¹³C Probing of Ambient Photo-Fenton Reactions Involving Iron and Oxalic Acid: Implications for Oceanic Biogeochemistry

The atmospheric abundance of oxalic acid significantly influences the supply of soluble iron (Fe_ws), an essential micronutrient for the phytoplankton growth, to the ocean surface. We used the linear relationship between stable carbon isotopic composition of oxalic acid (δ¹³C_oxalic) and Fe_ws in marine aerosols collected from a coastal ocean, the Bay of Bengal (BoB), to probe the likely photochemical cycling of total aerosol iron (Fe_Tot). In winter, the BoB is influenced by continental pollution from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA). Despite higher levels of oxalic acid, anthropogenic SO₄^2–, and Fe_Tot in IGP outflow, we observe higher fractional solubility of aerosol iron [Fe_ws (%) = Fe_ws/Fe_Tot × 100] in the SEA outflow (11.4–49.7%). Coincidently, we observe an inverse linear relationship between δ¹³C_oxalic (from −18.4 to −8.8‰) and Fe_ws (3.5–38.0 ng m^–3) in the SEA outflow (slope = −0.18; R² = 0.42; p < 0.05) but not in the IGP outflow. This relationship indicates the catalytic behavior of Fe_Tot in the oxidation of precursor water-soluble organics to oxalic acid, yielding Fe_ws. Unlike the IGP outflow, SEA outflow aerosols (i.e., having excess free inorganic acidity and low aerosol pH) are more acidic and experience higher solar influx over the remote BoB. These findings emphasize the concept that oxalic acid formation from the precursor organics in deliquescent aerosols is catalyzed by Fe_Tot, contributing to an overall increase of Fe_ws (%).