Abstract
Iron (Fe) is a key micronutrient regulating primary productivity in many parts of the global ocean. Dust deposition is an important source of Fe to the surface ocean, but most of this Fe is biologically unavailable. Atmospheric processing and reworking of Fe in dust aerosol can increase the bioavailable Fe inputs to the ocean, yet the processes are not well understood. Here, we experimentally simulate and model the cycling of Fe-bearing dust between wet aerosol and cloud droplets. Our results show that insoluble Fe in dust particles readily dissolves under acidic conditions relevant to wet aerosols. By contrast, under the higher pH conditions generally relevant to clouds, Fe dissolution tends to stop, and dissolved Fe precipitates as poorly crystalline nanoparticles. If the dust-bearing cloud droplets evaporated again (returning to the wet aerosol stage with low pH), those neo-formed Fe nanoparticles quickly redissolve, while the refractory Fe-bearing phases continue to dissolve gradually. Overall, the duration of the acidic, wet aerosol stage ultimately increases the amount of potentially bioavailable Fe delivered to oceans, while conditions in clouds favor the formation of Fe-rich nanoparticles in the atmosphere.
Highlights
Iron (Fe) is a limiting micronutrient for phytoplankton growth in large parts of the global ocean.[1,2] A major external source ofFe to the open ocean is atmospheric aerosols, Fe from dust.[3]
The duration of the acidic, wet aerosol stage increases the amount of potentially bioavailable Fe delivered to oceans, while conditions in clouds favor the formation of Fe-rich nanoparticles in the atmosphere
We report the results of a series of experiments that simulated the pH controlled processing of mineral dust as wet aerosols and as cloud droplets
Summary
Iron (Fe) is a limiting micronutrient for phytoplankton growth in large parts of the global ocean.[1,2] A major external source of. There is strong evidence that atmospheric processing can at least partly explain the enhanced fractional Fe solubility in aerosols compared to fresh dust.[4,5] One of the important atmospheric reactions controlling these processes involves the changing chemical conditions in the water around mineral dust. Besides the changes in water content, these condensation/ evaporation cycles induce large variations in the chemistry of the water around the aerosol particles. We monitored the change in fractional Fe solubility in dust samples during cycling from low, wet aerosol pH to near-neutral, cloud droplet pH. On the basis of this and previous data, we developed a model to predict the changes in fractional Fe solubility as a function of water content surrounding dust particles, replicating in silico the transition from wet aerosols to cloud droplets
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