18 Feb 2021

18 Feb 2021

Review status: this preprint is currently under review for the journal ACP.

Ice nucleation on surrogates of boreal forest SOA particles: effect of water content and oxidative age

Ana A. Piedehierro1, André Welti1, Angela Buchholtz2, Kimmo Korhonen2, Iida Pullinen2, Ilkka Summanen2, Annele Virtanen2, and Ari Laaksonen1,2 Ana A. Piedehierro et al.
  • 1Finnish Meteorological Institute, Helsinki, Finland
  • 2Department of Applied Physics, University of Eastern Finland, Kuopio, Finland

Abstract. α-pinene is an abundant volatile organic compound (VOC) emitted by boreal forests and a source of atmospheric Secondary Organic Aerosol (SOA). This precursor is commonly used as a model compound for SOA studies representing boreal forest emissions. α-pinene SOA particles can have a highly viscous solid or semi-solid phase state depending on water content, temperature and oxidative age. The phase state (or viscosity) of SOA particles has multiple effects on the chemical and physical processes in which SOA particles are involved; one of the affected processes is ice formation.

We investigate the effect of water content and oxidative age on ice nucleation using 100 nm quasi-monodisperse particles of boreal forest SOA surrogates . Ice nucleation experiments are conducted in the temperature range between 210 and 240 K and from ice to water saturation using the Spectrometer for Ice Nuclei (SPIN). The effect of the particle water content on the ice nucleation process is tested by preconditioning α-pinene SOA at different humidity (40 %, 10 % and <1 % RHw). The influence of the particle oxidative age is tested by varying their O : C ratio (oxygen-to-carbon ratios, O : C ∼ 0.45, 0.8, 1.1). To assess the suitability of α-pinene as a model compound to study the ice nucleation properties of boreal forest SOA and to confirm the atmospheric relevance of our findings, we compare them to measurements of SOA using pine-needle oil or Scots pine tree emissions as precursors.

The ice nucleation measurements show that surrogates of boreal forest SOA particles promote only homogeneous ice formation. An effect of preconditioning humidity on homogeneous ice nucleation could be observed. Contrary to the expected behavior, homogeneous freezing is suppressed for SOA particles with high water content (preconditioned at 40 % RHw) and was only observed for SOA preconditioned at low RHw (≤ 10 %). No dependence of homogeneous freezing on the SOA oxidative age was observed. The results can be explained by a significant change of particulate water diffusivity as a function of humidity (from 10 % to 40 % RHw) at 293 K, where the aerosol is preconditioned. On dry SOA particles, water diffusion into the particle is slow enough to form a core-shell morphology with an outer layer that can equilibrate within the timescale of the experiment and freeze homogeneously. On SOA particles with higher water content, water diffuses faster into the particle, delaying equilibration at the particle surface and preventing the formation of a diluted shell, which can delay homogeneous freezing. To predict if a core-shell develops, we propose that the partial water vapor pressure particles are exposed to prior to an experiment can serve as an indicator.

Ana A. Piedehierro et al.

Status: open (until 15 Apr 2021)

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Ana A. Piedehierro et al.

Ana A. Piedehierro et al.


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