Yun-Hong Zhang

ATR-FTIR spectroscopic studies on aqueous LiClO4, NaClO4, and Mg(ClO4)2 solutions

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.

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.

Observation of Conformational Changes in 1-Propanol−Water Complexes by FTIR Spectroscopy

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 approximately 1053 and approximately 1068 cm(-1), assigned to the vibrational modes of the gauche (v(C-C-C-O-G)) and the trans (v(C-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 (A(vC-C-C-O-G)/A(vC-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 (A(vC-C-C-O-G)/A(vC-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 A(vC-C-C-O-G)/A(vC-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.

Molecular events in deliquescence and efflorescence phase transitions of sodium nitrate particles studied by Fourier transform infrared attenuated total reflection spectroscopy

The NaNO(3) droplets with sizes of 1-5 microm generated from a nebulizer were deposited on a ZnSe substrate in a Fourier transform infrared attenuated total reflection (FTIR-ATR) chamber. After solidification of the droplets with dry N(2) gas passing through the chamber, the solid NaNO(3) particles were monitored by in situ FTIR-ATR spectra in cycles of deliquescence and efflorescence processes with varying relative humidities (RHs). With an increase in the RH, a dominant peak at approximately 3539 cm(-1), together with three relatively weak peaks at approximately 3400, approximately 3272, and approximately 3167 cm(-1), in the O-H stretching band of water was resolved by the high signal-to-noise ratio FTIR-ATR spectra. The dominant peak and the three relatively weak peaks were contributed by the water monomers and the aggregated water molecules adsorbed on the surfaces of solid NaNO(3) particles, respectively. When the RH approached approximately 72%, slightly lower than the deliquescence RH (74.5%), the band component at approximately 3400 cm(-1) became the main peak, indicating that the water monomers and the aggregated water molecules aggregated to form a thin water layer on the surfaces of solid NaNO(3) particles. A splitting of the nu(3)-NO(3)(-) band at 1363 and 1390 cm(-1) at the RH of approximately 72%, instead of the single nu(3)-NO(3)(-) band at 1357 cm(-1) for the initial solid NaNO(3), was observed. We suggested that this reflected a phase transition from the initial solid to a metastable solid phase of NaNO(3). The metastable solid phase deliquesced completely in the region from approximately 87% to approximately 96% RH according to the fact that the nu(3)-NO(3)(-) band showed two overlapping peaks at 1348 and 1405 cm(-1) similar to those of bulk NaNO(3) solutions. In the efflorescence process of the NaNO(3) droplets, the nu(1)-NO(3)(-) band presented a continuous blueshift from 1049 cm(-1) at approximately 77% RH to 1055 cm(-1) at approximately 36% RH, indicating the formation of contact ion pairs between Na(+) and NO(3)(-). Moreover, in the RH range from approximately 53% down to approximately 26%, two peaks at 836 and 829 cm(-1) were observed in the nu(2)-NO(3)(-) band region, demonstrating the coexistence of NaNO(3) solid particles and droplets.

Kinetics of heterogeneous reaction of ozone with linoleic acid and its dependence on temperature, physical state, RH, and ozone concentration.

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.

Micro-Raman and FTIR Spectroscopic Observation on the Phase Transitions of MnSO4 Droplets and Ionic Interactions between Mn2+ and SO42−

Micro-Raman and FTIR spectroscopy have been used to investigate the micrometer-sized MnSO(4) 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 MnSO(4) droplets, when the RH decreased from approximately 94% to approximately 76%, the full width of the peak at half-maximum (fwhm) of the v(1)-SO(4)(2-) band at 983 cm(-1) initially increased from 95.7 to 104.2 cm(-1). Two shoulders at approximately 993 and approximately 1002 cm(-1) occurred in the v(1)-SO(4)(2-) band at approximately 76% RH, while the v(2)-SO(4)(2-) 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 MnSO(4) droplets. From component band analysis of the v(1)-SO(4)(2-) band, four peaks at 983, 993, 1002, and 1010 cm(-1) were identified and assigned to the free SO(4)(2-), monodentate CIPs, bidentate CIPs, and more complex ion aggregates, respectively. Signatures of monodentate and bidentate CIPs reached their maximum values at approximately 76% and 60% RH, respectively. With further decreasing the RH, great abundance of various ion pairs presented in the droplet, corresponding the v(1)-SO(4)(2-) band steadily blue-shifted, and the fwhm increased continuously until it went through a plateau at approximately 60% RH. When the RH was between approximately 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 MnSO(4).2.8H(2)O. Further decreasing the RH below 13% RH, monodentate CIPs changed to bidentate ones. In the meantime, MnSO(4).2.8H(2)O was deduced to transform into another amorphous phase with a stoichiometry of MnSO(4).1.7H(2)O. This transition was also supported by the observation of the FTIR spectra according to a sharp increase of the fwhm of the v(3)-SO(4)(2-) band (at approximately 1091 cm(-1)) because of the appearance of a shoulder at 1132 cm(-1).

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.

Probing the time scale for bulk equilibration and mass transport of water in amorphous inorganic aerosol.

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 D(2)O by H(2)O in droplets initially composed of MgSO(4)/D(2)O. 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) m(2) 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.

Theoretical understanding on the v1‐SO band perturbed by the formation of magnesium sulfate ion pairs

The factors determining the spectroscopic characteristics of the v1‐SO42− band of the MgSO4 ion pairs are discussed via ab initio calculation, including coupling effect, hydrogen bonding effect, and direct contact effect of Mg2+ with SO42−. With the calculation of the heavy water hydrated contact ion pairs (CIP), the overlap between the librations of water and the v1‐SO42− band can be separated, and thus the coupling effect is abstracted, and this coupling effect leads to a blue shift for the v1‐SO42− band of 5.6 cm−1 in the monodentate CIP and 3.6 cm−1 in the bidentate CIP. The hydrogen bonding between each water molecule without relation to Mg2+ and the sulfate ion makes the v1‐SO42− band blue shift of 3.7 cm−1. When the outer‐sphere water around Mg2+ are hydrogen bonded between SO42− and Mg2+, it will make the largest disturbance to the v1‐SO42− band. Moreover, the inner‐sphere water can affect the v1‐SO42− band conjunct with the direct contact of Mg2+ with SO42−, showing a blue shift of 14.4 cm−1 in the solvent‐shared ion pair, 22.6 cm−1 in the monodentate CIP, 4.3 cm−1 in the bidentate CIP, and 21.4 cm−1 in the tridentate CIP. At last, the Raman spectral evolution in the efflorescence production process is tried to be rationalized. The shoulder at 995 cm−1 is attributed to the monodentate CIP with 2–3 outer‐sphere water molecules, whereas the new peak at 1021 cm−1 at high concentration is assigned to the formation of aqueous triple ion.

A new approach to determine vapour pressures and hygroscopicities of aqueous aerosols containing semi-volatile organic compounds

We present a new approach to study the equilibrium gas-particle partitioning of volatile and semi-volatile organic components in aqueous aerosol, deriving a correlational analysis method that examines and interprets simultaneous and correlated fluctuations in particle size and composition. From this approach, changes in particle size driven by organic component evaporation can be clearly resolved from size changes driven by hygroscopicity and fluctuations in environmental conditions. The approach is used to interpret measurements of the evaporation of semi-volatile organic components from binary aqueous/organic aerosol and the hygroscopic growth of involatile inorganic aerosol. The measurements have been made by the aerosol optical tweezers technique, which allows the simultaneous retrieval of particle size and refractive index with high accuracy. We suggest that this approach will be particularly valuable for investigating the thermodynamic behaviour of mixed component aqueous aerosol and will allow the accurate derivation of solution phase equilibrium properties that are prone to large uncertainties when measurements are made simply of the change in particle size with gas phase relative humidity.