Estimating Viscosity of Individual Substrate-Deposited Particles from Measurements of Their Height-to-Width Ratios

AbstractAirborne 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 semi-solid 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 semi-solid (>1010 Pa·s), viscous (104-108 Pa·s), and liquid (10°-101 Pa·s) particles within ensembles of ambient particles collected for microscopy studies.DisclaimerAs a service to authors and researchers we are providing this version of an accepted manuscript (AM). Copyediting, typesetting, and review of the resulting proofs will be undertaken on this manuscript before final publication of the Version of Record (VoR). During production and pre-press, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal relate to these versions also. AcknowledgementsThis work was supported by the U. S. Department of Energy’s (DOE) Atmospheric System Research program, Office of Biological and Environmental Research (OBER), award DE-SC0021977. 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.Author contributionsF.R. and A.L. devised the project. F.R., J.T., M.F., 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.