资源环境科技发展态势分析平台

The effective density (rho(eff)) of refractory black carbon (rBC) is a key parameter relevant to its mixing state that imposes great uncertainty in evaluating the direct radiation forcing effect. In this study, a tandem differential mobility analyzer-centrifugal particle analyzer-single-particle soot photometer (DMA-CPMA-SP2) system was used to investigate the relationship between the effective density (rho(eff)) and the mixing state of rBC particles during the winter of 2018 in the Beijing megacity. During the experiment, aerosols with a known mobility diameter (D-mob) and known rho(eff) values (0.8, 1.0, 1.2, 1.4, 1.6, and 1.8 g cm(-3)) were precisely selected and measured by the SP2 to obtain their corresponding mixing states. The results showed that the rho(eff) well represented the morphological variation in rBC-containing particles. The rBC-containing particles changed from an irregular structure to a compact spherical structure with the increase in rho(eff). A rho(eff) value of 1.4 g cm(-3) was the morphological transition point. The morphology and rho(eff) value of the rBC-containing particles were intrinsically related to the mass ratio of non-refractory matter to rBC (M-R). As the rho(eff) values of the rBC-containing particles increased from 0.8 to 1.8 g cm(-3), the M-R of the rBC-containing particles significantly increased from 2 up to 6-8, indicating that atmospheric aging processes were likely to lead to the reconstruction of more compact and regular particle shapes. During the observation period, the rho(eff) of the majority of rBC-containing particles was smaller than the morphology transition point independent of the pollution conditions. This suggested that the major rBC-containing particles did not have a spherical structure. Simulation based on an aggregate model considering the morphological information of the particles demonstrated that absorption enhancement of rBC-containing particles could be overestimated by similar to 17 % by using a core-shell model. This study highlights the strong dependence of the morphology of ambient rBC-containing particles on rho(eff ) and will be helpful for elucidating the microphysical characteristics of rBC and reducing uncertainty in the evaluation of rBC climate effects and health risks.