Laboratory measurements of contact freezing by dust and bacteria at temperatures of mixed-phase clouds

Laboratory measurements of freezing by aerosol particles in contact mode are presented. The fraction of particles catalyzing freezing is quantified for three mineral dusts and three strains of bacteria. This is the most comprehensive such dataset to date for temperatures greater than 220°C, relevant for warm, mixed-phase clouds. For Arizona Test Dust, feldspar, or rhyolitic ash, more than 10 particles are required to initiate a freezing event at 220°C in the contact mode. At 215°C, more than 10 particles are required. An icenegative strain of Pseudomonas fluorescens is an order of magnitude more effective than the mineral dusts at every temperature tested. To the best of the authors' knowledge, this is the first measurement of contact-mode freezing by an ice-negative bacterium. An ice-positive strain of Pseudomonas syringae reaches its maximum nucleating efficiency, E = 0.1, 12°C higher than does Pseudomonas fluorescens. This is consistent with the behavior of ice-negative and ice-positive bacteria in the immersion mode, as discovered 40 years ago. Surprisingly, cells of the ice-positive strain Pseudomonas syringae CC94 that do not express the ice nucleation active gene showed no contact-freezing activity, whereas the cells of the ice-negative strain of Pseudomonas fluorescens showed significant activity.