Aethalometer multiple scattering correction Cref for mineral dust aerosols

In this study we provide a first estimate of the Aethalometer multiple scattering correction C-ref for mineral dust aerosols. C-ref is an empirical constant used to correct the aerosol absorption coefficient measurements for the multiple scattering artefact of the Aethalometer; i.e. the filter fibres on which aerosols are deposited scatter light and this is miscounted as absorption. The C-ref at 450 and 660 nm was obtained from the direct comparison of Aethalometer data (Magee Sci. AE31) with (i) the absorption coefficient calculated as the difference between the extinction and scattering coefficients measured by a Cavity Attenuated Phase Shift Extinction analyser (CAPS PMex) and a nephelometer respectively at 450 nm and (ii) the absorption coefficient from a MAAP (Multi-Angle Absorption Photometer) at 660 nm. Measurements were performed on seven dust aerosol samples generated in the laboratory by the mechanical shaking of natural parent soils issued from different source regions worldwide. The single scattering albedo (SSA) at 450 and 660 nm and the size distribution of the aerosols were also measured. C-ref for mineral dust varies between 1.81 and 2.56 for a SSA of 0.85-0.96 at 450 nm and between 1.75 and 2.28 for a SSA of 0.98-0.99 at 660 nm. The calculated mean for dust is 2.09 (+/- 0.22) at 450 nm and 1.92 (+/- 0.17) at 660 nm. With this new C-ref the dust absorption coefficient by the Aethalometer is about 2% (450 nm) and 11% (660 nm) higher than that obtained by using C-ref = 2.14 at both 450 and 660 nm, as usually assumed in the literature. This difference induces a change of up to 3% in the dust SSA at 660 nm. The C-ref seems to be independent of the fine and coarse particle size fractions, and so the obtained C-ref can be applied to dust both close to sources and following transport. Additional experiments performed with pure kaolinite minerals and polluted ambient aerosols indicate C-ref of 2.49 (+/- 0.02) and 2.32 (+/- 0.01) at 450 and 660 nm (SSA = 0.96-0.97) for kaolinite, and C-ref of 2.32 (+/- 0.36) at 450 nm and 2.32 (+/- 0.35) at 660 nm for pollution aerosols (SSA = 0.62-0.87 at 450 nm and 0.42-0.76 at 660 nm).