Estimating viscosity of individual substrate-deposited particles from measurements of their height-to-width ratios

Abstract

Airborne particles alter the radiative forcing of climate and have further consequences on air visibility, atmospheric chemistry, and human health. Recent studies reported the existence of highly viscous semisolid and even solid amorphous organic aerosol (OA) particles. Particle viscosity has an impact on the heterogeneous chemistry, gas-particle partitioning, and ice nucleation properties. Consequently, variations in particle viscosity must be considered when predicting the atmospheric impact of OA. Here, we use scanning electron microscopy (SEM) and scanning transmission X-ray microscopy (STXM) to estimate the viscosity of individual particles deposited on substrates based on their characteristic height-to-width ratios, which are affected by changes in morphology upon deposition. The height-to-width ratios obtained from SEM and STXM exhibit a strong correlation, demonstrating that both imaging approaches can be applied separately for viscosity assessment of the substrate-deposited particles. While these metrics are largely qualitative, this method enables rapid assessment of particle viscosity ranges, distinguishing between semisolid (>10¹⁰ Pa·s), viscous (10^4–10⁸ Pa·s), and liquid (10°–10¹ Pa·s) particles within ensembles of ambient particles collected for microscopy studies.

Copyright © 2023 American Association for Aerosol Research

Graphical Abstract

Editor:

• Cari Dutcher

Acknowledgments

The SEM imaging for this project was performed at the Life Science Microscope Facility at Purdue University. The STXM imaging was performed at beamline 5.3.2.2 of the Advanced Light Source at Lawrence Berkeley National Laboratory. We thank Mr. Mark Carlsen, instrumentation specialist from Purdue’s Jonathan Amy Facility for Chemical Instrumentation, for assembling the drying system used for particle generation and collection.

Authors’ contributions

F.R. and A.L. devised the project. F.R., J.T., M.F., and R.M. conducted STXM measurements. F. R. and E.M. conducted laboratory experiments, collected samples of particle standards, performed SEM measurements, analyzed and integrated all datasets. M.L. provided geometry derivations. F.R. and A.L. wrote the manuscript with contributions from all coauthors. The authors report there are no competing interests to declare.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

U. S. Department of Energy, Office of Biological and Environmental Research, Atmospheric System Research program, award DE-SC0021977.