Detonation Soot: A New Class of Ice Nucleating Particle

Temperatures and pressures from high explosive detonations far exceed atmospheric conditions in typical combustion reactions, and consequently, detonation soot forms with physiochemical properties distinct from soot formed by combustion. In this study, samples of detonation soot from two high explosives, PBX 9502 and Composition B‐3, were analyzed. Ice nucleation experiments on soot collected after controlled detonations were conducted in the laboratory to probe immersion and contact mode freezing. Samples nucleated ice at temperatures warmer than commercially available nanodiamonds, which has a mean nucleation temperature of −20.7°C. Ice nucleation rate coefficients increase rapidly by two to three orders of magnitude below −20°C for every sample. Size‐selected 137 μm diameter particles produced during detonation in an ambient air atmosphere yield bimodal distributions of freezing temperature with primary and secondary nucleation modes centered at −20°C and −13°C, respectively. The presence of a secondary mode allows for enhanced ice nucleation rate coefficients (one to two orders of magnitude greater than samples without a secondary mode) at temperatures outside the influence of the primary mode (>−17°C). Given the observed onset nucleation temperatures of −9.2°C, our results imply that detonation soot of the type studied here would only need to reach an altitude of approximately 4 km to facilitate ice formation. Plain Language Summary Tiny suspended particles can modify the earth's energy balance. Some of these particles influence cloud formation through a process called the indirect effect of aerosol. One aspect of the indirect effect involves a subset of aerosol known as ice nucleating particles (INPs) that enhance ice formation in cold regions of the atmosphere. Because of the impact of INPs on climate, it is important to identify the types of aerosols that efficiently form ice. One understudied aerosol is a unique soot with distinct properties that is formed by the extreme temperatures and pressures achieved within high explosive detonations. This detonation soot can be lofted high into the atmosphere where it could impact ice formation. The ice nucleation activity of detonation soot is examined here for the first time. Through experimentation, we measured the freezing temperature of detonation soot originating from two different high explosives, PBX‐9502 and Composition B‐3. Detonation soot does act as an INP with a small portion of efficient parti