Understanding the nature of atmospheric acid processing of mineral dusts in supplying bioavailable phosphorus to the oceans

Acidification of airborne dust particles can dramatically increase the amount of bioavailable phosphorus (P) deposited on the surface ocean. Experiments were conducted to simulate atmospheric processes and determine the dissolution behaviour of phosphorus compounds in dust and dust precursors oils. Acid dissolution occurs rapidly (seconds to minutes) and is controlled by the amount of H + ions present. For H + 10-4 mol per gram of dust, the amount of phosphorus (and Ca) released follows a power law dependent on the amount of H + consumed until all inorganic phosphorus minerals are exhausted and the final pH remains acidic. Once dissolved, phosphorus will stay in solution due to slow precipitation kinetics. Dissolution of apatite-P, the major mineral phase in dust (79-96%), occurs whether CaCO 3 is present or not, though the increase in dissolved phosphorus is greater if CaCO 3 is absent or if the particles are externally mixed. The system was modelled adequately as a simple mixture of apatite-P and calcite. Phosphorus dissolves readily by acid processes in the atmosphere in contrast to iron, which dissolves slower and is subject to re-precipitation at cloud water pH. We show that acidification can increase bioavailable phosphorus deposition over large areas of the globe, and may explain much of the previously observed patterns of variability in leachable phosphorus in oceanic areas where primary productivity is limited by this nutrient (e.g. Mediterranean)