Representation of iron aerosol size distributions is critical in evaluating atmospheric soluble iron input to the ocean

Abstract. Atmospheric aerosol deposition acts as a major source of soluble (bioavailable) iron in open ocean regions where it limits phytoplankton growth and primary production. The aerosol size distribution of emitted iron particles, along with particle growth from mixing with other atmospheric components, is an important modulator of its long-range transport potential. There currently exists a large uncertainty in the particle size distribution of iron aerosol, and the role of aerosol size in shaping global soluble iron deposition is thus unclear. In this study, we couple a sophisticated microphysical, size-resolved aerosol model with an iron-speciated and -processing module to disentangle the impact of iron emission size distributions on soluble iron input to the ocean, with a focus on anthropogenic combustion and metal smelting sources. We first evaluate our model results against a global-scale flight measurement dataset for anthropogenic iron concentration and find that the different representations of iron size distribution at emission, as adopted in previous studies, introduces a variability in modeled iron concentrations over remote oceans of a factor of 10. Shifting the iron aerosol size distribution toward finer particle sizes (<1 μm) enables longer atmospheric lifetime (a doubling), promoting atmospheric processing that enhances the soluble iron deposition to ocean basins by up to 50 % on an annual basis. Importantly, the monthly enhancements reach 110 % and 80 % over the Southern Ocean and North Pacific Ocean, respectively. Compared with emission flux uncertainties, we find that iron emission size distribution plays an equally important role in regulating soluble iron deposition, especially to the remote oceans. Our findings provide implications for understanding the effects of atmospheric nutrients input on marine biogeochemistry, including but not limited to iron, phosphorus, and others.