Ice nucleating properties of bare, coated, coagulated, and thermally treated diesel soot particles

Combustion exhaust from diesel engines contains submicron size soot particles that are commonly found in the ambient atmosphere. These particles may accumulate compounds such as sulfates and oxidized organic species as they are transported in the atmosphere and lofted via deep convection to high altitudes where they can influence mixed-phase and cirrus clouds. Ice formation on aged diesel soot particles under these cloud conditions is still largely unresolved. Previous non-diesel soot source studies indicate that ice nucleation properties are sensitive to composition and combustion technique. However, the combined effects of composition, morphology, and aging of soot particles representative of carbonaceous particles found in the ambient atmosphere have not been systematically investigated. During the recent Soot Aerosol Aging Study (SAAS) laboratory campaign, conducted at the Atmospheric Measurement Laboratory at PNNL, we investigated ice nucleating properties of size-selected diesel soot particles that were subjected to different physical and chemical treatments. Detailed characterization of soot particles was obtained by several state-of-the-art instruments that analyzed the particles in both real time and off-line modes. We found that freshly formed bare (untreated) diesel soot particles nucleate ice at water sub-saturation conditions. In contrast, soot coated with oxidized organic species required high humidity nearly at and above homogeneous freezing threshold conditions. Furthermore, organic coating led to compaction of the initially fractal-like structure of bare soot, but the ice nucleation efficiency of the thermally denuded particles was similar to that of bare soot particles, suggesting marginal influence of soot structure on ice nucleation. Soot particles that had coagulated with secondary organic aerosols for more than 10 hours appeared to have no major effects on their ice nucleation properties when compared to bare soot particles. In summary, our results show that ice nucleation properties of soot particles are more sensitive to their mixing state (coating material) than their morphological structure. In addition to these results, soot ice nucleation parameterizations for cirrus cloud simulations and soot properties will be discussed and presented.