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

Abstract. The effective density (ρeff) 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 (ρeff) 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 (Dmob) and known ρeff 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 ρeff 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 ρeff. A ρeff value of 1.4 g cm−3 was the morphological transition point. The morphology and ρeff value of the rBC-containing particles were intrinsically related to the mass ratio of non-refractory matter to rBC (MR). As the ρeff values of the rBC-containing particles increased from 0.8 to 1.8 g cm−3, the MR 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 ρeff 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 ∼ 17 % by using a core–shell model. This study highlights the strong dependence of the morphology of ambient rBC-containing particles on ρeff and will be helpful for elucidating the microphysical characteristics of rBC and reducing uncertainty in the evaluation of rBC climate effects and health risks.

Mixing state and effective density of aerosol particles during the Beijing 2022 Olympic Winter Games

“…8c), including porous aggregates with low ρ eff peaking at 0.8 g cm -3 and dense particles with higher ρ eff peaking at 1.4 g cm -3 . The results are consistent with the conclusions of previous studies (Rissler et al, 2014;Liu et al, 2019;Ma et al, 2020).…”

“…After dried with a diffusion dryer, the particles with mobility diameters (D m ) of 150, 200, 250 and 300 nm were selected by DMA, and those with aerodynamic diameter (D a ) of 300 nm were selected by AAC, and then the highly monodispersed particles were measured by SPAMS. The detailed description of AAC is given elsewhere (Liu et al, 2020;Liu et al, 2019). Before the campaign, the size calibration of SPAMS was performed using polystyrene latex spheres (PSLs) with known sizes (0.23, 0.32, 0.51, 0.74, 0.96, 1.4, and 2 μm).…”

Vertical distribution of black carbon and its mixing state in urban boundary layer in summer

“…for the BC core (Bond and Bergstrom, 2006) and 1.50+0i for the coatings (Liu et al, 2015). The core-shell assumption may overestimate the Eabs Wu et al, 2018;Liu et al, 2019c) because of the complex morphology of BC Hu et al, 2021b). Liu et al (2020b) estimated that such overestimates could be ~17% in summer in Beijing.…”

Mixing state and effective density of aerosol particles during the Beijing 2022 Olympic Winter Games

“…After being dried with a diffusion dryer, the particles with mobility diameters (D m ) of 150, 200, 250, and 300 nm were selected by the DMA, and those with an aerodynamic diameter (D a ) of 300 nm were selected by the AAC, and then the highly monodispersed particles were measured by the SPAMS. A detailed description of the AAC is given elsewhere (Liu et al, 2019(Liu et al, , 2020Yu et al, 2022). Before the campaign, the size calibration of the SPAMS was performed using polystyrene latex spheres (PSLs) with known sizes (0.23, 0.32, 0.51, 0.74, 0.96, 1.4, and 2 µm).…”