Laboratory studies of immersion mode ice nucleation on five types of black carbon samples

Heterogeneous ice nucleation onto black carbon (BC) particles has been previously studied in the deposition and condensation mode for temperatures relevant to mixed phase and ice cloud regimes in the troposphere. Deposition mode ice nucleation onto BC particles is only found to occur at temperatures below approximately 235 K, whereas at warmer temperatures BC is found to be an inactive ice nucleus (IN) in the condensation mode. However, some field studies that have sampled ice crystal residual compositions have frequently found BC to be associated with ice particles in both the cirrus and mixed phase cloud regimes. In addition, the types of BC used in the previous laboratory studies and comparisons varied significantly in composition and preparation methods and in some cases not being representative of atmospheric soot. In this work, using the Immersion Mode Cooling chAmber (IMCA), we present the ice nucleation abilities of 5 black carbon samples investigated in the framework of the SP-AMS intercomparison campaign (SIC): fuel-rich and fuel-lean propane flame soot, graphite spark-generated black carbon, Regal Black (a carbon black), and Fullerene-Enriched Soot. Ice nucleation was studied in the temperature range 243 – 233 K. The chemical and physical properties of the individual samples such as organic carbon content and morphology were investigated by aerosol mass spectrometry and single particle mass analysis in parallel. Suspended soot particles were forced to activate into water droplets in IMCA followed by supercooling to observe freezing. A depolarization detector was used to distinguish droplets from ice crystals. Frozen fractions as well as nucleation rates are reported for the 5 samples in order to evaluate the freezing mechanism (heterogeneous or homogeneous) of the BC samples over the temperature range studied. Of the 5 samples investigated, we observed little (fullerene enriched sample) to no heterogeneous ice formation for temperatures above 237 K. We conclude that freshly emitted BC particles will have a small (if any) contribution to ice nucleation in mixed-phase clouds in the temperature range studied.