Single-Particle Analysis for Structure and Iron Chemistry of Atmospheric Particulate Matter

As a representative transition metal, iron plays a key role in chemical activities of atmospheric particulate matter (PM), being involved in particle-related free radical generation and adverse health effects. However, limited understanding of the structure and properties of individual micrometer-sized particulates obscures investigating the contributions of iron toward chemical activities. Here, we describe multidimensional analytical strategies to characterize the mass, spatial distribution, and chemical forms of iron in single haze particles using synchrotron radiation techniques. We first used X-ray fluorescence imaging to quantify the masses of multiple metals and yielded distribution maps of transition metals, which revealed the types of elements that tend to occur together. Additionally, we employed nanocomputed tomography to assess the spatial distribution of iron and observed that iron exists as small aggregates and is concentrated primarily in subsurface regions. We also combined X-ray absorption near structures with scanning transmission X-ray microscopy to quantify the ferrous and ferric forms and mapped their distributions in individual particles, which probably attribute chemical activity of iron. In conclusion, we demonstrated the power of synchrotron radiation-based techniques to study heretofore inaccessible chemical information in single haze particles, which may provide important clues about iron chemistry as a source of Fenton reactions and health effects. The multifaceted analytical approaches exhibit high sensitivity (subfemtogram per particle or ∼0.2 fg/μm²) toward multiple elements and are promising to be used for studying other concepts such as the solubility of aerosol iron, the heterogeneous oxidation of organic matters and SO₂, and the formation and the aging of haze particles.