This study provides unique insights into the properties of iron (Fe) in the marine atmosphere over the late summertime Arctic Ocean. Atmospheric deposition of aerosols can deliver Fe, a limiting micronutrient, to the remote ocean. Aerosol particle size influences aerosol Fe fractional solubility and air-to-sea deposition rate. Size-segregated aerosols were collected during the 2015 US GEOTRACES cruise in the Arctic Ocean. Results show that aerosol Fe had a single-mode size distribution, peaking at 4.4 µm in diameter, suggesting regional dust sources of Fe around the Arctic Ocean. Estimated dry deposition rates of aerosol Fe decreased from 6.1 µmol m−2 yr−1 in the areas of ~56°N–80°N to 0.73 µmol m−2 yr−1 in the areas north of 80°N. Aerosol Fe solubility was higher in fine particles (<1 µm) which were observed mainly in the region north of 80°N and coincided with relatively high concentrations of certain organic aerosols, suggesting interactions between aerosol Fe and organic ligands in the high-latitude Arctic atmosphere. The average molar ratio of Fe to titanium (Ti) was 2.4, substantially lower than the typical crustal ratio of 10. We speculate that dust sources around the Arctic Ocean may have been altered because of climate warming.

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Organic ligands, especially oxalate, play an important role in iron dissolution from iron-containing minerals. To study the effects of organic acid ligands on the dissolution of iron-containing minerals, the dissolution kinetics of hematite in the presence of oxalate, acetate, and formate were studied under ultraviolet radiation with varying ligand concentrations (10-3 mM). The results indicate that for adsorption dissolution, oxalate is the dominating ligand for producing soluble iron (III) from hematite; for photoreductive dissolution under ultraviolet radiation and in oxic conditions, the production of iron (II) is highly proportional to the concentrations of oxalate, whereas the effects of varying concentrations of formate and acetate are not significant. At low oxalate concentrations (10-500 lM), the photoreductive dissolution of iron (II) is substantially low, while at high oxalate concentrations (3 mM), oxalate is equally effective as formate and acetate for producing photoreduced iron (II) from hematite. Combining with field data from other works, it is likely that the ratios of oxalate to total iron need to be higher than a threshold range of *1.2-5.5 in order for oxalate to effectively produce photoreduced iron (II) from hematite. This study demonstrates that the iron (II) yield from photoreduction of hematite is significantly lower when the hematite surface is pre-coated with organic ligands versus when it is exposed to ultraviolet radiation instantaneously.

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