Li-Jun Zhao

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.

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

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

Micro-Raman study on the conformation behavior of succinate in supersaturated sodium succinate aerosols

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-CO(2)(-)) band on the rise in more concentrated droplets was believed indicative of conformation transformations. The intensity ratio (I(963)/I(997)) of the v(C-CO(2)(-)) 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 I(963)/I(997) retains a value of approximately 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 approximately 2.6 to approximately 1.1 at WSR = 9.6, which was attributed to the formation of contact ion pairs (CIPs).

State of Water in Supersaturated Nitrate Aerosols Disclosed by the Raman Difference Spectra

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(NO(3))(2) and NaNO(3) 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 (W(free)) as a function of RH, as well as the solvated water-to-solute ratio (W(solvated)SR) and the free water-to-solute ratio (W(free)SR) 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(NO(3))(2) aerosols, three phases were identified with distinct mechanisms for the transition of the state of water. One unique structure with W(solvated)SR = 4 was proposed to occur in supersaturated Mg(NO(3))(2) aerosols at low RHs. In NaNO(3) 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 NaNO(3) aerosols, but also in relatively diluted droplets.