Using an electrodynamic balance (EDB), single droplets of laboratory generated particles  can be levitated stationary for unambiguous in-situ characterization.

The EDB basically consists of two DC electrodes and a ring of an AC electrode. A charged particle inside the EDB experiences an electrostatic force due to the DC field, a time varying force due to the AC field, gravitational force, and any drag force due to relative movement with the ambient air. Because of the inertia of the particle, any particle off the center of the balance will experience a force, which on an AC cycle average, points to the center of the balance. When the electrostatic force due to the DC balances the weight and the drag force of the particle, the particle can be held stationary.    Effectively, the EDB serves as a highly sensitive gravimetric balance, with the mass (m) of the stationary droplet being proportional to the applied balancing DC voltage (Vdc). Any mass change of a levitated particle with the change of RH will result in the change of the balancing DC voltage. The mass fraction of solute of the particle (mfs = mass of solute / mass of (solute + water)) can be determined by measuring the ratio of Vdc to that of the same particle at a reference state of known composition.

Figure 1: A particle oscillating in the EDB because its weight is not balanced by the electrostatic force it experiences

Figure 2: A particle is held stationary in the EDB because its weight is balanced by the electrostatic force it experiences

Much of the work has been focused in studying the hygroscopic properties of atmospheric aerosols, which are important to many aspects of air pollution such as visibility, radiative transfer, cloud and fog particle formation, and atmospheric reaction and removal processes.  Hygroscopic properties are also important to the deposition, processing and storage of many pharmaceutical products. 

A fast technique for measuring the water activities of aerosols
Typical EDB measurements at ambient conditions take about 10 hours to complete a set of water activity - composition data. We have developed a method which enables the measurement of a set of water activity data within an hour.  This method facilitates the study of hygroscopic properties of semi-volatile species.

EDB data

Inorganic systems
1) NH₄NO₃ – (NH₄)₂SO₄ – H₂O at ambient temperatures
2) (NH₄)₂SO₄ at 5 –35 °C
3) Na – Cl – NO₃ – SO₄  at ambient temperatures
4) Mg – Cl – NO₃ – SO₄  at ambient temperatures
5) Na – NH₄ – Cl – NO₃–SO₄at ambient temperatures
6) Na – Mg – Cl – NO₃ –SO₄ at ambient temperatures
7 ) Ca - Mg - Na - Cl – NO₃ 

Organic systems
1) Glucose, Citric acid, and Sorbitol(pharmaceutical additives) at ambient temperatures. Peng C., Chow A. H. L. and Chan C. K. (2001)  “Hygroscopic Study of Glucose, Citric Acid and Sorbitol Using an Electrodynamic Balance: Comparison with UNIFAC Predictions”, Aerosol Science and Technology, 35, 753-758.

b. maleic acid + malic acid mixture; malonic acid + glutaric acid mixture.  Choi M. Y. and Chan C. K. (2002) “Continuous measurements of the water activities in aqueous droplets of water-soluble organic compounds”, J. Phys. Chem. A, 106, 4566-4572.

c. maleic acid/water. Choi M. Y. and Chan C. K. (2002) “Continuous measurements of the water activities in aqueous droplets of water-soluble organic compounds”, J. Phys. Chem. A, 106, 4566-4572.

3) Mixtures of NaCl or (NH₄)₂SO₄ with organics (glycerol, succinic acid, malonic acid, citric acid and glutaric acid) - see special notes on the excel file.   Choi M. Y. and Chan C. K. (2002) “Effect of organic species on the hygroscopic behaviors of inorganic aerosols”,  Environmental Science Technology, 36, 2422-2428.

4) Sodium and ammonium salts of organic acids: Na formate, Na acetate, NH4 oxalate, Na malonate, Na succinate, Na maleate, Na pyruvate, Na MSA. Peng C. and Chan C. K. (2001) “The water cycles of water soluble organic salts of atmospheric importance”, Atmospheric Environment, 35, 1183-1192.

5) Fulvic and Humic acids.   Chan, M. N. and Chan, C. K. (2003) “Hygroscopic properties of two model humic-like substances and their mixtures with inorganics of atmospheric importance”, Environ. Sci. Tech., 37, 5109-5115.

6) Levoglucosan, mannosan, and galactosan. Chan M. N., Chan C. K. (2005) “The hygroscopicity of water-soluble organic compounds in atmospheric aerosols: amino acid and biomass burning derived organic species”, Environmental Science and Technology, 39, 1555-1562.

7) Amino acids:  glycine, alanine, serine, glutamine, and threonine. Chan M. N., Chan C. K. (2005) “The hygroscopicity of water-soluble organic compounds in atmospheric aerosols: amino acid and biomass burning derived organic species”, Environmental Science and Technology, 39, 1555-1562.

8) Ammonium sulfate - glutaric acid; Ammonium sulfate + malonic acid; Ammonium sulfate + succinic acid. Ling, T. Y., and C. K. Chan (2008), Partial crystallization and deliquescence of particles containing ammonium sulfate and dicarboxylic acids, J. Geophys. Res., doi:10.1029/2008JD009779, in press.

If you are interested in downloading the measured data for research purpose, please click the item in the list above.  Data are in the form of  water activity of the droplet (a_w=RH/100) as a function of the total mass fraction of solute (mfs), defined as the ratio of the dry solute mass to the solution droplet mass. If you decide to use the data in your publication, references to this website or the relevant paper would be appreciated.

Projects

Humic Like Substances in Atmospheric Aerosols: Abundance, Size Distributions and Hygroscopic Properties
Sponsor: Hong Kong Research Grants Council Earmarked Grant (HKUST6056/02P)
PM & PI:  Dr. Chak K. Chan
CI: FANG, Ming, Director, Institute for Environment and Sustainable Development, HKUST
Status: Completed

In this study, the hygroscopicity of HULIS and the mixture of HULIS and sodium chloride (NaCl) and that of HULIS and ammonium sulfate (AS) aerosols at a mass ratio of 1:1 are studied using two natural FA: the Nordic Aquatic Fulvic Acid (NAFA) and the Suwannee River Fulvic Acid (SRFA) as model compounds in an electrodynamic balance. NAFA and SRFA both absorbed and desorbed water reversibly without crystallization and retained water at a relative humidity (RH) < 10%. NAFA and SRFA have a mass growth ratio of 1.25 and 1.45 from RH = 10% to RH = 90%, respectively. However, these results are different from those of another natural FA (the Nordic River Fulvic Acid Reference) in the literature. The differences are possibly due to the differences in the chemical composition of natural FA, which depends on their sources and the isolation methods. These results suggest that a standardization of the isolation methods of HULIS is needed for better understanding of their atmospheric properties and environmental impacts. In general, the deliquescence and crystallization RH of FA-inorganic mixtures are comparable with those of their respective pure inorganic species.

Amino acids and organic species derived from biomass burning can potentially affect the hygroscopicity and cloud condensation activities of aerosols. The hygroscopicity of 7 amino acids (glycine, alanine, serine, glutamine, threonine, arginine, and asparagine) and 3 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 UNIFAC 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 and the RH dependence. A generalized growth law (G_f = (1-RH/100)^-0.095 ) is proposed for levoglucosan, mannosan, and galactosan particles.

Publications: Journal Paper

Chan M.N. and Chan C.K. (2005) “The Hygroscopicity of Water-Soluble Organic Compounds in Atmospheric Aerosols: Amino Acid and Biomass Burning Derived Organic Species”, Environmental Science and Technology, 39, 1555-1562.
Chan M. N. and Chan C. K. (2003) “Hygroscopic Properties of Two Model Humic-Like Substances and Their Mixtures with Inorganics of Atmospheric Importance”, Environmental Science and Technology, 37, 5109 - 5115.

Conference Presentation

Chan, M. N. and Chan, C. K. (2003) “Hygroscopicity of a model humic-like substance and its mixture with sodium chloride”, presented at the European Aerosol Conference, 31 August-5 September, Madrid, Spain.
Chan M. N. and Chan C. K. (2004) “Hygroscopicity of biomass burning derived organic species in atmospheric aerosols”, Abstract of the European Aerosol Conference 2004, Sept 6-10, Budapest, Hungary, Journal of Aerosol Science, S475-476.
Chan M. N. and Chan C. K. (2004) “Hygroscopicity of biomass burning derived organic species in atmospheric aerosols”, Abstract of the 16^th International Conference on Nucleation and Atmospheric Aerosols, July 26-30, Kyoto, Japan, 686-689.
Chan M. N. and Chan C. K. (2004) “Hygroscopic properties of a natural occurring fulvic acid – a representative humic-like substance in atmospheric aerosols”, Abstracts of the 3^rd Asian Aerosol Conference, Jan 6-9, Hong Kong, p. 42-43.

The role of water-soluble organic compounds (WSOC) on the hygroscopic growth of atmospheric aerosols
Sponsor: Hong Kong Research Grants Council Earmarked Grant (HKUST6039/00P)
PM & PI:  Dr. Chak K. Chan
Status: Completed

In this study, the water cycles of nine WSOCs of atmospheric interest were studied using an electrodynamic balance (EDB) at 25^oC. Sodium formate, sodium acetate, sodium succinate, sodium pyruvate and sodium methanesulfonate (Na-MSA) particles crystallize as the relative humidity (RH) decreases and they deliquesce as the RH increases. Sodium oxalate and ammonium oxalate form supersaturated particles at low RH before crystallization but they do not deliquesce even at RH = 90%. Sodium malonate and sodium maleate particles neither crystallize nor deliquesce and they absorb and evaporate water reversibly without hysteresis. In most cases, the solid formed from the crystallization of supersaturated droplets is not the most thermodynamically stable state found in bulk studies. Sodium formate, sodium oxalate, ammonium oxalate, sodium succinate, sodium pyruvate and Na-MSA form anhydrous particles after crystallization. Sodium acetate forms particles with a water/salt molar ratio of 0.5 after crystallization. The hygroscopic organic salts have a growth factor of 1.76-2.18 from RH = 10% to 90%, comparable to that of typical hygroscopic inorganic salts such as NaCl and (NH₄)₂SO₄.

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 DRH of NaCl and ammonium sulfate (AS) were also studied using scanning EDB (SEDB) (Choi and Chan, 2002). This SEDB method reduces the experimental from more than 10 hours to within an hour, which significantly reduces the evaporation loss, making it suitable for semi-volatile organic water activity measurement. 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. The ZSR model was found to be useful in predicting the water activity of the mixtures and the growth ratios. Efforts in detailed thermodynamic modeling of inorganic and organic mixed systems are needed to understand the deliquescence properties of these mixtures.

Publications: Journal Paper

Peng, C. and Chan, C. K. (2001) “The water cycles of water soluble organic salts of atmospheric importance”, Atmospheric Environment, 35, 1183-1192.

Peng C., Chan M. N. and Chan C. K. (2001) “The hygroscopic properties of dicarboxylic and multifunctional acids: measurements and UNIFAC predictions” Environmental Science Technology, 35, 4495-4501.

Choi M. Y. and Chan C. K. (2002) “Effect of organic species on the hygroscopic behaviors of inorganic aerosols”, Environmental Science Technology, 36, 2422-2428.

Choi M. Y. and Chan C. K. (2002) “Continuous measurements of the water activities in aqueous droplets of water-soluble organic compounds”, J. Phys. Chem. A., 106(18), 4566-4572.

Hygroscopic Properties of Atmospheric Aerosols
Sponsor: Hong Kong Research Grants Council Earmarked Grant (HKUST665/96P)
PI: Chak K. Chan
Status: Completed

We have measured the water activities of the following systems:

1) MgCl2, Mg(NO3)2 and MgSO4 and their mixtures,
2) Mixtures of NH4+ and Na+ salts of Cl-, NO3-, and SO42-,
3) Mixtures of Na+ and Mg2+salts of Cl-, NO3-, and SO42-
4) Mixture of NaCl and Disodium Fluorescein (a non-deliquescent salt)

The measured data were compared with available literature data and the predictions of the ZSR and KM equations.  In general, they give accurate predictions.  The AIM model is especially useful in predicting the water activities of partially crystallized droplets of Na+-NH4+ mixtures.

One of the most interesting results out of the above measurements is that MgSO4 and Na2SO4/MgSO4 mixtures exhibit significant mass transfer limitations, not found in other Na+ and Mg2+ salts we studied, in our hygroscopic measurements.  While these mass transfer effects may not be a problem in ambient aerosols since their life times are usually longer than a few hours, which is the time scale of the mass transfer found, they complicate measurements of hygroscopic properties of ambient aerosols using instruments such as the Tandem Differential Mobility Analyzer, in which particles stay for a few seconds. To understand the special behavior of MgSO4, we have used Raman spectroscopy to study the molecular structures of aqueous MgSO4 (and (NH4)2SO4) droplets at high concentrations.  Spectral changes in the sulfate peak at 980 cm-1 were found in MgSO4 droplets, not found in (NH4)2SO4 droplets even at supersaturation, at a water-to-solute molar ratio smaller than 6. These spectral changes are attributed to direct contact pairs without water of hydration between the magnesium and sulfate ions at low water-to-solute ratios. Mass transfer limitations have also been found in the hygroscopic measurements of other species such as sodium pyruvate and magnesium acetate.

Our measurements of water activities of mixtures of NaCl and Disodium Fluorescein, which behaves like a non-deliquescent salt in single particle studies, suggest that organic salts can profoundly alter the hygroscopic properties of typical inorganic aerosols.  We have started a series of hygroscopic measurements involving atmospheric water soluble organic compounds earlier this year (RGC CERG earmarked grant HKUST 6039/00P).

We have also developed a simple method to determine water activities of binary solutions based on ternary solution data. We are able to estimate the water activities of (NH4)2SO4 solutions for ionic strengths up to 112 mol kg-1, of NaCl solutions for ionic strengths up to 25 mol kg-1 and of NH4Cl solutions up to 42 mol kg-1.  These are new water activity relations that can be used to improve modeling tools for multicomponent aerosols.

Publications

Chan C. K., Choi M. Y., and Zhang Y. (2000) “Observation of mass transfer limitation in evaporation of single levitated droplets”, J. Aerosol Sci., 31(S1) S989-990.
Zhang Y. and Chan C. K. (2000) “Study of Contact Ion Pairs of Supersaturated Magnesium Sulfate Solutions using Raman Scattering of Levitated Single Droplets”, J. Physical Chemistry A., 104, 9191-1916.
Chan, C. K., Ha, Z., and Choi, M. Y. (2000) “Study of water activities of supersaturated aerosols of mixtures of sodium and magnesium salts”, Atmospheric Environment, 34, 4795-4803.
Ha, Z., Choy L. and Chan C. K. (2000) “Study of water activities of supersaturated aerosols of sodium and ammonium salts”, J. Geophysical Research, 105(D9), 11,699-11,709.
Chan C. K. and Ha, Z. (1999) “A simple method to derive the water activities of highly supersaturated binary electrolyte solutions from ternary solution data”, J. Geophysical Research, 104(D23), 30,193-30,200.
Ha, Z. and Chan C. K. (1999) “The water activities of MgCl2, Mg(NO3)2, MgSO4 , and their mixtures”,  Aerosol Sci Tech. 31, 154-169.
Chan, C. K., Kwok, C. S., A. H. L. Chow (1997) “Water activity of mixed organic and inorganic aerosols”, J. Aerosol Sci., 28, Suppl. 1, S77-S78.
Chan, C. K., Kwok, C. S., and Chow, A, H.-L. (1997) “Study of hygroscopic properties of aqueous mixtures of disodium fluorescein and sodium chloride using an electrodynamic balance”, Pharm. Res.,14(9), 1171-1175.

Conference papers
Ha Z, Choy, L. and Chan C. K. (1999) “Water activity of mixtures of sodium and ammonium salts”, paper 3D1, presented at the 18th Annual meeting of the American Association of Aerosol Research, Oct 11-15, Tacoma, WA, USA.
Ha, Z. and Chan C. K. (1999) “Water activity of mixtures of sodium and magnesium salts”, paper 12PA6, presented at the 18th Annual meeting of the American Association of Aerosol Research, Oct 11-15, Tacoma, WA, USA.
Chan, C. K. and Ha Z. (1999) “Extending water activity data of single component electrolyte solutions to lower values”, paper 13C2, presented at the 18th Annual meeting of the American Association of Aerosol Research, Oct 11-15, Tacoma, WA, USA.
Peng, C. G., Choy, L., and Chan C. K. (1999) “Study on hygroscopic properties of low molecular weight carboxylic acids/salts”, paper 13C3, presented at the 18th Annual meeting of the American Association of Aerosol Research, Oct 11-15, Tacoma, WA, USA.
Chan C. K. and Ha Z. (1999) Hygroscopic Property of Seasalt and Crustal Aerosols, paper S3-1, presented at the 1st Asian Aerosol Conference, July 27-29, Nagoya, Japan.

Sponsor: Hong Kong Research Grants Council Earmarked Grant ( HKUST6121/97P)
PM and PI: Dr. Chak K. Chan; CI: Dr. Albert Chow (CUHK) and Dr. Guo Hua Chen (HKUST)
Status: Completed

Key Results:

The water cycles of the three common tablet drug additives glucose, citric acid and sorbitol at 25^oC and the four respiratory drugs disodium cromoglycate (DSCG), atropine sulfate (AS), isoproterenol hydrochloride (IPHC) and isoproterenol hemisulfate (IPHS) at 37^oC were studied using single levitated droplets in an electrodynamic balance (EDB). The water activities of bulk samples were also measured. Crystallization and deliquescence were not observed for any the chemicals studied except for DSCG. Glucose and sorbitol contains no water at relative humidity below 20% but citric acid contains 5% of residual water even at RH=10% at 25^oC. Glucose, citric acid and sorbitol can increase 106%, 108% and 114% of weight as a result of water absorption from a RH of 10% to 90% at 25^oC, respectively. UNIFAC gives accurate predictions of water activities of the additives. AS, IPHC, IPHS and DSCG increase in mass by 114%, 80%, 66%, and 43% from a RH of 10% to 90% at 37^oC, respectively. DSCG is in solid state in the RH range of 10-90% and is less hygroscopic compared with the other three drugs.  (Peng et al., 2000ab). We have also developed a method to continuously study the growth of levitated particles of semi-volatile species. This method can shorten the time of measurement of a water cycle from over 10 hours to within an hour (Choi and Chan, 2002). 

Publications: Journal Papers

Choi M. Y. and Chan C. K. (2002) “Continuous measurements of the water activities in aqueous droplets of water-soluble organic compounds”, J. Phys. Chem. A., 106(18), 4566-4572.

Peng, C., Chow, A. H. L. and Chan, C. K. (2001). Hygroscopic Study of Glucose, Citric Acid and Sorbitol Using an Electrodynamic Balance: Comparison with UNIFAC Predictions. Aerosol Sci. Tech., 35,753-758.

Peng, C., Chow, A. H. L. and Chan, C. K. (2000). Study of the Hygroscopic Properties of Selected Pharmaceutical Aerosols Using Single Particle Levitation. Pharmaceutical Research, 17, 1104-1109.

 Student Graduated:

Peng, C.G., MPhil in Chemical Engineering, 2000.

 Student Under Training:

Choi, M. Y., PhD in Chemical Engineering