RUI: Temporal Variability of Iron Speciation in Rain

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The specific objective of this research is to determine the speciation of iron in various types of rain events. Speciation is the precise chemical form of the iron in rain, including free and complexed ferrous and ferric iron. This information is crucial to the understanding of several important chemical processes in the atmosphere which depend on the concentration and speciation of iron, as well as the impacts of rainwater iron on aquatic ecosystems. Any change in iron speciation in rain may alter the rate of marine primary productivity, because speciation affects the bioavailability of iron as a nutrient, as well as the residence time for iron in seawater. Marine primary productivity plays a significant role in carbon removal from the atmosphere, and hence impacts global warming.Preliminary data indicate that approximately half of the iron in rain passes through a fine filter (nominally dissolved), with the remaining fraction in the larger particulate phase. Data from a June thunderstorm in Wilmington, NC, suggest that part of the dissolved iron occurred as an organic complex. Data from an early summer rain suggest that photochemical reduction of iron proceeds as the day progresses, and changes are large enough and rapid enough to be observed in rainwater.The presence of organically bound iron in rain, and of large variations in Fe(II) concentration, indicate that iron speciation is highly variable in rain. This research will assess how the relative abundance of iron species vary both temporally with time of day, season and storm history, and as a function of concentrations of ligands capable of complexation. Ligands to be investigated include oxalate, formate, sulfate, amino acids, and soil humics. This will be accomplished by careful sampling, analysis and laboratory experiments. The variability of Fe(II) in rain suggests that controlled photochemical experiments will give a useful indication of the potential for generation of Fe(II) in rain, which concentration analyses alone cannot provide.The sampling site in Wilmington, NC, a coastal location, is ideal for this study because continental and maritime storms bring rain with a wide range in rainwater composition. A twelve year record of rainwater composition exists for this site; and supporting rain data essential for a study of iron speciation (pH, hydrogen peroxide, organic acids, amino acids, aldehydes, nitrite and inorganic anions) are currently being collected by the same laboratory.This project is supported under the Research in Undergraduate Institutions (RUI) program and will involve, among other participants, undergraduate and Master's students.