Abstract
We studied the solubility, in real sea water, of iron present in the African dust outbreaks that traverse the Atlantic. Based on measurements of soluble iron (sFe) and aerosol chemistry, we found iron solubilities within the range of 0.4–1.8% in Tenerife, 0.4–3.1% in Barbados and 1.6–12% in Miami. We apportioned the concentrations of sFe between the three sources and processes that we identified: (1) dust, (2) heavy fuel oil combustion emissions, associated with an excess of vanadium and nickel, and (3) atmospheric processing, which is influenced by acidic pollutants. We tracked the propagation of the dust-front of the African dust outbreaks across the Atlantic, which are associated with dust peak events at the impacting sites. During the westward transport across the Atlantic, the contribution to sFe from dust decreased (63%, 43% and 9% in Tenerife, Barbados and Miami, respectively), whereas the contribution due to atmospheric processing increased (26%, 45% and 74% in Tenerife, Barbados and Miami, respectively). In these Saharan-dust outbreaks, the concentrations of sFe due to heavy fuel oil combustion were significantly lower (mostly < 5 ng/m3) than those in the polluted marine atmosphere (10–200 ng/m3). The overall results are consistent with the idea that the mixing of dust with acid pollutants increases the solubility of iron during the African-dust outbreaks that traverse the Atlantic.
Highlights
Marine microorganisms account for ~ half of Earth’s primary pro ductivity, i.e. the conversion of carbon from carbon dioxide to organic molecules via photosynthesis and chemosynthesis, a process that replenishes the oxygen budget (Chapman, 2013)
We present the results of an experiment focused on studying the change of iron solubility after African dust plumes had crossed the Atlantic
From 30 June to September 1, 2015 we collected daily samples of aerosols in three sites of the North Atlantic: (i) at Izana Observatory in Tenerife island (28.308 ◦N, 16.500 ◦W), located at ~2400 m above sea level (m.a.s.l.), directly exposed to the dusty air arriving from the inner Sahara, (ii) Ragged Point, on Barbados (13.165 ◦N, 59.432 ◦W), ~45 m. a.s.l., exposed to the easterly trade winds blowing in the tropical central North Atlantic and (iii), Virginia Key (25.732 ◦N, 80.162 ◦W), an island 4 km east off the coast at Miami, ~30 m.a.s.l., directly exposed to the trade winds arriving from the tropical Atlantic and the eastern Carib bean
Summary
Marine microorganisms account for ~ half of Earth’s primary pro ductivity, i.e. the conversion of carbon from carbon dioxide to organic molecules via photosynthesis and chemosynthesis, a process that replenishes the oxygen budget (Chapman, 2013). This process requires nutrients to support the growth of primary producers. In many regions of the open ocean, atmospheric deposition is the main supplier of those nutrients. Ice core records suggest that increased dust inputs to the ocean may have reduced atmospheric CO2 in glacial versus interglacial periods, linked to iron modulated primary produc tivity (Martínez-Garcia et al, 2009; Ridgwell and Watson, 2002)
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