Direct observation and assessment of phase states of ambient and lab-generated sub-micron particles upon humidification

Abstract

We present a new analytical platform that uses a tilted and Peltier cooling stage interfaced with an environmental scanning electron microscope to directly observe and assess the phase state of individual particles as a function of relative humidity.

Aerosol Composition, Mixing State, and Phase State of Free Tropospheric Particles and Their Role in Ice Cloud Formation

“…The distribution of organic matter affects the phase state of atmospheric aerosol. , We evaluated the particle phase state from tilted SEM images (75°) and STXM/NEXAFS. , Figure a–c shows the direct observation of phase state from tilted SEM images where the phase state was obtained from particles’ aspect ratio (particle width/height). The viscosity boundary shown is defined from a previous study . High viscosity particles (solid/semi-solid-like) maintain a low aspect ratio (1–1.85) upon impaction on the substrate (Figure a).…”

“…The viscosity boundary shown is defined from a previous study. 71 High viscosity particles (solid/semi-solid-like) maintain a low aspect ratio (1−1.85) 71 upon impaction on the substrate (Figure 5a). Low viscosity particles (liquid-like) become more oblate upon impaction, exhibiting higher aspect ratios (>1.85) 71 (Figure 5b,c).…”

“…High viscosity particles (solid/semi-solid-like) maintain a low aspect ratio (1–1.85) upon impaction on the substrate (Figure a). Low viscosity particles (liquid-like) become more oblate upon impaction, exhibiting higher aspect ratios (>1.85) (Figure b,c). Particles in SA1 have a lower aspect ratio (more viscous).…”

“…Phase state of the particles. (a–c) Aspect ratio violin plots for individual particles measured from tilted (75°) SEM images with different viscosity regions . (d–f) Optical thickness of total carbon was obtained from the STXM/NEXAFS with the phase state boundaries. ,, (g–i) RH-dependent T g values for the last 5 days of transport for the maximum, mean, and minimum RH.…”

Studying Ice with Environmental Scanning Electron Microscopy

“…The technique is continuously evolving and, as a recent example, a new system combining ESEM capabilities with an X-ray spectroscopy analytical platform was developed to directly observe the evolution of organic particles with increasing RH [ 38 ]. The new setup allowed for the probing of the chemical state, morphology, and functional groups of individual particles with two combined techniques: computer-controlled scanning electron microscopy with energy dispersive X-ray spectroscopy (CCSEM\EDX) and scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM\NEXAFS).…”