Spatial distribution of dust's optical properties over the Sahara and Asia inferred from Moderate Resolution Imaging Spectroradiometer

Abstract

Abstract. There is great uncertainty regarding the role of mineral dust aerosols in Earth's climate system. One reason for this uncertainty is that the optical properties of mineral dust, such as its single scattering albedo (the ratio of scattering to total extinction), are poorly constrained because ground observations are limited to a few locations and satellite standard products are not available due to the excessively bright surface of the desert in the visible wavelength, which makes robust retrievals difficult. Here, we develop a method to estimate the spatial distributions of the aerosol single scattering albedo (ω 0 ) and optical depth (τ a ), with daily 1 • × 1 • spatial resolution using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) as well as model simulations of radiative transfer. This approach is based on the "critical surface reflectance" method developed in the literature, which estimates ω 0 from the top of the atmospheric radiance. We estimate the uncertainties in ω 0 over the Sahara (Asia) to be approximately 0.020 and 0.010 (0.023 and 0.017) for bands 9 and 1, respectively, while the uncertainty in τ a is approximately 0.235 and 0.228 (0.464 and 0.370) for bands 9 and 1, respectively. The 5-95 % range of the spatial distribution of ω 0 over the Sahara (Asia) is approximately 0.90-0.94 and 0.96-0.99 (0.87-0.94 and 0.89-0.97) for bands 9 and 1, respectively, and that of τ a over the Sahara (Asia) is approximately 0.8-1.4 and 0.8-1.7 (0.7-2.0 and 0.7-1.9) for bands 9 and 1, respectively. The results for the Sahara indicate a good correlation between ω 0 and the surface reflectance, and between ω 0 and τ a . However, the relationships between ω 0 , τ a , and surface reflectance are less clear in Asia than in the Sahara, and the ω 0 values are smaller than those in the Sahara. The regions with small ω 0 values are consistent with the regions where coal-burning smoke and carbonaceous aerosols are reported to be transported in previous studies. Because the coal-burning and carbonaceous aerosols are known to be more absorptive and have smaller ω 0 values than dust aerosols, our results indicate that the dust aerosols in Asia are contaminated by these anthropogenic aerosols. The spatial distribution of dust optical properties obtained in our work could be useful in understanding the role of dust aerosols in Earth's climate system, most likely through future collaboration with regional and global modelling studies.

A Climatological Satellite Assessment of Absorbing Carbonaceous Aerosols on a Global Scale

“…These are more specifically observed from December to April (Figure 4), with frequencies up to about 9 days/month (maxima in December and January),and originate from fires taking place over shrubland and cropland areas between November and April [98,101,107,108].Very low frequencies of occurrence of ACA, lower than 5-10 days/year (on an 11-year basis), are found over the Sahara. Such low frequencies of ACA aerosols over this world region have been reported in the literature [109][110][111] and are associated with a northward transport of biomass-burning aerosol (emitted by anthropogenic activities south of 11 • N) in warm ascending air that overrides the cooler, drier mineral dust laden air, reaching altitudes higher than 4 km [109].…”

An outstanding Saharan dust event at Mt. Cimone (2165 m a.s.l., Italy) in March 2004

“…8 highlights that the change in the extinction properties of the aerosol was linked to a marked change in the scattering as compared to the absorption, confirming once more the arrival of the Saharan dust. It is in fact known that the absorptivity of dust is smaller compared to anthropogenic aerosols (e.g., Yoshida et al, 2013), whereas the efficiency of scattering by particles increases when the size parameter is comparable with the wavelength (thus the scattering of small particles is more pronounced at the short wavelengths, whereas the scattering of large particles (dust) is more pronounced at long ones).…”

Complex refractive indices and single-scattering albedo of global dust aerosols in the shortwave spectrum and relationship to size and iron content

“…As an example, the retrieval of the dust SSA and optical depth over bright desert surfaces with the MODIS (Moderate Imaging Resolution Spectroradiometer) Deep Blue algorithm (Hsu et al, 2004) applies the critical surface reflectance method (Kaufman, 1987) to retrieve dust properties from measured top-of-atmosphere (TOA) spectral reflectance. This algorithm depends critically on a priori information on the spectral refractive index (Kaufman et al, 2001;Yoshida et al, 2013). Similarly, active remote sensing techniques (lidar, light detection and ranging) require knowledge of the extinction-to-backscatter ratio (the lidar ratio), which is also a strong function of the complex index of the refraction or SSA of the aerosol particles (e.g., Gasteiger et al, 2011;Shin et al, 2018).…”