Unraveling the Origin of Atmospheric Soluble Iron during Asian Dust Storms

East Asian dust storms are a critical source of iron (Fe) to the North Pacific Ocean, where soluble Fe (Fe_s) supports marine primary productivity, playing an important role in global carbon cycling and ultimately influencing the climate. However, the relative contributions of natural mineral dust versus those of anthropogenic sources to Fe_s remain unclear. In this study, stable isotope composition (δ⁵⁶Fe) of total Fe (Fe_T) and Fe_s, together with mineralogical (STEM) and geochemical analysis for PM_2.5 and PM₁₀ size fractions of atmospheric particles collected during two dust storm events in Tianjin (China) were performed, with a goal to elaborate dynamics of Fe in dust storms and trace their sources. The results show that Fe solubility (1.7-30%) and δ⁵⁶Fe vary significantly across storm stages, with the natural dust-dominated culmination stage exhibiting δ⁵⁶Fe_T close to crustal values (∼+0.09‰), while antecedent stages influenced by local emissions show extremely low δ⁵⁶Fe_s (down to -3.65‰), characteristic of high-temperature combustion sources. Various evidence confirm minimal Fe isotope fractionation during transport, indicating the preservation of isotopic signatures of primary sources. Isotope mixing models quantify large anthropogenic contribution of 1-85% (average 24%) to Fe_s, significantly higher than pristine dust (<1% solubility). Our study demonstrates the usefulness of the isotope approach for discriminating atmospheric Fe sources, and highlights the high contribution of anthropogenic emissions to the oceanic soluble Fe budget that may control the primary productivity and thus the global carbon cycle.