Evaluation of aerosol iron solubility over Australian coastal regions based on inverse modeling: implications of bushfires on bioaccessible iron concentrations in the Southern Hemisphere

Akinori Ito,Michal Strzelec,Bernadette C Proemse,Philip W Boyd,Andrew R Bowie,Morgane M G Perron,Melanie Gault-Ringold

doi:10.1186/s40645-020-00357-9

Abstract

Mineral dust is the major source of external micro-nutrients such as iron (Fe) to the open ocean. However, large uncertainties in model estimates of Fe emissions and aerosol-bearing Fe solubility (i.e., the ratio of labile Fe (LFe) to total Fe (TFe)) in the Southern Hemisphere (SH) hampered accurate estimates of atmospheric delivery of bioavailable Fe to the Southern Ocean. This study applied an inverse modeling technique to a global aerosol chemistry transport model (IMPACT) in order to optimize predictions of mineral aerosol Fe concentrations based on recent observational data over Australian coastal regions (110°E–160°E and 10°S–41°S). The optimized (a posteriori) model did not only better capture aerosol TFe concentrations downwind from Australian dust outbreak but also successfully reproduced enhanced Fe solubility (7.8 ± 8.4%) and resulted in much better agreement of LFe concentrations with the field measurements (1.4 ± 1.5 vs. 1.4 ± 2.3 ng Fe m–3). The a posteriori model estimates suggested that bushfires contributed a large fraction of LFe concentrations in aerosols, although substantial contribution from missing sources (e.g., coal mining activities, volcanic eruption, and secondary formation) was still inferred. These findings may have important implications for the projection of future micro-nutrient supply to the oceans as increasing frequency and intensity of open biomass burning are projected in the SH.

Highlights

Atmospheric deposition has been widely recognized as one of the major pathways for delivering essential micro-nutrients such as iron (Fe) to the open ocean (Fung et al 2000; Jickells et al 2005; Mahowald et al 2005)
The scaling factor is extremely sensitive to sporadic dust outbreak and the intensity of dust chemistry models, highlighting the need for better constraining key parameters controlling atmospheric Fe cycling including the magnitude of atmospheric emissions and the processes leading to enhanced aerosol Fe solubilities over the Southern Ocean (SO) (Myriokefalitakis et al 2018; Ito et al 2019)
If deposition fluxes calculated by these models were to be used to estimate dissolved Fe in marine biogeochemistry models, this could contribute to the underestimation of marine primary production in the SO

Summary

Introduction

Atmospheric deposition has been widely recognized as one of the major pathways for delivering essential micro-nutrients such as iron (Fe) to the open ocean (Fung et al 2000; Jickells et al 2005; Mahowald et al 2005). Such external supply of Fe is important in high-nutrient low-chlorophyll (HNLC) oceanic regions where even a small addition of this limiting. To avoid any confusion in comparing model estimates to observations in this study, we consider LFe as bioaccessible Fe (Myriokefalitakis et al 2018; Ito et al 2019; Perron et al 2020a)

Methods

Results

Conclusion