Aitken mode particles as CCN in aerosol- and updraft-sensitive regimes of cloud droplet formation

Pöhlker, Mira L.; Zhang, Minghui; Campos Braga, Ramon; Krüger, Ovid O.; Pöschl, Ulrich; Ervens, Barbara

The high variability of aerosol particle concentrations, sizes and chemical composition makes their description challenging in atmospheric models. Aerosol–cloud interaction studies are usually focused on the activation of accumulation mode particles as cloud condensation nuclei (CCN). However, under specific conditions Aitken mode particles can also contribute to the number concentration of cloud droplets (inline-formulaNd), leading to large uncertainties in predicted cloud properties on a global scale. We perform sensitivity studies with an adiabatic cloud parcel model to constrain conditions under which Aitken mode particles contribute to inline-formulaNd. The simulations cover wide ranges of aerosol properties, such as total particle number concentration, hygroscopicity (inline-formulaκ) and mode diameters for accumulation and Aitken mode particles. Building upon the previously suggested concept of updraft (inline-formulaw)- and aerosol-limited regimes of cloud droplet formation, we show that activation of Aitken mode particles does not occur in inline-formulaw-limited regimes of accumulation mode particles. The transitional range between the regimes is broadened when Aitken mode particles contribute to inline-formulaNd, as aerosol limitation requires much higher inline-formulaw than for aerosol size distributions with accumulation mode particles only. In the transitional regime, inline-formulaNd is similarly dependent on inline-formulaw and inline-formulaκ. Therefore, we analyze the sensitivity of inline-formulaNd to inline-formulaκ, inline-formulaξ(κ), as a function of inline-formulaw to identify the value combinations above which Aitken mode particles can affect inline-formulaNd. As inline-formulaξ(κ) shows a minimum when the smallest activated particle size is in the range of the “Hoppel minimum” (0.06 inline-formulaµm inline-formula≤inline-formulaDmininline-formula≤0.08inline-formulaµm), the corresponding (inline-formulawinline-formulaκ) pairs can be considered a threshold level above which Aitken mode particles have significant impact on inline-formulaNd. This threshold is largely determined by the number concentration of accumulation mode particles and by the Aitken mode diameter. Our analysis of these thresholds results in a simple parametric framework and criterion to identify aerosol and updraft conditions under which Aitken mode particles are expected to affect aerosol–cloud interactions. Our results confirm that Aitken mode particles likely do not contribute to inline-formulaNd in polluted air masses (urban, biomass burning) at moderate updraft velocities (inline-formulaw≤3 m sinline-formula−1) but may be important in deep convective clouds. Under clean conditions, such as in the Amazon, the Arctic and remote ocean regions, hygroscopic Aitken mode particles can act as CCN at updrafts of inline-formulaw<1 m sinline-formula−1.

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Pöhlker, Mira L. / Zhang, Minghui / Campos Braga, Ramon / et al: Aitken mode particles as CCN in aerosol- and updraft-sensitive regimes of cloud droplet formation. 2021. Copernicus Publications.

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