Enhanced Iron Solubility at Low pH in Global Aerosols

Ellery Ingall,Amelia Longo,Kalliopi Violaki,Barry Lai,Shivraj Sahai,Erin Castorina,Yuan Gao,Athanasios Nenes,Rachel Shelley,Nikolaos Mihalopoulos,William Landing,Peter Morton,Yan Feng

doi:10.3390/atmos9050201

Abstract

The composition and oxidation state of aerosol iron were examined using synchrotron-based iron near-edge X-ray absorption spectroscopy. By combining synchrotron-based techniques with water leachate analysis, impacts of oxidation state and mineralogy on aerosol iron solubility were assessed for samples taken from multiple locations in the Southern and the Atlantic Oceans; and also from Noida (India), Bermuda, and the Eastern Mediterranean (Crete). These sampling locations capture iron-containing aerosols from different source regions with varying marine, mineral dust, and anthropogenic influences. Across all locations, pH had the dominating influence on aerosol iron solubility. When aerosol samples were approximately neutral pH, iron solubility was on average 3.4%; when samples were below pH 4, the iron solubility increased to 35%. This observed aerosol iron solubility profile is consistent with thermodynamic predictions for the solubility of Fe(III) oxides, the major iron containing phase in the aerosol samples. Source regions and transport paths were also important factors affecting iron solubility, as samples originating from or passing over populated regions tended to contain more soluble iron. Although the acidity appears to affect aerosol iron solubility globally, a direct relationship for all samples is confounded by factors such as anthropogenic influence, aerosol buffer capacity, mineralogy and physical processes.

Highlights

Biogeochemical studies of aerosol chemistry have been motivated in part by the “iron hypothesis”that links availability of the nutrient iron to marine primary productivity, and carbon dioxide sequestration [1,2,3,4]
Similar to Finokalia Research Station, North African influenced samples from the GEOTRACES cruise contained the highest levels of soluble iron
North-American-influenced samples from the GEOTRACES collection averaged the same magnitude of soluble iron (1.3 ± 0.78 ng/m3 ) as the year-long collection of BIOS samples

Summary

Introduction

Biogeochemical studies of aerosol chemistry have been motivated in part by the “iron hypothesis”that links availability of the nutrient iron to marine primary productivity, and carbon dioxide sequestration [1,2,3,4]. Iron (Fe) availability influences primary productivity in vast ocean regions [5]. The factors that control the solubility (a proxy for potential bioavailability) of aerosol iron remain uncertain, especially in environments relatively unaffected by anthropogenic influences [11,12,13]. This has hampered understanding of the role of aerosol iron in marine primary productivity and the associated uptake and sequestration of carbon dioxide by marine algae [1,14], in addition to other biogeochemical impacts [15]

Results

Discussion

Conclusion