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We have investigated the formation of cloud droplets under pyro-convectiveconditions using a cloud parcel model with detailed spectral microphysicsand with the κ-Köhler model approach for efficient and realisticdescription of the cloud condensation nucleus (CCN) activity of aerosolparticles. Assuming a typical biomass burning aerosol size distribution(accumulation mode centred at 120 nm), we have calculated initial clouddroplet number concentrations (NCD) for a wide range of updraftvelocities (w=0.25–20 m s−1) and aerosol particle numberconcentrations (NCN=200–105 cm−3) at the cloud base.Depending on the ratio between updraft velocity and particle numberconcentration (w/NCN), we found three distinctly different regimes ofCCN activation and cloud droplet formation: (1) An aerosol-limited regime that is characterized by high w/NCNratios (>≈10−3 m s−1 cm3), high maximum valuesof water vapour supersaturation (Smax>≈0.5%), and highactivated fractions of aerosol particles (NCN/NCN>≈90%).In this regime NCD is directly proportional to NCN andpractically independent of w. (2) An updraft-limited regime that is characterized by low w/NCN ratios(<≈10−4 m s−1 cm3), low maximum values of watervapour supersaturation (Smax<≈0.2%), and low activatedfractions of aerosol particles (NCD/NCN<≈20%). Inthis regime NCD is directly proportional to w and practicallyindependent of NCN. (3) An aerosol- and updraft-sensitive regime (transitional regime), which ischaracterized by parameter values in between the two other regimes andcovers most of the conditions relevant for pyro-convection. In this regimeNCD depends non-linearly on both NCN and w. In sensitivity studies we have tested the influence of aerosol particle sizedistribution and hygroscopicity on NCD. Within the range of effectivehygroscopicity parameters that is characteristic for continental atmosphericaerosols (κ≈0.05–0.6), we found that NCD dependsrather weakly on the actual value of κ. A compensation of changes inκ and Smax leads to an effective buffering of NCD. Onlyfor aerosols with very low hygroscopicity (κ<0.05) and also inthe updraft-limited regime for aerosols with higher than averagehygroscopicity (κ>0.3) did the relative sensitivities ∂lnNCD/∂lnκ≈(ΔNCD/NCD)/(Δκ/κ) exceed values of ~0.2,indicating that a 50% difference in κ would change NCDby more than 10%. The influence of changing size distribution parameters was stronger thanthat of particle hygroscopicity. Nevertheless, similar regimes of CCNactivation were observed in simulations with varying types of sizedistributions (polluted and pristine continental and marine aerosols withdifferent proportions of nucleation, Aitken, accumulation, and coarse modeparticles). In general, the different regimes can be discriminated withregard to the relative sensitivities of NCD against w and NCN(∂lnNCD/∂lnw and ∂lnNCD/∂lnNCN).We propose to separate the different regimes by relativesensitivity ratios, (∂lnNCD/∂lnw)/(∂lnNCD/∂lnNCN)of 4:1 and 1:4, respectively. The results of this and related studies suggest that the variability ofinitial cloud droplet number concentration in convective clouds is mostlydominated by the variability of updraft velocity and aerosol particle numberconcentration in the accumulation and Aitken mode. Coarse mode particles andthe variability of particle composition and hygroscopicity appear to playmajor roles only at low supersaturation in the updraft-limited regime of CCNactivation (Smax<0.2%)

atmospheric chemistry and physics

Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN)