Preprints
https://doi.org/10.5194/acp-2021-221
https://doi.org/10.5194/acp-2021-221

  15 Mar 2021

15 Mar 2021

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

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

Mira L. Pöhlker1, Minghui Zhang2, Ramon Campos Braga1, Ovid O. Krüger1, Ulrich Pöschl1, and Barbara Ervens2 Mira L. Pöhlker et al.
  • 1Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany
  • 2Université Clermont Auvergne, CNRS, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, 63000 Clermont-Ferrand, France

Abstract. 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 also Aitken mode particles can contribute to the number concentration of cloud droplets (Nd), 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 Nd. The simulations cover wide ranges of aerosol properties, such as total particle number concentration, hygroscopicity (κ) and mode diameters for accumulation and Aitken mode particles. Building upon the previously suggested concept of updraft (w)- and aerosol-limited regimes of cloud droplet formation, we show that activation of Aitken mode particles does not occur in w-limited regimes of accumulation mode particles. The transitional range between the regimes is broadened when Aitken mode particles contribute to Nd as aerosol-limitation requires much higher w than for aerosol size distributions with accumulation mode particles only. In the transitional regime, Nd is similarly dependent on w and κ. Therefore, we analyze the sensitivity of Nd to κ, ξ(κ), as a function of w to identify the value combinations, above which Aitken mode particles can affect Nd. As ξ(κ) shows a minimum when the smallest activated particle size is in the range of the Hoppel minimum (0.06 μm ≤ Dmin ≤ 0.08 μm), the corresponding (w,κ) pairs can be considered a threshold level, above which Aitken mode particles have significant impact on Nd. 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 Nd in polluted air masses (urban, biomass burning) at moderate updraft velocities (w ≤ 3 m s−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 w < 1 m s−1.

Mira L. Pöhlker et al.

Status: open (until 10 May 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-221', Anonymous Referee #1, 01 Apr 2021 reply

Mira L. Pöhlker et al.

Mira L. Pöhlker et al.

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Short summary
Clouds cool our atmosphere. The role of small aerosol particles in affecting them represent one of the largest uncertainties in current estimates of climate change. Traditionally it is assumed that cloud droplets form only particles of diameters ~100 nm (accumulation mode). Previous studies suggest that this can also occur on smaller particles (Aitken mode). Our study provides a general framework to estimate under which aerosol and cloud conditions Aitken mode particles affect clouds.
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