Over the past decade it has become apparent that the atmosphere is a significant pathway for the transport of many natural and pollutant materials from the continents to the ocean. The atmospheric input of many of these species can have an impact (either positive or negative) on biological processes in the sea and on marine chemical cycling. For example, there is now evidence that the atmosphere may be an important transport path for such essential nutrients as iron and nitrogen in some regions. In this report we assess current data in this area, develop global scale estimates of the atmospheric fluxes of trace elements, mineral aerosol, nitrogen species, and synthetic organic compounds to the ocean; and compare the atmospheric input rates of these substances to their input via rivers. Trace elements considered were Pb, Cd, Zn, Cu, Ni, As, Hg, Sn, Al, Fe, Si, and P. Oxidized and reduced forms of nitrogen were considered, including nitrate and ammonium ions and the gaseous species NO, NO2, HNO3, and NH3. Synthetic organic compounds considered included polychlorinated biphenyls (PCBs), hexachlorocyclohexanes (HCHs), DDTs, chlordane, dieldrin, and hexachlorobenzenes (HCBs). Making this assessment was difficult because there are very few actual measurements of deposition rates of these substances to the ocean. However, there are considerably more data on the atmospheric concentrations of these species in aerosol and gaseous form. Mean concentration data for 10° × 10° ocean areas were determined from the available concentration data or from extrapolation of these data into other regions. These concentration distributions were then combined with appropriate exchange coefficients and precipitation fields to obtain the global wet and dry deposition fluxes. Careful consideration was given to atmospheric transport processes as well as to removal mechanisms and the physical and physicochemical properties of aerosols and gases. Only annual values were calculated. On a global scale atmospheric inputs are generally equal to or greater than riverine inputs, and for most species atmospheric input to the ocean is significantly greater in the northern hemisphere than in the southern hemisphere. For dissolved trace metals in seawater, global atmospheric input dominates riverine input for Pb, Cd, and Zn, and the two transport paths are roughly equal for Cu, Ni, As, and Fe. Fluxes and basin‐wide deposition of trace metals are generally a factor of 5‐10 higher in the North Atlantic and North Pacific regions than in the South Atlantic and South Pacific. Global input of oxidized and reduced nitrogen species are roughly equal to each other, although the major fraction of oxidized nitrogen enters the ocean in the northern hemisphere, primarily as a result of pollution sources. Reduced nitrogen species are much more uniformly distributed, suggesting that the ocean itself may be a significant source. The global atmospheric input of such synthetic organic species as HCH,PCBs, DDT, and HCB completely dominates their input via rivers.

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[1] We report empirical estimates of the fractional solubility of aerosol iron over the Sargasso Sea during periods characterized by high concentrations of Saharan dust (summer 2003) and by low concentrations of aerosols in North American/maritime North Atlantic air masses (spring 2004 and early summer 2004). We observed a strong inverse relationship between the operational solubility of aerosol iron (defined using a flow-through deionized-water leaching protocol) and the total concentration of aerosol iron, whereby the operational solubility of aerosol iron was elevated when total aerosol iron loadings were low. This relationship is consistent with source-dependent differences in the solubility characteristics of our aerosol samples and can be described by a simple mixing model, wherein bulk aerosols represent a conservative mixture of two air mass end-members that carry different aerosol types: ''Saharan air,'' which contains a relatively high loading of aerosol iron (27.8 nmol Fe m À3 ) that has a low fractional solubility (0.44%), and ''North American air,'' which contains a relatively low concentration of aerosol iron (0.5 nmol Fe m À3 ) that has a high fractional solubility (19%). Historical data for aerosols collected on Bermuda indicate that the low iron loadings associated with North American air masses are typically accompanied by elevated V/ Al, Fe/Al, and V/Mn mass ratios in the bulk aerosol, relative to Saharan dust, which are indicative of anthropogenic fuel-combustion products. The identification of similar compositional trends in our Sargasso Sea aerosol samples leads us to suggest that the elevated solubility of iron in the aerosols associated with North American air masses reflects the presence of anthropogenic combustion products, which contain iron that is readily soluble relative to iron in Saharan soil dust. We thus propose that the source-dependent composition of aerosol particles (specifically, the relative proportion of anthropogenic combustion products) is a primary determinant for the fractional solubility of aerosol iron over the Sargasso Sea. This hypothesis implies that anthropogenic combustion emissions could play a significant role in determining the atmospheric input of soluble iron to the surface ocean.

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.