Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN) 1Max Planck Institute for Chemistry, Biogeochemistry Department, Mainz, Germany
2Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Mainz, Germany
3Leibniz Institute for Tropospheric Research, Leipzig, Germany
*now at: German Weather Service, DWD Offenbach, Germany
Received: 03 Mar 2009 – Published in Atmos. Chem. Phys. Discuss.: 01 Apr 2009 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-Köhler 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:
Revised: 24 Jul 2009 – Accepted: 17 Aug 2009 – Published: 24 Sep 2009
(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
(<≈10−4 m s−1 cm3), low maximum values of water
vapour supersaturation (Smax<≈0.2%), and low activated
fractions of aerosol particles (NCD/NCN<≈20%). In
this regime NCD is directly proportional to w and practically
independent of NCN.
(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<0.05) and also in
the updraft-limited regime for aerosols with higher than average
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
Citation: Reutter, P., Su, H., Trentmann, J., Simmel, M., Rose, D., Gunthe, S. S., Wernli, H., Andreae, M. O., and Pöschl, U.: Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN), Atmos. Chem. Phys., 9, 7067-7080, doi:10.5194/acp-9-7067-2009, 2009.