1Aerosol deposition provides a major input of the essential micronutrient iron to 2 the open ocean. A critical parameter with respect to biological availability is the 3 proportion of aerosol iron that enters the oceanic dissolved iron pool -the so-called 4 fractional solubility of aerosol iron (%Fe S ). Here we present a global-scale compilation 5 of total aerosol iron loading (Fe T ) and estimated %Fe S values for ~1100 samples 6collected over the open ocean, the coastal ocean, and some continental sites, including a 7 new data set from the Atlantic Ocean. Despite the wide variety of methods that have 8 been used to define 'soluble' aerosol iron, our global-scale compilation reveals a 9 remarkably consistent trend in the fractional solubility of aerosol iron as a function of 10 total aerosol iron loading, with the great bulk of the data defining an hyperbolic trend. 11The hyperbolic trends that we observe for both global-and regional-scale data are 12 adequately described by a simple two-component mixing model, whereby the fractional 13 solubility of iron in the bulk aerosol reflects the conservative mixing of 'lithogenic' 14 mineral dust (high Fe T and low %Fe S ) and non-lithogenic 'combustion' aerosols (low Fe T 15 and high %Fe S ). An increasing body of empirical and model-based evidence points to 16 anthropogenic fuel combustion as the major source of these non-lithogenic 'combustion' 17 aerosols, implying that human emissions are a major determinant of the fractional 18 solubility of iron in marine aerosols. The robust global-scale relationship between %Fe S 19 and Fe T provides a simple heuristic method for estimating aerosol iron solubility at the 20 regional to global scale.

Atmospheric deposition contributes potentially significant amounts of the nutrients iron, nitrogen, and phosphorus (via mineral dust and anthropogenic aerosols) to the oligotrophic tropical North Atlantic Ocean. Transport pathways, deposition processes, and source strengths contributing to this atmospheric flux are all highly variable in space and time. Atmospheric sampling was conducted during 28 research cruises through the eastern tropical North Atlantic (ETNA) over a 12-yr period, and a substantial dataset of measured concentrations of nutrients and trace metals in aerosol and rainfall over the region was acquired. This database was used to quantify (on a spatial and seasonal basis) the atmospheric input of ammonium, nitrate, soluble phosphorus, and soluble and total iron, aluminum, and manganese to the ETNA. The magnitude of atmospheric input varies strongly across the region, with high rainfall rates associated with the intertropical convergence zone contributing to high wet deposition fluxes in the south, particularly for soluble species. Dry deposition fluxes of species associated with mineral dust exhibited strong seasonality, with the highest fluxes associated with wintertime low-level transport of Saharan dust. Overall (wet plus dry) atmospheric inputs of soluble and total trace metals were used to estimate their soluble fractions. These also varied with season and were generally lower in the dry north than in the wet south. The ratio of ammonium plus nitrate to soluble iron in deposition to the ETNA was lower than the N:Fe requirement for algal growth in all cases, indicating the importance of the atmosphere as a source of excess iron.