Atmospheric Nanoparticles

Below 50 nm, the surface energy of the particles provides a significant contribution to their overall free energy. As a result, the deliquescence and crystallization relative humidities can significantly change. In addition, due to the Kelvin effect the hygroscopic growth of atmospheric nanoparticles is expected to be lower compared to that of their larger counterpart. Understanding these differences is important because particles having diameters between 1 and 100 nm are the ubiquitous and abundant precursors to the larger particles that strongly influence global climate. We are investigating how physical state depends on particle diameter by using two experimental approaches (namely hygroscopic TDMA measurements and Environmental-TEM observations) and computational predictions.

Hygroscopic Tandem nano-DMA Experiments

A tandem nano-DMA (TnDMA) has been built to investigate the hygroscopic properties of common atmospheric nanoparticles. As shown in the figure, the apparatus consists of two nano-DMAs (TSI Model 3085), an ultrafine condensation particle counter (CPC) (TSI Model 3025), and a set of Nafion tube conditioners. DMA-1 produces a monodisperse sample from a dry, polydisperse input aerosol. The aerosol is then exposed to one of two RH histories, depending upon whether deliquescence- or efflorescence-mode experiments are being conducted. DMA-2, in conjunction with the ultrafine CPC, measured the number size distributions of the particles subsequent to the applied RH history.

Using this setup, we have measured the deliquescence and efflorescence relative humidity values of 6- to 60-nm NaCl particles. The deliquescence relative humidity (DRH) increased when the dry particle mobility diameter decreased below approximately 40 nm. The efflorescence relative humidity (ERH) similarly increased. For example, the DRH and ERH of 6-nm particles were 87% and 53%, respectively, compared to 75% and 45% for particles larger than 40 nm. The growth factors steadily decreased within detection limit for dry sizes below 40 nm. The decrease was found to be quantitatively predicted by a model that includes the Kelvin effect and a size-dependent shape factor. This factor is not tuned to the data but rather is grounded in theoretical predictions from literature.

Two independent methods were used to generate the aerosol particles, namely by vaporizing and condensing granular sodium chloride and by electrospraying a high purity sodium chloride aqueous solution, to investigate possible effects of impurities on the results. The DRH and ERH values were found to be the same within experimental uncertainty for the particles generated by the two methods. Agreement in growth factors for particles generated by two independent methods (namely, vaporization-condensation and electrospray), as well as observations of prompt deliquescence, indicates the absence of significant chemical impurities.

In addition to sodium chloride, the deliquescence and efflorescence of ammonium sulfate, sea salts, and a variety of other inorganic binary salts have been studied using the nano-TDMA. In contrast to the sodium chloride data, no size dependence was found for the DRH (80%) and ERH (35%) of ammonium sulfate, while the behavior of sea salt was consistent with that expected from an "impure" sample of sodium chloride.

Other salts tested included potassium iodide, which showed a decrease in both the DRH and ERH with decreasing particle size, and potassium chloride, which showed an increase in DRH but a decrease in ERH with decreasing particle size. We are currently working on developing a theoretical explanation for why these particles exhibit behavior deviant from the DRH/ERH increase predicted by current theories.

We are also currently conducting experiments with letovicite [(NH₄)₃H(SO₄)₂] in an effort to investigate the effect of increased acidity on the hygroscopic behavior of ammonium sulfate particles.

Environmental TEM Observations

The Environmental-TEM (E-TEM) at ASU is employed by Matthew Wise and Peter Buseck for the observations of the hygroscopicity of nanoparticles. This instrument allows us to observe phase transitions and behavior of nanoparticles at relative humidities between 0 and 100 %. Monodisperse NaCl particles in the size range of 15 to 60 nm are generated by the vaporization-condensation method and passing the sample through a nano-DMA. The monodisperse particles are deposited on TEM grids in an electrostatic precipitator.

The hygroscopic behavior of 15 to 60-nm NaCl particles have been monitored using the E-TEM. Panel A of the following figure shows a TEM grid laden with ~ 30-nm NaCl particles inserted into the ETEM. The particles are imaged at 291 K with 0% RH in the environmental cell. Following the initial imaging, the electron beam is blocked to avoid particle heating and damage. Panel B shows the behavior of the particles as the RH in the environmental cell is increased (denoted by the up arrow). If the particles remain crystalline, the electron beam is blocked and the water vapor pressure in the environmental cell is increased. At 77 % RH (panel C), an enlargement and rounding in particle morphology occur. We interpret these phenomena as resulting from deliquescence. Although we have successfully imaged the deliquescence of particles as small as 15-nm, we do not see a dependence of the DRH on particle size. The apparent lack of dependence of the DRH on NaCl nanoparticle size may be due to the interaction of the particle with the TEM grid. We are currently performing experiments to determine the extent of the effect the substrate has on NaCl nanoparticle deliquescence.

People Involved

• Current Group Members

  • Amanda L. Mifflin
  • Mackenzie Smith
  • Scot Martin
  
  
  

• Collaborators

  • Peter A. Buseck (Arizona State University)
  • Lynn M. Russell (Scripps Institute)

Publications

1. G. Biskos, P.R. Buseck, and S.T. Martin, "Hygroscopic growth of nucleation-mode acidic sulfate particles," Journal of Aerosol Science, accepted December 2008.

2. E.J. Freney, S.T. Martin, and P.R. Buseck, "Deliquescence and efflorescence of potassium salts relevant to biomass-burning aerosol particles," Aerosol Science and Technology, In press.

3. M.E. Wise, C.A. Tyree, J.O. Allen, S.T. Martin, L.M. Russell, and P.R. Buseck, "Hygroscopic behavior and liquid-layer composition of aerosol particles generated from natural and artificial seawater," Journal of Geophysical Research, 2009, 114, D03201. PDF file.

4. Wise, M.E., S.T. Martin, L.M. Russell, and P.R. Buseck, "Water uptake by NaCl particles prior to deliquescence and the phase rule," Aerosol Science and Technology, 2008, 42, 281-294. PDF file.

5. T.A. Semeniuk, M.E. Wise, S.T. Martin, L.M. Russell, and P.R. Buseck, "Water uptake characteristics of individual atmospheric particles having coating," Atmospheric Environment, 2007, 41, 6225-6235. PDF File.

6. Wise, M.E., T.A. Semeniuk, R. Bruintjes, S.T. Martin, L.M. Russell, and P.R. Buseck, "Hygroscopic behavior of NaCl-bearing natural aerosol particles using environmental transmission electron microscopy," Journal of Geophysical Research, 2007, 112, D10224. PDF File.

7. T.A. Semeniuk, M.E. Wise, S.T. Martin, L.M. Russell, and P.R. Buseck, "Hygroscopic behavior of aerosol particles from biomass fires using environmental transmission electron microscopy," Journal of Atmospheric Chemistry, 2007, 56, 259-273. PDF File.

8. G. Biskos, D. Paulsen, L.M. Russell, P.R. Buseck, and S.T. Martin, "Prompt Deliquescence and Efflorescence of Aerosol Nanoparticles," Atmospheric Chemistry and Physics, 2006, 6, 4633. PDF File. Supplement. Table 1 Erratum.

9. Bahadur, R., Russell, L.M., Alavi, S., Martin, S.T., Buseck, P.R., "Void-induced Dissolution in MD Simulations of NaCl and Waters," Journal of Chemical Physics, 2006, 124, 154713. PDF File.

10. Biskos, G., Russell, L.M., Buseck, P.R., and Martin, S.T., "Nanosize Effect on the Hygroscopic Growth of Aerosol Particles," Geophysical Research Letters, 2006, 33, L07801. PDF File. Supporting Material A. Supporting Material B.

11. Biskos, G., Malinowski, A., Russell, L.M., Buseck, P.R., and Martin, S.T., "Nanosize Effect on the Deliquescence and the Efflorescence of Sodium Chloride Nanoparticles," Aerosol Science and Technology, 2006, 40, 97-106. PDF File. Supporting Material A. Supporting Material B.

12. Wise, M.E., Biskos, G., Martin, S.T., Russell, L.M., and Buseck, P.R., "Phase Transitions of Single Salt Particles Studied Using a Transmission Electron Microscope with an Environmental Cell," Aerosol Science and Technology, 2005, 39, 849-856. PDF File.