Optical properties of Saharan dust aerosol and contribution from the coarse mode as measured during the Fennec 2011 aircraft campaign (original) (raw)
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Saharan dust absorption and refractive index from aircraft-based observations during SAMUM 2006
Tellus B, 2009
A B S T R A C T During the Saharan Mineral Dust Experiment (SAMUM) conducted in summer 2006 in southeast Morocco, the complex refractive index of desert dust was determined from airborne measurements of particle size distributions and aerosol absorption coefficients at three different wavelengths in the blue (467 nm), green (530 nm) and red (660 nm) spectral regions. The vertical structure of the dust layers was analysed by an airborne high spectral resolution lidar (HSRL). The origin of the investigated dust layers was estimated from trajectory analyses, combined with Meteosat 2nd Generation (MSG) scenes and wind field data analyses. The real part n of the dust refractive index was found almost constant with values between 1.55 and 1.56, independent of the wavelength. The values of the imaginary part k varied between the blue and red spectral regions by a factor of three to ten depending on the dust source region. Absolute values of k ranged from 3.1 × 10 −3 to 5.2 × 10 −3 at 450 nm and from 0.3 × 10 −3 to 2.5 × 10 −3 at 700 nm. Groupings of k values could be attributed to different source regions.
Atmospheric Chemistry and Physics, 2007
This work will present aerosol size distributions measured in a Saharan dust plume between 0.9 and 12 km altitude during the ACE-2 campaign 1997. The distributions contain a significant fraction of large particles of diameters from 4 to 30 µm. Radiative transfer calculations have been performed using these data as input. Shortwave, longwave as well as total atmospheric radiative effects (AREs) of the dust plume are investigated over ocean and desert within the scope of sensitivity studies considering varied input parameters like solar zenith angle, scaled total dust optical depth, tropospheric standard aerosol profiles and particle complex refractive index. The results indicate that the large particle fraction has a predominant impact on the optical properties of the dust. A single scattering albedo of ω o =0.75−0.96 at 550 nm was simulated in the entire dust column as well as 0.76 within the Saharan dust layer at ∼4 km altitude indicating enhanced absorption. The measured dust leads to cooling over the ocean but warming over the desert due to differences in their spectral surface albedo and surface temperature. The large particles absorb strongly and they contribute at least 20% to the ARE in the dusty atmosphere.
Atmospheric Chemistry and Physics Discussions, 2010
As part of the international project entitled "African Monsoon Multidisciplinary Analysis (AMMA)", NAMMA (NASA AMMA) aimed to gain a better understanding of the relationship between the African Easterly Waves (AEWs), the Sahara Air Layer (SAL), and tropical cyclogenesis. The NAMMA airborne field campaign was based out of the Cape Verde Islands during the peak of the hurricane season, i.e., August and September 2006. Multiple Sahara dust layers were sampled during 62 encounters in the eastern portion of the hurricane main development region, covering both the eastern North Atlantic Ocean and the western Saharan desert (i.e., 5-22 • N and 10-35 • W). The centers of these layers were located at altitudes between 1.5 and 3.3 km and the layer thickness ranged from 0.5 to 3 km. Detailed dust microphysical and optical properties were characterized using a suite of in-situ instruments aboard the NASA DC-8 that included a particle counter, an Ultra-High Sensitivity Aerosol Spectrometer, an Aerodynamic Particle Sizer, a nephelometer, and a Particle Soot Absorption Photometer. The NAAMA sampling inlet has a size cut (i.e., 50% transmission efficiency size) of approximately 4 µm in diameter for dust particles, which limits the representativeness of the NAMMA observational findings. The NAMMA dust observations showed relatively low particle number densities, Correspondence to: G. Chen (gao.chen@nasa.gov) ranging from 268 to 461 cm −3 , but highly elevated volume density with an average at 45 µm 3 cm −3 . NAMMA dust particle size distributions can be well represented by tri-modal lognormal regressions. The estimated volume median diameter (VMD) is averaged at 2.1 µm with a small range of variation regardless of the vertical and geographical sampling locations. TheÅngström Exponent assessments exhibited strong wavelength dependence for absorption but a weak one for scattering. The single scattering albedo was estimated at 0.97 ± 0.02. The imaginary part of the refractive index for Sahara dust was estimated at 0.0022, with a range from 0.0015 to 0.0044. Closure analysis showed that observed scattering coefficients are highly correlated with those calculated from spherical Mie-Theory and observed dust particle size distributions. These values are generally consistent with literature values reported from studies with similar particle sampling size range.
Journal of Geophysical Research Atmospheres, 2003
The physical and optical properties of Saharan dust aerosol measured by the Met Office C-130 during the Saharan Dust Experiment (SHADE) are presented. Additional radiation measurements enable the determination of the aerosol optical depth, τ aerλ, and the direct radiative effect (DRE) of the mineral dust. The results suggest that the absorption by Saharan dust is significantly overestimated in the solar spectrum if standard refractive indices are used. Our measurements suggest an imaginary part of the refractive index of 0.0015i is appropriate at a wavelength λ of 0.55 μm. Different methods for determining τ aerλ=0.55 are presented, and the accuracy of each retrieval method is assessed. The value τ aerλ=0.55 is estimated as 1.48 +/- 0.05 during the period of heaviest dust loading, which is derived from an instantaneous DRE of approximately -129 +/- 5 Wm-2 or an enhancement of the local planetary albedo over ocean of a factor of 2.7 +/- 0.1. A comparison of the DRE derived from the C-130 instrumentation and from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite is presented; the results generally showing agreement to within a factor of 1.2. The results suggest that Saharan dust aerosol exerts the largest local and global DRE of all aerosol species and should be considered explicitly in global radiation budget studies.
Spectral aerosol optical depth characterization of desert dust during SAMUM 2006
Tellus B, 2009
The aerosol optical depth (AOD) in the range 340-1550 nm was monitored at Ouarzazate (Morocco) during the Saharan Mineral Dust Experiment (SAMUM) experiment in May-June 2006. Two different sun photometers were used for this purpose. The mean AOD at 500 nm was 0.28, with a maximum of 0.83, and the mean Ångström exponent (AE) was 0.35. The aerosol content over the site changed alternatively from very low turbidity, associated to Atlantic air masses, to moderate dust load, associated to air masses arriving in the site from Algeria, Tunisia and Libya. The dusty conditions were predominant in the measurement period (78% of data), with AOD (500 nm) above 0.15 and AE below 0.4. The spectral features of the AOD under dusty conditions are discussed. Air mass back trajectory analysis is carried out to investigate the origin and height patterns of the dust loaded air masses. The advection of dust occurred mainly at atmospheric heights below 3000 m, where east flow is the predominant. At the 5000 m level, the air masses originate mainly over the Atlantic Ocean. Finally the Optical Properties of Aerosols and Clouds (OPAC) model is used to perform a set of simulations with different aerosol mixtures to illustrate the measured AOD and AE values under varying dust concentrations, and a brief comparison with other measurement sites is presented.
Measurements of desert dust optical characteristics at Porte au Sahara during SAMUM
Tellus B, 2009
Main optical characteristics of desert dust, such as phase function and single scattering albedo, have been derived from combinations of sun-/sky-radiometer and satellite measurements during the SAMUM experiment (10 May-10 June 2006) at the site Porte au Sahara (30.237 • N, 5.607 • W) in South Morocco. Scattering phase functions have been retrieved using combined data of spectral aerosol optical thickness (AOT) and spectral sky brightness in the almucantar, considering non-spherical light scattering. Intercomparisons of modelled top-of-atmosphere (TOA) reflectance with satellite observations of the Medium Resolution Imaging Spectrometer (MERIS) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography () instrument have been used for the estimation of spectral single scattering albedo. For the radiative transfer calculations scattering phase functions and AOT from ground-based observations have been used. The spectral single scattering albedo ranges from 0.93 in the blue to 0.98 at 753 nm.
Tellus B, 2009
A B S T R A C T The Saharan Mineral Dust Experiment (SAMUM) was conducted in May/June 2006 in southern Morocco. As part of SAMUM, airborne in situ measurements of the particle size distribution in the diameter range 4 nm < D p < 100 μm were conducted. The aerosol mixing state was determined below D p < 2.5 μm. Furthermore, the vertical structure of the dust layers was investigated with a nadir-looking high spectral resolution lidar (HSRL). The desert dust aerosol exhibited two size regimes of different mixing states: below 0.5 μm, the particles had a non-volatile core and a volatile coating; larger particles above 0.5 μm consisted of non-volatile components and contained light absorbing material. In all cases, particles larger than 10 μm were present, and in 80% of the measurements no particles larger than 40 μm were present. The abundance of large particles showed almost no height dependence. The effective diameter D eff in the dust plumes investigated showed two main ranges: the first range of D eff peaked around 5 μm and the second range of D eff around 8 μm. The two ranges of D eff suggest that it may be inadequate to use one average effective diameter or one parametrization for a typical dust size distribution.