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. In this study, for the first time, we interactively combined laboratory kinetic experiments with global aerosol modeling to more accurately quantify anthropogenic soluble Fe due to air pollution. Firstly, we determined 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). Then, by using acidity as a master variable, we constructed a new empirical scheme for Fe release from mineral dust due to inorganic and organic anions in aerosol water. 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 northern-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 the preindustrial era to 0.11–0.12 Tg Fe yr−1 in the present day, due to air pollution. Over the high-nitrate, low-chlorophyll (HNLC) regions of the ocean, the modeled Fe solubility remains low for mineral dust (

Climate engineering by mimicking natural dust climate control: the iron salt aerosol method

“…Combustion processes currently contribute from 20 to 100 % of the soluble-iron deposition over many ocean regions (Luo et al, 2008). 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 and that deposition of soluble iron from combustion sources contributes more than 40 % of the total soluble-iron deposition over significant portions of the open ocean in the Southern Hemisphere (Ito, 2015). Anthropogenic aerosol associated with coal burning is maybe the major bioavailable iron source in the surface water of the oceanic regions (Lin et al, 2015).…”

Insight into the in-cloud formation of oxalate based on in situ measurement by single particle mass spectrometry

“…Oxalic acid is globally the most abundant dicarboxylic acid (Mochida et al, 2007;Ho et al, 2010;Kawamura and Bikkina, 2016), accounting for as high as 5 % of water-soluble organic compounds downwind of the mainland China Kawamura and Bikkina, 2016). In addition, oxalate has great impact on the solubility, photochemistry and bioavailability of transition metals in aerosols (Johnson and Meskhidze, 2013;Ito and Shi, 2016).…”

The GESAMP atmospheric iron deposition model intercomparison study

“…Soils may include a small fraction of LFe -roughly 0.1%; e.g., Ito and Shi (2016) -considered as impurities attached on minerals such as illite, smectite, kaolinite and feldspars (e.g., Ito 25 and Xu, 2014). Fe-containing fly ash has been observed to be present as ferric sulfate salts or nanoparticulate Fe and thus is highly soluble (Fu et al, 2012;Schroth et al, 2009), since it is mainly formed via high-temperature combustion followed by sulfuric acid condensation (Sippula et al, 2009).…”

“…State-of-the-art global models clearly indicate a strong spatial and temporal variability of atmospheric LFe supply to the global ocean, that can be partly attributed to atmospheric processing. The global LFe deposition flux is currently estimated in the range of 0.4-1.1 Tg-Fe yr −1 (Ito and Kok, 2017;Ito and Shi, 2016;Ito and Xu, 2014;Johnson and Meskhidze, 2013;25 Luo et al, 2008;Myriokefalitakis et al, 2015;Wang et al, 2015).…”

“…The aerosol acidity depends on the aerosol types, mineralogy, particle size, meteorological conditions, and transport pathway of aerosols (Ito and Feng, 2010;Ito and Xu, 2014;Ito, 2015). A more detailed description of the IMPACT model used 5 for this work can be found in (Ito, 2015;Ito and Kok, 2017;Ito and Shi, 2016) and references therein. …”