10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Cleaning up Our Act: Assessment of Background Freezing Caused by Impurities and Substrates and Strategies to Reduce These Interferences in Droplet Freezing Assays
Michael Polen, Thomas Brubaker, Josh Somers, Perry Cheng, RYAN SULLIVAN, Carnegie Mellon University
Abstract Number: 871 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract Droplet freezing techniques have been utilized for decades to measure the concentration of ice nucleating particles (INP) and assess the freezing properties of atmospheric particles. However, these methods suffer from contamination issues, especially in regards to impurities in the water or on the substrates used that induce freezing at temperatures warmer than the homogeneous freezing limit. Published results often do not present a sufficiently in-depth discussion of how the pure water was generated or its quality, or an assessment of background freezing due to surface interactions or matrix impurities. We present a detailed assessment of the droplet freezing temperature spectra and inferred INP concentrations obtained using different substrates in the CMU cold stage system, as well as the effects of different approaches to water purification and the resulting matrix impacts on the background freezing spectrum. We have found that bottled HPLC water along with filtration provides the lowest background signal and the most consistent pure water freezing spectra, though many in the ice nucleation community use Milli-Q generated water instead. Typical Milli-Q purification systems do not filter particles smaller than 200 nm, which could explain the inconsistent freezing spectra we observed. We have also observed the quality of the Milli-Q produced water to change suddenly with use. Substrates tested include silicon wafers, Vaseline, gold-coated surfaces, hydrophobic cover slips, and polydimethylsiloxane polymer. Our findings suggest that gold-coated substrates induce freezing at the lowest temperatures, while Vaseline consistently causes samples to freeze at higher temperatures. However, nearly all substrates induce freezing at higher temperatures than previous literature suggests, and warmer than the homogenous freezing temperature threshold. We also lay out our recommendations for future ice nucleation studies to present, in detail, how different research groups control for contamination and background freezing of a rare phenomenon that is highly sensitive to contamination. In this vein, we encourage the community to regularly publish droplet volume-normalized ice nucleating particle concentrations for pure water tests, as well as the raw droplet freezing temperature spectra. This will enable a direct comparison between methods so the community can identify the best practices and materials for droplet freezing techniques.