Modelling the optical properties of fresh biomass burning aerosol produced in a smoke chamber: results from the EFEU campaign

A better characterisation of the optical properties of biomass burning aerosol as a function of the burning conditions is required in order to quantify their effects on climate and atmospheric chemistry. Controlled laboratory combustion experiments with different fuel types were carried out at the combustion facility of the Max Planck Institute for Chemistry (Mainz, Germany) as part of the "Impact of Vegetation Fires on the Composition and Circulation of the Atmosphere" (EFEU) project. The combustion conditions were monitored with concomitant CO2 and CO measurements. The mass scattering efficiencies of 8.9 +/- 0.2 m(2) g(-1) and 9.3 +/- 0.3 m(2) g(-1) obtained for aerosol particles from the combustion of savanna grass and an African hardwood (musasa), respectively, are larger than typically reported mainly due to differences in particle size distribution. The photoacoustically measured mass absorption efficiencies of 0.51 +/- 0.02 m(2) g(-1) and 0.50 +/- 0.02 m(2) g(-1) were at the lower end of the literature values. Using the measured size distributions as well as the mass scattering and absorption efficiencies. Mie calculations provided effective refractive indices of 1.60-0.010i (savanna grass) and 1.56-0.010i (musasa) (lambda=0.55 mu m). The apparent discrepancy between the low imaginary part of the refractive index and the high apparent elemental carbon (ECa) fractions (8 to 15%) obtained front the thermographic analysis of impactor samples can be explained by a positive bias in the elemental carbon data due to the presence of high molecular weight organic substances. Potential artefacts in optical properties due to instrument bias, non-natural burning conditions and unrealistic dilution history of the laboratory smoke cannot be ruled out and are also discussed in this study.