AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Characterizing the Organic Ice Nuclei in Soils
THOMAS C. HILL, Paul DeMott, Yukata Tobo, Janine Froelich-Nowoisky, William L. Stump, Gary D. Franc, University of Wyoming
Abstract Number: 597 Working Group: Bioaerosols: Characterization and Environmental Impact
Abstract Organic ice nuclei (IN) are abundant and active at warm temperatures in most soils. However, their identities are not yet known. We characterized the organic IN present in a range of Wyoming and Colorado topsoils. This included soil beneath semi-arid sagebrush shrubland, a plant community widespread across the North American Western Range that can become a major dust source after wildfires; wind erosion events generate strong vertical fluxes of PM$_10 particles and even large plumes visible from space (Wagenbrenner et al. 2012).
Methods used included physical, chemical and enzymatic tests, and quantitative PCR of the ina gene to enumerate ice nucleation active bacteria. All soils contained 10$^6 to >10$^7 IN active at -10°C. Reductions in IN following heating (105°C) or digestion with hydrogen peroxide indicated that organic IN predominated above -15°C. Treatment with lysozyme, which digests bacterial cell walls and hydrolyzes fungal chitin, reduced IN active at -5°C to -8°C by 80-90% in non-forest soils. Ice nuclei active at colder temperatures were resistant to most challenges.
We also isolated individual IN from the sagebrush topsoil. In this soil, 70% of IN active at -7°C were >5 micro-meters in diameter. Ice nuclei active at -7°C were obtained by selective concentration, using repeated cycles of freeze-testing and subdivision of droplets of dilute soil suspensions. The identity of the first IN isolated is not obvious. They were multi-component structures, were seemingly entirely organic and survived repeated freeze-thaw events intact. However, one particle fragmented after several cycles, generating at least two smaller progeny IN. Sagebrush topsoils freeze and thaw almost daily during shoulder seasons. Their organic IN will both nucleate, and be modified by, the repeated freezing events.
Wagenbrenner, N.S., et al. Wind erosion from a sagebrush steppe burned by wildfire: Measurements of PM$_10 and total horizontal sediment flux. Aeolian Research (2012), http://dx.doi.org/10.1016/j.aeolia.2012.10.003.