Aerosol Phosphorus Composition: New Insights from Synchrotron X-ray Spectroscopy

Biological productivity in many ocean regions is controlled by the availability of the nutrient phosphorus (P), including the eastern Mediterranean Sea. Aerosol deposition is a key source of P in the Mediterranean. Understanding the composition of this P is critical for determining its solubility and therefore potential bioavailability. We investigated aerosol P composition in European and North African air masses, the main sources of aerosol deposition to the Mediterranean Sea, using Phosphorus Near Edge X-ray Fluorescence Spectroscopy (P-NEXFS). We show that European aerosols are a significant source of soluble P to the Mediterranean Sea. The traditionally studied North African sourced air masses carry more total P to the Mediterranean Sea than European sourced air masses and contain mostly apatite, a largely insoluble calcium phosphate mineral. However, European aerosols deliver P that is on average 3.5 times more soluble than North African aerosols and furthermore are dominated by organic P compounds. The increased solubility of European aerosols leads to comparable amounts of soluble P by mass delivered to the Mediterranean Sea by both air masses. The ultimate origin of organic P does not stem from common primary emission sources, such as coal fly ash and diesel emissions. Rather, P associated with bacteria best explains the presence of organic P in Mediterranean aerosols. The soluble organic nature of the P in European sourced air masses underscores the need to examine aerosols from this region as a source of P to the Mediterranean Sea. This study also demonstrates that synchrotron-based techniques are effective tools for characterizing aerosols and gaining new insights.