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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2017-134
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
15 Feb 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
On the Limits of Köhler Activation Theory: How do Collision and Coalescence Affect the Activation of Aerosols?
Fabian Hoffmann Institute of Meteorology and Climatology, Leibniz Universität Hannover, Hannover, Germany
Abstract. Activation is necessary to form a cloud droplet from an aerosol, and it occurs as soon as a wetted aerosol grows beyond its critical radius. Traditional Köhler theory assumes that this growth is driven by the diffusion of water vapor. However, if the wetted aerosols are large enough, the coalescence of two or more particles is an additional process for accumulating sufficient water for activation. This transition from diffusional to collectional growth marks the limit of traditional Köhler theory and it is studied using a Lagrangian cloud model in which aerosols and cloud droplets are represented by individually simulated particles within large-eddy simulations of shallow cumuli. It is shown that the activation of aerosols larger than 0.1 μm in dry radius can be affected by collision and coalescence, and its contribution increases with a power-law relation toward larger radii and becomes the only process for the activation of aerosols larger than 0.4–0.8 μm depending on aerosol concentration. Due to the natural scarcity of the affected aerosols, the amount of aerosols that are activated by collection is small with a maximum of 1 in 10 000 activations. The fraction increases as the aerosol concentration increases, but decreases again as the number of aerosols becomes too high and the particles too small to cause collections. Moreover, activation by collection is found to affect primarily aerosols that have been entrained above the cloud base.

Citation: Hoffmann, F.: On the Limits of Köhler Activation Theory: How do Collision and Coalescence Affect the Activation of Aerosols?, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-134, in review, 2017.
Fabian Hoffmann
Fabian Hoffmann
Fabian Hoffmann

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Short summary
This study analyzes at which aerosol radius the predominantly diffusional mass growth switches to collectional, which can be identified as the limit of Köhler theory (the so-called activation of aerosols). It is found that collection becomes increasingly important for aerosols larger than 0.1 µm and is responsible for all activations of aerosols larger than 1.0 µm. A novel particle-based cloud modeling approach is applied, in which activation can be represented without parameterizations.
This study analyzes at which aerosol radius the predominantly diffusional mass growth switches...
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