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

We present a new analytical platform that uses a tilted stage (60°) integrated to the Peltier cooling stage interfaced with an Environmental Scanning Electron Microscope (ESEM) to directly observe and assess the phase state of particles as a function of RH at a controlled temperature. Three types of organic particles have been studied: (a) Suwannee River Fulvic Acid (SRFA) particles, (b) lab generated soil organic particles, and (c) field-collected ambient particles. The chemical composition, morphology, and functional groups of individual particles were probed using 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). Results show that all three types of particles are organic-rich, but soil organic particles and ambient particles contain a considerable amount of inorganic species. The phase state can be determined based on the particle's aspect ratio (particle width/height), which we proposed for solid, semisolid, and liquid particles are 1.00-1.30, 1.30-1.85, and >1.85, respectively. We found that solid SRFA particles transition to a semisolid state at ∼90% RH and to the liquid state at ∼97% RH, in agreement with the literature. The solid soil organic particles transition to a semisolid state at ∼85% RH and to the liquid state at ∼97% RH. The solid ambient organic particles transition to a semisolid state at ∼65% RH and the liquid state at ∼97% RH. Our results indicate that this new platform can directly observe and quantitatively indicate the phase transition of field-collected particles under different ambient conditions.