Reduced radiative absorption of aged black carbon aerosols due to liquid-liquid phase separation

Reduced radiative absorption of aged black carbon aerosols due to liquid-liquid phase separation
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My group has been engaged in the microanalysis of individual aerosol particles for a long time. The liquid-liquid phase separation (LLPS) of secondary aerosols and black carbon (BC, also called soot particle) have been the two major focuses in my group because of their important roles in climate and atmospheric chemistry. BC aerosols not only aggravate air pollution and pose significant health risks to the general public, but also change atmospheric radiative forcing and influence global climate1. The mixing structure of BC particles strongly influences their optical properties and their climate effects2. In 2012, You et al.3 proposed the LLPS phenomenon of organic and inorganic species in individual ambient particles, which affects hygroscopic and optical properties of aerosol particles4. However, the LLPS is not recognized to connect with the optical properties of aged BC particles in the ambient air until this study.

In the past, scientists worked on internally mixed BC particles and generally believed that BC was mainly distributed in inorganic particles or mixtures of inorganic and organic compounds in ambient air5 that are classified into partially coated BC, fully-coated BC and bare-like BC particles (Fig. 1). In 2017, our microscopic observations found that airborne BC particles were commonly distributed in organic coatings of individual phase-separated particles collected in background air instead of the inorganic aerosols collected in urban air. This unique phenomenon has attracted our attention and brought our minds to the connection between BC particles and LLPS (Fig. 1). We therefore raised questions whether the LLPS can change position of BC core and further influence “lensing effect” of the aged BC particles. Coincidentally, we noticed that a laboratory study suggested that the distribution of BC inside individual particles can change to organic coatings following LLPS, which was called “BC redistribution”6. However, there is a lack of field data to support such statement and conditions favoring the BC redistribution are unclear and not elucidated.

Fig. 1 A schematic diagram of a bridge between aged black carbon particles and liquid-liquid phase separation (LLPS).

In 2017, I joined Zhejiang University and established my own aerosol laboratory. To clarity the BC redistribution, we carried out field mountain campaigns to collect the suitable samples for microscopic analysis. We noticed that the common transmission electron microscopy (TEM) can only obtain morphology and mixing states of dry particles and cannot well observe secondary nitrate and sulfate under the electron beam and semi-volatile aerosol components because of the restriction of vacuum conditions. Therefore, we inquired into other technologies and found that the cryogenic transmission electron microscopy (Cryo-TEM) can overcome the difficulties. Center of Cryo-Electron Microscopy in Zhejiang University has advanced Cryo-TEM. In this work, we utilize the Cryo-TEM integrating with a humidity chamber for the first time to obtain the direct evidence that the LLPS influences BC redistribution from inorganic cores to organic coatings under a wide range of relative humidity conditions (Fig. 2). Moreover, we recently established a novel Electron-Microscope-to-BC-Simulation (EMBS) tool to construct the realistic BC shape models with various morphology and mixing structures for optical calculation using Discrete Dipole Approximation (DDA)2,7. After all these conditions prepared, we thought that we can achieve the idea which has been started in the past five years ago.

Fig. 2 Cryogenic transmission electron microscopy analysis and optical simulation of black carbon particles by Electron-Microscope-to-BC-Simulation (EMBS).

This study found that the ratio of organic coating thickness to BC size determines the BC redistribution. This is the first evidence to provide the conditions for BC redistribution in airborne aerosols in the world. Our findings change the conventional cognition that internally mixed BC particles are often in the central cores of individual particles. Morphology and mixing states of BC particles which influence their optical absorption dynamically change following particle aging. As we known, the Mie theory calculation assuming the core-shell model and homogeneous mixing model is not enough to accurately obtain optical absorption of BC. To well estimate optical absorption of BC, we used the new developed EMBS to construct the real shape and mixing models of BC particles for DDA calculation (Fig. 2). Interestingly, we found that the BC redistribution due to the LLPS reduces absorption enhancement effect of BC particles by 28-34%. Further analysis suggests that the current climate models assuming a core-shell particle structure probably overestimate radiative absorption of BC aerosols by approximately 18% due to the LLPS in the rural or remote air, highlighting that the microscopic redistribution of BC in the LLPS particles should be incorporated into the current climate models.

We are grateful to our co-authors: Jian Zhang, Yuanyuan Wang, Xiaomi Teng, Lei Liu, Yisheng Xu, Lihong Ren, Zongbo Shi, Yue Zhang, Jingkun Jiang, Dantong Liu, Min Hu, Longyi Shao, Jianmin Chen, Scot T. Martin, and Xiaoye Zhang, for their assistance in the experimental analysis, model simulation, and detailed guidance. This study is funded by the National Natural Science Foundation of China (42075096 and 91844301) and Zhejiang Provincial Natural Science Foundation of China (LZ19D050001).

The detailed information about our BC redistribution study can be found in Communications Earth & Environment: https://www.nature.com/articles/s43247-022-00462-1

References

1          Bond, T. C. et al. Bounding the role of black carbon in the climate system: A scientific assessment. J. Geophys. Res.-Atmos. 118, 5380-5552 (2013).

2          Wang, Y. et al. Nonlinear Enhancement of Radiative Absorption by Black Carbon in Response to Particle Mixing Structure. Geophys. Res. Lett. 48, e2021GL096437 (2021).

3          You, Y. et al. Images reveal that atmospheric particles can undergo liquid-liquid phase separations. Proc. Natl. Acad. Sci. U. S. A. 109, 13188-13193 (2012).

4          Li, W. et al. Organic Coating Reduces Hygroscopic Growth of Phase-Separated Aerosol Particles. Environ. Sci. Technol. 55, 16339-16346 (2021).

5          Cappa, C. D. et al. Radiative Absorption Enhancements Due to the Mixing State of Atmospheric Black Carbon. Science 337, 1078-1081 (2012).

6          Brunamonti, S., Krieger, U. K., Marcolli, C. & Peter, T. Redistribution of black carbon in aerosol particles undergoing liquid-liquid phase separation. Geophys. Res. Lett. 42, 2532-2539 (2015).

7          Wang, Y. et al. Constructing Shapes and Mixing Structures of Black Carbon Particles With Applications to Optical Calculations. J. Geophys. Res.-Atmos. 126, e2021JD034620 (2021).

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