Abstract View
Direct Observation and Assessment of Phase States of Ambient and Lab-generated Sub-micron Particles upon Humidification
ZEZHEN CHENG, Noopur Sharma, Kuo-Pin Tseng, Libor Kovarik, Swarup China, Pacific Northwest National Laboratory
Abstract Number: 144
Working Group: Aerosol Physics
Abstract
Ambient organic particles (AOP) exhibit a wide range of viscosities and different phase states, altering their roles in atmospheric processes and complicating the prediction of their climate effects. Our current knowledge of phase states of AOP is limited due to challenges in the field measurements. Thus, we present a new analytical platform that uses a tilted stage integrated to the Peltier cooling stage interfaced with an Environmental Scanning Electron Microscope (ESEM) to directly observe and assess the phase state of organic particles as a function of RH. Three types of organic particles have been studied: Suwannee River fulvic acid (SRFA) particles, lab generated soil organic particles (SOP), and field-collected AOP. 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 SOP and AOP 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 semisolid state at ~90% RH and to liquid state at ~97% RH, agree with the literature. The solid SOP transition to the semisolid state at ~85% RH and the liquid state at ~97% RH. The solid AOP transition to semisolid state at ~65% RH and liquid state at ~97% RH. Our results indicate that this new platform can directly observe and quantitatively indicate the phase transition of field-collected organic particles at different ambient conditions.