Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 115, Issue 30, Pages 7717-7722Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1804481115
Keywords
soot; photochemistry; aging; elemental carbon
Categories
Funding
- Strategic Priority Research Program of the CAS [XDA09030200]
- National Natural Science Foundation of China [21525729, 21590811, 21521062, 2177168]
- Key Research Program of Frontier Sciences Grant of the CAS [QYZDY-SSW-SLH028]
- CAS Interdisciplinary Innovation Team Program
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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 lambda = 648 nm (L-648). However, separated OC (extracted from soot by n-hexane) and EC exhibit little reactivity under L-648. 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.
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