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

Ana A Piedehierro,Ari Laaksonen,Ilkka Summanen,Annele Virtanen,André Welti,Iida Pullinen,Angela Buchholz,Kimmo Korhonen

doi:10.5194/acp-21-11069-2021

Abstract

Abstract. We investigate the effect of water content and oxidative age on ice nucleation using 100 nm monodisperse particles of boreal forest secondary organic aerosol (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 humidities (40 %, 10 % and <1 % RHW). The influence of the particle oxidative age is tested by varying their O:C ratio (oxygen-to-carbon ratio, 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. The measurements suggest that at low temperatures, water diffusion into dry SOA particles is slow enough to form a core-shell morphology. The liquid outer layer 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. We propose that the partial water vapor pressure to which the particles are exposed prior to an experiment can serve as an indicator of whether a core-shell structure is developing.

Highlights

Heterogeneous ice formation by ice-nucleating particles (INPs) allows for the formation of cirrus clouds at lower humidity than required for ice formation by homogeneous freezing of solution droplets, which is determined by the water activity criterion (Koop et al, 2000)
The O : C ratio appears to have a minor impact on the conditions of ice formation of the α-pinene secondary organic aerosol (SOA) particles
We have investigated the influence of particle water content and oxidative age on the ice nucleation ability of boreal forest SOA surrogates, using α-pinene SOA generated in a potential aerosol mass reactor (PAM) reactor as a model compound

Summary

Introduction

Heterogeneous ice formation by ice-nucleating particles (INPs) allows for the formation of cirrus clouds at lower humidity than required for ice formation by homogeneous freezing of solution droplets, which is determined by the water activity criterion (Koop et al, 2000). In contrast to low-viscosity (more liquid-like) particles, highly viscous aerosols are slow to take up or lose water or other vapors in response to variations in gas-phase composition (Mikhailov et al, 2009; Koop et al, 2011; Shiraiwa et al, 2013; Yli-Juuti et al, 2017) This resistance is pronounced under dry conditions or low temperatures, where SOA particles can exist in a highly viscous or glassy state (Koop et al, 2011; Virtanen et al, 2010; Zobrist et al, 2008)

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Conclusion