The speciation, oxidation states, and relative abundance of iron (Fe) phases in PM2.5 samples from two locations in urban Los Angeles were investigated using a combination of bulk and spatially resolved, element-specific spectroscopy and microscopy methods. Synchrotron X-ray absorption spectroscopy (XAS) of bulk samples in situ (i.e., without extraction or digestion) was used to quantify the relative fractions of major Fe phases, which were corroborated by spatially resolved spectro-microscopy measurements. Ferrihydrite (amorphous Fe(III)-hydroxide) comprised the largest Fe fraction (34-52%), with hematite (α-Fe2O3; 13-23%) and magnetite (Fe3O4; 10-24%) identified as major crystalline oxide components. An Fe-bearing phyllosilicate fraction (16-23%) was fit best with a reference spectrum of a natural illite/smectite mineral, and metallic Fe(0) was a relatively small (2-6%) but easily identified component. Sizes, morphologies, oxidation state, and trace element compositions of Fe-bearing PM from electron microscopy, electron energy loss spectroscopy (EELS), and scanning transmission X-ray microscopy (STXM) revealed variable and heterogeneous mixtures of Fe species and phases, often associated with carbonaceous material with evidence of surface oxidation. Ferrihydrite (or related Fe(III) hydroxide phases) was ubiquitous in PM samples. It forms as an oxidation or surface alteration product of crystalline Fe phases, and also occurs as coatings or nanoparticles dispersed with other phases as a result of environmental dissolution and re-precipitation reactions. The prevalence of ferrihydrite (and adsorbed Fe(III) has likely been underestimated in studies of ambient PM because it is non-crystalline, non-magnetic, more soluble than crystalline phases, and found in complex mixtures. Review of potential sources of different particle types suggests that the majority of Fe-bearing PM from these urban sites originates from anthropogenic activities, primarily abrasion products from vehicle braking systems and engine emissions from combustion and/or wear. These variable mixtures have a high probability for electron transfer reactions between Fe, redox-active metals such as copper, and reactive carbon species such as quinones. Our findings suggest the need to assess biological responses of specific Fe-bearing phases both individually and in combination to unravel mechanisms of adverse health effects of particulate Fe.

中文翻译：

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使用光谱显微镜法分析洛杉矶市区颗粒物 (PM2.5) 中的铁形态

结合使用体积和空间分辨、元素特异性光谱学和显微镜方法，研究了来自洛杉矶市区两个地点的 PM2.5 样品中铁 (Fe) 相的形态、氧化态和相对丰度。大块样品的同步加速器 X 射线吸收光谱 (XAS) 原位（即没有提取或消化）用于量化主要 Fe 相的相对分数，这通过空间分辨光谱显微镜测量得到证实。水铁矿（无定形 Fe(III)-氢氧化物）占铁的最大比例（34-52%），赤铁矿（α-Fe2O3；13-23%）和磁铁矿（Fe3O4；10-24%）被确定为主要的结晶氧化物成分. 含铁层状硅酸盐部分 (16-23%) 最适合天然伊利石/蒙脱石矿物的参考光谱，金属 Fe(0) 是一种相对较小 (2-6%) 但易于识别的成分。来自电子显微镜、电子能量损失光谱 (EELS) 和扫描透射 X 射线显微镜 (STXM) 的含铁 PM 的尺寸、形态、氧化态和微量元素组成揭示了 Fe 物种和相的可变和异质混合物，通常与碳质材料有关，并有表面氧化的迹象。水铁矿（或相关的氢氧化铁（III）相）在 PM 样品中无处不在。它形成为结晶铁相的氧化或表面改变产物，并且由于环境溶解和再沉淀反应，还以与其他相分散的涂层或纳米颗粒的形式出现。