PNNL

Abstract

The phase state of atmospheric particles impacts several atmospheric processes, such as heterogeneous reactions, cloud droplet activation and heterogeneous ice nucleation, all of which affect the Earth’s climate. Particle phase state is governed by the characteristics of the particles (e.g., chemical composition) and atmospheric conditions (e.g., temperature and relative humidity). The Arctic atmosphere often is stratified, with different particle chemical compositions in different layers. Our knowledge about the vertical profile of the phase state of individual submicron-size ambient particles is limited due to few aloft measurements . To this end, particle samples were collected via the tethered balloon system (TBS) at the U.S. Department of Energy Atmospheric Radiation Measurement Program’s facility at Oliktok Point, Alaska, on November 19, 2020. Using an environmental scanning electron microscope with a tilted Peltier stage to mimic the atmospheric relevant temperature and relative humidity during sampling we probed the phase states of ambient particles, which were found to be possessed a combination of near-spherical, dome-like, and flat shapes when they impact the surface of a substrate. Particles at an altitude at 300 m contained similar, high number fractions of viscous particles (79 ±9%) compared to ground-level (74 ±5%). The size-resolved chemical composition obtained from computer-controlled scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy indicates variability in chemical composition across altitudes. The ground sample is dominated by carbonaceous-rich (71 ±1%, by number) and carbonaceous sulfate-rich (20 ±1%) particles, whereas the aloft sample collected at 300 m is dominated by carbonaceous-rich particles (77 ±2%) and carbonaceous-coated dust (15 ±2%). The chemical mixing state obtained from scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS) indicates that the ground sample is dominated by organic carbon and inorganic mixed particles (45 ± 3%), and organic carbon, elemental carbon, and inorganic mixed particles (27 ± 3%). In comparison, the aloft sample at 300 meters is predominantly composed of organic carbon (47 ± 7%), organic carbon and inorganic mixed particles (24 ± 6%), and organic carbon and elemental carbon mixed particles (20 ± 6%). Combining the chemical composition and direct observation of the phase state will improve our understanding of the vertical profile of the phase state and incorporate this variation in climate model to predict the climate effects accurately.