Emissions and modeling of atmospheric iron in China

Iron (Fe) in the atmosphere is of critical importance to atmospheric chemistry and supplies essential nutrients to the ocean once deposited, thus affecting the global carbon cycle. However, reliable modeling of atmospheric Fe has been hindered by incomplete representations of both fine- and coarse-mode Fe emissions and by scarce measurements of Fe content data at their sources. Here, we construct a gridded Fe emission inventory in China at a spatial resolution of 25 km, separating soluble and insoluble fractions and covering the years from 1990 to 2023. This upgraded emission inventory includes Fe from anthropogenic, dust, and open-fire sources, constrained by more than 370 China-specific and size-resolved Fe mass fractions across 14 major emission sectors. Integrated into the GEOS-Chem model with source-explicit transport tracers, we correct the default model’s severe underestimation of fine- and coarse mode Fe, reasonably reproducing multi-season observed total and soluble Fe concentrations at inland, coastal, and shipborne sites. Source apportionment reveals that land ambient Fe is predominantly anthropogenic in origin. While natural dust dominates marine total Fe, marine soluble Fe is overwhelmingly controlled by anthropogenic combustion. Driven by national Clean Air Actions, anthropogenic combustion Fe emissions decreased by 51% over the decade, primarily from the Fe-making sector and concentrated in eastern China. This decline reduced soluble Fe deposition over the subtropical Northwest Pacific in winter, which aligns with satellite-observed decreases in chlorophyll-a, implying a weakened Fe fertilization effect on marine productivity. Our study underscores an important environmental trade-off and calls for reevaluating the impacts of changing Fe emissions on atmospheric chemistry and ocean carbon sequestration.