Abstract. Primary ice formation,
which is an important process for mixed-phase clouds with an 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 the
droplet 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 PBAPs are assumed
to act as INPs. 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 modeling 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 significant,
suggesting that PBAPs 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 modeled number of
cloud droplets and hence the cloud structure.