Trace metal dissolution kinetics of East Asian size-fractionated aerosols in seawater: The effect of a model siderophore

Aerosol soluble metals are considered bioavailable to marine phytoplankton and may influence phytoplankton growth, their community structure, and material cycling in the ocean. The dissolution is controlled by the physical and chemical properties of aerosols and various atmospheric, physical, and biogeochemical processes before and after depositing in the surface water. Among these complicated processes, the interaction of aerosol metals with organic ligands in seawater is one of the major factors. In this study, we systematically investigated the dissolution kinetics of trace metals from fine (0.45–0.95 μm) and coarse (>7.2 μm) aerosols in seawater for 30 days under conditions with or without adding a model Fe organic ligand, siderophore desferrioxamine B (DFB). We found that most of the soluble metals in the fine aerosols were leached rapidly within the first hour. In terms of the coarse aerosols, with DFB addition, the fractional solubility of Fe increased substantially from 0.1 to 10%, and the solubilities of most other metals generally increased by about 2-fold within 30 days. Without DFB addition, we found that most soluble metals were still gradually released over time in coarse aerosols, except particle-reactive metals, Fe and Pb. We further observed strong linear correlations between dissolved metals (Al, Fe, Mn, Co, Ni, and Zn) and silicate concentrations for DFB addition with coarse aerosols, with metal to silicate ratios comparable to those of typical aluminosilicates. Quantitatively, the aluminosilicate-associated dissolved metals accounted for >85% of total dissolved Fe, Al, and Co, and 40–60% for total dissolved Mn and Ni with the addition of DFB for 30 days. By comparing dissolved metal to Al ratios with lithogenic ratios, the metals leached in the first hour for both coarse and fine aerosols originated from anthropogenic sources, and lithogenic aerosols gradually became the dominant soluble metal source for coarse aerosols after 1 h, extending to 720 h. The findings of this study exhibit the importance of the time-dependent interaction of lithogenic aerosols with organic ligands in seawater. The residence time of aerosol particles and the concentrations of available organic ligands in the euphotic zone are key factors affecting aerosol dissolved metal availability to marine phytoplankton in the surface ocean.