Determination of various types of labile atmospheric iron over remote oceans

Abstract

Atmospheric deposition of iron (Fe) is a major source of the micronutrient to the remote ocean. Most studies have focused on the total atmospheric Fe fluxes to the oceans while fewer studies have focused on the chemistry and chemical speciation of atmospheric Fe. This speciation of Fe in the atmosphere is critical to understanding the fraction of Fe that will be labile in surface waters after deposition and consequently has implications for the bioavailability of this atmospherically derived Fe. In this study, 24‐hour aerosol samples were collected using a high‐volume dichotomous virtual impactor (HVDVI) that collected coarse (Dp > 2.5 μm) and fine (Dp< 2.5 μm) aerosol fractions on two 90‐mm Teflon membrane filters, over the tropical and subtropical North Atlantic Ocean. A sequential aqueous extraction procedure using a pH 4.5 buffer solution and a chemical reductant (hydroxylamine hydrochloride (HA)) was used to measure various labile Fe fractions. The extraction procedure was performed immediately after aerosol sample collection and used time series measurements of Fe(II) using long path length absorbance spectroscopy (LPAS) for analysis of Fe(II). The method measured both the quantities of labile Fe and also the dissolution and reduction kinetics of the labile Fe. Comparisons of HA‐reducible Fe and photoreducible Fe concentrations were conducted on board and showed that both reduction processes had similar reduction kinetics and final Fe(II) concentrations during the initial 90 min. The average pseudo‐first‐order rate constants for the increase in Fe(II) were 0.020 and 0.0076 min−1 for the photoreducible Fe extraction and HA‐reduction extraction, respectively. This HA‐reducible Fe amount could potentially be used to determine the maximum amount of labile atmospheric Fe that is deposited into the ocean.