Collaborative Research: Laboratory and Ground-Based Studies Addressing Unresolved Aspects of Atmospheric Ice Nucleation

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).This project focuses on advancing understanding of critical issues in atmospheric ice nucleation, one of the most basic processes affecting precipitation and impacting the radiative properties of cold clouds. Incomplete understanding of ice initiation processes in clouds results in large uncertainties in the ability to model clouds and precipitation. Under prior support, the Principal Investigators (PIs) have made advancements in the area of real-time measurement of heterogeneous ice nuclei (IN) and in understanding the role of different aerosol particle types capable of initiating ice formation in the atmosphere. These measurements highlight significant remaining questions. For example, atmospheric studies have emphasized the importance of mineral dust as a significant IN source, but laboratory studies of natural dust particles up to 1 micron in size have shown no evidence for IN activity warmer than about -15 degrees Celsius. The second most prevalent IN composition that has been identified in the atmosphere are carbonaceous particles, but their sources are unresolved. Biological aerosols represent potential sources for primarily carbonaceous IN, but their number concentrations are poorly observed by existing measurement methods. Finally, apparent degradation of IN efficiency in polluted air has been observed but there is little fundamental basis for understanding the impacts of atmospheric processing on IN activation. The Principal Investigators will use laboratory studies to investigate ice formation by natural dusts at sizes up to 5 microns to quantify the relation between IN activation temperature and mineral dust particle size for different mineralogical types. They also will quantify the changes in IN activation properties after exposure to realistic degrees of atmospheric processing. Examinations of supermicron-sized particles as ice nuclei in the PIs' continuous flow ice nuclei instrument is made possible by application of a new particle phase-discrimination detector (PPD) that identifies ice crystals and aerosol particles by their spatial scattering patterns rather than optical size alone. Use of the PPD will permit exploration of the impact of evaporation of IN containing droplets on ice nucleation, of interest due to inferences that this process spawns primary or secondary ice nucleation. The PIs will investigate the number concentrations of biological ice nuclei through studies that utilize a real-time IN detector and application of real-time mass spectrometric or post-application of microbiological methods (DNA analyses) on activated and collected IN. Using this approach, IN-active bacteria are identified at specific temperature and humidity conditions, enabling an assessment of their role across the full temperature regime of tropospheric clouds. After refining procedures through laboratory studies, The PIs will apply these methods to quantify the proportion of ambient IN, as a function of temperature, that are of biological origin. The opportunity also exists to identify heretofore unrecognized biological IN. The intellectual merit of the project lies in the opportunity to confirm and augment present understanding and quantification of ice initiation in clouds and its relation to the specific properties of key atmospheric source populations of ice nuclei. The PIs will quantify ice formation by IN across their atmospherically-relevant size and temperature range, apply advanced techniques for detecting IN composition, determine the impacts of atmospheric processing on IN activation, and explore ice formation mechanisms. Nucleation parameterization development for numerical modeling studies may be advanced using results from this research. This work will have broader impacts through promoting graduate education and training, enhancing research infrastructure, development and testing of new instrumentation and methods, application of results toward global climate change issues through associated numerical modeling studies, and fostering cross-disciplinary research between the atmospheric and biological sciences. The PIs will encourage application of results through related modeling studies, collaboration with other researchers, and participation in working groups. Graduate students and a postdoctoral scientist will participate and the work will involve collaborations across multiple disciplines and universities. Results will be disseminated via a project web site, publications, and participation in conferences. Finally, these data are of critical importance to unresolved impact of aerosols on ice clouds and global climate.