Atmospheric deposition of mineral dust supplies much of the essential nutrient iron to the ocean. Presumably only the readily soluble fraction is available for biological uptake. Previous ocean ...models assumed this fraction was constant. Here the variable solubility of Fe in aerosols and precipitation is parameterized with a two‐step mechanism, the development of a sulfate coating followed by the dissolution of iron (hydr)oxide on the dust aerosols. The predicted soluble Fe fraction increases with transport time from the source region and with the corresponding decrease in dust concentration. The soluble fraction is ∼1 percent near sources, but often 10–40 percent farther away producing a significant increase in soluble Fe deposition in remote ocean regions. Our results may require more rapid biological and physicochemical scavenging of Fe than used in current ocean models. We further suggest that increasing SO2 emission alone could have caused significant Fe fertilization in the modern northern hemisphere oceans.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Aerosol transport from the Sahara desert to the North Atlantic Ocean generates the largest annual flux of mineral dust and total Fe found in the global oceans, enriching the mixed layer with soluble ...iron. We use the Geophysical Fluid Dynamics Laboratory Global Chemical Transport model to examine the transport and deposition of bioavailable iron on time scales ranging from seasonal to daily. The model is compared with observed mineral dust concentrations, depositions, and soluble Fe fractions. It is shown that simulated cumulative soluble Fe deposition (SFeD) employing a variable Fe solubility parameterization compares well with observed short‐term changes of dissolved iron within a thermally stratified surface mixed layer, while assuming a constant 2% solubility does not. The largest year‐to‐year variability of seasonal SFeD (45 to 90%) occurs throughout winter and spring in the central and northeast Atlantic Ocean. It is strongly linked to the North Atlantic Oscillation (NAO) index, producing substantially more SFeD during the positive phase than the negative phase. The ratio of wet to total SFeD increases with distance from the Saharan source region and is especially large when concentrations are small during the negative NAO. In summer, the relatively steady circulation around the Azores high results in low interannual variability of SFeD (<30%); however, regional short‐term events are found to be highly episodic, and daily deposition rates can be a factor of 4 or more higher than the monthly mean flux. Three‐dimensional backward trajectories are used to determine the origin and evolution of a specific SFeD event. We show that the dust mass‐mean sedimentation rate should be incorporated into the air parcel dynamical vertical velocity for a more precise transport path.
Aeolian dust input may be a critical source of dissolved iron for phytoplankton growth in some oceanic regions. We used an atmospheric general circulation model (GCM) to simulate dust transport and ...removal by dry and wet deposition. Model results show extremely low dust concentrations over the equatorial Pacific and Southern Ocean. We find that wet deposition through precipitation scavenging accounts for ∼40% of the total deposition over the coastal oceans and ∼60% over the open ocean. Our estimates suggest that the annual input of dissolved Fe by precipitation scavenging ranges from 0.5 to 4 × 1012 g yr−1, which is 4–30% of the total aeolian Fe fluxes. Dissolved Fe input through dry deposition is significantly lower than that by wet deposition, accounting for only 0.6–2.4 % of the total Fe deposition. Our upper limit estimate on the fraction of dissolved Fe in the total atmospheric deposition is thus more than three times higher than the value of 10% currently considered as an upper limit for dissolved Fe in Aeolian fluxes. As iron input through precipitation may promote episodic phytoplankton growth in the ocean, measurements of dissolved iron in rainwater over the oceans are needed for the study of oceanic biogeochemical cycles.
Mineral dust aerosol is the main ice nucleus (IN) in the Arctic. Observed dust concentrations at Alert, Canada, are lowest in winter and summer and highest in spring and autumn. In this study, we ...simulate transport and deposition of dust in a global chemical transport model. The model predicts the spring maximum caused by natural dust from desert sources in Asia and Sahara but underestimates the observations in autumn. Both natural and pollution sources contribute to the wintertime dust burden, as suggested by previous measurements of elemental compositions. Cloud parcel model simulations were carried out to study the impact of dust aerosol on the formation of mixed‐phase and ice clouds in the Arctic lower troposphere. The liquid water path of low‐level cloud is most sensitive to dust aerosol concentration from winter to early spring when air temperature is at its lowest in the annual cycle. The global and parcel models together suggest that low concentrations and acid coating of dust particles are favorable conditions for occurrence of mixed‐phase clouds and that anthropogenic pollution can cause significant perturbations to Arctic IN and clouds in winter.
Key Points
Anthropogenic pollution is a major source of dust in the Arctic in winter
Acid coating of dust aerosols may influence Arctic clouds
LWP of low‐level Arctic clouds is sensitive to dust aerosol in winter
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Mineral dust aerosols originating from arid regions are simulated in an atmospheric global chemical transport model. Based on model results and observations of dust concentration, we hypothesize that ...air pollution increases the scavenging of dust by producing high levels of readily soluble materials on the dust surface, which makes dust aerosols effective cloud condensation nuclei (CCN). This implies that air pollution could have caused an increase of dust deposition to the coastal oceans of East Asia and a decrease by as much as 50% in the eastern North Pacific.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
A time-dependent chemistry model is used to predict reactive oxygen species (ROS
=
H
2O
2
+
O
2
−) and dissolved Fe (DFe) speciation in the surface ocean. A new feature of the model is inclusion of ...biological sources of superoxide. The model suggests that biochemistry mediated by phytoplankton cells is as important as photochemistry for the formation of ROS. Formation of stable organic Fe(III) complexes (FeL) maintains the concentration of DFe in seawater. Iron speciation in the model is also controlled by biochemical and photochemical processes, and is far from thermodynamic equilibrium. During light periods, photo-reduction of FeL produces dissolved inorganic iron much more than thermal decomposition and cell-surface reduction of FeL, thus facilitating phytoplankton uptake of iron in the ocean. During the nighttime, O
2
− produced by reductases on cell surfaces both reacts with FeL, producing Fe(II), and retards the oxidation of Fe(II) and subsequent formation of FeL; therefore significant levels of bio-available Fe is maintained through this period. Photo-reduction nearly balances the formation of FeL in the model, and may control bioavailability of dissolved iron. This suggests a possible extracellular mechanism of iron and light colimitation to primary productivity. A phytoplankton growth limitation by FeL photo-reduction depends on its rate coefficient for which we need extensive measurements in natural seawater.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Surface ocean iron speciation is simulated using a time‐dependent box‐model of light‐mediated redox cycling over a range of aeolian inputs of soluble iron in the stratified epipelagic ocean. At ...steady‐state, Dissolved iron (DFe) concentration increases with aeolian input of soluble iron up to 0.1 μmol m−2 d−1, and is limited by the solubility of ferric hydroxide at higher fluxes which causes the formation of colloidal iron. We demonstrate that even in the presence of ample excess ligand, rapid conversion of dissolved iron between oxidized and reduced forms in the tropical surface ocean exposes DFe to colloid formation and scavenging. This result provides an explanation for the much smaller range of interregional variability in DFe measurements (0.05–0.4 nM) than soluble Fe fluxes (0.01–1 μmol m−2 d−1) and dust fluxes (0.1–10 g m−2 yr−1) predicted by atmospheric models. We incorporate the critical behavior of the full chemical speciation model into a reduced, computationally efficient model suitable for large scale calculations.
Key Points
Dust deposition influences dissolved Fe in the epipelagic ocean
Dissolved iron may be buffered by colloidal iron formation and scavenging
An efficient iron model is derived for use in ocean circulation models
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In order to study the internal cross-linking mechanism between cellulose and polysaccharides as wet-end additives in papermaking reconstituted tobacco, crystalline cellulose (MCC) was used as the ...target in this paper, sodium alginate (SA) , pullulan (Pul) , κ-carrageenan (κ-C), carboxymethyl chitosan (CMCS), hydroxypropyl chitosan (HPCS) and tamarind polysaccharide (TSP) were selected, the structure and composition of the monomer polysaccharides were determined by dynamic light scattering and ion chromatography, at the same time, the characteristics of the above-mentioned polysaccharides and MCC blend were analyzed, the results showed that SA contained a lot of negative charge, which resulted in weak adhesion and poor adsorption ability in cellulose blend; Pul was the most compact chain conformation, effectively improve the adhesion of cellulose, can be quickly adsorbed on cellulose; κ-C increased the viscosity of MCC blend and could adhere the 30μm size fragments of cellulose CMCS had strong adhesion to cellulose edge; due to the introduction of carboxymethyl group, the hydrophilicity of HPCS and the apparent viscosity of MCC blend increased greatly; the complex branched-chain structure of TSP results in high storage modulus of TSP/cellulose blend, and the amount of TSP adsorbed on cellulose is large and lasts long. Pul, TSP, κ-c, CMCS and TSP could adsorb cellulose well.
We estimate the annual net air‐sea fluxes of oxygen for 13 regions on the basis of a steady state inverse modeling technique that is independent of air‐sea gas exchange parameterizations. The ...inverted data consist of the observed oceanic oxygen concentration after a correction has been applied to account for biological cycling. We find that the tropical oceans (13°S−13°N) emit ∼212 Tmol O2 yr−1, which is compensated by uptake of 148 Tmol yr−1 in the Northern Hemisphere (>13°N) and by uptake of 65 Tmol yr−1 in the Southern Hemisphere (<13°S). These results imply that the dominant feature of oxygen transport in the combined ocean‐atmosphere system is the existence of a closed circulation cell in each hemisphere. These two cells consist of O2 uptake by the ocean in the middle and high latitudes of both hemispheres and transport in the ocean toward the tropics, where O2 is lost to the atmosphere and transported in the atmosphere back toward the poles. We find an asymmetry in the two cells involving O2 uptake in the temperate regions of the Northern Hemisphere versus loss of O2 in the temperate regions of the Southern Hemisphere. There is an additional asymmetry between the Atlantic basin, which has a net southward transport at all latitudes north of 36°S, in agreement with independent transport estimates, versus the Indian and Pacific Oceans, which have a strong equatorward transport everywhere. We find that these inverse estimates are relatively insensitive to details in the inversion scheme but are sensitive to biases in the ocean general circulation model that provides the linkage between surface fluxes and ocean interior concentrations. Forward simulations of O2 in an atmospheric tracer transport model using our inversely estimated oxygen fluxes as a boundary condition agree reasonably well with observations of atmospheric potential oxygen (APO ≈ O2 + CO2). Our results indicate that the north‐south asymmetry in the strength of the two hemispheric cells coupled with a strong asymmetry in fossil fuel emissions can explain much of the observed interhemispheric gradient in APO. Therefore it might not be necessary to invoke the existence of a large southward interhemispheric transport of O2 in the ocean, such as proposed by Stephens et al. 1998. However, we find that uncertainties in the modeled APO distribution stemming from seasonal atmospheric rectification effects and the limited APO data coverage prevent the currently available APO data from providing strong constraints on the magnitude of interhemispheric transport.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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