ammonium sulfate (AS) and ammonium nitrate (AN) aerosol droplets were performed by confocal Raman spectroscopy combined with high-speed photography. Information was obtained on the dynamic process of morphological changes and the heterogeneous distribution of efflorescence products. The supersaturated AS/AN mixed droplet was found to first precipitate as the double salt 3AN·AS when the relative humidity was 50%. The excess AS was irregularly distributed on the surface of the double salt as an inclusion structure. No formation of 2AN·AS was observed throughout the efflorescence process.

Adsorption and desorption are critical to crystal engineering for the protein crystal applications. In this paper, we present a new method to study the adsorption and desorption kinetics of waters in lysozyme crystals. H2O and D2O were used as ideal indicators for the purpose. The lysozyme crystals were prepared in a complete D2O environment to ensure that all the water molecules in crystal were D2O. The H2O in the gas phase directly exchanges with the D2O in the crystal by exposing the crystal to an ambiance with saturated water vapor. Using in situ confocal Raman microscopy, H2O adsorption and D2O desorption in the lysozyme crystals were found to follow first-order kinetics. The rate constants of H2O adsorption and D2O desorption were obtained to be equal to each other. The kinetic rates were found to linearly depend on the surface area-to-volume ratio of the crystals.

New techniques are required to explore directly the kinetics of water transport in aerosol between the gas and condensed phases, both at high relative humidity (RH) close to saturation and at low RH where the role of amorphous states must be considered. Here, we present micro-Raman measurements of the kinetics of water transport between the bulk of a particle and the surrounding gas phase by examining the rate of exchange of D2O by H2O in droplets initially composed of MgSO4/D2O. The formation of an amorphous gel inhibits the response of the droplet composition to changes in the ambient RH and leads to a substantial reduction of the mass transfer rate of water in the droplet bulk with an apparent diffusion constant of 10–15 to 10–14 m2 s–1. These measurements are consistent with the imposition of a kinetic limitation on the time response of the aerosol particle size to changes in RH.

We report an in situ strategy for the quantitative analysis of association equilibria in a single NaClO4 droplet of nanogram mass deposited on a quartz substrate. In the new approach, the single droplet was forced to enter into a supersaturated state by decreasing the relative humidity (RH) of the environment, allowing accurate control over the concentration of the solute within the droplet. An analysis of the solvated structure of the ClO4− anion with change in molar water-to-solute ratio (WSR) was performed by micro-Raman spectroscopy within the confines of a single droplet. The symmetric stretching v1-ClO4− band was observed to shift from a Stokes frequency of 935 to 944 cm−1 and was accompanied by a change in the full width at half-maximum (FWHM) from 11.4 to 16.6 cm−1 as the WSR decreased from 16.1 to 2.3. From component band analysis of the spectral range of 900–970 cm−1, four peaks at 933.3, 938.6, 944.1 and 946.0 cm−1 were identified and assigned to the free solvated perchlorate anion, the solvent-shared ion pair, the contact ion pair and complex ion aggregates, respectively. As expected, the signature of the free solvated ClO4− ion was observed to decrease in intensity with a decrease in RH over the full range from 94 to 27%. The intensity of the signature from solvent-shared ion pairs was observed to rise with decrease in RH from 94% to 75% before decreasing as the RH was further reduced to 27%. Signatures of the contact ion pair and of complex ion aggregates were shown to increase over the full range of RH as the RH was reduced. Based upon the Eigen mechanism, three association equilibria were used to describe the transformations between the free solvated perchlorate anion, the solvent-shared ion pair, the contact ion pair and complex ion aggregates. The overall association constant K and the stepwise association constants Ki (i = 1 to 3) were determined separately (0.27 ± 0.01, 0.03 ± 0.01, 5.49 ± 0.95, 0.76 ± 0.06).

Micro-Raman and FTIR spectroscopy have been used to investigate the micrometer-sized MnSO4 droplets. The concentration of solute within the droplet is controlled accurately by decreasing the relative humidity (RH) of the surroundings. According to the Raman spectra of MnSO4 droplets, when the RH decreased from ∼94% to ∼76%, the full width of the peak at half-maximum (fwhm) of the v1-SO42− band at 983 cm−1 initially increased from 95.7 to 104.2 cm−1. Two shoulders at ∼993 and ∼1002 cm−1 occurred in the v1-SO42− band at ∼76% RH, while the v2-SO42− band at 450 cm−1 split into two bands at 442 and 462 cm−1, indicating the formation of monodentate and bidentate contact ion pairs (CIPs) in supersaturated MnSO4 droplets. From component band analysis of the v1-SO42− band, four peaks at 983, 993, 1002, and 1010 cm−1 were identified and assigned to the free SO42−, monodentate CIPs, bidentate CIPs, and more complex ion aggregates, respectively. Signatures of monodentate and bidentate CIPs reached their maximum values at ∼76% and 60% RH, respectively. With further decreasing the RH, great abundance of various ion pairs presented in the droplet, corresponding the v1-SO42− band steadily blue-shifted, and the fwhm increased continuously until it went through a plateau at ∼60% RH. When the RH was between ∼44% and 13%, the intensity of the signature from four species of ion pairs was almost invariant, corresponding to the formation of an amorphous phase of MnSO4·2.8H2O. Further decreasing the RH below 13% RH, monodentate CIPs changed to bidentate ones. In the meantime, MnSO4·2.8H2O was deduced to transform into another amorphous phase with a stoichiometry of MnSO4·1.7H2O. This transition was also supported by the observation of the FTIR spectra according to a sharp increase of the fwhm of the v3-SO42− band (at ∼1091 cm−1) because of the appearance of a shoulder at 1132 cm−1.

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements were carried out on the 1-propanol−water (abbreviated as 1PA−W) mixtures over the entire 1-propanol molar fraction range at 298 K. The two bands at ∼1053 and ∼1068 cm−1, assigned to the vibrational modes of the gauche (vC−C−C−O−G) and the trans (vC−C−C−O−T) conformational isomers, respectively, which both include C−O and C−C stretching motions, were used to monitor the structural changes of the mixtures. When the water to 1-propanol molar ratio (WPR) is smaller than 0.2, the absorbance ratio of the two bands (AvC−C−C−O−G/AvC−C−C−O−T) remains constant at 1.42, characteristic of the existence of the 1-propanol aggregate chains, hydrogen-bonded by the O−H groups of 1-propanol in gauche conformations. When increasing the WPR from 0.2 to 20, there is an abrupt decrease in the absorbance ratio (AvC−C−C−O−G/AvC−C−C−O−T) from 1.42 to 1.01, corresponding to penetration of water molecules into the gauche-aggregate chains. The penetrated water molecules disrupt the 1PA chains and transform these gauche-aggregate 1PA chains to trans-aggregate chains, which are 1PA dimers of trans-conformation. The structural change induces complicated spectroscopic changes, including the red shifts of the series of bands 1016, 1053, and 1098 cm−1 and blue shifts of the bands 2877, 2937, and 2961 cm−1. With further increase of WPR up to 100, the absorbance ratio of AvC−C−C−O−G/AvC−C−C−O−T increases from 0.98 to 1.07, indicating a transformation of partial 1PA dimers to single molecules with gauche-conformation in the water hydrogen-bonding network. Together with results from quantum calculations at the B3L YP/6-31G (d, p) level, and two-dimensional infrared correlation and excess spectroscopy analysis, the structural evolution of water and 1PA molecules in 1PA−W mixtures has been inferred.

We present here a study of the phase behavior of mixed component NaNO3−Na2SO4 (SNS) droplets with NaNO3 to Na2SO4 molar ratios of 1:1, 3:1, and 10:1, comparing observations with thermodynamic predictions. Measurements are made by Fourier transform infrared attenuated total reflection and micro-Raman spectroscopy for SNS droplets deposited on ZnSe and quartz substrates, respectively. The conventional deliquescence/efflorescence hysteresis in phase behavior is observed. On drying, heterogeneous crystallization leads to phase behavior that is consistent with bulk solution thermodynamics, with the formation of the mixed salt NaNO3·Na2SO4·H2O, Na2SO4 (s), and NaNO3 (s) all observed to form at relative humidities that coincide with predictions by the aerosol inorganics model. However, conditioning of the droplet at high relative humidity prior to drying is observed to lead to quantitative differences between the fractions of different salts formed. When substrate effects do not influence the crystallization process, supersaturated solutions are formed, and this leads to the observation of contact ion pairs. Such measurements of the phase behavior of mixed component droplets are important for testing the reliability of thermodynamic models.

In this paper, three kinds of solid lysozyme samples with different water contents were investigated by confocal Raman spectroscopy. For the rod-like lysozyme crystal with highest water content, a sudden decrease of the intensity ratio of the doublet at 1338 and 1360 cm−1 was observed when the ambient relative humidity (RH) was lower than 86%, indicating the destruction of hydrophobic clusters of lysozyme induced by the expulsion of the hydration water from the crystal. In contrast to the rod-like crystal, tetragonal crystal and floor-like precipitate with a smaller amount of water showed no change of the structures of the hydrophobic clusters when the relative humidity was decreasing. The presence of bulk water in the rod-like crystal is believed a necessary factor for the function of the hydration water which promotes the hydrophobicity of hydrophobic clusters.

The hygroscopic properties of supersaturated aerosols as a function of relative humidity (RH) can be determined on the molecular level by the solutes with varied structures, as well as the solvent in the state of solvated water or free water. Although the former has been investigated by FTIR and Raman spectroscopy, the latter has gone mostly unnoticed. In this work, the state of water in supersaturated Mg(NO3)2 and NaNO3 aerosols were investigated through the application of the Raman difference spectra with respect to pure water. This technique could be developed from the observation that the Raman scattering and infrared absorbance cross sections of the molecular vibrations of interest remain practically unchanged from diluted solutions to supersaturated aerosols at low RHs. The results were expressed in terms of the percentage of free water (Wfree) as a function of RH, as well as the solvated water-to-solute ratio (WsolvatedSR) and the free water-to-solute ratio (WfreeSR) as a function of the total water-to-solute ratio (WSR). Solvated water observed in the Raman difference spectra was primarily related to the first hydration layers. In Mg(NO3)2 aerosols, three phases were identified with distinct mechanisms for the transition of the state of water. One unique structure with WsolvatedSR = 4 was proposed to occur in supersaturated Mg(NO3)2 aerosols at low RHs. In NaNO3 aerosols, it was found that the equality of solvated and free water could not provide a necessary condition for efflorescence, in contrast to the recent investigations by fluorescence spectroscopy. According to this investigation, solvated water could be more abundant than free water not only prior to the efflorescence of supersaturated NaNO3 aerosols, but also in relatively diluted droplets.

By using the measuring system previously designed by the authors, the conductance of KCl, NaCl and NH4Cl microdroplets is obtained in the whole measuring RH range, especially in the supersaturation region, which cannot be acquired from the bulk solutions and fills the gap of lack of experimental data of conductance under the supersaturated state. The ERH and DRH of these three kinds of microdroplets observed from a microscope are 80.5% and 95.4% (KCl), 75.7% and 93.3% (NaCl), and 69.9% and 96.6% (NH4Cl), respectively. In addition, it can be found from the dependence of conductance on RH that conductance is very sensitive to the existence of water molecules inside the microdroplet and the threshold of the deliquescence process can be predicted by the variation of conductance.

We report on the investigation of the phase separation of individual seawater droplets in the efflorescence processes with the spatially resolved Raman system. Upon decreasing the relative humidity (RH), CaSO4·0.5H2O separated out foremost from the droplet at an unexpectedly high RH of ∼90%. Occasionally, CaSO4·2H2O substituted for CaSO4·0.5H2O crystallizing first at ∼78% RH. Relatively large NaCl solids followed to crystallize at ∼55% RH and led to the great loss of the solution. Then, the KMgCl3·6H2O crystallites separated out from the residual solutions, adjacent to NaCl at ∼44% RH. Moreover, a shell structure of dried sea salt particle was found to form at low RHs, with the NaCl crystals in the core and minor supersaturated solutions covered with MgSO4 gel coating on the surface. Ultimately, the shielded solution partly effloresced into MgSO4 hydrates at very dry state (<5% RH).

Micro-Raman spectra of supersaturated aerosols of sodium succinate were obtained. The conformation behavior of the succinate dianion as a function of relative humidity (RH) was investigated by combining micro-Raman spectroscopy with theoretical calculations. A shoulder at 968 cm−1 of the v(C–CO2−) band on the rise in more concentrated droplets was believed indicative of conformation transformations. The intensity ratio (I963/I997) of the v(C–CO2−) band at 963 cm−1 to the v(C–C) band at 997 cm−1, versus the molar water-to-solute ratio (WSR), was used to fathom the equilibrium between gauche and trans conformations. Before saturation (WSR = 25.8) for the droplets, the ratio of I963/I997 retains a value of ∼2.6 independent of WSR, indicating that the equilibrium was not disturbed in the dilute droplets. In supersaturated droplets (WSR < 25.8), however, the ratio sharply decreases from ∼2.6 to ∼1.1 at WSR = 9.6, which was attributed to the formation of contact ion pairs (CIPs).

The luminescence compound (Yx,Gd0.95–x)BO3:Eu0.053+ with a single phase hexagonal crystal structure has been synthesized by the solid state reaction. The c/a value is linearly decreasing when the ratio of Y3+/Gd3+ is increasing, which indicates that the solid-solution of YBO3 and GdBO3 is forming. The two symmetric stretching vibration peaks of the BO4 anion at 866 and 912 cm−1 in the FTIR spectrum exist in two forms in the solid solutions. The local symmetric environment surrounding the Eu3+ ion was lowered with the decrease of Y3+/Gd3+ ratio. The host absorption band and Eu3+–O2− charge transfer band were observed in the excitation spectra of (Yx,Gd0.95−x)BO3:Eu0.053+. In particular, the relative intensity of Eu3+–O2− charge transfer band sharply decreased when the ratio of Y3+/Gd3+ increased was identified by the variation of local symmetric environment around Eu3+. The ratio of the fluorescence intensity of 5D0 → 7F2 transition and 5D0 → 7F1 transition (R/O ratio) is decreasing with the increasing of Y3+ content.

The Mg–Sr–Ba hexagonal aluminates (MgSrBa)Al12O19 doped with either Ce3+ or Tb3+ ion have been synthesized by solid-state reaction. Crystal structure variations with the increasing Ba2+ concentration have been thoroughly studied. When the concentration of Ba2+ reaches 0.39, the crystal structure becomes the ideal magnetoplumbite-type (No. PDF #80-1195), however, it becomes the same structure as BaMgAl10O17 with space group P63/mmc when Sr2+ is completely replaced by Ba2+ ions. In Ce3+ doped compounds, the emission peak of the Ce3+ ion is gradually red-shifted when the concentration of Mg2+ ion increases, while the 4f–5d transition of Ce3+ at 260 nm splits into five components. In the Tb3+ doped compounds, the spin-allowed and spin-forbidden 4f–5d transitions of Tb3+ have been observed. The intensity of the spin-forbidden 4f–5d transition is gradually increased with decreasing Sr2+ concentration and becomes dominant when Sr2+ is completely replaced by Ba2+. These experimental observations are also discussed in the context and match well with the theoretical calculations of 4f–5d transitions of Tb3+.

The efflorescence and deliquescence processes of Mg(NO3)2 aerosol particles deposited on ZnSe substrate have been investigated through in situ Fourier transform infrared-attenuated total reflection (FTIR-ATR) technique at the molecular level. At relative humidity (RH) of ∼3%, Mg(NO3)2 particles existed as amorphous states. The amorphous Mg(NO3)2 particles were transformed into crystalline Mg(NO3)2 · nH2O (n ≤ 5) with slight increasing of RH. Thermodynamically stable Mg(NO3)2·6H2O crystals were gradually formed on the particle surface and started to be dissolved at the saturation point (∼53% RH). At the same time, a continuous phase transition from Mg(NO3)2 · nH2O (n≤5) to Mg(NO3)2·6H2O occurred on the particle surface. This led the solid particles to completely deliquesce at 76% RH, which was much higher than the saturation point of 53% RH. In the efflorescence process, Mg(NO3)2 droplets entered into the supersaturated region due to the gradual evaporation of water. Finally, amorphous particles were formed when RH decreased below 5%. In the FTIR-ATR spectra of the supersaturated Mg(NO3)2 droplets, the absorbance of the symmetric stretching vibration of NO3− (v1- NO3−) clearly became stronger. It resulted from the continuous formation of solvent share ion pairs (SIPs), and even the contact ion pairs (CIPs) between Mg2+ and NO3−.

Single supersaturated NaClO4 aerosol droplets on a quartz substrate were observed by a high-speed video camera for their morphologic changes with decreasing relative humidity (RH). The supersaturated droplets were found to form anhydrous NaClO4 at ∼10% RH. Three stages were roughly observed in this process, i.e., the main stem appearance, primary and secondary branches growth, and the deep dehydration. The main stem grew steadily and finished in 120 ms. Affected by both crystal precipitation and water evaporation, primary branches grew for 1200 ms, accompanied by a slower growth of secondary branches. The deep dehydration was more complicated, and lasted for a longer time.

Spherical MgSO4 droplets were deposited by a syringe on the hydrophobic Teflon substrate. Using micro-Raman technique, the laser beam was highly focused twice on the surface and in the center of spherical droplets. The Raman spectra for the surface and the inner of MgSO4 droplets were accordingly obtained, suggesting formation of a thin layer of gels on MgSO4 droplets at low relative humidity. The gel layer covered the surface and exhibited a significant delay in response to the change of ambient relative humidity, resulting in the structural difference between the surface and the inner of MgSO4 droplets.

This paper demonstrates an approach for investigating the hygroscopic growth of MgSO4 aerosols deposited on ZnSe substrate by using attenuated total reflection Fourier transform infrared (FTIR/ATR) spectroscopy. The experimental setup based upon a refitted standard FTIR/ATR accessory was adopted for the hygroscopic study of aerosols. It has been found that the “predeliquescence” of fine MgSO4 aerosol particles with the mean spreading diameter around 500 nm occurred before roughly 15% RH. In contrast, the abrupt water absorption of coarse MgSO4 particles with the mean diameter larger than about 10 μm was reported to occur at roughly 42% RH, which was reproduced in this study. Up to now, both theoretical and experimental investigations were rare and immature for fine particles, but the method we used in this study worked very well for the fine particles of MgSO4 aerosols. It has been found that the possible reason for the “predeliquescence” fine particles of MgSO4 aerosols is that the initial state or phase of fine particles is different from coarse ones after desiccation. This research demonstrates that the approach based on the ATR technique is very convenient, accurate and requires only a little amount of lab supplies.

Supersaturated NaClO4 aerosols have been studied using a Fourier transform infrared (FTIR) spectrometer coupled with an aerosol flow tube (AFT). Compared with previous Raman results, the water O–H stretching envelope in the supersaturated solutions of NaClO4 aerosols was more structured in response to changing RH, revealing at the same time the existence of water monomers weakly hydrogen-bonded with ClO4− at extremely high concentrations. Due to enhanced ion interactions in the supersaturated solutions of NaClO4 aerosols, the formation of contact ion pairs (CIPs) could be observed without component decomposition for the nondegenerate ν1 band of ClO4−, and the degenerate ν3 band of ClO4− was successfully related to the formation of CIPs in NaClO4 solutions. Based on these observations, a new mechanism featured by the attack of ClO4− upon hydrated Na+ for CIPs formation in the supersaturated solutions of NaClO4 aerosols was further proposed. The anhydrous NaClO4, characterized by the upper limit deliquescence relative humidity (DRH) of ∼43% and the disappearance of the ν1 band of ClO4− in the infrared spectra, was observed to form on the silicon windows at low RHs.

The ATR-FTIR spectra of aqueous LiClO4, NaClO4, and Mg(ClO4)2 solutions with ClO4− concentrations ranging from 0 to 3.00 mol dm−3 were obtained. After subtracting the spectra of pure water, positive peaks on the high wavenumber side and negative peaks on the low wavenumber side of the O–H stretching bands are observed in the difference spectra. The positive peaks appear constantly at about 3580 cm−1 independent of cation, which are assigned to the water molecules weakly hydrogen-bonded with ClO4−. However, the negative peaks appear at 3203, 3196, and 3254 cm−1 for LiClO4, NaClO4, and Mg(ClO4)2 solutions, respectively, and the peak areas show significant difference with increasing the concentration of perchlorate anions and are dependent on cations. The negative peaks are attributed to the “structure breaking” effect of perchlorate ions on the hydrogen bond network of water, which is in agreement with Raman spectroscopic studies. Besides the “structure breaking” effect of ClO4− on destroying the strong hydrogen bonds of the water molecules with fully hydrogen-bonded five-molecule tetrahedral nearest neighbor structure, the difference of the negative peaks are the results of the different “structure making” effect of the three cations, which is consistent with the ability of the polarization and hydration, in the order of Na+<Li+≪Mg2+. The overall shifting of the v3 band of perchlorate ions towards low wavenumber with increasing the concentration of perchlorates is attributed to the presence of solvent separated ion pairs, i.e., M⋯(H2O)n⋯ClO4−. The symmetric stretching vibration (v1) of perchlorate ions, which is an infrared inactive mode for free perchlorate ions, shows a weak band at ∼930 cm−1 in a wide concentration range of the three systems. The appearance of the weak band is considered as the perturbation of the ZnSe/water interface on perchlorate ions.

•Vacuum FTIR was used to observe hygroscopic property and phase transition of malonic acid aerosols.•The evolution of hydrogen-bonding structures of malonic acid aerosols took place from (H2O)n-MA to MA-MA accompanying with phase transition in the dehumidifying process.•The stepwise efflorescence of MA aerosols and nucleation rates at different RHs are first reported.•Nucleation rates of MA aerosols increased with decreasing RH.A novel approach based on a combination of a pulse relative humidity (RH) controlling system and a rapid scan vacuum FTIR spectrometer was utilized to investigate the hygroscopic property and phase transition of malonic acid (MA) aerosols. By using this approach, both water vapor amount around the aerosols and water content within aerosols with sub-second time resolution were obtained. Based on the features of FTIR absorbing bands, it can be known that the evolution of hydrogen-bonding structures of malonic acid aerosols took place from (H2O)n-MA to MA-MA accompanying with phase transition in the dehumidifying process. And in present paper, the stepwise efflorescence of MA aerosols and nucleation rates at different RHs are first reported. Our observation has shown that the efflorescence of MA started at ∼17% RH and the nucleation rates increased with decreasing RH.

Micro-Raman spectra of supersaturated aerosols of sodium succinate were obtained. The conformation behavior of the succinate dianion as a function of relative humidity (RH) was investigated by combining micro-Raman spectroscopy with theoretical calculations. A shoulder at 968 cm−1 of the v(C–CO2−) band on the rise in more concentrated droplets was believed indicative of conformation transformations. The intensity ratio (I963/I997) of the v(C–CO2−) band at 963 cm−1 to the v(C–C) band at 997 cm−1, versus the molar water-to-solute ratio (WSR), was used to fathom the equilibrium between gauche and trans conformations. Before saturation (WSR = 25.8) for the droplets, the ratio of I963/I997 retains a value of ∼2.6 independent of WSR, indicating that the equilibrium was not disturbed in the dilute droplets. In supersaturated droplets (WSR < 25.8), however, the ratio sharply decreases from ∼2.6 to ∼1.1 at WSR = 9.6, which was attributed to the formation of contact ion pairs (CIPs).

The effect of gel formation on the mass transfer of water during evaporation or condensation from MgSO4 droplets is studied using aerosol optical tweezers coupled with Raman spectroscopy. In particular, the kinetics of water transport during hydration and dehydration are followed for variable step changes in relative humidity and compared with previous measurements using different methodologies. Slow diffusion of water in the particle bulk is shown to limit water evaporation and condensation from the aerosol. Desorption of water continues over a long time at the very low RH region and this is validated with complementary studies made by FTIR-ATR and measurements of water adsorption isotherms. The observations can be rationalized when considering the possible phase transformation of the gel structure at very low RHs. Finally, the influence of the duration of the drying time (RH ≤ 10%) on the kinetics of condensation during hydration is investigated. Apparent diffusion coefficients of water molecules in the gel are obtained, showing little dependence on the water activity and droplet composition, and are consistent with the slow removal of water during drying from pores formed at the gel transition RH.