Observed Interactions Between Black Carbon and Hydrometeor During Wet Scavenging in Mixed-Phase Clouds

Wet scavenging of black carbon (BC) has been subject to large uncertainty, which importantly determines its atmospheric lifetime and indirect forcing impact on cloud microphysics. This study reveals the complex BC-hydrometeor interactions in mixed-phase clouds via single particle measurements in the real-world environment, by capturing precipitation processes throughout cloud formation, cold rain/graupel, and subsequent snow events at a mountain site influenced by anthropogenic sources in wintertime. We found highly efficient BC wet scavenging during cloud formation, with large and thickly coated BC preferentially incorporated into droplets. During snow processes, BC core sizes in the interstitial phase steadily increased. A mechanism was proposed whereby the BC mass within each droplet was accumulated through droplet collision, leading to larger BC cores, which were then released back to the interstitial air through the Wegener-Bergeron-Findeisen processes when ice dominated. These results provide fundamental basis for constraining BC wet scavenging. Plain Language Summary The wet removal of black carbon (BC) is crucial to determine its atmospheric lifetime and indirect radiative effects. However, the mechanism of BC-cloud particle interactions is still largely unknown, especially for mixed-phase clouds. This study conducted in situ single particle measurements to reveal this microphysical process throughout an entire precipitation event. The BC particles with large core sizes and thick coatings were observed to be preferentially incorporated into liquid droplets when the cloud was formed. The subsequent droplet collision further resulted in fewer but larger droplets with larger BC cores inside. Afterward, when a significant amount of ice was present, the water vapor could be transferred from droplets to ice crystals. As a result, the BC contained in the droplet could be released back to the interstitial phase, leading to larger BC cores in the air than those before the precipitation event. This is an important process when considering the wet scavenging of BC in mixed-phase clouds, which is first observed in the real atmosphere in this study.