Saharan and Namib Dust Phosphorus Fluxes in the North and South Atlantic Regions

Abstract

Phosphorus (P) is essential for marine ecosystems, particularly in nutrient-poor regions of the Northeastern and Southeastern Tropical Atlantic (NETA/SETA). However, P deposition and bioavailability remain poorly understood, limiting predictions of its role in marine productivity. To address this, P speciation, including organic, inorganic, and soluble contents in aerosol particles, was analyzed over 19 months at the Cape Verde (CVAO) and Namib Desert (NDAO) Atmospheric Observatories. P concentrations at NDAO (56.1 ± 62 ng/m³) were 47% higher than those at CVAO (29.8 ± 76 ng/m³) with dominant coarse mode contents and fine-to-coarse ratios (PM_1.2/PM₁₀) of 0.42–0.57 at NDAO and 0.17–0.32 at CVAO. Phosphorus sources at both sites include mineral dust and biomass burning with additional biogenic aerosols at NDAO. Organic P made up 19% and 39% of total P at NDAO and CVAO, respectively. Soluble P was 20% more abundant at NDAO, linked to higher biomass burning source provenance and aerosol acidity, confirming previous reports that atmospheric processing enhances P solubility. P solubility was lower during Saharan and Namib dust events, indicating comparatively reduced solubility from mineral-dust sources. This first report of annual deposition fluxes from these regions reveals somewhat higher average values in the SETA (2.05 ± 2.8 μmol/m²d at NDAO) than the NETA (1.3 ± 3.4 μmol/m²d at CVAO), with pronounced differences in the austral winter months. Elevated dissolved inorganic nitrogen to dissolved inorganic P ratios at CVAO indicated a smaller contribution of atmospheric P deposition in mitigating nutrient limitation in the nearby waters. These findings offer new insights into atmospheric P solubility and fluxes, crucial for improving ocean-atmosphere models and understanding its ecological impacts in the tropical Atlantic.