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

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 accumulation mode particles as cloud condensation nuclei (CCN). However, under specific conditions Aitken can also contribute to number concentration droplets (Nd), leading large uncertainties predicted properties a global scale. We perform sensitivity with an adiabatic parcel model constrain which Nd. simulations cover wide ranges properties, such total concentration, hygroscopicity (κ) diameters for particles. Building upon previously suggested concept updraft (w)- aerosol-limited regimes droplet formation, we show that does not occur w-limited transitional range between is broadened when Nd, limitation requires much higher w than size distributions only. In regime, Nd similarly dependent κ. Therefore, analyze κ, ξ(κ), function identify value combinations above affect As ξ(κ) shows minimum smallest activated “Hoppel minimum” (0.06 µm ≤ Dmin ≤0.08 µm), corresponding (w–κ) pairs be considered threshold level have significant impact This largely determined by diameter. Our analysis these thresholds results simple parametric framework criterion expected aerosol–cloud interactions. confirm likely do polluted air masses (urban, biomass burning) at moderate velocities (w≤3 m s−1) but may important deep convective clouds. Under clean conditions, Amazon, Arctic remote ocean regions, hygroscopic act CCN updrafts w<1 s−1.