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A New Method to Determine the Number Concentrations of Refractory Black Carbon...

Levin, E., G. R. McMeeking, P. J. DeMott, C. S. McCluskey, C. E. Stockwell, R. Yokelson, and S. M. Kreidenweis (2014), A New Method to Determine the Number Concentrations of Refractory Black Carbon Ice Nucleating Particles, Aerosol Sci. Tech., 48, 1264-1275, doi:10.1080/02786826.2014.977843.

Ice nucleating particles (INP) initiate heterogeneous ice nucleation in mixed-phase clouds, influencing cloud phase and onset temperatures for ice formation. Determination of particle types contributing to atmospheric INP populations requires isolation of the relatively rare INP from a total particle sample, typically followed by time-consuming single-particle characterization. We propose a method to estimate the contributions of light-absorbing, primarily refractory black carbon (rBC), particles to INP populations by selectively removing them prior to determination of INP concentrations. Absorbing particles are heated to their vaporization temperature using laser induced incandescence in a single particle soot photometer (SP2) and the change in INP number concentrations, compared to unheated samples, is assessed downstream in the CSU Continuous Flow Diffusion Chamber (CFDC). We tested this approach in the laboratory using stronglyabsorbing and nonabsorbing aerosol types to confirm effective removal of rBC INP and to explore the impact of the processing on non-light-absorbing INP. An INP-active rBC particle type was efficiently removed, while nonabsorbing kaolinite and a soil-based INP were not affected by laser exposure. Results for the products of wiregrass combustion indicated that absorbing particles, primarily rBC, accounted for about 40% of all INP, consistent with electron microscopy of INP emitted during prescribed burns of this fuel type. However, kaolinite internally mixed with rBC exhibited reduced activity after passing through the SP2, suggesting that the validity of the method for realistic internal mixtures needs additional research. The sensitivity of the CFDC presently limits applicability of the method to relatively high INP number concentration samples.

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