Airborne bacteria as cloud condensation nuclei

Abstract

Bacteria cultivated from aerosol and cloud water samples collected at a remote Austrian mountain site under wintry conditions were tested for their ability to act as cloud condensation nuclei (CCN). The experiment was carried out with a cloud condensation nuclei counter (CCNC) operating on the principle of a static thermal diffusion chamber. Average concentrations of cultivable airborne bacteria amounted to 8 colony forming units (CFU) m −3 in aerosol samples and to 79 CFU mL −1 in cloud water. The set of tested bacteria comprised Gram positive and Gram negative but no known ice nucleating species. At supersaturations between 0.07 and 0.11% all types of bacteria were activated as CCN. As the sizes of the bacteria were smaller than the Kelvin diameters for the respective supersaturations, the physico‐chemical properties of their outer cell walls must have enhanced their CCN activity.