Preprints
https://doi.org/10.5194/acp-2021-53
https://doi.org/10.5194/acp-2021-53

  10 Feb 2021

10 Feb 2021

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

Ice nucleation ability of Ammonium Sulfate aerosol particles internally mixed with Secondary Organics

Barbara Bertozzi1, Robert Wagner1, Kristina Höhler1, Joschka Pfeifer2, Harald Saathoff1, Junwei Song1, Thomas Leisner1, and Ottmar Möhler1 Barbara Bertozzi et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 2CERN, Geneva, 1211, Switzerland

Abstract. The abundance of aerosol particles and their ability to catalyze ice nucleation are key parameters to correctly understand and describe the aerosol indirect effect on the climate. Cirrus clouds strongly influence the Earth's radiative budget, but their effect is highly sensitive to their formation mechanism, which is still poorly understood. Sulfate and organics are among the most abundant aerosol components in the troposphere and have also been found in cirrus ice crystal residuals. Most of the studies on ice nucleation at cirrus cloud conditions looked at either purely inorganic or purely organic particles. However, particles in the atmosphere are mostly found as internal mixtures, the ice nucleation ability of which is not yet fully characterized.

In this study, we investigated the ice nucleation ability of internally mixed particles composed of crystalline ammonium sulfate (AS) and secondary organic material (SOM) at temperatures between −50 °C and −65 °C. The SOM was generated from the ozonolysis of α-pinene. The experiments were conducted in a large cloud chamber, which also allowed to simulate various aging processes that the particles may experience during their transport in the atmosphere, like cloud cycling and redistribution of the organic matter. We found that the ice nucleation ability of the mixed AS/SOM particles is strongly dependent on the particle morphology. If the organic matter is evenly distributed around the AS crystals in a core-shell morphology, small organic mass fractions of 5–8 wt% are sufficient to completely mask the heterogeneous ice nucleation ability of the inorganic core. In this case, the ice nucleation onset increased from a saturation ratio with respect to ice Sice ~ 1.30 for the pure AS crystals to ≥ 1.45 for the SOM-coated AS crystals. However, if such SOM-coated AS crystals are subjected to the mentioned aging processes, they show an improved ice nucleation ability with the ice nucleation onset at Sice ~ 1.35. We suggest that the aging processes change the particle morphology. The organic matter might redistribute on the surface to form a partially engulfed structure, where the ice-active sites of the AS crystals are no longer completely masked by the organic coating, or the morphology of the organic coating layer might transform from a compact to a porous structure.

Our results underline the complexity to represent the ice nucleation ability of internally mixed particles in cloud models. They also demonstrate the need to further investigate the impact of atmospheric aging and cloud processing on the morphology and related ice nucleation ability of internally mixed particles.

Barbara Bertozzi et al.

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Barbara Bertozzi et al.

Barbara Bertozzi et al.

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Short summary
Internally mixed particles composed of sulfate and organics are among the most abundant aerosol types. Their ice nucleation (IN) ability influences the formation of cirrus and, thus, the climate. We show that the presence of a thin organic coating suppresses the heterogeneous IN ability of crystalline ammonium sulfate particles. However, the IN ability of the same particle can substantially change if subjected to atmospheric processing, mainly due to differences in the resulting morphology.
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