Optical properties of Saharan dust aerosol and contribution from the coarse mode
Abstract
Saharan mineral dust has an effect on the climate system by altering the radiative balance of the atmosphere, affecting tropical cyclone development and depositing nutrients to the Atlantic Ocean. New in-situ aircraft measurements from the Fennec 2011 aircraft campaign over a remote part of the Sahara Desert will be presented and compared to previous airborne measurements. Size distributions extending to 300 μm will be shown, representing measurements extending further into the coarse mode than previously published for Saharan dust. The dust sampled by the aircraft covered a wide variety of loadings, dust source regions (Mali, Mauritania and Algeria) and dust ages (from fresh uplift to several days old). A significant coarse mode was present in the size distribution measurements with effective diameter up to 23 μm, and the mean size distribution showed greater concentrations of coarse mode than previous aircraft measurements. Single scattering albedo (SSA) values at 550nm calculated from these size distributions revealed high absorption from 0.77 to 0.95, with a mean of 0.85. Directly measured SSA values were higher (0.91 to 0.99) but new instrumentation revealed that these direct measurements, behind Rosemount inlets, overestimate the SSA by 0.02 to 0.20 depending on the concentration of coarse particles present. This is caused by inlet inefficiencies and pipe losses. Previous measurements of SSA from aircraft measurements may also have been overestimates for this reason. The imaginary part of the refractive index was found to vary between 0.001 to 0.007 at 550nm. No clear relationship between effective diameter and dust altitude was found. Number concentration, mass loading and extinction coefficient showed strong inverse relationships to dust age, and showed some sensitivity to the type of uplift for cases of fresh dust. Effects of the SSA measurements on atmospheric heating rates will be presented.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A23F0287R
- Keywords:
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- 0305 ATMOSPHERIC COMPOSITION AND STRUCTURE / Aerosols and particles;
- 3359 ATMOSPHERIC PROCESSES / Radiative processes