Key Laboratory of Photochemistry, Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, 100190 Beijing, People's Republic of China.
School of Chemical Sciences, University of Chinese Academy of Sciences, 100049 Beijing, People's Republic of China.
Proc Natl Acad Sci U S A. 2018 Jul 24;115(30):7717-7722. doi: 10.1073/pnas.1804481115. Epub 2018 Jul 9.
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 (L). However, separated OC (extracted from soot by n-hexane) and EC exhibit little reactivity under L 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.
煤烟由有机碳（OC）和元素碳（EC）组成，是大气总气溶胶质量的重要组成部分。光化学氧化是煤烟老化的重要途径。人们普遍认为 OC 具有光活性，但 EC 尽管具有很强的光吸收性，但在光化学上是惰性的。在这里，我们利用 OC 和 EC 的不同光吸收特性，提供了直接的实验证据，证明 EC 即使在红光照射下，也通过引发 OC 的氧化，在煤烟的光化学老化中起着重要作用。我们表明，初生煤烟除了在波长为 440nm 的蓝光下发生光化学反应外，在 λ = 648nm（L）的红光照射下也会发生类似的氧化。然而，分离出的 OC（通过正己烷从煤烟中提取）和 EC 在 L 下几乎没有反应性。这些观察结果表明，EC 通过吸收光来吸附光，从而在煤烟的光老化中起着关键作用，引发 OC 的氧化。原位红外光谱和光电化学行为的比较表明，EC 引发的 OC 光氧化通过电子转移途径进行，这与 OC 光吸收引起的光老化不同。由于 EC 的吸收光谱与太阳光谱的重叠比 OC 的吸收光谱大得多，因此我们的结果为大气现场测量中观察到的有机物种导致快速煤烟老化的化学机制提供了新的认识。
