Light Absorption Enhancement of Black Carbon Aerosol Constrained by Particle Morphology

Wu, Yu; Cheng, Tianhai; Liu, Dantong; Allan, J. D.; Zheng, Lijuan; Chen, Hao

doi:10.1021/acs.est.8b00636

Cited by 99 publications

(98 citation statements)

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“…Firstly, the assumption of sphericity used in the Mie calculations to derive E sca introduces significant bias for larger BC cores with little or no coating. The resulting scattering for uncoated BC will be lower than that derived by Mie scattering due to geometrical influences as has been shown when comparing with T-matrix calculations (He et al, 2015;Wu et al, 2018), and this increases with rBC core size. Secondly, the BC at larger D c largely results from coal combustion (Sect.…”

Section: Bc Segregation By Size-resolved Mixing Statementioning

confidence: 83%

Contrasting physical properties of black carbon in urban Beijing between winter and summer

et al. 2019

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Abstract. Black carbon (BC) is known to have major impacts on both human health and climate. The populated megacity represents the most complex anthropogenic BC emissions where the sources and related impacts are very uncertain. This study provides source attribution and characterization of BC in the Beijing urban environment during the joint UK–China APHH (Air Pollution and Human Health) project, in both winter (November–December 2016) and summer (May–June 2017). The size-resolved mixing state of BC-containing particles was characterized by a single-particle soot photometer (SP2) and their mass spectra was measured by a soot particle aerosol mass spectrometer (SP-AMS). The refractory BC (rBC) mass loading was around a factor of 2 higher in winter relative to summer, and more variable coatings were present, likely as a result of additional surface emissions from the residential sector and favourable condensation in the cold season. The characteristics of the BC were relatively independent of air mass direction in summer, whereas in winter air masses from the Northern Plateau were considerably cleaner and contained less-coated and smaller BC, but the BC from the Southern Plateau had the largest core size and coatings. We compare two online source apportionment methods using simultaneous measurements made by the SP2, which measures physical properties of BC, and the chemical approach using the positive matrix factorization (PMF) of mass spectra from the SP-AMS for the first time. A method is proposed to isolate the BC from the transportation sector using a mode of small BC particles (core diameter Dc<0.18 µm and coating thickness ct < 50 nm). This mode of BC highly correlated with NOx concentration in both seasons (∼14 ng m−3 BC ppb−1 NOx) and corresponded with the morning traffic rush hour, contributing about 30 % and 40 % of the total rBC mass (35 % and 55 % in number) in winter and summer respectively. The BC from coal burning or biomass burning was characterized by moderate coatings (ct = 50–200 nm) contributing ∼20 %–25 % of rBC mass. Large uncoated BC particles (Dc>0.18 µm and ct < 50 nm) were more likely to be contributed by coal combustion, as these particles were not present in urban London. This mode was present in Beijing in both winter (∼30 %–40 % rBC mass) and summer (∼40 % rBC mass) but may be dominated by the residential and industrial sector respectively. The contribution of BC thickly coated with secondary species (ct > 200 nm) to the total rBC mass increased with pollution level in winter but was minor in summer. These large BC particles importantly enhanced the absorption efficiency at high pollution levels – in winter when PM1 > 100 µg m−3 or BC > 2 µg m−3, the absorption efficiency of BC increased by 25 %–70 %. The reduction of emissions of these large BC particles and the precursors of the associated secondary coating will be an effective way of mitigating the heating effect of BC in urban environments.

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Section: Bc Segregation By Size-resolved Mixing Statementioning

confidence: 83%

Contrasting physical properties of black carbon in urban Beijing between winter and summer

et al. 2019

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“…When emitted to the surroundings, BC particles transform their morphology from fractal to spherical and then grow as fully compact particles with other components depositing on the BC aerosol (Peng et al, 2016). The variation in the shapes of BC aerosols, together with the variation in the mixing states, can lead to substantial changes in aerosol optical properties (Liu et al, 2017;China et al, 2013;Wu et al, 2016Wu et al, , 2018. BC aerosols also have a significant influence on the climate by interacting with clouds (Koch and Del Genio, 2010;Roberts et al, 2008;Stevens and Feingold, 2009), ice and snow (Bond et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

Role of black carbon mass size distribution in the direct aerosol radiative forcing

et al. 2019

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Large uncertainties exist when estimating radiative effects of ambient black carbon (BC) aerosol. Previous studies about the BC aerosol radiative forcing mainly focus on the BC aerosols' mass concentrations and mixing states, while the effects of BC mass size distribution (BCMSD) were not well considered. In this paper, we developed a method of measuring the BCMSD by using a differential mobility analyzer in tandem with an Aethalometer. A comprehensive method of multiple charging corrections was proposed and implemented in measuring the BCMSD. Good agreement was obtained between the BC mass concentration integrated from this system and that measured in the bulk phase, demonstrating the reliability of our proposed method. Characteristics of the BCMSD and corresponding radiative effects were studied based on a field measurement campaign conducted in the North China Plain by using our own measurement system. Results showed that the BCMSD had two modes and the mean peak diameters of the modes were 150 and 503 nm. The BCMSD of the coarser mode varied significantly under different pollution conditions with peak diameter varying between 430 and 580 nm, which gave rise to significant variation in aerosol bulk optical properties. The direct aerosol radiative forcing was estimated to vary by 8.45 % for different measured BCMSDs of the coarser mode, which shared the same magnitude with the variation associated with assuming different aerosol mixing states (10.5 %). Our study reveals that the BCMSD as well as its mixing state in estimating the direct aerosol radiative forcing matters. Knowledge of the BCMSD should be fully considered in climate models.

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“…However, debate exists among researchers. For instance, several studies (Lack et al, 2012;Wang et al, 2014Wu et al, 2016) observed a large absorption enhancement of rBC caused by mixing with the coating material, whereas other studies found negligible absorption enhancement (Cappa et al, 2012;Lan et al, 2013). Liu et al (2017) proposed that the mixing structure of rBCcontaining particles depended on the proportion of coating material.…”

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confidence: 99%

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

Pan

et al. 2019

Atmos. Chem. Phys.

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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.

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Light Absorption Enhancement of Black Carbon Aerosol Constrained by Particle Morphology

Cited by 99 publications

References 50 publications

Contrasting physical properties of black carbon in urban Beijing between winter and summer

Contrasting physical properties of black carbon in urban Beijing between winter and summer

Role of black carbon mass size distribution in the direct aerosol radiative forcing

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

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