Direct Quantification of Droplet Activation of Ambient Black Carbon Under Water Supersaturation
Dawei Hu, Dantong Liu, Shaofei Kong, Delong Zhao, Yangzhou Wu, Siyuan Li, Shuo Ding, Shurui Zheng, Yi Cheng, Kang Hu, Zhaoze Deng, Yunfei Wu, Ping Tian, Quan Liu, Mengyu Huang, Deping Ding
Abstract
Abstract Understanding the droplet activation of black carbon (BC) under water supersaturation (SS) conditions is crucial to estimate its lifetime (through activation scavenging) and indirect radiative impacts. However, directly probing the droplet activation of ambient BC is challenging because of the difficulty in discriminating the BC‐containing particles (BCc) from other components. Here, we coupled the measurements of number concentrations between size‐resolved cloud condensation nuclei (CCN) and individual BCc, achieving a direct quantification of droplet activation for ambient BCc. The activation diameters of BCc (D50_BCc), determined as the dry diameter at which 50% of BCc were activated, were 144 ± 21 nm under SS 0.2% corresponding to activated number fraction of total BCc of 60% ± 10%. D50_BCc was determined as a function of the coating hygroscopicity, and the organic content was observed as a limiting factor in controlling BCc activation. The activated BCc number concentration was predicted using the volume‐weighted compositions within particle. The predicted CCN agreed well with the measurements for all particles, while overestimating the activated BCc number concentration by 71%, 27%, and 27% at SS = 0.1%, 0.2%, and 0.3%, respectively. The higher discrepancy under lower SS may result from the higher non‐sphericity of BCc at large sizes. Therefore, only knowing the hygroscopicity distribution of bulk compositions may not be sufficient for estimating the activation of BCc, but considering additional factors such as the non‐sphericity of BCc is necessary to estimate droplet activation.