Iron Speciation in Urban Atmospheric Aerosols: Comparison between Thermodynamic Modeling and Direct Measurements

Metals are important components of atmospheric aerosols for both health and atmospheric reactivity. Coordination chemistry, leading to the formation of metal-ligand complexes, can alter metal solubility and their redox activity in solution; however, such processes have been so far predominantly studied via thermodynamic modeling approaches alone. Such approaches have indicated iron-oxalate complexes as major species of interest in urban environments. In this study, aerosol samples collected in the urban environment of Padua (Italy) are used to compare the speciation picture of iron obtained by thermodynamic modeling with that measured experimentally using X-ray absorption spectroscopy (XAS). The results showed broadly consistent speciation pictures between the two approaches, however, with some quantitative differences probably due to a discrepancy between bulk and single particle chemical composition of the aerosol samples. The XAS results showed the presence of iron-oxalate complexes in the samples, with both Fe(III) and Fe(II), and also suggested that most of the Fe may be mixed with organic matter on an atomic level. The thermodynamic modeling results indicated malonate as an additional important ligand besides oxalate and a potential candidate for explaining the mixed iron-organic nature of the aerosol samples.