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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Mar 2018

Research article | 07 Mar 2018

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This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.

Simulating the Influence of Primary Biological Aerosol Particles on Clouds by Heterogeneous Ice Nucleation

Matthias Hummel1,a, Corinna Hoose1, Bernhard Pummer2, Caroline E. Schaupp1, Janine Fröhlich-Nowoisky2, and Ottmar Möhler1 Matthias Hummel et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 2Department of Multiphase Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
  • anow at: Department of Geosciences, University of Oslo, Norway

Abstract. Primary ice formation, which is an important process for mixed-phase clouds with impact on their lifetime, radiative balance and hence the climate, strongly depends on the availability of ice nucleating particles (INPs). Supercooled droplets within these clouds remain liquid until an INP immersed in or colliding with to the droplet gets reaches its activation temperature. Only a few aerosol particles are acting as INPs and the freezing efficiency varies among them. Thus, the fraction of supercooled water in the cloud depends on the specific properties and concentrations of the INPs. Primary biological aerosol particles (PBAPs) have been identified as very efficient INPs at high subzero temperatures, but their very low atmospheric concentrations make it difficult to quantify their impact on clouds.

Here we use the regional atmospheric model COSMO-ART to simulate the heterogeneous ice nucleation by PBAPs during a 1-week case study on a domain covering Europe. We focus on three highly ice nucleation active PBAP species, Pseudomonas syringae bacteria cells and spores from the fungi Cladosporium sp. and Mortierella alpina. PBAP emissions are parameterized in order to represent the entirety of bacteria and fungal spores in the atmosphere. Thus, only parts of the simulated PBAP are assumed to act as INP. The ice nucleation parameterizations are specific for the three selected species and are based on a deterministic approach. The PBAP concentrations simulated in this study are within the range of previously reported results from other modelling studies and atmospheric measurements. Two regimes of PBAP INP concentrations are identified: a temperature-limited and a PBAP-limited regime, which occur at temperatures above and below a maximal concentration at around −10°C, respectively. In an ensemble of control and disturbed simulations, the change in the average ice crystal concentration by biological INPs is not statistically 1significant, suggesting that PBAP have no significant influence on the average state of the cloud ice phase. However, if the cloud top temperature is below −15°C, PBAP can influence the cloud ice phase and produce ice crystals in the absence of other INPs. Nevertheless, the number of produced ice crystals is very low and it has no influence on the modelled number of cloud droplets and hence the cloud structure.

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Matthias Hummel et al.
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Matthias Hummel et al.
Matthias Hummel et al.
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