Observation-Based Quantification of Pollution-Dependent Activation and Empirical Activation Boundaries of Black-Carbon-Containing Particles

The activation of black-carbon-containing particles (BCc) as cloud condensation nuclei influences their incorporation into clouds, wet removal, and atmospheric lifetime, yet remains challenging to quantify under ambient conditions. Using a coupled size-resolved CCNc–SP2 measurement system, the critical activation diameter and activation fraction of BCc (D50,BCc and AFBCc, respectively) were quantified at supersaturations (SS) of 0.1–0.3% under different pollution conditions during wintertime in urban Beijing. At SS = 0.1%, D50,BCc remained nearly constant at ∼ 222 nm as pollution increased, whereas AFBCc rose from 32% to 44%, consistent with an increased abundance of larger BCc. At SS = 0.3%, D50,BCc increased from 128 to 146 nm with increasing pollution, accompanied by a greater contribution of small, relatively less-coated BCc (<150 nm), while AFBCc remained consistently around 77%. Logistic regression was used to derive empirical activation boundaries relating D50,BCc to BC core size (Dc). At a given SS and D50,BCc within the overlapping observed range, the corresponding Dc generally increased from light to heavy pollution. Boundary slopes decreased with increasing SS, indicating a smaller change in D50,BCc per unit change in Dc along the fitted boundaries. These observations reveal contrasting pollution-dependent activation characteristics and empirical BC core-size relationships across supersaturations.