Field Observation of Important Nonactivation Scavenging of Black Carbon by Clouds

The in-cloud scavenging of black carbon containing (BCc) aerosols by cloud condensation nuclei (CCN) activation is considered to be the primary mechanism for wet scavenging. By measuring the particle-resolved mixing state of BCc in the residues of orographic clouds downstream of a ground-based counter-flow virtual impactor, we found that a fraction of 30-90% of BCc could be in clouds by a collision/coalescence process without satisfying the conventional Köhler theory for CCN activation. These non-CCN BCc in clouds occupied a larger fraction of total in-cloud BCc when cloud droplets were larger, e.g., the non-CCN BCc fraction in clouds increased from 40% to 80% when the effective diameter of droplets increased from about 7.6 to 15.8 μm. This collision/coalescence process among droplets and BCc tended to be more efficient for larger droplets, which is also supported by microscopy images that one droplet could capture more than one BC cores. Larger droplets corresponded to lower ambient water supersaturation, under which conditions the removal of BCc in clouds by collision/coalescence may override CCN activation. By in situ measurements, the results here for the first time separated and quantified the BCc in clouds between activation and nonactivation processes, which provides insights into evaluating the wet removal and aging of atmospheric BC.