Determination of various types of labile atmospheric iron over remote oceans

Abstract

[1] 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 (D p > 2.5 mm) and fine (D p < 2.5 mm) 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 À1 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.

Role of dust alkalinity in acid mobilization of iron

“…As a result, the global model simulates higher iron fractional solubility for the fine mode (diameter <2.5 µm) than that for the coarse mode (diameter >2.5 µm) along the atmospheric transport to the eastern North Pacific Ocean. The onboard cruise measurements have reported that smaller particles have higher iron fractional solubility over the tropical Pacific Ocean between 24 • N-28 • N and 170 • E-155 • W from 9 to 26 April 2001 (Chen and Siefert, 2003;Hand et al, 2004). Similarly, the simulated iron fractional solubility for the fine mode is also higher than that for the coarse mode averaged over the cruise tracks (Table 3).…”

Fractional iron solubility of atmospheric iron inputs to the Southern Ocean

“…Reported values of the Southern Ocean fractional Fe solubility (i.e., the ratio of water leachable Fe to total Fe) range from 0.01 to 90% (Edwards and Sedwick, 2001;Mahowald et al, 2005;Bowie et al, 2009;Baker and Croot, 2010;Heimburger et al, 2013a). This large range may reflect differences in mineral dust concentrations, particle size, atmospheric weathering, cloud chemistry and aerosol leaching methods (Zhuang et al, 1990(Zhuang et al, , 1992Spokes and Jickells, 1995;Chen and Siefert, 2003;Meskhidze et al, 2003;Bonnet and Guieu, 2004;Mackie et al, 2005;Buck et al, 2006;Meskhidze and Nenes, 2006;Aguilar-Islas et al, 2010;Trapp et al, 2010). Laboratory studies have indicated that aerosol Fe solubility is enhanced by acid processing (Spokes et al, 1994;Desboeufs et al, 1999), although this relationship was not observed in the remote Atlantic and Pacific ocean (Hand et al, 2004;).…”

Mega fire emissions in Siberia: potential supply of bioavailable iron from forests to the ocean

“…The initial conditions for soluble iron fractions from the dust (0.45 %) and combustion sources (4 %) are prescribed (Luo et al, 2008). The iron dissolution in dust aerosols due to atmospheric chemical processing is calculated from the online simulation of aerosol-phase chemistry, which quantitatively reproduces higher iron solubility in smaller particles (Ito and Feng, 2010), as suggested by the onboard cruise measurements over the Pacific Ocean (Chen and Siefert, 2003). To examine the potential impact of extreme fire events on soluble iron deposition, the biomass burning emission rates estimated in this paper are used as revised input data for the aerosol chemistry transport model.…”