Role of elemental carbon in the photochemical aging of soot

Soot, which consists of organic carbon (OC) and elemental carbon (EC), is a significant component of the total aerosol mass in the atmosphere. Photochemical oxidation is an important aging pathway for soot. It is commonly believed that OC is photoactive but EC, albeit its strong light absorption, is...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2018-07, Vol.115 (30), p.7717-7722
Main Authors: Li, Meng, Bao, Fengxia, Zhang, Yue, Song, Wenjing, Chen, Chuncheng, Zhao, Jincai
Format: Article
Language:English
Subjects:
Absorption
Absorption spectra
Aerosols
Aging
Carbon
Electromagnetic absorption
Electron transfer
Infrared spectroscopy
Irradiation
Light irradiation
n-Hexane
Organic carbon
Organic chemistry
Oxidation
Photochemicals
Photochemistry
Photooxidation
Physical Sciences
Solar spectra
Soot
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Summary:Soot, which consists of organic carbon (OC) and elemental carbon (EC), is a significant component of the total aerosol mass in the atmosphere. Photochemical oxidation is an important aging pathway for soot. It is commonly believed that OC is photoactive but EC, albeit its strong light absorption, is photochemically inert. Here, by taking advantage of the different light absorption properties of OC and EC, we provide direct experimental evidence that EC also plays an important role in the photochemical aging of soot by initiating the oxidation of OC, even under red light irradiation. We show that nascent soot, in addition to undergoing photochemical oxidation under blue light with a wavelength of 440 nm, undergoes similar oxidation under red light irradiation of λ = 648 nm (L648). However, separated OC (extracted from soot by n-hexane) and EC exhibit little reactivity under L648. These observations indicate that EC plays a pivotal role in photoaging of soot by adsorbing light to initiate the oxidation of OC. Comparison of in situ IR spectra and photoelectrochemical behaviors suggests that EC-initiated photooxidation of OC proceeds through an electron transfer pathway, which is distinct from the photoaging induced by light absorption of OC. Since the absorption spectra of EC have a much larger overlap with the solar spectra than those of OC, our results provide insight into the chemical mechanism leading to rapid soot aging by organic species observed from atmospheric field measurements.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1804481115