The solubility of ferric ion in marine mineral aerosol solutions at ambient relative humidities

The solubility of ferric ion from Fe 2O 3, FeO(OH) and Fe(OH) 3 in hygroscopic marine aerosol solutions was estimated from free energies at the pHs and ionic strengths that are characteristic of ambient trade wind aerosol particles collected at Barbados, West Indies, under conditions when significant concentrations of Saharan dust were present. The chemical composition of these mineral-containing particles was based on measurements made with cascade impactor samples with the assumptions that the particles in a given size fraction were internally mixed and that the NO 3 − and nonsea-salt (nss) SO 4 2− in the particles were initially in the form of HNO 3 and H 2SO 4. At typical local relative humidities (RH) above 80%, the pH of the aerosol solution could be lower than 1.0 and the ionic strength could be higher than 3.0 even when part of the acid has been neutralized by NH 3 and CaCO 3. The concentration of ferric ion increases by three orders of magnitude for each unit decrease in pH. By considering changes in speciation and ionic strength, the ferric ion solubilities at pHs of 2, 5 and 8 are estimated to be, respectively, 6.67 × 10 −5 mol kg −1, 5.53 × 10 −14 mol kg −1, 3.17 × 10 −15 mol kg −1 from Fe 2O 3, 1.10 × 10 −4 mol kg −4, 8.68 × 10 −14 mol kg −1, 5.04 × 10 −15 mol kg −1 from FeO(OH) and 1.55 × 10 1 mol kg −1, 1.12 × 10 −8 mol kg −1, 6.61 × 10 −10 mol kg −1 from Fe(OH) 3. The ferric ion solubilities in aerosol solutions are far higher than those normally expected for seawater, fresh water and even rain water in which the pHs are usually higher. Thus aerosol solution processes may be a major factor affecting the fraction of iron that will readily dissolve when the atmospheric mineral particles enter the ocean. These same aerosol solution processes may explain the unusually large solubilities that have been observed for other aerosol metals in aqueous solutions.