Cloud Processing of Soot Particles and the Effect on Ice Nucleation in Subsequent Cloud Formation Cycles

A quantitative understanding of the aerosol-cloud interactions of soot particles, especially their potential to form cirrus clouds, is a key factor to reduce uncertainties in the estimates of the net radiative forcing of black carbon. We test the ice nucleation ability on size-selected propane soot aerosol using the Horizontal Ice Nucleation Chamber (HINC). We focus on the temperature range 233 – 218 K. In parallel, we investigate changes in soot particle morphology before and after formation of ice crystals using transmission electron microscopy (TEM). Finally, we demonstrate how the ice nucleation abilities of the soot particles change in subsequent cloud formation cycles by coupling two HINC chambers in series. We hypothesize that the change in ice nucleation ability in subsequent cloud cycles is due to morphological changes of the soot particles after the initial ice crystal formation event.

We observe negligible heterogeneous freezing of the unprocessed propane soot particles in the first cloud formation cycle, attributing ice formation to homogeneous freezing conditions. However, our TEM results reveal a compaction of the soot particles upon ice nucleation. When the compacted soot particles are re-exposed to a second ice nucleation cycle within HINC, their ice formation ability is significantly altered resulting in an enhanced ice formation ability with the humidity (with respect to ice) conditions required to trigger ice formation being as much as 15% lower than the first cycle. The results are related to different cloud processing conditions and to the changes in morphology.