Environmentally persistent free radicals (EPFRs) are a new class of pollutants that are long-lived in fine particles (PM_2.5), i.e., their 1/e lifetime ranges from days to months (or even infinite). They are capable of producing harmful reactive oxygen species such as hydroxyl radicals. The redox cycling of EPFRs is considered as an important pathway for PM_2.5 to induce oxidative stress inside the humans, causing adverse health effects such as respiratory and cardiovascular diseases. Consequently, research regarding their toxicity, formation and environmental occurrences in PM_2.5 has attracted increasing attentions globally during the past two decades. However, literature data in this field remain quite limited and discrete. Hence, an extensive review is urgently needed to summarize the current understanding of this topic. In this work, we systematically reviewed the analytical methods and environmental occurrences, e.g., types, concentrations, and decay behaviors, as well as possible sources of EPFRs in PM_2.5. The types of pretreatment methods, g-values of common EPFRs and categories of decay processes were discussed in detail. Moreover, great efforts were made to revisit the original data of the published works of EPFRs in airborne particulate matter and provided additional useful information for comparison where possible, e.g., their mean and standard deviation of g-values, line widths (∆H_p-p), and concentrations. Finally, possible research opportunities were highlighted to further advance our knowledge of this emerging issue.

中文翻译：

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PM 2.5中的环境持久性自由基：综述

持久性环境自由基（EPFRs）是一类新型的污染物，它们长期存在于细颗粒（PM _2.5）中，即它们的1 / e寿命为数天至数月（甚至无限）。它们能够产生有害的活性氧，例如羟基自由基。EPFR的氧化还原循环被认为是PM _2.5诱导人体内氧化应激，引起不良健康影响（如呼吸系统疾病和心血管疾病）的重要途径。因此，有关其在PM _2.5中的毒性，形成和环境事件的研究在过去的二十年中，它已引起全球越来越多的关注。但是，该领域的文献数据仍然非常有限且分散。因此，迫切需要进行广泛的综述以总结对这一主题的当前理解。在这项工作中，我们系统地审查了分析方法和环境事件，例如类型，浓度和衰减行为，以及PM _2.5中EPFR的可能来源。预处理方法的种类，g讨论了常见EPFR的-值和衰减过程的类别。此外，还付出了巨大的努力重新审视了空气颗粒物中EPFR的已发表著作的原始数据，并提供了更多有用的信息以进行比较，例如它们的g值的平均值和标准偏差，线宽（∆ H _p-p）和浓度。最后，强调了可能的研究机会，以进一步提高我们对这一新出现问题的认识。