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

By Philipp Reutter, H. Su, Jörg Trentmann, M. Simmel, D. Rose, S. S. Gunthe, Heini Wernli, M. Andreae, Ulrich Pöschl. 2009

359 citations recorded by the source corpus.

Abstract. We have investigated the formation of cloud droplets under pyro-convective conditions using a cloud parcel model with detailed spectral microphysics and with the κ-Kohler model approach for efficient and realistic description of the cloud condensation nucleus (CCN) activity of aerosol particles. Assuming a typical biomass burning aerosol size distribution (accumulation mode centred at 120 nm), we have calculated initial cloud droplet number concentrations (NCD) for a wide range of updraft velocities (w=0.25–20 m s−1) and aerosol particle number concentrations (NCN=200–105 cm−3) at the cloud base. Depending on the ratio between updraft velocity and particle number concentration (w/NCN), we found three distinctly different regimes of CCN activation and cloud droplet formation: (1) An aerosol-limited regime that is characterized by high w/NCN ratios (>≈10−3 m s−1 cm3), high maximum values of water vapour supersaturation (Smax>≈0.5%), and high activated fractions of aerosol particles (NCN/NCN>≈90%). In this regime NCD is directly proportional to NCN and practically independent of w. (2) An updraft-limited regime that is characterized by low w/NCN ratios ( (3) An aerosol- and updraft-sensitive regime (transitional regime), which is characterized by parameter values in between the two other regimes and covers most of the conditions relevant for pyro-convection. In this regime NCD depends non-linearly on both NCN and w. In sensitivity studies we have tested the influence of aerosol particle size distribution and hygroscopicity on NCD. Within the range of effective hygroscopicity parameters that is characteristic for continental atmospheric aerosols (κ≈0.05–0.6), we found that NCD depends rather weakly on the actual value of κ. A compensation of changes in κ and Smax leads to an effective buffering of NCD. Only for aerosols with very low hygroscopicity (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 NCD by more than 10%. The influence of changing size distribution parameters was stronger than that of particle hygroscopicity. Nevertheless, similar regimes of CCN activation were observed in simulations with varying types of size distributions (polluted and pristine continental and marine aerosols with different proportions of nucleation, Aitken, accumulation, and coarse mode particles). In general, the different regimes can be discriminated with regard to the relative sensitivities of NCD against w and NCN (∂lnNCD/∂lnw and ∂lnNCD/∂lnNCN). We propose to separate the different regimes by relative sensitivity ratios, (∂lnNCD/∂lnw)/(∂lnNCD/∂lnNCN) of 4:1 and 1:4, respectively. The results of this and related studies suggest that the variability of initial cloud droplet number concentration in convective clouds is mostly dominated by the variability of updraft velocity and aerosol particle number concentration in the accumulation and Aitken mode. Coarse mode particles and the variability of particle composition and hygroscopicity appear to play major roles only at low supersaturation in the updraft-limited regime of CCN activation (Smax

Subfields: Atmospheric Science, Global and Planetary Change

View Paper: https://www.semanticscholar.org/paper/65ba78561ebce7e4dc60b5f6025b3b7d8ed8d7d1

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Citations in the corpus

Developments in the CSU-RAMS Aerosol Model: Emissions, Nucleation, Regeneration, Deposition, and Radiation (2013, positive): “Ward et al. (2010) and Reutter et al. (2009) represented aerosol solubility with the kappa parameter k (Petters and Kreidenweis 2007).”
Cloud condensation nuclei in pristine tropical rainforest air of Amazonia: size-resolved measurements and modeling of atmospheric aerosol composition and CCN activity (2009, positive): “…of the enhanced sensitivity of cloud droplet formation to aerosol hygroscopicity at low supersaturation (updraft limited regime of CCN activation; Reutter et al., 2009), this may be a potentially important difference between " blue ocean " and " green ocean " regimes (Andreae et al., 2004;…”
Cloud condensation nuclei production associated with atmospheric nucleation: a synthesis based on existing literature and new results (2012, background): “This quantity, in turn, is a function of the aerosol loading and meteorological conditions (McFiggans et al., 2006; Reutter et al., 2009; Ghan et al., 2011).”
Global observations of aerosol‐cloud‐precipitation‐climate interactions (2014, background): “Simulations show that almost all CCN are nucleated in an aerosol-limited regime and evenmore so for smaller CCN concentrations [Reutter et al., 2009].”