The dissolution of atmospheric iron in surface seawater of the open ocean

A series of leaching experiments designed to determine the soluble fraction of atmospheric iron in seawater was conducted. Aerosol samples collected in 1986 at four island stations (Midway, Oahu, Enewetak, and Fanning) of SEAREX (Sea‐Air Exchange) and in May 1988 at station 4 (33.3°N, 139.1°W) of VERTEX (Vertical Transport and Exchange) were leached by surface seawater collected from the North Pacific and Sargasso Sea and at a pH range of 3–8. The results from these experiments indicated that for all marine aerosol samples examined, the dissolution of atmospheric iron was limited by a similar saturated concentration range. Using the operationally defined concept of a dissolved fraction that could pass through a 0.4‐μm filter, the “saturated concentration” of dissolved atmospheric iron in seawater was found to be ∼10–17 nmol/kg. If 0.05‐μm Millipore filter was used to separate the soluble fraction from the particulate fraction, the “saturated concentration” of dissolved atmospheric iron in seawater was found to be ∼5–8 nmol/kg. The dissolution of atmospheric iron in seawater takes place within a few minutes. If the total iron concentration was less than 2 nmol/kg (the open Pacific Ocean is in this category), ∼50% of the atmospheric iron dissolved in seawater. A comparison of the atmospheric dissolved iron flux into the North Pacific with the dissolved iron profile in this area suggests that more than 99% of the dissolved iron is provided by atmospheric input.