Role of Oxalic acid on Fractional Solubility of Aerosol Iron over Coastal Ocean: Evidence from compound-specific stable carbon isotopic composition and diagnostic mass ratios

Atmospheric transport and the subsequent air-to-sea deposition of water-soluble iron (Fe_ws), an essential micronutrient for the phytoplankton growth, have a profound influence on the biogeochemical cycles of carbon and nitrogen. Sources of Fe_ws include contributions from poorly soluble natural mineral dust and highly soluble anthropogenic aerosols from biomass burning emissions and fossil-fuel combustion in the continental outflows. Apart from the source/emission contributions, atmospheric processing of aerosol iron (Fe_Tot) by inorganic acidic species (e.g., non-sea-salt or nss-SO₄^2- and NO₃^-) and/or organic acids also affect the supply of Fe_ws to the surface waters that are downwind of pollution sources. Among these, the least understood process is the oxalic acid-mediated photochemical cycling of Fe_ws. Laboratory studies have clearly demonstrated an enhancement in the fractional solubility of aerosol iron (i.e., Fe_ws (%) = Fe_ws/Fe_Tot ×100) via the oxalic acid complexation with Fe_Tot and subsequent photochemical reduction process. However, lacking support from the field measurements limits our ability to incorporate the proposed mechanism in the current biogeochemistry models. This study is designed with the overarching goal of investigating the role of oxalic acid on the Fe_ws (%) over a coastal ocean (i.e., the Bay of Bengal: BoB) influenced by the atmospheric outflow from the Indo-Gangetic Plain (IGP) and South-east Asia (SEA) during the winter season. We analysed 31 PM_2.5 samples for the mass concentrations of Fe_Tot, Fe_ws and other chemical composition including nss-SO₄^2-, NO₃^-, oxalic acid and related polar compounds as well as stable carbon isotopic composition of oxalic acid (δ¹³C_oxalic). Strong positive linear relationship of oxalic acid with Fe_Tot and significant inverse linear relationship between δ¹³C_oxalic and Fe_ws over the BoB clearly emphasize the role of oxalic acid on the Fe_ws (%).  These findings comply with the notion that oxalic acid formed from the precursor water-soluble organic acids in the deliquescent aerosols, is complexed with aerosol-Fe and undergoes through successive photochemical reactions, contributing to an overall increase in the Fe_ws (%).