Global-scale constraints on the origins of aerosol iron using stable iron isotopes: A review

Understanding the origins of aerosol iron (Fe) is crucial for comprehending its influence on Earth's climate and the global biogeochemical cycle. Fe isotopes (δ⁵⁶Fe) serve as a distinctive and effective tool for constraining aerosol Fe sources and transport routes. In this work, we comprehensively compiled a global dataset (n = 195) of recent aerosol Fe isotopes, spanning diverse atmospheric environments such as urban areas, remote glacier areas, and oceans in order to elucidate the distribution of aerosol Fe isotopes and conduct a quantitative assessment of atmospheric Fe sources on a global scale. We first summarized the spatiotemporal distribution of aerosol δ⁵⁶Fe and its partitioning pattern in various aerosol size fractions. On the spatial scale, the field observations of aerosol Fe isotopes were predominantly concentrated in the Northern Hemisphere locations. Aerosol δ⁵⁶Fe exhibited a pronounced decreasing trend from glaciers to oceans to human-influenced urban areas. On the temporal change, aerosol δ⁵⁶Fe showed lower values during non-dust periods compared to those of dust periods, along with greater variability. The partitioning pattern of δ⁵⁶Fe in various aerosol size fractions is characterized by a notable enrichment of lighter isotopes in PM_2.5 compared to that of bulk samples and PM_>2.5. Secondly, the current quantification study of aerosol Fe sources using Fe isotopes remains uncertain due to the exclusion of heavy-isotope anthropogenic endmembers in calculations. Therefore, here we re-categorized the global aerosol-Fe sources into three endmembers along with their representative δ⁵⁶Fe values, including natural dust (0.09 ± 0.03 ‰), steel smelting+automobile exhaust (−2.9 ± 1.3 ‰), as well as coal combustion (0.46 ± 0.16 ‰). Finally, utilizing the MixSIAR model and complied isotope dataset, we identified coal combustion as the predominant anthropogenic source of aerosol Fe on the hemisphere scale. To enhance our understanding of the atmospheric Fe cycle, future research will necessitate broader large-scale observations of aerosol Fe isotopes, with a particular emphasis on the Southern Hemisphere.