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

Liu, Hang; Pan, Xiaole; Wu, Yu; Wang, Dawei; Tian, Yu; Liu, Xiaoyong; Lei, Lu; Sun, Yele; Fu, Pingqing; Wang, Zifa

The effective density (inline-formulaρeff) 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 (inline-formulaρeff) 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 (inline-formulaDmob) and known inline-formulaρeff values (0.8, 1.0, 1.2, 1.4, 1.6, and 1.8 g cminline-formula−3) were precisely selected and measured by the SP2 to obtain their corresponding mixing states. The results showed that the inline-formulaρeff 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 inline-formulaρeff. A inline-formulaρeff value of 1.4 g cminline-formula−3 was the morphological transition point. The morphology and inline-formulaρeff value of the rBC-containing particles were intrinsically related to the mass ratio of non-refractory matter to rBC (inline-formulaMR). As the inline-formulaρeff values of the rBC-containing particles increased from 0.8 to 1.8 g cminline-formula−3, the inline-formulaMR 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 inline-formulaρeff 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 inline-formula∼ 17 % by using a core–shell model. This study highlights the strong dependence of the morphology of ambient rBC-containing particles on inline-formulaρeff and will be helpful for elucidating the microphysical characteristics of rBC and reducing uncertainty in the evaluation of rBC climate effects and health risks.

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Liu, Hang / Pan, Xiaole / Wu, Yu / et al: 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. 2019. Copernicus Publications.

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