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Heterogeneous ice nucleation of α ‐pinene SOA particles before and after ice cloud processing
Author(s) -
Wagner Robert,
Höhler Kristina,
Huang Wei,
Kiselev Alexei,
Möhler Ottmar,
Mohr Claudia,
Pajunoja Aki,
Saathoff Harald,
Schiebel Thea,
Shen Xiaoli,
Virtanen Annele
Publication year - 2017
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2016jd026401
Subject(s) - ice nucleus , nucleation , supercooling , aerosol , cirrus , materials science , chemical physics , ice crystals , chemical engineering , chemistry , atmospheric sciences , meteorology , geology , physics , organic chemistry , engineering
The ice nucleation ability of α ‐pinene secondary organic aerosol (SOA) particles was investigated at temperatures between 253 and 205 K in the Aerosol Interaction and Dynamics in the Atmosphere cloud simulation chamber. Pristine SOA particles were nucleated and grown from pure gas precursors and then subjected to repeated expansion cooling cycles to compare their intrinsic ice nucleation ability during the first nucleation event with that observed after ice cloud processing. The unprocessed α ‐pinene SOA particles were found to be inefficient ice‐nucleating particles at cirrus temperatures, with nucleation onsets (for an activated fraction of 0.1%) as high as for the homogeneous freezing of aqueous solution droplets. Ice cloud processing at temperatures below 235 K only marginally improved the particles' ice nucleation ability and did not significantly alter their morphology. In contrast, the particles' morphology and ice nucleation ability was substantially modified upon ice cloud processing in a simulated convective cloud system, where the α ‐pinene SOA particles were first activated to supercooled cloud droplets and then froze homogeneously at about 235 K. As evidenced by electron microscopy, the α ‐pinene SOA particles adopted a highly porous morphology during such a freeze‐drying cycle. When probing the freeze‐dried particles in succeeding expansion cooling runs in the mixed‐phase cloud regime up to 253 K, the increase in relative humidity led to a collapse of the porous structure. Heterogeneous ice formation was observed after the droplet activation of the collapsed, freeze‐dried SOA particles, presumably caused by ice remnants in the highly viscous material or the larger surface area of the particles.