Variations of the density of ambient black carbon retrieved by a new method: importance to CCN prediction

Abstract. The effective density of black carbon (BC) is a crucial factor relevant to its morphology and mixing state that would add uncertainty in evaluating its climate effect. Here, we develop a new method to retrieve the effective density of ambient BC combining field observations with the Köhler theory. The uncertainty of the new retrieval method was evaluated within ±30 %, which is primarily caused by assumptions of the hygroscopic parameter of organics and the fraction of primary organic aerosols in non-hygroscopic or hygroscopic mode. Using the new method, we obtain that the ambient BC density during the campaign varies widely from 0.14 to 2.1 g cm⁻³, with a campaign mean density of 1.11±0.54 g cm⁻³ for internally-mixed BC that accounts for 79±18 % of total BC particles. The retrieved values fall within the range of typical density of internally-mixed BC reported in the literatures. We further examined the sensitivity of cloud condensation nuclei (CCN) number concentrations (NCCN) prediction to variations of BC density, showing an uncertainty of −28 %~11 % in calculating NCCN at supersaturations of 0.2 % and 0.4 % by varying the BC density within the retrieved ranges. We also find that the NCCN is more sensitive to the variations of BC density when it is <1.0 g cm⁻³, illustrating a necessity of accounting for such effect closer to source regions where the BC particles are mostly freshly emitted. The CCN closure achieves when introducing the retrieved real-time BC density and mixing state. This study provides a unique way of utilizing field measurements to infer ambient BC density and highlights the importance of applying varying BC density values in models when predicting CCN and assessing its relevant climate effect.