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

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From: Atmospheric Chemistry and Physics(Vol. 21, Issue 14)
Publisher: Copernicus GmbH
Document Type: Article
Length: 408 words

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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 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 % RH.sub.W) and was only observed for SOA preconditioned at low RH.sub.W ([less than or equal to]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 % RH.sub.W) 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.

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Gale Document Number: GALE|A669226883