Article Cite This: Environ. Sci. Technol. 2019, 53, 8227−8234
pubs.acs.org/est
Mixing State and Fractal Dimension of Soot Particles at a Remote Site in the Southeastern Tibetan Plateau Qi Yuan,† Jianzhong Xu,‡ Yuanyuan Wang,† Xinghua Zhang,‡ Yuner Pang,† Lei Liu,† Lei Bi,† Shichang Kang,‡,§ and Weijun Li*,† †
Department of Atmospheric Sciences, School of Earth Sciences, Zhejiang University, Hangzhou 310027, Zhejiang, China State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences (CAS), Lanzhou 730000, Gansu, China § CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China Downloaded via KEAN UNIV on July 23, 2019 at 08:46:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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S Supporting Information *
ABSTRACT: The mixing state and fractal dimension (Df) of soot particles are two major factors affecting their absorption capacity and their climate effects. Here we investigated these factors of soot particles found in a typical valley of the southeastern Tibetan Plateau where wood burning in local villages was one major source of soot particles. Our motivation revealed Df and the aging property of soot particles in remote air and discussed their regional climatic implications. We found that 64% of total analyzed particles by number were soot-bearing particles and most of them aged with sulfate or organic coating. The Df sequence is bare-like soot (1.75 ± 0.08) < partly coated soot (1.82 ± 0.05) < embedded soot (1.88 ± 0.05). The aging process enlarged the overall size of the soot-bearing particles and increased the compactness of soot. Soot aging critically depended on high relative humidity (RH) during nighttime. Besides emission sources and coating processes, the coating aerosol phase under different RHs is another important factor affecting the soot Df.
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μg/m3, as well as a mass median diameter of 160 nm (70−300 nm) at the Lulang site in the southeastern TP.12 Moreover, large number fractions of soot particles have also been observed in some urban areas (>40% by number) and at a glacial site (∼25% by number) in the southeastern TP.13 Wang et al.15 found that soot particles were overall thickly coated with biomass burning organic aerosols in the TP and that these secondary organic coatings further enhanced their light absorption capacity. However, physical and chemical data of soot particles were limited in the vast TP, as demonstrated by a literature review.16 Moreover, none of these studies quantified the soot aging processes through morphology changes and their mixing states with other aerosols in the southeastern TP. The data scarcity about soot particles serves as an obstacle toward an accurate simulation in atmospheric models of their optical absorption in the southeastern TP. The fractal dimension (Df) parameter has been extensively used to represent soot compactness and to evaluate their optical properties.17−19 Fresh soot particles usually exhibit lacy
INTRODUCTION The Tibetan Plateau (TP), often known as the “third pole” or the “Roof of the World”, is the highest and largest plateau on earth. Due to its special terrain, large geographic scale, and intense solar radiation, the TP plays a crucial role in the formation of the Asian monsoon system and even influences overall global climate change.1 Owing to its high altitude and long distance from dense anthropogenic emission sources, the TP has been taken as one of the most clean and pristine regions on earth, but at the same time, it could prove to be sensitive to external human interferences.2 Some previous studies have already suggested that large amounts of aerosols (e.g., black carbon) and trace gaseous pollutants have been found in the TP atmosphere and glaciers.3−6 Black carbon (BC or soot) is a typical light-absorbing aerosol component of fine particles (
