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

Abstract

Abstract.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 CO 2 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) (λ=0.55 µm). The apparent discrepancy between the low imaginary part of the refractive index and the high apparent elemental carbon (EC a ) fractions (8 to 15%) obtained Correspondence to: from 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.

Brown carbon in tar balls from smoldering biomass combustion

“…The derived refractive indices of the observed combustion particles are fuel and time dependent, and agree with recently reported results (Guyon et al, 2003;Hoffer et al, 2006;Hungershoefer et al, 2008). The imaginary part of refractive index corresponding to the fuel types in Table 1 can be fitted with a simple function of wavelength λ as: …”

Measured and modeled humidification factors of fresh smoke particles from biomass burning: role of inorganic constituents

“…Values corresponding to each fuel are listed in Table 2. These values are consistent with previous reported estimates of biomass burning refractive indices (Reid et al, 2005bHungershoefer et al, 2008) and are consistent with values reported by Levin et al (2010) for FLAME 2006 and 2007 studies. More discussion of these values can be found in Sect.…”

The importance of plume rise on the concentrations and atmospheric impacts of biomass burning aerosol

“…They obtained dry mass extinction efficiency ranging from 1.64 to 6.64 m 2 g −1 at a wavelength of 0.532 µm. Hungershoefer et al (2008) found mass extinction efficiencies on the order of 9.0 m 2 g −1 for savanna grass and African hardwood.…”

“…We are not aware of any available in situ characterizations of the aerosol particles. Instead we compare the model results with the size distribution measured during a small-scale laboratory experiment performed by Hungershoefer et al (2008). For their experiment savanna grass and African hardwood were burnt in a smoke chamber in order to characterize the optical properties of biomass burning aerosol.…”