Microphysical and optical properties of dust and tropical biomass burning aerosol layers in the Cape Verde region—an overview of the airborne in situ and lidar measurements during SAMUM-2

Abstract

A B S T R A C TIn the framework of the Saharan Mineral Dust Experiment (SAMUM) airborne High Spectral Resolution Lidar and in situ measurements of the particle size, aerosol mixing state and absorption coefficient were conducted. Here, the properties of mineral dust and tropical biomass burning layers in the Cape Verde region in January/February 2008 are investigated and compared with the properties of fresh dust observed in May/June 2006 close the Sahara. In the Cape Verde area, we found a complex stratification with dust layers covering the altitude range below 2 km and biomass burning layers aloft. The aerosol type of the individual layers was classified based on depolarization and lidar ratios and, in addition, on in situ measured Ångström exponents of absorptionå ap . The dust layers had a depth of 1.3 ± 0.4 km and showed a medianå ap of 3.95. The median effective diameter D eff was 2.5 μm and the dust layers over Cape Verde yielded clear signals of aging: large particles were depleted due to gravitational settling and the accumulation mode diameter was shifted towards larger sizes as a result of coagulation. The tropical biomass layers had a depth of 2.0 ± 1.1 km and were characterized by a medianå ap of 1.34. They always contained a certain amount of large dust particles and showed a median D eff of 1.1 μm and a fine mode D eff,fine of 0.33. The dust and biomass burning layers had a median aerosol optical depth (AOD) of 0.23 and 0.09, respectively. The median contributions to the AOD of the total atmospheric column below 10 km were 75 and 37%, respectively.

In situ measurements of desert dust particles above the western Mediterranean Sea with the balloon-borne Light Optical Aerosol Counter/sizer (LOAC) during the ChArMEx campaign of summer 2013

“…They report an average volume median diam- eter of the coarse mode of 15.5 ± 10.9 µm near dust source region and a maximum value larger than 60 µm in a case of strong convection. Weinzierl et al (2011) also report that particles more than 20 µm in diameter (but < 30 µm) are still found at concentrations > 10 −2 cm −3 in 1/3 of 24 dust transport cases documented over the eastern tropical North Atlantic. The better sensitivity of LOAC may explain why we report more systematically a coarse mode of dust particles above 20 µm in diameter.…”

“…Some aircraft observations were also conducted in situ inside dust plumes, but they are expensive and scarce (e.g. Schmid et al, 2000;Dulac and Chazette, 2003;Haywood et al, 2003;Reid et al, 2003a;Formenti et al, 2008;Weinzierl et al, 2009Weinzierl et al, , 2011Chen et al, 2011;Denjean et al, 2016), and were often limited to several micrometres in terms of the particle size range covered and did not explore the same dust plume along its transport. One of the incompletely resolved issues is the evolution of the dust particle size distribution during long-range transport .…”

Optical properties of long-range transported Saharan dust over Barbados as measured by dual-wavelength depolarization Raman lidar measurements

“…For example, biomass burning aerosols and mineral dust may become internally mixed when aging together (Hand et al, 2010) and thus change their size distribution, optical properties, hygroscopicity and ability to act as cloud condensation nuclei. From measurements close to the dust source regions in comparisons to measurements in dust plumes over Cape Verde, Weinzierl et al (2011) found an indication of sedimentation of large particles in Saharan dust plumes during transport although sedimentation of large super-micron dust particles was less pronounced than expected from Stokes gravitational settling. Yang et al (2013) assume a shape-induced particle sedimentation from measurements of transported dust with the space-based lidar system onboard the CALIOP satellite mission .…”

Characterisation of the artificial neural network CiPS for cirrus cloud remote sensing with MSG/SEVIRI

“…Therefore we only acknowledge aerosol layers with an AOT > 0.2. These aerosols come mainly from desert dust (Weinzierl et al, 2011;Groß et al, 2015) but also from biomass burning (Rosário et al, 2011;Ten Hoeve et al, 2012) or, sometimes, sea salt (Toth et al, 2013). We assume that AOT ≤ 0.2 is a good approximation for the AOT of typical aerosol loads.…”