Delivery of anthropogenic bioavailable iron from mineral dust and combustion aerosols to the ocean

Abstract

Abstract. Atmospheric deposition of anthropogenic soluble iron (Fe) to the ocean has been suggested to modulate primary ocean productivity and thus indirectly affect the climate. A key process contributing to anthropogenic sources of soluble Fe is associated with air pollution, which acidifies Fe-containing mineral aerosols during their transport and leads to Fe transformation from insoluble to soluble forms. However, there is large uncertainty in our estimate of this anthropogenic soluble Fe. Here, we, for the first time, interactively combined laboratory kinetic experiments with global aerosol modeling to more accurately quantify anthropogenic soluble Fe due to air pollution. We firstly examined Fe dissolution kinetics of African dust samples at acidic pH values with and without ionic species commonly found in aerosol water (i.e., sulfate and oxalate). We then constructed a new empirical scheme for Fe release from mineral dust due to inorganic and organic anions in aerosol water, by using acidity as a master variable. We implemented this new scheme and applied an updated mineralogical emission database in a global atmospheric chemistry transport model to estimate the atmospheric concentration and deposition flux of soluble Fe under preindustrial and modern conditions. Our improved model successfully captured the inverse relationship of Fe solubility and total Fe loading measured over the North Atlantic Ocean (i.e., 1–2 orders of magnitude lower Fe solubility in North African- than combustion-influenced aerosols). The model results show a positive relationship between Fe solubility and water soluble organic carbon (WSOC)/Fe molar ratio, which is consistent with previous field measurements. We estimated that deposition of soluble Fe to the ocean increased from 0.05–0.07 Tg Fe yr−1 in preindustrial era to 0.11–0.12 Tg Fe yr−1 in present days, due to air pollution. Over the High Nitrate Low Chlorophyll (HNLC) regions of the ocean, the modeled Fe solubility remains low for mineral dust (< 1 %) in a base simulation but is substantially enhanced in a sensitivity simulation, which permits the Fe dissolution for mineral aerosols in the presence of excess oxalate under low acidity during daytime. Our model results suggest that human activities contribute to about half of the soluble Fe supply to a significant portion of the oceans in the Northern Hemisphere, while their contribution to oceans in high latitudes remains uncertain due to limited understanding of dust Fe dissolution under pristine conditions.

Do dust emissions from sparsely vegetated regions dominate atmospheric iron supply to the Southern Ocean?

“…This study uses the Integrated Massively Parallel Atmospheric Chemical Transport model [ Rotman et al , ; Liu et al , ; Feng and Penner , ; Ito et al , , , , , ; Lin et al , ; Xu and Penner , ; Ito , ; Ito and Shi , ]. The model is driven by assimilated meteorological fields from the Goddard Earth Observation System (GEOS) of the NASA Global Modeling and Assimilation Office with a horizontal resolution of 2.0° × 2.5° and 59 vertical layers.…”

“…Subsequently, aging processes for Fe‐containing aerosols are dynamically simulated for the size‐segregated dust and combustion aerosols in the model, accounting for the formation of soluble Fe in aerosol water due to proton‐promoted, oxalate‐promoted, and photo‐reductive Fe dissolution schemes [ Ito , ; Ito and Shi , ]. While the Fe dissolution scheme for mineral dust was developed using laboratory measurements for Saharan dust samples, the calculation (blue triangles) reproduced the Fe release from Australian dust aerosols in acidic solution (Figure S1 in the supporting information) [ Mackie et al , ; Ito and Xu , ; Ito and Shi , ]. It should be noted that the Fe dissolution rates from mineral dust are much slower than those from combustion aerosols (red circles) [ Chen and Grassian , ; Ito , ].…”

“…However, significant uncertainties remain regarding the magnitude of the dust emissions and thus the effect of dust deposition on the oceans, especially in the Southern Hemisphere (SH) [ Shao et al , ; Schulz et al , ; Hajima et al , ]. The major source regions of atmospheric Fe to the Southern Ocean include southern South America (Patagonia), Australia, and southern Africa [ Mahowald, ; Li et al , ; Johnson et al , ; Ito and Shi , ]. Large parts of these regions are (sparsely) vegetated, which causes dust emissions to be highly spatially variable and particularly susceptible to climate and land use changes, further enhancing the relevance of Southern Hemispheric dust emissions to ecosystems and climate change [ McConnell et al , ; Bhattachan et al , ; Bhattachan and D'Odorico , ].…”

Aerosol Deposition Impacts on Land and Ocean Carbon Cycles

“…Previously, most studies assumed 3.5% of dust was iron with a mean dust solubility of 2% [5]; however, recent studies have highlighted the importance of the various chemical states of iron in dust minerals on the solubilization rate of iron [102][103][104][105]106••]. Different mechanisms for solubilization of iron have been proposed, including photochemical-, acidic-, and organic ligand-mediated reactions [107][108][109][110][111]. Studies have also identified the potential importance of more soluble forms of iron from biomass burning, coal, and other combustion sources [84,85,112], since in some regions of the globe, long-range transported combustion iron may be quite important (for example, parts of the Southern Ocean) [44,113].…”

“…This large increase in dust may in part be explained by an anthropogenic land use source, which detailed model-data comparisons based on satellite-retrieved aerosol distributions suggest is 25% of current dust sources [120], and/or by increased aridity associated with climate change [42]. In addition, the increase in combustion iron, as well as increased acidity [110,111] from S and N emissions likely has increased the soluble iron deposition to the oceans 3-4-fold over the twentieth century [41]. Improvements in air quality in the future may reduce soluble iron deposition from combustion iron [113] although future scenarios for whether iron deposition from dust sources will increase or decrease is subject to competing processes such as increased dust due to land use change [121,122] or increased aridity [42,123], decreased dust due to wind changes [124], or some combination of these processes (Fig.…”