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

Ice initiated at temperatures greater than ~ -34 °C, must be formed heterogeneously. There are three known heterogeneous mechanisms, or modes – deposition, immersion/condensation, and contact. The contact mode causes freezing at temperatures where the other two are inactive. Though contact freezing was investigated in the late 1960s, quantitative measurements of the fraction of aerosol particles which are active in that mode remain scarce. We have quantified the fraction of aerosol particles which induce freezing in the contact mode for six mineral dusts and three strains of bacteria. This is the most comprehensive such dataset to date for temperatures greater than -20 °C. For all of the mineral dusts that we have tested, more than 10³ particles are required to initiate a freezing event at -20 °C in the contact mode. At -15 °C, more than 10⁵ particles are required. An ice negative strain of Pseudomonas fluorescens is an order of magnitude more effective than the mineral dusts at every temperature tested. To the best of our knowledge, this is the first measurement of contact mode freezing by an ice negative bacterium. We find that an ice positive strain of Pseudomonas syringae reaches its maximum nucleating efficiency, E = 0.1, twelve degrees higher than does Pseudomonas fluorescens. Surprisingly, cells of the ice positive strain Pseudomonas syringae CC94 which 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. We will discuss these measurements in the context of ice initiation in warm, mixed phase clouds.