Abstract

Mineral dust is the largest source of aerosol iron (Fe) to the offshore global ocean, but acidic processing of coal fly ash (CFA) in the atmosphere may result in a disproportionally higher contribution of bioavailable Fe. Here, we determined the Fe speciation and dissolution kinetics of CFA from Aberthaw (United Kingdom), Krakow (Poland), and Shandong (China) in solutions which simulate atmospheric acidic processing. In CFA-PM10 fractions, 8 %–21.5 % of the total Fe was as hematite and goethite (dithionite extracted Fe), 2 %–6.5 % as amorphous Fe (ascorbate extracted Fe), while magnetite (oxalate extracted Fe) varied from 3 %–22 %. The remaining 50 %–87 % of Fe was associated with aluminosilicates. High concentration of ammonium sulphate ((NH4)2SO4), often found in wet aerosols, increased Fe solubility of CFA up to 7 times at low pH (2–3). Our results showed a large variability in the effects of oxalate on the Fe dissolution rates at pH 2, from no impact in Shandong ash to doubled dissolution in Krakow ash. However, this enhancement was suppressed in the presence of high concentration of (NH4)2SO4. Dissolution of highly reactive Fe was insufficient to explain the high Fe solubility at low pH in CFA, and the modelled dissolution kinetics suggests that other Fe phases such as magnetite may also dissolve rapidly under acidic conditions. Overall, Fe in CFA dissolved up to 7 times faster than in Saharan dust samples at pH 2. Based on these laboratory data, we developed a new scheme for the proton- and oxalate- promoted Fe dissolution of CFA, which was implemented into the global atmospheric chemical transport model IMPACT. The revised model showed a better agreement with observations of surface concentration of dissolved Fe in aerosol particles over the Bay of Bengal, due to the rapid Fe release at the initial stage at highly acidic conditions. The improved model also enabled us to predict sensitivity to a more dynamic range of pH changes, particularly between anthropogenic combustion and biomass burning aerosols.

Highlights

  • The availability of iron (Fe) limits primary productivity in high-nutrient low-chlorophyll (HNLC) regions of the global ocean including the subarctic North Pacific, the East Equatorial Pacific and the Southern Ocean (Boyd et al, 2007; Martin, 1990)

  • Fe in Coal fly ash (CFA) dissolved up to 7 times faster than in Saharan dust samples at pH 2. Based on these laboratory data, we developed a new scheme for the proton- and oxalate- promoted Fe dissolution of CFA, which was implemented into the global atmospheric chemical transport model Integrated Massively Parallel Atmospheric Chemical Transport (IMPACT)

  • Atmospheric Fe is largely derived from lithogenic sources, which contribute around 95% of the total Fe in suspended particles (e.g., Myriokefalitakis et al, 2018) and most studies concentrate on atmospheric processing of mineral dust (e.g., Cwiertny et al, 2008; Fu et al, 2010; Ito and Shi, 2016; Shi et al, 2011a; Shi et al, 2015)

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Summary

Introduction

The availability of iron (Fe) limits primary productivity in high-nutrient low-chlorophyll (HNLC) regions of the global ocean including the subarctic North Pacific, the East Equatorial Pacific and the Southern Ocean (Boyd et al, 2007; Martin, 1990). In other regions of the global ocean such as the subtropical North Atlantic, the Fe input may affect primary productivity by stimulating nitrogen fixation (Mills et al, 2004; Moore et al, 2006). These areas are sensitive to changes in the supply of bioavailable Fe. Atmospheric aerosols are an important source of soluble (and, potentially bioavailable) Fe to the offshore global ocean. Bioavailable Fe consists of aerosol dissolved Fe, and Fe-nanoparticles which can be present in the original particulate matter and/or formed during atmospheric transport as a result of cycling into and out of clouds (Shi et al, 2009). It is in addition possible that other more refractory forms of Fe could be solubilised in the surface waters by zooplankton (Schlosser et al, 2018) or the microbial community (Rubin et al, 2011)

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