The effect of aerosol size on Fe solubility and deposition flux: A case study in the East China Sea

Abstract

Aerosol sizes are highly associated with the solubilities and the deposition fluxes of aerosol Fe in the surface ocean since the sizes may reflect the sources and decide the deposition velocities. However, systematic studies for the association of the solubilities and fluxes have been limited. In this study, five size-fractions of dry aerosols were collected monthly for a year at two islets in the East China Sea, where large amounts of both fine anthropogenic and coarse lithogenic aerosols deposit. Both pure water and buffer leached methodologies were applied to determine the two operationally defined dissolvable Fe fractions, instantly dissolved Fe (DFe) and supposedly Fe-ligand complexed labile Fe (LFe), respectively. We found that the solubilities of DFe varied up to 4 orders of magnitude with the size spectrum and exhibited a highly linear correlation with non-sea-salt sulfur, indicating that the solubilities of DFe were closely associated with the acidity. Finer aerosols (PM 3) accounted for 90% of total DFe but coarser aerosols (>PM 3) contributed 66% of the difference between LFe and DFe (LFe-DFe). The increasing trend of the difference with increasing sizes indicates that the residence time of coarse aerosol particles and their interaction with Fe-ligands are critical factors deciding the total fluxes of LFe in the ocean. Considering the deposition velocities of each size of aerosols, the averaged fluxes of aerosol Fe of the fine and coarse aerosols were 1.8 and 5.9 nmol m−2 d−1 for DFe; and 2.8 and 62 nmol m−2 d−1 for LFe in the East China Sea, respectively. Attributed to the relatively low deposition velocities of fine aerosols, we found that either single or two averaged deposition velocities (fine/coarse) that were used in most of the previous studies would significantly overestimate dissolvable Fe fluxes in regions where the contribution of fine anthropogenic aerosols is dominant, such as the open ocean. In conclusion, this study demonstrates that aerosol sizes are essential and powerful parameters to accurately estimate the solubility and the fluxes of aerosol dissolvable Fe.