Radiative Properties of mixed Biomass/ Mineral Dust Aerosol in the Thermal Infrared

Abstract

The Saharan Mineral Dust Experiment (SAMUM) is a 7-year national German project dedicated to the radiative and micro-physical properties of mixed biomass and mineral dust aerosol. It comprised two field experiments in Morocco (May-June, 2006) and Cape Verde (January-February, 2008). We intend to present results of ground-based radiance measurements in the thermal infrared (TIR) obtained during the 2008 field experiment at Praia with a D\&P Model 102 FTIR . These measurements are complemented by space borne measurements with the IASI FTIR on board METOP-A. Furthermore we conducted radiative transfer simulations based on the data measured by other SAMUM-2 groups including LIDAR derived aerosol layer heights, airborne measurements of aerosol size distribution and off-line chemical analysis of aerosol samples collected by the DLR Falcon research aircraft. A comparison of simulated and measured radiances at top and bottom of the atmosphere (TOA, BOA) reveals insight into the validity of several approximations commonly made in TIR radiative transfer, such as the assumption of spherical model particles or the representation of the complex mineral mixture in terms of a single (wavelength-dependant) effective refractive index. Figure 1 is an example of a comparison of measured and simulated data obtained during a dust event on January 25th, 2008. The dust radiative effect is clearly visible when comparing the measurement to the clear-sky simulation. Apparently the OPAC (Hess {\it et al}, 1998) and Volz (1973) models agree relatively well with the Maxwell-Garnett effective medium theory, which uses the refractive indices of pure mineral constituents identified in the chemical analysis, while the model of Shettle and Fenn (1979) seems to underestimate the aerosol emission. An interesting effect occurs around 1100 cm$^{-1}$, where a considerably different behaviour between measurement and simulation is observable. Based on similar observations by Thomas and Gautier (2009) we suspect this to be attributable to the non-spherical shape of the aerosols, which is not accounted for in the simulations. We plan to conduct further investigations adressing this issue using spheroidal model particles and intend to discuss the results in our presentation. This work contributes to the development of a generally accepted benchmark model of aerosol TIR radiative properties, which is essential for validation of fast radiation models commonly found in climate simulations, weather prediction or satellite retrieval algorithms.