The Mechanisms Responsible for the Interactions among Oxalate, pH, and Fe Dissolution in PM2.5

In this study, 10-year records of PM2.5 oxalate concentrations in six urban and rural Canadian monitoring sites are analyzed along with the time series of other water-soluble ionic components, water-soluble metals, and aerosol pH to identify the major factors influencing its abundance and speciation. Oxalate was found to have higher mass loadings relative to sulfate and nitrate when the temperature was higher and the aerosol was more acidic, which is counter to the gas/particle phase partitioning expected from the effective Henry's Law constant. However, a strong correlation was found between oxalate and water-soluble Fe (WS-Fe), with a slope between 1.3 and 4.6, suggesting that the major forms of the particulate oxalate were in complexes with WS-Fe, whose concentration was also found to be significantly impacted by aerosol pH. When aerosol exhibits pH around 2, the proton-promoted dissolution of Fe leads to a strong relationship between WS-Fe and pH, indirectly supporting higher oxalate levels in more acidic particles. Meanwhile, higher aerosol pH in wintertime favors oxalate to fully partition into the particle phase, but higher WS-Fe concentration in aerosol liquid water was also often observed when aerosol had higher oxalate concentration. This phenomenon suggests that the chelating-promoted dissolution mechanism induced by oxalate can enhance Fe dissolution in wintertime. Our results reveal that the interactions among pH, oxalate. and Fe exhibit strong seasonality, and an improved understanding of these mechanisms is important for the environmental fate studies of both organic acids and metals.