Journal Article
Atmospheric Chemistry and Physics, vol. 18, iss. 23, pp. 17497-17513, 2018
Authors
Kaitlyn J. Suski, David M. Bell, Naruki Hiranuma, Ottmar Möhler, Dan Imre, Alla Zelenyuk
Abstract
Abstract. Biological particles, including bacteria and bacterial fragments, have been
of much interest due to the special ability of some to nucleate ice at
modestly supercooled temperatures. This paper presents results from a recent
study conducted on two strains of cultivated bacteria which suggest that
bacterial fragments mixed with agar, and not whole bacterial cells, serve as
cloud condensation nuclei (CCN). Due to the absence of whole bacteria cells
in droplets, they are unable to serve as ice nucleating particles (INPs) in
the immersion mode under the experimental conditions. Experiments were
conducted at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA)
cloud chamber at the Karlsruhe Institute of Technology (KIT) by injecting
bacteria-containing aerosol samples into the cloud chamber and inducing cloud
formation by expansion over a temperature range of −5 to −12 ∘C.
Cloud droplets and ice crystals were sampled through a pumped counterflow
virtual impactor inlet (PCVI) and their residuals were characterized with a
single particle mass spectrometer (miniSPLAT). The size distribution of the
overall aerosol was bimodal, with a large particle mode composed of intact
bacteria and a mode of smaller particles composed of bacterial fragments
mixed with agar that were present in higher concentrations. Results from
three expansions with two bacterial strains indicate that the cloud droplet
residuals had virtually the same size distribution as the smaller particle
size mode and had mass spectra that closely matched those of bacterial
fragments mixed with agar. The characterization of ice residuals that were
sampled through an ice-selecting PCVI (IS-PCVI) also shows that the same
particles that activate to form cloud droplets, bacteria fragments mixed with
agar, were the only particle type observed in ice residuals. These results
indicate that the unavoidable presence of agar or other growth media in
all laboratory studies conducted on cultivated bacteria can greatly
affect the results and needs to be considered when interpreting CCN and IN
activation data.
