Fractional solubility of aerosol iron: Synthesis of a global-scale data set

Aerosol deposition provides a major input of the essential micronutrient iron to the open ocean. A critical parameter with respect to biological availability is the proportion of aerosol iron that enters the oceanic dissolved iron pool - the so-called fractional solubility of aerosol iron (%Fe-S). Here we present a global-scale compilation of total aerosol iron loading (Fe-T) and estimated %FeS values for similar to 1100 samples collected over the open ocean, the coastal ocean, and some continental sites, including a new data set from the Atlantic Ocean. Despite the wide variety of methods that have been used to define 'soluble' aerosol iron, our global-scale compilation reveals a remarkably consistent trend in the fractional solubility of aerosol iron as a function of total aerosol iron loading, with the great bulk of the data defining an hyperbolic trend. The hyperbolic trends that we observe for both global- and regional-scale data are adequately described by a simple two-component mixing model, whereby the fractional solubility of iron in the bulk aerosol reflects the conservative mixing of 'lithogenic' mineral dust (high Fe-T and low %Fe-S) and non-lithogenic 'combustion' aerosols (low Fe-T and high %Fe-S). An increasing body of empirical and model-based evidence points to anthropogenic fuel combustion as the major source of these non-lithogenic 'combustion' aerosols, implying that human emissions are a major determinant of the fractional solubility of iron in marine aerosols. The robust global-scale relationship between %Fe-S and Fe-T provides a simple heuristic method for estimating aerosol iron solubility at the regional to global scale. (C) 2012 Elsevier Ltd. All rights reserved.