Single Scattering Properties of Biomass Burning Particles for Radiative Transfer

Abstract

Our work as subproject P3 in the joint project EFEU aims for a better understanding of the interaction between biomass burning aerosol and radiation. The two main points are the single scattering properties and the multiple scattering between these particles, as described through the radiative transfer equation. The scattering and absorption properties of the single particles depend on the particle size, their morphology and composition. The regarded wavelength plays an important role, too. Information about the optical properties can be received either from experiments, like the ones that have been carried out by our EFEU project partners or by calculation with the help of our models. A comparison between the measured and modelled properties showed partially a good agreement. While we were able to demonstrate that the temporal development of the number size distribution is substantial, it has been shown that its temporal variation does not capture the resulting optical properties alone. After adapting our radiative transfer model to the project-specific requirements, radiative transfer calculations had been carried out for the Quinault fire. Thereby the influence on the photolysis frequencies and the heating rates were in the center of attention. These calculations showed a strong backscattering of the solar radiation through the emitted particles. The result was an increase of the photolysis frequencies up to 20% above the plume compared to the clear case. Within and under the smoke layer a stronger decrease was obtained. In case of the net heating rates a maximum heating of the atmosphere was reached above the center of the fire. Responsible for this was the absorption of solar radiation through the black carbon at wavelengths below 1 micron. This also explained the somewhat higher heating in case of an internal mixture of organic and black carbon compared with an external mixture