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05/2006;
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ABSTRACT: Amino acids and organic species derived from biomass burning can potentially affect the hygroscopicity and cloud condensation activities of aerosols. The hygroscopicity of seven amino acids (glycine, alanine, serine, glutamine, threonine, arginine, and asparagine) and three organic species most commonly detected in biomass burning aerosols (levoglucosan, mannosan, and galactosan) were measured using an electrodynamic balance. Crystallization was observed in the glycine, alanine, serine, glutamine, and threonine particles upon evaporation of water, while no phase transition was observed in the arginine and asparagine particles even at 5% relative humidity (RH). Water activity data from these aqueous amino acid particles, except arginine and asparagine, was used to revise the interaction parameters in UNIQUAC functional group activity coefficients to give predictions to within 15% of the measurements. Levoglucosan, mannosan, and galactosan particles did not crystallize nor did they deliquesce. They existed as highly concentrated liquid droplets at low RH, suggesting that biomass burning aerosols retain water at low RH. In addition, these particles follow a very similar pattern in hygroscopic growth. A generalized growth law (Gf = (1 - RH/100)-0.095) is proposed for levoglucosan, mannosan, and galactosan particles.
Environmental Science and Technology 04/2005; 39(6):1555-62. · 5.23 Impact Factor
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ABSTRACT: The phase transition is one of the most fundamental phenomena affecting the physical and chemical properties of atmospheric aerosols. Efflorescence, in particular, is not well understood, partly because the molecular interactions between the solute and water molecules of saturated or supersaturated solution droplets have not been well characterized. Recently, we developed a technique that combines the use of an electrodynamic balance and a fluorescence dye, 8-hydroxyl-1,3,6-pyrenetrisulfonate (pyranine), to study the distributions of solvated and free water in aqueous droplets (Choi, M. Y.; Chan, C. K.; Zhang, Y. H. J. Phys. Chem. A 2004, 108, 1133). We found that the equality of the amounts of solvated and free water is a necessary but not sufficient condition for efflorescence. For efflorescing compounds such as Na2SO4, (NH4)2SO4, and a mixture of NaCl and Na2SO4, the amount of free water decreases, while that of solvated water is roughly constant in bulk measurements and decreases less dramatically than that of free water in single-particle measurements as the relative humidity (RH) decreases. Efflorescence of the supersaturated droplets of these solutions occurs when the amounts of free and solvated water are equal, which is consistent with our previous observation for NaCl. For nonefflorescing compounds in single-particle levitation experiments such as MgSO4 and Mg(NO3)2, the amounts of free and solvated water are equal at a water-to-solute molar ratio of about 6, at which spectral changes due to the formation of contact ion pairs between magnesium and the anions occur as shown by Raman spectroscopy. Fluorescence imaging shows that the droplets of diluted Mg(NO3)2 (at 80% RH) and MgSO4 are homogeneous but those of NaCl, Na2SO4, (NH4)2SO4, and supersaturated Mg(NO3)2 (at 10% RH) are heterogeneous in terms of the solvated-to-free water distribution. The solvated-to-free water ratios in NaCl, Na2SO4, and (NH4)2SO4 droplets are higher in the outer regions by about half a radius deep than at the center of the droplets.
The Journal of Physical Chemistry A 03/2005; 109(6):1042-8. · 2.95 Impact Factor
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ABSTRACT: The formation of contact ion pairs in Mg(NO3)2 solutions and their effects on the hygroscopic properties of the solutions were studied using Raman spectroscopy of Mg(NO3)2 droplets levitated in an electrodynamic balance. Upon reduction in the ambient relative humidity (RH), Mg(NO3)2 droplets lose water but do not effloresce. The molar water-to-solute ratio (WSR) decreases exponentially with decreasing RH, but it decreases linearly with RH when it is less than 6. This transition of hygroscopicity at WSR = 6 coincides with an abrupt blue shift of the ν1 band of NO3- from 1049 to 1055 cm-1 in the Raman spectra, which is due to nitrate ions entering into the primary solvation shell of Mg2+ ions to form direct contact ion pairs. As the WSR further decreases, a shoulder on the higher wavenumber side of the ν1 band appears near 1060 cm-1 and the separation of ν3 bands increases because of the formation of more complex contact ion pairs. Raman images of supersaturated droplets at WSR = 2.8 show structural heterogeneity not found in diluted droplets. Overall, the NO3- to Mg2+ ratio increases and the H2O to Mg2+ ratio decreases as the surface of the droplet is approached. However, droplets at low concentration (WSR = 9.6) show a uniform distribution of monodentates.
02/2004;
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ABSTRACT: The knowledge of the state of water molecules, particularly the amounts of solvated water and free water in aqueous droplets, is valuable in understanding the hydration properties of atmospheric aerosols. A novel technique combining the use of an electrodynamic balance and a fluorescence dye, 8-hydroxyl-1,3,6-pyrenetrisulfonate (pyranine), was used to study the state of the water molecules in single levitated aqueous droplets from subsaturation to supersaturation concentrations. The steady-state fluorescence spectra of sucrose, glucose, and NaCl solutions doped with 100 ppm pyranine were measured. The fluorescence emission of pyranine is sensitive to the proton-transfer capacity of its microenvironment. When excited by radiation at around 345 nm, pyranine fluoresces and the spectrum consists of two peaks, one at about 440 nm and the other at about 510 nm, which correspond to the presence of solvated and free water, respectively. The fluorescence peak intensity ratios of the 440-nm peak to the 510-nm peak and the hygroscopic measurements were used to calculate the amounts of solvated and free water in the droplets as a function of relative humidity. The amount of free water equals the amount of solvated water when crystallization or saturation (for noncrystallizating chemicals) occurs. Imaging analysis has revealed that the solvated to free water ratio oscillates within the droplets, which indicates the spatial heterogeneity of aqueous droplets. This study demonstrates that fluorescence spectroscopy is a unique tool in understanding the hydration properties, the efflorescence, and the structural heterogeneity of aqueous droplets.
01/2004;
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ABSTRACT: Condensational growth and deliquescence are important characteristics of atmospheric aerosols. A scanning electrodynamic balance (SEDB), based on a well-characterized increase of relative humidity (RH) in the vicinity of a levitated particle in an electrodynamic balance, was used to measure the water activity of an equal molar aqueous solution of CaCl2 + Ca(NO3)2, MgCl2 + CaCl2, Mg(NO3)2 + Ca(NO3)2, NaCl + Ca(NO3)2, and NaNO3 + Ca(NO3)2 mixtures in both evaporation and growth modes. These mixtures were selected because of the lack of literature data at high concentrations. CaCl2 + Ca(NO3)2, MgCl2 + CaCl2, and Mg(NO3)2 + Ca(NO3)2 mixtures neither crystallize nor deliquesce because the solutes are nondeliquescent in single particle measurements. The presence of nitrate suppresses the crystallization of NaCl, since efflorescence was not observed even at RH = 20%. The water cycles of NaCl + Ca(NO3)2 (molar ratio = 3:1) and NaNO3 + Ca(NO3)2 (3:1) mixtures were also measured. The results show that the presence of Ca(NO3)2 lowers the deliquescence relative humidity values of NaCl and NaNO3 and alters their hygroscopicity. The Zdanovskii−Stokes−Robinson (ZSR) and Simulating Composition of Atmospheric Particles at Equilibrium (SCAPE) predictions are consistent with the water activity measurements except for the Mg(NO3)2 + Ca(NO3)2 mixtures, for which the predictions show significant deviations from the measurements at low RH. For the Mg(NO3)2 + Ca(NO3)2, NaCl + Ca(NO3)2, and NaNO3 + Ca(NO3)2 mixtures, the ZSR model prediction shows larger deviation when compared with the measurements than does the SCAPE model throughout the range of water activity studied.
10/2002;
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ABSTRACT: Water-soluble organic compounds have recently received much attention because of their ability to absorb water and alter the hygroscopic properties of inorganic aerosols. The effects of glycerol, succinic acid, malonic acid, citric acid, and glutaric acid on the water cycles (water activities during particle evaporation and growth), crystallization relative humidities (CRH), and deliquescence relative humidities (DRH) of sodium chloride (NaCl) and ammonium sulfate (AS) were studied using an electrodynamic balance (EDB). The growth factors of these inorganic and organic mixtures were lower than those of the pure inorganic species. The presence of all these organics in the mixed particle reduce the water absorption of NaCl but enhance that of AS relative to that of the pure inorganic salts. Glycerol and succinic acid did not affect the deliquescence properties of NaCl and AS, although succinic acid increased the CRH of NaCl and AS. Malonic acid and citric acid, behaving as nondeliquescent species in single particle studies, caused NaCl and AS particles to absorb a significant amount of water before deliquescence. Glutaric acid caused NaCl and AS to deliquesce gradually, spanning a wide range of relative humidity. The ZSR model was found to be useful in predicting the water activity of the mixtures and the growth ratios. However, the detailed crystallization and deliquescence behaviors of the organic/inorganic mixtures cannot be easily predicted from the hygroscopic properties of the individual components.
Environmental Science and Technology 07/2002; 36(11):2422-8. · 5.23 Impact Factor
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ABSTRACT: Water-soluble organic compounds have recently received much attention because of their ability to absorb water and affect the radiation balance and the climate. Partly because of their relatively high volatility, thermodynamic data on water-soluble organic compounds are scarce. Recently, we have developed a method based on the scanning electrodynamic balance (SEDB) that enables the measurement of water activity data of evaporating droplets within an hour, which can potentially be used to measure volatile species. This paper demonstrates the use of the SEDB to study the hygroscopic growth of selected atmospheric species, including semivolatile organic species with vapor pressure up to 1 × 10-4 mmHg. We also measured the water activities, the crystallization relative humidity, and the deliquescence relative humidity (DRH) of aqueous solutions of maleic acid and glutaric acid. The DRHs of maleic acid and glutaric acid are in general agreement with the literature, except that glutaric acid shows a small delay in the completion of deliquescence due to mass-transfer limitation. The water activities of equal molar mixtures of maleic acid and malic acid and of malonic acid and glutaric acid were also measured. The Zdanovskii−Stokes−Robinson (ZSR) predictions agree well with the measurements of the mixtures. The UNIFAC (UNIQUAC functional group activity coefficients) predictions, using the modified functional group interaction parameters of COOH, OH, and H2O derived from our earlier measurements of the water activities of aqueous droplets of a list of dicarboxylic and multifunctional acids, are also in agreement with the mixture data.
04/2002;
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ABSTRACT: Sodium and magnesium are major components of sea salt aerosols. Their salts of chlorides, sulfates and nitrates are hygroscopic. In this research, water activities of aqueous solutions of NaCl–MgCl2 (molar ratio 1 : 1), NaNO3–Mg(NO3)2 (1 : 1), Na2SO4–MgSO4 (1 : 1), NaCl–Mg(NO3)2 (1 : 1 and 3 : 1), NaCl–MgSO4 (1 : 1), and NaNO3–MgSO4 (1 : 1) were measured from dilute concentrations to high supersaturations using an electrodynamic balance. We employed a dynamic method, which is based on continuous measurements of the balancing voltage of the particle as a result of a step decrease of the relative humidity (RH) of the feed to the electrodynamic balance. Good agreements are found between the predictions of the ZSR (Zdanovskii–Stokes–Robinson) and the KM (Kusik and Meissner) models and the experimental data, with the exception of Na2SO4–MgSO4 solutions. Analysis of the time-series data reveals that Na2SO4–MgSO4 solution has a lower evaporation rate than other solutions. We postulate that this is due to internal mass transfer limitation when the particle forms a gel at high concentration. Partial crystallization of NaCl–MgCl2, NaCl–Mg(NO3)2 (3 : 1), and NaCl–MgSO4 mixtures occurs in the measurements taken at low RHs. The particles continue to lose water as RH decreases further. Other systems, including NaCl–Mg(NO3)2 (1 : 1), do not crystallize even at RH=30%. Crystallization of NaCl is suppressed when sufficient amount of nitrate is present.
Atmospheric Environment.
