EVS atmospheric scientist Yan Feng is collaborating with beamline scientist Barry Lai of Argonne’s Advanced Photon Source (APS), as well as university researchers, to unravel the chemical transformation mechanisms of iron in atmospheric dust. This effort is an extension of the 3‑year research project, ​“Probing the Chemistry of Atmospheric Dust Particles using X‑ray Spectromicroscopy: Implications for Climate Science” funded by the Laboratory Director’s Competitive Grant (2014–2016), and strives to address both variations in composition of iron compounds and the dissolution mechanisms involved in the atmospheric transport of dust particles. It combines synchrotron-based X-ray techniques developed at APS with modeling studies using the high-performance computing capabilities of the Argonne Leadership Computing Facility (ALCF) and the Argonne Laboratory Computing Resource Center (LCRC).

Iron is an essential micronutrient for marine ecosystems. In many ocean regions, it controls the primary production (as organic compounds produced by photosynthetic organisms) and sequestration of carbon dioxide from the atmosphere. Iron in atmospheric dust is a major source of iron (about 95%) for marine ecosystems; therefore, it is critical to understand the factors controlling its solubility and associated bioavailability. Although many hypotheses have been proposed, the chemistry surrounding the dissolution of atmospheric (aerosol) iron is still not well understood. Earth system models conventionally assume a constant rate of iron solubility in dust, which could lead scientists to miscalculate carbon dioxide sequestration, especially in iron-limited oligotrophic waters. Our study addresses the uncertainties in modeled global soluble iron inputs to ocean ecosystems. Its results will improve the effectiveness of Earth system models, making them more effective tools in understanding ocean biogeochemical cycles and their impact on climate change.

Modern microscopy and microanalysis techniques, such as the X-ray fluorescence (XRF) micro‑spectroscopy developed and advanced at the APS, provide scientists the unique capability of probing the mineral composition of individual dust particles. These synchrotron-based techniques have been used mostly to study interplanetary stardust and have only recently been applied to the study of atmospheric particles. Elements such as iron, silicon, calcium, and aluminum in micron-sized atmospheric dust particles (10^-7 to 10^-5 meters) can easily be measured using hard X-ray microscopy at the APS beamlines in X-ray fluorescence mode for quantities down to the trace level in the real atmosphere. Using the APS micro-spectroscopy beamlines (e.g., 2-ID-D), we can further determine the oxidization states of iron (iron (II) or iron (III)) in dust particles, which is linked to the solubility and bioavailability of iron.

Over the last several years, EVS and collaborators have performed beamline tests on more than 100 dust samples provided by university collaborators from Rutgers, Georgia Tech, Amity University, University of Hawaii, and University of Paris. These samples were collected from seven locations: the Southern Ocean, the Mediterranean, the Atlantic Ocean, Hawaii, Bermuda, Patagonia, and India. They represent a wide range of dust aerosol characteristics from different source regions and remote oceans. To date, we have analyzed the chemical information for a few hundred individual dust particles.

Strong evidence of reductive iron transformation by acidic pollutants is found in these samples, as well as indicators of the proton-induced reactions that are typically considered in atmospheric models. Because iron reduction reactions occur in different environments from acid processing, our study highlights the importance of modeling the reductive mechanism to capture variations in dust iron dissolution. Several manuscripts have been published on the main findings of this project.

The developed dataset has been utilized by a broad science community to develop and improve the model representation of dust iron dissolution processes and bioavailable iron cycle in the Earth system models. We also developed a computational efficient iron dissolution model for the Community Earth System Model (CESM). These modeling studies are documented in recent high‑impact journal publications including the Science Advances.

Our ongoing effort is to build on the iron dissolution model developed for CESM and develop a modeling capability of bioavailable iron in the DOE Energy Exascale Earth System model (E3SM). This research has now been integrated as part of the E3SM project funded by the DOE, Office of Science, Office of Biological and Environmental Research.