Processes that solubilize iron in mineral dust aerosol may increase the amount of iron supplied to ocean surface waters, and thereby stimulate phytoplankton productivity. In particular, the uptake of acids such as H2SO4 and HNO3 on mineral dust surfaces can produce extremely acidic environments that promote iron dissolution. Here, four samples that represent source materials for mineral dust (Saudi Beach sand (SB), Inland Saudi sand (IS), Saharan Sand (SS) and China Loess (CL)) and one commercial reference material (Arizona Test Dust (AZTD)) were characterized, and examined in dissolution studies in solutions of sulfuric, nitric and hydrochloric acid ranging from pH 1 to 3. Mössbauer spectroscopy revealed Fe(III) in all samples, whereas SB, CL and AZTD also contained appreciable Fe(II). Spectra suggest that both Fe(II) and Fe(III) were substituted into aluminosilicates, although CL, AZTD and IS also contained Fe(III) oxide phases. Total iron solubility measured after 24 h ranged between 4–16% of the initial iron content for each material, but did not scale with either the specific surface area or the total iron content of the samples. Instead, we show that Fe(II)‐containing solid phases such as Fe(II)‐substituted aluminosilicates represent a significant, and sometimes dominant, source of soluble Fe in acidic environments. Results of dissolution studies also show that the nature of the acid influences iron solubilization, as elevated concentrations of nitrate encountered from nitric acid at pH 1 suppressed Fe(II) formation. We propose a surface‐mediated, non‐photochemical redox reaction between nitrate and Fe(II), which may contribute to Fe(II)/Fe(III) cycling in the atmosphere.

Although recent evidence suggests that particle size plays an important role in the dissolution of iron from mineral dust aerosol, a fundamental understanding of how particle size influences the rate and extent of iron oxide dissolution processes remains unclear. In this study, surface spectroscopic methods are combined with solution phase measurements to explore ligand-promoted dissolution and photochemical reductive dissolution of goethite (R-FeOOH) of different particle sizes in the presence of oxalate at pH 3 and 298 K. Both X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) revealed differences between R-FeOOH particles in the nanometer-size range as compared to R-FeOOH particles in the micrometer-size range (nanorods and microrods, respectively). ATR-FTIR spectra showed a significant presence of surface hydroxyl groups as well as differences in surface complexes formed on nanorod surfaces. Furthermore, the saturation coverage of oxalate adsorbed on nanorods relative to microrods is ∼30% less as determined from solution phase batch adsorption isotherms. Despite less oxalate uptake per unit surface area, the surface-area-normalized rate of oxalate-promoted dissolution was ∼4 times greater in nanorod suspensions, suggesting this process is particle-size-dependent. Photochemical dissolution experiments revealed only a moderate increase in the rate of oxalate oxidation per gram of R-FeOOH with decreasing particle size. However, concentration profiles of photochemically generated Fe(II) and Fe(III) suggest differences in the dominant mechanisms controlling nanorod and microrod dissolution. Although loss of reactive surface area arising from oxalate-induced particle aggregation can contribute to size-dependent reactivity trends toward oxalate, our data, taken collectively, suggest unique surface chemistry of nanorods as compared to larger microrods. Results from ligand-promoted and photochemical dissolution experiments also highlight the important, and sometimes dominant, role that iron oxides on the nanoscale may play in iron mobilization relative to the larger oxide phases present in mineral dust aerosol. † Part of the special section "Physical Chemistry of Environmental Interfaces".

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The use of worm egg counts for observational and experimental purposes in naturally infested sheep has become very popular since Whitlock and Gordon (1989) introduced the simple and efficient McMaster slide technique. When using a dilution technique such as this, the accuracy of the count increases as more eggs are counted. In order to count more eggs when using the McMaster slide it is usually more convenient, in practice, to take more samples (i.e. count more cells) than to alter the dilution. It is therefore desirable to know the optimum number of samples which should be counted for each sheep when comparing the counts of naturally infested sheep. The usefulness of sampling on one or more days is another problem concerning experiments of this nature.

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