Direct Measurement of Aerosol Surface Tension As a Function of Humidity

Aerosols are small solid particles suspended in a gas. Atmospheric aerosols are particles suspended in the atmosphere. Atmospheric aerosols are known to directly influence the global climate [Boucher et al., 2013]. Additionally, these aerosols can indirectly affect climate when they form cloud seeds [Seinfeld and Pandis, 2006]. Over time, these cloud seeds, also called cloud condensation nuclei (CCN), aggregate and grow to form clouds which have immense implications for our atmosphere and weather patterns.

Particle surface tension is a necessary parameter to model this cloud formation process. It is difficult to collect enough of the aerosol to use common surface tension instruments. This research focuses on directly measuring atmospherically relevant aerosol surface tension using the atomic force microscope (AFM). This technique allows surface tension measurements on nano to pico liter volumes of collected aerosol. The method oxidizes volatile organic compounds and then uses particle impaction to collect a film of the aerosols on a small metal puck and then the AFM probes the puck with specialty tips.

This method uses specifically designed AFM tips to measure forces involved with submersing and withdrawing the tip in the collected sample. Initial testing on liquids with known standard surface tension values verified the validity of the method. Further testing proved that oxidized α -pinene particles formed in dry conditions have a surface tension of that similar to the pure liquid, while oxidized α -pinene particles formed in more humid conditions have a significantly higher surface tension than that of the pure α -pinene liquid [Hritz et al, 2016]. Further experiments are being conducted in an attempt to clearly uncover the relationship between surface tension of atmospherically relevant aerosols and the humidity at which they were formed.

References:

Boucher, O., Randall, D., Artaxo, P., Bretherton, C., Feingold, G., Forster, P., Kerminen, V. M., Kondo, Y., Liao, H., Lohmann, U., Racsch, P., Satheesh, S. K., Sherwood, S., Stevens, B. and Zhang, X. Y.: Clouds and Aerosols, in : Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom, 571-657, 2013.

Hritz, A. D., Raymond, T. M., and Dutcher, D. D.: A method for the direct measurement of surface tension of collected atmospherically relevant aerosol particles using atomic force microscopy, Atmos. Chem. Phys., 16, 9761-9769, doi:10.5194/acp-16-9761- 2016, 2016.

Seinfeld, J. H. and Pandis, S. N.: Atmospheric Chemistry and Physics, Wiley, Hoboken, NJ, 2006.