Stable isotopes as a tool to apportion atmospheric iron

Identification of atmospheric iron is a key parameter to understanding the source of iron in urban and remote areas. Atmospheric deposition of desert dust, which also can include an anthropogenic component, is a primary nutrient source for most of the open ocean. To better assess particulate matter (PM) sources specific to iron, we measured the iron isotopic compositionofaerosolsintwosizefractions:PMwithaerodynamic diameters less than 2.5 μm and less than 10 μm (PM2.5 and PM10, respectively). Using colocated samplers, atmospheric aerosol samples were collected in the U.S. desert Southwest at a mixed suburban/agricultural site near Phoenix, AZ. The measurements are presented as δ⁵⁶Fe relative to the IRMM-014 (Institute for Reference Materials and Measurements) standard. Using multiple collector inductively coupled plasma mass spectrometry, we found differences in iron isotopic composition within the PM10 aerosol. Half of the PM10 samples had an iron isotopic signature similar to crustal material (+0.03 ‰), which implicates wind-blown soil-dust as the primary source. The other PM10 samples showed a lighter iron isotopic composition, centered at -0.18 ‰. Further analysis showedthat the lighter ironwasassociated with winds originating from the southwest. This strongly suggests that there is a different PM10 source in this direction, with a distinct iron isotopic composition. The iron in the PM2.5 samples was usually substantially lighter than the corresponding PM10 samples, which is consistent with coarse and fine particles having different sources, again with distinctively different isotopic compositions. The magnitude of the iron isotopic difference between the PM10 and the PM2.5 size fractions (δ⁵⁶Fe^PM10-δ⁵⁶Fe^PM2.5) correlated with the PM2.5 concentrations of elements known to be emitted from industrial sources (Pb, Cd, As, V, and Cr). This observation implies that the isotopically light iron is created or emitted alongside industrial processes. Our data demonstrate that iron isotope compositioncanbeavaluable tool in the source-apportionment of iron in atmospheric particles.