Estimation of iron solubility from observations and a global aerosol model

Abstract

Mineral aerosol deposition is the dominant source of iron to the open ocean. Soil iron is typically insoluble and understanding the atmospheric processes that convert insoluble iron to the more soluble forms observed over the oceans is crucial. In this paper, we model several proposed processes for the conversion of Fe(III) to Fe(II), and compare with cruise observations. The comparisons show that the model results in similar averaged magnitudes of iron solubility as measured during 8 cruises in 2001–2003. Comparisons show that results of cases including cloud, SO2 and hematite processing are better than the other approaches used using the reaction rates we assume in this paper; unfortunately the reaction rates are not well known, and this hampers our ability to conclusive show one process is more likely than another. The total soluble iron deposited to the global ocean is estimated by the model to range from 0.36 to 1.6 Tg y−1, with 0.88 Tg y−1 being the mean estimate; however there are large uncertainties in these estimates. Comparison shows that the regions with largest differences between the model simulations and observations of iron solubility are in the Southern Atlantic near South America coast and North Atlantic near Spain coast. More observations in these areas or in the South Pacific will help us identify the most important processes. Additionally, laboratory experiments that constrain the reaction rates of different compounds that will result in a net solubilization of iron in aerosols are required to better constrain iron processing in the atmosphere. Additionally, knowing what forms of iron are most bioavailable will assist atmospheric scientists in providing better budgets of iron deposited to the ocean surfaces.

Characterization of Atmospheric Iron Speciation and Acid Processing at Metropolitan Newark on the US East Coast

“…On the other side, it was impossible to isolate the fraction of dissolved iron directly emitted with anthropogenic sources from the fraction of dust dissolved iron formed in acidic air masses rich in anthropogenic pollutants, so the correlation between iron solubility and other acid species was complicated, which may not be simply presented as a linear correlation [64]. Other factors may also affect the iron dissolution reactions: Luo et al (2005) [65] concluded that in-cloud processing was a dominant factor for iron dissolution. In-cloud photochemical processes reducing iron to a more soluble state (Fe(II)) have been the focus of several experiments.…”

“…Other factors may also affect the iron dissolution reactions: Luo et al (2005) [65] concluded that in-cloud processing was a dominant factor for iron dissolution. In-cloud photochemical processes reducing iron to a more soluble state (Fe(II)) have been the focus of several experiments.…”

Atmospheric processing of iron carried by mineral dust

“…Yet, there is a necessity to understand modeling performance in direct comparison against observations, because large gaps in ironrelated observations could be at least partly compensated by information from model simulations. Several studies report on direct point-to-point model-observations comparisons (Luo et al, 2005(Luo et al, , 2008Hand et al, 2004;Mahowald et al, 2009). Based on several model studies used to extrapolate observation data, Mahowald et al (2009) concluded that annual averaged model iron surface concentrations and daily averaged observations differ by a factor of 50-1000 %.…”

“…The iron solubility in dust increases due to clouds, solar radiation and polluted air (Gao et al, 2003;Luo et al, 2005;Jickells and Spokes, 2001;Desboeufs et al, 2001;Hand et al, 2004), but it could also depend on the time that dust spends in the atmosphere (Zhuang et al, 1992;. The atmospheric chemical processing of iron occurs in clouds because they provide a relatively high acidic environment.…”

“…Hand et al (2004) considered that the decay rate coefficient is dependent on the incoming solar radiation and on the cloud cover. Luo et al (2005) combined the radiation and cloud effects with the influence of the sulfate concentration of polluted air on the iron solubility. In their study, the iron fraction in the dust and the soluble iron fraction in the soils were kept constant.…”