In-cloud processing as a possible source of isotopically light iron from anthropogenic aerosols: New insights from a laboratory study

Wet atmospheric deposition can account for up to 50% of the total iron input to surface waters, so establishing the extent to which kinetic and equilibrium isotope effects can influence aerosol soluble delta Fe-56 values is imperative to trace and constrain aerosol sources using Fe isotopes and to understand the differences found between delta Fe-56 values for bulk and soluble phases of aerosols. In this context, changes in iron solubility and isotopic composition of dissolved Fe during simulated atmospheric processing of industrial ash was investigated. Kinetic and equilibrium experiments were performed under UV/VIS light using ash from a Fe-Mn alloy metallurgical plant and a synthetic solution that mimics cloud water chemistry. The nature of the Fe species of the industrial ash was investigated by Mossbauer Spectroscopy, whereas ash and dissolved delta Fe-56 values were measured by MC-ICP-MS. Mossbauer Spectroscopy revealed that alpha-hematite, magnetite, and poorly crystallized manganoferrite nanoparticles are the main Fe species. In the early-stage dissolution (until 60 min) a Fe isotope fractionation (Delta Fe-56(solution-bulk ash)) of -0.284 +/- 0.103 parts per thousand was found at the minimum contact time evaluated herein (i.e., 5 min) due to kinetic isotopic effects. In the late-stage dissolution (after 60 min) a Delta Fe-56(solution-ash) of 0.227 +/- 0.091 parts per thousand was found due to equilibrium isotopic effects. The kinetic isotope effect within one ash surface monolayer was modeled with an enrichment factor (epsilon) of -1 parts per thousand in Fe-56/Fe-54 ratio. Iron fractional dissolution undergone during different atmospheric processing time scales may release Fe with contrasted isotope compositions to solution, changing the original soluble Fe isotope signature (which is linked to its source). This might be especially important when the dissolution process goes from kinetic to near-equilibrium conditions, in which higher amounts of Fe are progressively released from ash surface.

Automated summary

The study investigated changes in iron solubility and isotopic composition during atmospheric processing of industrial ash, revealing both kinetic and equilibrium isotope effects. Iron dissolution during different time scales may release Fe with varied isotope compositions, potentially altering the original soluble Fe isotope signature.