Multi-wavelength scattering and absorption of freshly emitted dust during field observations in Morocco

Abstract:

Dust produced in arid and semi-arid regions is likely the most abundant aerosol in terms of mass with an estimated, yet uncertain net cooling direct effect. The emitted dust particle size distribution (PSD) affects the duration of particle transport and thus each mineral’s global distribution, along with its specific effect upon climate. Measurements of in-situ scattering and absorption properties of dust at 1-minute temporal resolution were collected during a dust emission field campaign near M’hamid El Ghizlane in Morocco, at the edge of the Sahara Desert, within the FRAGMENT project (FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe). Scattering coefficients at 450, 525 and 635 nm and seven different angles (from 0º to 90º) were obtained with a polar nephelometer and absorption coefficients at 370, 470, 520, 590, 660, 880 and 950 nm with an aethalometer. This work presents the results of the assessment of extensive and intensive optical parameters, such as scattering and absorption Ångström exponents (SAE and AAE), backscatter fraction (BF) and multi-wavelength single scattering albedo (SSA) for both PM2.5 and PM10 dust fractions. Our analyses show marked differences in extensive and intensive properties between local freshly emitted particles (episode type A) and regionally transported dust (episode type B). SAE, AAE, BF (525 nm), and SSA (370 nm), were in the range of 0.5 – 1.25, 1.2 – 2.2, 0.08 – 0.16, and 0.70 – 0.98, respectively for episode A; and around 0, 2.5, 0.1 and 0.95, respectively, for episode B. Our analysis of both extensive and intensive properties is combined with size distribution measurements and meteorological observations.