Aethalometer multiple scattering correction Cref for mineral dust aerosols

Abstract

In this study we provide a first estimate of the Aethalometer multiple scattering correction Cref for mineral dust aerosols. Cref 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 Cref at 450 and 660nm 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 450nm and (ii) the absorption coefficient from a MAAP (Multi-Angle Absorption Photometer) at 660nm. 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 660nm and the size distribution of the aerosols were also measured. Cref for mineral dust varies between 1.81 and 2.56 for a SSA of 0.85-0.96 at 450nm and between 1.75 and 2.28 for a SSA of 0.98-0.99 at 660nm. The calculated mean for dust is 2.09 (±0.22) at 450nm and 1.92 (±0.17) at 660nm. With this new Cref the dust absorption coefficient by the Aethalometer is about 2% (450nm) and 11% (660nm) higher than that obtained by using Cref Combining double low line 2.14 at both 450 and 660nm, as usually assumed in the literature. This difference induces a change of up to 3% in the dust SSA at 660nm. The Cref seems to be independent of the fine and coarse particle size fractions, and so the obtained Cref can be applied to dust both close to sources and following transport. Additional experiments performed with pure kaolinite minerals and polluted ambient aerosols indicate Cref of 2.49 (±0.02) and 2.32 (±0.01) at 450 and 660nm (SSA Combining double low line 0.96-0.97) for kaolinite, and Cref of 2.32 (±0.36) at 450nm and 2.32 (±0.35) at 660nm for pollution aerosols (SSA Combining double low line 0.62-0.87 at 450nm and 0.42-0.76 at 660nm).