A critical review of the use of iron isotopes in atmospheric aerosol research

Abstract. Deposition of atmospheric aerosols is recognized as a major source of iron (Fe) to the surface oceans, where it acts as a key micronutrient for primary productivity and metabolic functions of marine microbes. Initially, natural desert dust was thought to be the main source of aerosol Fe, albeit largely insoluble; however, in the last few decades, the role of anthropogenic and wildfire sources in providing soluble Fe to aerosols has been increasingly recognized. The stable isotope ratio of Fe (δ⁵⁶Fe) has emerged as a potential tracer for discriminating and quantifying sources of aerosol Fe. In this review, we examine the state of the field for using δ⁵⁶Fe as an aerosol source tracer, and constraints on endmember signatures. We begin with an overview of the methodology of δ⁵⁶Fe analysis for aerosol samples. We then describe knowledge of δ⁵⁶Fe endmember signatures of different source materials, and review existing knowledge of the δ⁵⁶Fe signature of ambient aerosols collected from around the globe, and how these measurements can be used to enhance atmospheric Fe deposition modelling. We also examine the various chemical processing mechanisms which might influence δ⁵⁶Fe source signatures of aerosol Fe during its transport in the atmosphere. This review paper is concluded with a perspective on the state of the field and a call for future work. Overall, we find aerosol δ⁵⁶Fe to be a promising tracer, but highlight that greater constraints on both source endmembers and processing mechanisms are needed to fully utilize this tracer.