Single scattering albedo of fine mineral dust aerosols controlled by iron concentration

Sign and magnitude of solar aerosol direct radiative forcing are largely determined by the aerosol single scattering albedo (SSA) and the albedo of the underlying scene (e.g., surface albedo). On a global scale, mineral dust aerosol has the largest mass emission rate, average column mass burden, and average optical depth of all aerosol types. Therefore, better understanding of its optical properties with a focus on SSA is of great importance. Here, we entrain ten bulk soil samples from deserts and semi-arid regions in the Arabian Peninsula, from locations representative of the Sahara and Sahel in North Africa, and from Northeast Africa and South-Central Asia. We segregate the fine particle fraction (aerodynamic diameter < 2.5 mm) and measure its iron content with an x-ray fluorescence (XRF) spectrometer and its aerosol SSA and absorption, scattering, and extinction Ångström coefficients for two wavelengths (405 nm and 870 nm) with a photoacoustic instrument with integrating reciprocal nephelometer. Results show that SSA is much lower at 405 nm than at 870 nm and that SSA at both wavelengths is dominated by and linearly correlated with the iron content of the entrained mineral dust. The second result points toward potential use of SSA remote sensing to measure aerosol iron content and vice versa, measurements of iron content with simple filter sampling followed by XRF analysis, providing an assessment of SSA for aerosol radiative forcing calculations and modeling. Average Ångström coefficients were ≈3.2 for absorption, ≈-0.4 for scattering, and ≈-0.3 for extinction and showed little correlation with iron content.