Inferring Viscosity of Individual Particles from Chemical Imaging
FELIPE RIVERA-ADORNO, Jay Tomlin, Matthew Fraund, Erick Morgan, Kevin Jankowski, Mihail Laskin, Ryan Moffet, Alexander Laskin,
Purdue University Abstract Number: 374
Working Group: Aerosol Physical Chemistry and Microphysics
AbstractAirborne particles alter radiative forcing of climate and have further consequences on visibility and human health. Until recently, atmospheric organic aerosol (OA) was assumed to have a liquid-like consistency. However, recent studies reported the existence of highly viscous semi-solid and even solid amorphous OA particles. Particle viscosity have an impact on the heterogeneous chemistry, gas-particle partitioning, and ice nucleation property. Variations in particle viscosity must be considered when predicting the atmospheric impact of OA. Here we use spectro-microscopy measurements of particles deposited on substrates to infer viscosity properties based on their characteristic height-to-width (H/W) ratios, as particles experience changes in morphology during collection. Standards of saccharide particles, with known viscosities, were prepared and collected by impaction on substrates. A scanning electron microscope (SEM) was then used for tilted imaging, which facilitates measurements of particle height and width. Results showed that particles with viscosities of ≥ 10
10 Pa s (amorphous solids) retain their characteristic spherical morphology upon impaction and exhibit a H/W ratio of ~1. Particles with viscosities of 10
2-10
9 Pa s (semi-solids) were observed as dome-shaped (H/W~0.35-0.7), which indicates partial deformation in morphology during collection. Severe flattening of particles (H/W < 0.35) became prominent at viscosities of ≤ 10
2 Pa s (liquids). SEM results were correlated with quantitative X-ray spectro-microscopy measurements, demonstrating that both imaging approaches can be applied interchangeably to assess viscosity of atmospheric particles based on their dimensions after impaction. We demonstrate that chemical imaging provides practical opportunity to assess viscosity values of individual particles sampled in field studies.