Effective densities of soot particles and their relationships with the mixing state at an urban site in the Beijing megacity in the winter of 2018

Abstract

The effective density ( ρ 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 ( ρ 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 ρ 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 ρ 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 ρ eff. A ρ eff value of 1.4 g cm -3 was the morphological transition point. The morphology and ρ eff value of the rBC-containing particles were intrinsically related to the mass ratio of non-refractory matter to rBC ( M R). As the ρ 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 ρ 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 ĝ1/4 17 % by using a core-shell model. This study highlights the strong dependence of the morphology of ambient rBC-containing particles on ρ 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..