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Activation is necessary to form a cloud droplet from an aerosol, and it is widely accepted that it occurs as soon asa wetted aerosol grows beyond its critical radius. Traditional Köhler theory assumes that this growth is driven by thediffusion of water vapor. However, if the wetted aerosols are large enough, the coalescence of two or more particles is anadditional process for accumulating sufficient water for activation. This transition from diffusional to collectionalgrowth marks the limit of traditional Köhler theory and it is studied using a Lagrangian cloud model in which aerosolsand cloud droplets are represented by individually simulated particles within large-eddy simulations of shallow cumuli. Itis shown that the activation of aerosols larger than 0. 1 µm in dry radius can be affected bycollision and coalescence, and its contribution increases with a power-law relation toward larger radii and becomes theonly process for the activation of aerosols larger than 0. 4–0. 8 µm depending on aerosolconcentration. Due to the natural scarcity of the affected aerosols, the amount of aerosols that are activated bycollection is small, with a maximum of 1 in 10 000 activations. The fraction increases as the aerosol concentrationincreases, but decreases again as the number of aerosols becomes too high and the particles too small to causecollections. Moreover, activation by collection is found to affect primarily aerosols that have been entrained above thecloud base

On the limits of Köhler activation theory: how do collision and coalescence affect the activation of aerosols?