10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Historic and Unpublished Data on Biological Ice Nuclei: Terrestrial and Marine
RUSSELL SCHNELL, National Oceanic and Atmospheric Adminstration
Abstract Number: 1488 Working Group: Bioaerosols
Abstract History: By way of background, most precipitation over land (even in hot deserts) and in convective clouds over oceans, is initiated by ice crystals forming in supercooled cloud water (cloud water may supercool to -250C or colder). These freezing events are initiated by ice nuclei (IN), the best of which are of biological origin. In the late 1960s it was observed that some species of bacteria living on decaying plant litters nucleated ice at -1.30C in supercooled water drops. In the residue of well decayed plant litters a stable IN is produced that may initiate ice at -5.50C in concentrations of up to 109 at -100C per gram of litter. A sample of such decayed leaf litter collected in 1971 and stored at room temperature maintained appreciable ice nucleus activity over a 35 year period. In the marine environment, IN in plankton may nucleate supercooled water at -4.5 0C in concentrations of 108 active at -100C per gram of plankton. Some bacteria isolated from plankton are excellent sources of IN active at temperatures as warm as -30C. Natural mineral dusts free of organic particles initiate ice at much colder temperatures (-120C to -200C). As such, there is interest in determining the role of biological IN in precipitation formation.
Unpublished or Obscure Biogenic Ice Nucleus Data: In recent years a number of publications on biogenic IN show results similar to those observed up to half a century ago but unpublished or in conference proceedings and theses. Some of these earlier data may be of interest to current IN researchers. For instance, it has been observed that ocean derived IN (ODN) are better deposition IN (nucleated from supercooled water vapor) than leaf derived IN (LDN) or bacteria derived IN (BDN). But, immersion freezing activity (nucleants in water) of these biogenic nucleants show that up to 104 more IN are activated by immersion freezing than deposition freezing. In the contact freezing nucleation mode (nucleants introduced onto supercooled water), ODN exhibited more IN events than in the immersion freezing mode whereas LDN and BDN exhibited similar nucleation activity spectra in both modes. Well aged and dried LDN ground in a fluid energy mill was far superior as a contact IN than unground LDN. But, the milled LDN began to lose its advantage within hours of milling. As for IN stability, ODN can lose a majority of its IN activity within hours after being dried at room temperature. As noted above, LDN may maintain IN activity for many decades. BDN IN activity is rapidly deactivated under anaerobic stress but recovers within a few hours under an aerobic environment. Atmospheric IN measured with a combined filter and drop freezing technique has shown that atmospheric IN concentrations in Colorado are greatest in late summer and are depleted behind cold front systems that produced snow upwind of the a sampling site. Measurements of atmospheric IN in vertical profiles show higher concentrations near the surface in both terrestrial and marine environments. There are especially strong decreases above temperature inversions and above marine boundary layers compared to beneath. There are clear differences in the number and warm range IN in plant litters from different climate zones: Köppen D zones have the warmest IN and greatest concentrations per unit mass followed by C and then A zones. This is also reflected in the immersion freezing IN measured in precipitation falling in these zones. In the Sahel region of Africa during the drought of the early 1970s, IN were depleted in overgrazed regions compared to lesser grazed areas. This raises the question of what role, if any, the removal of local IN may have had in precipitation reduction?