AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Efficiency of Biological Aerosol for Contact Mode Freezing
JOSEPH NIEHAUS, Xin Xin Woodward, Will Cantrell, Michigan Technological University
Abstract Number: 700 Working Group: Aerosols, Clouds, and Climate
Abstract Biological particles such as pollen and bacteria have been implicated in cloud formation processes (Mohler et al. 2007). They are of particular interest due to their ability to nucleate ice at temperatures warmer than -20 C. Although mineral dust is the most abundant ice nucleus present in the atmosphere, it has not been found to be relevant for temperatures warmer than -25 C (see e.g. Neidermeier 2010). Contact nucleation, where particles catalyze the phase transition at the air/water interface, is one suspect for high temperature ice nucleation. In some laboratory tests, side by side comparisons of contact vs. immersion mode nucleation saw a difference in freezing temperatures of 5 C.
We have studied ice nucleation by Snomax, a commercially available form of the bacteria Pseudomonas syringae, in both the contact and immersion mode. The freezing efficiency, defined as the ratio of freezing events to the number of bacterial cells deposited to a supercooled water droplet, was greater in the contact mode for all temperatures in the range -3 to -8 C. In the contact mode at -3 C, there is one freezing event for every 80,000 cells that impact the surface of the supercooled water droplet. By -7 C, the efficiency has risen to one freezing event for every 100 cells. In contrast, in the immersion mode, the efficiency was one freezing event for every 10 million cells at -3 C and one in 3000 cells at -7 C. The difference between contact and immersion modes diminishes as temperatures decrease.