Optical properties of aerosol mixtures derived from sun-sky radiometry during SAMUM-2

Abstract

The colocated ground-based and airborne instruments allow the study of desert dust optical and microphysical properties in a closure experiment. The Meteorological Institute of the University of Munich deployed one sun-sky photometer and two tropospheric lidar systems. A travelling AERONET-Cimel sun-sky radiometer was also deployed. During the measurement period the aerosol scenario over Cape Verde mostly consisted of a dust layer below 2 km and a smoke-dust layer above 2-4 km a.s.l. The Saharan dust arrived at the site from the NE, whereas the smoke originated in the African equatorial region. This paper describes the main results of the Sun photometer observations, supported by lidar information. An analysis of the variations in the aerosol optical depth (AOD) in the range 340-1550 nm, the Ångström exponent, volume size distributions and single scattering albedo is presented. The aerosol mixtures are analysed by means of the fine mode fraction of the AOD provided by the sun-sky inversion data and the Spectral Deconvolution Algorithm. The mean AOD (500 nm) was 0.31, with associated low Ångström exponent of 0.46. Several types of events were detected within the data set, with prevalence of dust or mixtures as characterized by the Ångström exponents of extinction and absorption and the fine mode fraction. Aerosol properties derived from sunphotometry were compared to in situ measurements of size distribution, effective radius and single scattering albedo.

Improvements to the OMI near-UV aerosol algorithm using A-train CALIOP and AIRS observations

“…Differentiating between carbonaceous (fine particles) and dust (coarse particles), aerosols in ocean satellite retrieval algorithms that use visible and near IR observations, is generally done in terms of the well-known Ångström's wavelength exponent (AE) (Angstrom, 1929), whose magnitude is inversely related to the predominant particle size. Typical AE values vary from nearly zero for high concentrations of desert dust aerosols to values of 2.0 or greater associated with large AOD finesize carbonaceous aerosols (Eck et al, 1999;Toledano et al, 2011). Satellite-derived AE for aerosol type differentiation over land is unreliable due to uncertainties associated with surface reflectance characterization (Levy et al, 2010).…”

Impacts of Saharan dust and clouds on photosynthetically available radiation in the area off Northwest Africa

“…Often mixtures of mostly dust and maritime particles below 1500 m and lofted layers of dust and smoke up to about 5000 m height were found (e.g. Knippertz et al, 2011;Tesche et al, 2011;Toledano et al, 2011). Our derived aerosol components could not be more accurately determined because of the lack of on-site measurements in our observation periods.…”

“…The deviations of the linear fit to the new third-order polynomial fit were only in the order of few percent in the wavelength range 440Á675 nm (see Fig. 6b in Toledano et al (2011) Outliers in the MODIS data set were identified by strong deviations from the regression line. The mean value of area-averaged dust component of AOD BAOD dust !…”

“…1) and with the Deutsches Zentrum fu¨r Luft-und Raumfahrt Falcon research aircraft during the SAMUM project (e.g. Ansmann et al, 2011;Knippertz et al, 2011;Petzold et al, 2011;Toledano et al, 2011;Weinzierl et al, 2011). Different aerosol layers were found in January and (Knippertz et al, 2011, Weinzierl et al, 2011.…”

Aerosol Optical Properties and Direct Radiative Effects over Central China

“…Direct sun measurements are generally performed every 15 min (more frequently at sunrise and sunset), and sky radiances are measured every hour [32]. The instruments measure direct spectral solar radiation in eight channels of 340, 380, 440, 500, 675, 870, 940, and 1020 nm.…”