Aerosol water soluble organic matter characteristics over the North Atlantic Ocean: Implications for iron-binding ligands and iron solubility

Aerosol water soluble organic carbon (WSOC) and iron (Fe) loadings were examined in aerosols collected during the 2011 US GEOTRACES cruise in the North Atlantic Ocean to explore the feasibility of a role for water soluble organic matter (WSOM) organic complexation in determining the higher fractional solubility of Fe (%FeS) observed in North American-influenced relative to North African-influenced aerosols. Proton nuclear magnetic resonance spectroscopy (1H NMR) was employed to characterize the WSOM, and principal component analysis was then applied to differentiate North American-influenced from North African-influenced aerosol WSOM and provide clues to the molecular identities of organic ligands. As has been found in previous studies, North African-influenced samples had orders of magnitude higher total Fe (FeT) loadings (8.9–101nmol Fe m−3 air) and orders of magnitude lower %FeS (0.12–0.29%) than those found in North American-influenced (FeT: 0.03–1.4nmol Fe m−3 air; %FeS: 0.67–3.62%) and Marine (minimal recent continental influence; FeT: 0.02–0.25nmol Fe m−3 air; %FeS: 1.1–10.7%) samples. Aerosol %FeS values were positively correlated with WSOC/FeT ratios with North American combustion-influenced (39–540) and Marine (30–850) aerosol samples having orders of magnitude more moles of WSOC per mole of FeT than North African (0.42–3.8) aerosol samples. The high WSOC/FeT ratios in the North American combustion-influenced samples suggest a realistic role for organic ligands binding with Fe and accounting for some portion of the higher aerosol %FeS also observed for these samples. Principal component analyses of 1H NMR spectra show North American-influenced aerosol WSOM to have higher contributions from hydrogens in aliphatic carbon chains attached to heteroatomic functional groups such as those in nitrate, amine, ether, ester, and carboxylate compounds compared to North African mineral dust-influenced aerosol WSOM. These multifunctional aliphatic WSOM compounds are suggested as candidate Fe binding ligands to be explored with future work. •Marine aerosol fractional Fe solubility is positively correlated with WSOC/FeT.•Aerosol organic ligands may facilitate the delivery of soluble Fe to the ocean.•Aerosol WSOM ligands are suggested to be aliphatic, multifunctional compounds.