The Mechanisms Responsible for the Interactions amongOxalate, pH, and Fe Dissolution in PM 2.5

Abstract

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 ac...