The density of ambient black carbon retrieved by a new method: implications for cloud condensation nuclei prediction

The effective density of black carbon (BC) is a crucial factor that is relevant to its aging degree and adds uncertainty in evaluating its climate effect. Here, we have developed a new method to retrieve the effective density of internally mixed BC in the atmosphere, combining field observations conducted from 15 November to 14 December 2016 in urban Beijing with the Köhler theory. The uncertainty in the retrieval method was evaluated to be within ±30%, which was primarily caused by assumptions on both the hygroscopic parameter of organics and the proportional distribution of primary organic aerosols in different hygroscopic modes. Using the method, we find that the ambient internally mixed BC, accounting for 80%±20% of total BC aerosol particles, was retrieved with a campaign mean density of 1.1±0.6gcm-3 during the observed periods. The retrieved result was comparable with that reported in the literature. By applying a lower (0.14gcm-3) and upper (2.1gcm-3) limit of the retrieved BC density in the cloud condensation nuclei (CCN) number concentration (NCCN) estimation, we derived that the neglect of such variations in the BC density would lead to an uncertainty of -28%-11% in predicting NCCN at supersaturations of 0.23% and 0.40%. We also find that the NCCN was more sensitive to the variations in BC density when it was <1.0gcm-3. This illustrates the necessity of accounting for the effect of BC density on CCN activity closer to source regions where the BC particles are mostly freshly emitted. The CCN closure was achieved 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 variable BC density values in models when predicting CCN and assessing its relevant climate effect.