Some types of biological aerosol particles such as bacteria, fungal spores and pollen are known to be potent ice nuclei that form ice in the atmosphere at relatively warm temperatures. However, which types of particles are important in various regions around the world, and their molecular characteristics that make them effective ice nuclei, are not well known. Recent field measurements showed that fluorescent biological particles were present at a variety of altitudes in the atmosphere and in clouds over the U. S. Great Plains states. This work would help to identify the specific molecular characteristics of these aerosol particles, as well as which ones are most efficient as ice nuclei. Filter samples collected in the region will be analyzed with an array of EMSL capabilities, including high resolution liquid-chromatography mass spectrometry, scanning electron microscopy, Raman spectroscopy, and a continuous flow ice nucleation chamber. These state-of-the-art capabilities provide an unprecedented level of molecular details for these climatically important atmospheric particles that is currently not available elsewhere. Results will be critically important to understand and predict the formation of ice in mixed-phase clouds in the atmosphere.