Temporal variability of iron speciation in coastal rainwater

Iron occurs in rain as particulate and dissolved Fe and includes both Fe(II ) and Fe(III) species. Model calculations and correlation analysis indicate Fe(II)(aq) occurs almost exclusively as the free ion whereas Fe(III)(aq) o ccurs as both iron oxalate and Fe(OH)(2)(+)(aq) with large variations over the pH range from 4.0 to 5.0. Complexation with humic-like compounds may al so be important for Fe(III)(aq); however, the concentration and structural characteristics of these compounds have yet to be determined. 112 rain samp les were collected for iron analysis in Wilmington, North Carolina, between 1 July 1997, and 30 June 1999. Total iron, particulate iron and Fe(III)(aq ) were higher in concentration in summer and spring rain relative to winter and autumn rain. Fe(II)(aq) concentrations, in contrast, did not vary seas onally. Particulate iron, which was approximately half the total rainwater iron, was highest between noon and 6 p.m. (EST), probably due to more inten se regional convection including land-sea breezes during that time. The rat io of Fe(II)(aq)/Fe(III)(aq) was also highest in rain received between noon and 6 p.m., which most likely reflects photochemical reduction of Fe(III)( aq) complexes to form Fe(II)(aq). A conceptual model depicting the interpla y between iron species, light intensity and organic ligands in rainwater is presented.