School of Environmental Science and Engineering , Shaanxi University of Science and Technology , Xi'an 710021 , China.
Key Laboratory of Aerosol Chemistry and Physics, State Key Laboratory of Loess and Quaternary Geology , Institute of Earth Environment, Chinese Academy of Sciences , Xi'an 710061 , China.
Environ Sci Technol. 2019 Sep 3;53(17):10053-10061. doi: 10.1021/acs.est.9b02327. Epub 2019 Aug 21.
A secondary process may be an important source of environmentally persistent free radicals (EPFRs) in atmospheric particulates; yet, this process remains to be elucidated. This study demonstrated that secondary EPFRs could be generated by visible-light illumination of atmospheric particulate matter (PM), and their lifetimes were only 30 min to 1 day, which were much shorter than the lifetimes of the original EPFRs in PM. The yields of secondary EPFRs produced by PM could reach 15-60% of those of the original EPFRs. The extractable organic matter contributed to the formation of secondary EPFRs (∼55%), and a humic-like substance was the main precursor of the secondary EPFRs and was also the most productive precursor compared to the other aerosol components. The results of simulation experiments showed that the secondary EPFRs generated by the extractable and nonextractable PM components were similar to those produced by phenolic compounds and polycyclic aromatic hydrocarbons, respectively. We have found that oxygen molecules play an important role in the photochemical generation and decay of EPFRs. The reactive oxygen capture experiments showed that the original EPFRs may contribute to singlet oxygen generation, while the secondary EPFRs generated by photoexcitation may not produce singlet oxygen or hydroxyl radicals.
二次过程可能是大气颗粒物中持久性自由基（EPFRs）的一个重要来源，但这一过程仍有待阐明。本研究表明，大气颗粒物（PM）在可见光照射下可以产生二次 EPFRs，其半衰期仅为 30 分钟至 1 天，远短于 PM 中原生 EPFRs 的半衰期。PM 产生的次生 EPFRs 的产率可达原生 EPFRs 的 15-60%。可提取有机物有助于次生 EPFRs 的形成（约 55%），而类腐殖质物质是次生 EPFRs 的主要前体，与其他气溶胶成分相比，其产率也最高。模拟实验结果表明，可提取和不可提取 PM 成分产生的次生 EPFRs 分别类似于酚类化合物和多环芳烃产生的次生 EPFRs。我们发现氧分子在 EPFRs 的光化学产生和衰减中起着重要作用。活性氧捕获实验表明，原生 EPFRs 可能有助于单线态氧的产生，而光激发产生的次生 EPFRs 可能不会产生单线态氧或羟基自由基。
