Simulating the cloud processing of iron in Australian dust: pH and dust concentration

We report on the simulated cloud processing of an aerosol iron sample derived from an Australian dust storm. Primary factors influencing the extent and rate of Fe mobilization were pH, duration of extraction and dust concentration. Fe was significantly mobilized below a threshold of pH similar to 3.6. After initial rapid mobilization at low pH, the rate of Fe release was constant with constant pH. Between this threshold and pH similar to 7.1, dissolved Fe fell to a minimum but above this pH further dissolution of iron occurs, probably due to the formation of soluble ferrates. At our lowest dust concentrations ( 1 - 20 mg L(-1)) the rate of Fe extraction at low pH was constant, while at higher dust concentrations the rate was inversely proportional to dust concentration. Dissolution of iron from dust is thus a complex process and these factors must be considered when modeling the input of iron to the oceans.