Speciation of Iron in Aerosol Particles Entering the Irish Shelf at Mace Head measured by µXAS and cathodic-stripping voltammetry

Sarah L Nicholas, Brookhaven National Laboratory, Upton, NY, United States, Darius Ceburnis, National University of Ireland, School of Physics, Galway, Ireland, Maija Iris Heller, Pontificia Universidad Católica de Valparaíso Chile, Escuela de Ciencias del Mar, Valparaiso, Chile, Matthew A Marcus, Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, United States, Geraldine Sarthou, IUEM Institut Universitaire Européen de la Mer, LEMAR, CNRS, Plouzané, France and Peter L Croot, iCRAG (Irish Centre for Research in Applied Geosciences), Earth and Ocean Sciences, School of Natural Sciences and Ryan Institute, University of Galway, Galway, Ireland
We examined the iron (Fe) speciation of aerosol particles collected at Mace Head using X-ray absorption spectroscopy (XAS) and measured the solubility kinetics operationally via cathodic-stripping voltammetry.

Primary productivity in the oceans is strongly regulated by the availability of the micronutrient iron. Differences in chemical speciation, particle size, and temporal variablilty of atmopheric deposition can cause rapid changes in surface water Fe concentrations.

Aerosol particles were collected on Teflon filters over short time intervals in 2016 and 2017 at the Mace Head Atmospheric Research Station, County Galway, Ireland. Sampling dates for speciation work were chosen using Hysplit back-trajectories to limit the provenance of sampled particles to specific source areas. The source areas of interest were: Atlantic, European, North African, and a large gorse fire in Connemara near Mace Head.

We used micro X-ray fluorescence (µXRF) mapping to identify the distribution of elements within the particles, and micro X-ray Absorption Near Edge Structure (µXANES) spectroscopy to identify chemical speciation of Fe and sulfur (S) in the particles. Separate aliquots of the filters were examined using cathodic-stripping voltammetry to measure the kinetics of release of soluble Fe from the aerosols in seawater (Croot and Johansson 2000).

Measured Fe-bearing particles from all four source areas collected at Mace Head tended to contain multiple Fe phases. The most frequently observed Fe-bearing particles via µXANES from all four source areas were Fe(II) silicates with Fe(III) oxides/hydroxides. µXRF mapping and µXANES on S-bearing particles from all analyzed samples showed that most of the sulfur in all of the samples was sulfate, which we interpret as local sea-spray that was collected as droplets on the filters and crystalized dendriticly in place after deposition.

Cathodic-stripping voltammetry showed that filters that aerosols from the North African dust storm source at Mace Head had the most labile iron, while the least-reactive iron was found in the filters from the Atlantic source area.