workshops 24-May-2012     
de Graaf, Martin

 

Spectral analyses of desert dust and biomass burning aerosol scenes

Martin de Graaf(1)and Piet Stammes(1)

(1) Royal Netherlands Meteorological Institute (KNMI), Wilhelminalaan 30, 3732 GK, De Bilt, Netherlands

Abstract

Desert dust and smoke from biomass burning events are important climatic factors and monitoring of dust blowing and biomass burning events is important for our understanding of current climate changes. Space-borne sensors are ideally suited for monitoring of these wide-spread and short-lived aerosol events. However, satellite measurements of aerosol properties are often hampered by sensitivity to surface reflectivity and cloud contamination, resulting in distinct sea-land boundaries in satellite-measurements derived aerosol products and large gaps due to severe cloud screening. A product which lacks these problems is the Absorbing Aerosol Index (AAI), which is a measure of a radiance difference in the UV and is sensitive to UV-absorbing aerosols, i.e. desert dust, biomass burning aerosols and volcanic aerosols. Global AAI maps are ideal for tracking of events of these types of aerosols, although the AAI itself is not an aerosol quantity.

As yet, no distinction can be made between desert dust and biomass burning aerosol scenes. However, using the high spectral resolution of space-borne spectrometer SCIAMACHY, the top-of-atmosphere (TOA) reflectance spectra of scenes with these types of aerosols are shown to be completely different. DDA spectra over land are dominated by the surface albedo, because desert dust source regions can be very bright in the visible and the near-InfraRed (near-IR), but over the oceans aerosol characteristics dominate. Desert dust aerosols (DDA) are large, inert, UV-absorbing particles usually found in dry hot air and a desert dust scene can typically be identified by a fine yellowish haze in an otherwise clear sky.

Biomass burning aerosols (BBA) spectra on the other hand, are very variable, because BBA are chemically very active and BBA characteristics change rapidly over time. BBA are small, very hygroscopic aerosols with usually a black carbon core, which is an efficient light absorber, and over land BBA scenes are typically identified by a grey haze. However, once a smoke plume is transported over the oceans, BBA scenes are invariably found in the presence of clouds, as the BBA are efficient Cloud Condensation Nuclei (CCN), and cloud screening algorithms will fail to identify these plumes. The AAI is not sensitive to scattering aerosols and clouds and can still identify plumes of UV-absorbing aerosols within clouds.

The TOA reflectance spectra of two typical DDA and BBA scenes were analysed, using SCIAMACHY data and a radiative transfer model (RTM). In the desert dust case an aerosol model with a simple bimodal size distribution of mineral aerosols is sufficient to explain the reflectance spectrum of the scene. In the biomass burning case an external mixture of small smoke aerosols and large cloud droplets is necessary to explain the reflectance spectrum. The interaction between cloud droplets and BBA produce spectra that can help to identify biomass burning scenes within clouds. This might be used in the future to improve cloud screening based aerosol algorithms of space-based sensors.

 

 

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