Direct evidence of pyrogenic aerosol iron by intrusions of continental polluted air into the Eastern China Seas

Abstract

Atmospheric iron (Fe) is an important micronutrient controlling marine primary productivity, which affects marine ecosystems and biogeochemistry. However, few studies have reported the abundance of aerosol Fe and its associated mixing state and solubility in marine environments. Here, aerosol samples were collected over the Bohai Sea and the Yellow Sea near East China during a research cruise. The results show that anthropogenic Fe-containing particles accounted for 9.3% of the total observed marine particles with diameters from 0.1 to 7 μm. Almost all of the Fe-containing particles (94%) were internal mixtures of FeOx particles (i.e., Fe-rich/fly ash) and secondary aerosols (e.g., nitrate, sulfate, and organics). During their aging process, the FeOx particles grew approximately tenfold in size from 100 nm to 1 μm by acquiring a thick secondary aerosol coating after one to two days of transport. The elemental mapping on a per-particle basis showed that the secondary aerosol coating likely homogeneously contained little Fe, suggesting that the acidic coating solubilized Fe via an acid dissolution process in the marine air. Therefore, acid dissolution of metal aerosols in anthropogenic smog should be considered in studies on the biogeochemical cycle between coastal marine air and ocean productivity.