Abstract
Biomass burning (BB) aerosols contribute to climate forcing, but much is still unknown about the extent of this forcing, owing partially to the high level of uncertainty regarding BB aerosol optical properties. A key optical parameter is the refractive index (RI), which influences the absorbing and scattering properties of aerosols. This quantity is not measured directly, but it is obtained by fitting the measured scattering cross section and extinction cross section to a theoretical model using the RI as a fitting parameter. We used the Rayleigh–Debye–Gans (RDG) approximation to retrieve the complex RI of freshly emitted BB aerosol from two fuels (eucalyptus and olive) from Africa in the spectral range of 500–580 nm. Experimental measurements were carried out using cavity ring-down spectroscopy to measure extinction over the range of wavelengths of 500–580 nm and nephelometry to measure scattering at three wavelengths of 450, 550, and 700 nm for size-selected BB aerosol particles. The fuels were combusted in a tube furnace at a temperature of 800 °C, which is representative of the flaming stage of burning. Filter samples were collected and imaged using tunneling electron microscopy to obtain information on the morphology and size of the particles, which was used in the RDG calculations. The mean radii of the monomers were 27.8 and 31.5 nm for the eucalyptus and the olive fuels, respectively. The components of the retrieved complex RI were in the range of 1.31 ≤ n ≤ 1.56 and 0.045 ≤ k ≤ 0.468. The real and complex parts of the RI increase with increasing particle mobility diameter. The real part of the RI is lower, and the imaginary part is higher than what was recommended in literature for black carbon generated by propane or field measurements from fires of mixed wood samples. Fuel dependent results from controlled laboratory experiments can be used in climate modeling efforts and to constrain field measurements from biomass burning.
Highlights
Among the factors that affect the climate, few are as diverse or as challenging to understand as the impacts of aerosols [1]
Np = ko a where Np is the number of monomer spherules in the aggregate, R g is the radius of gyration, a is the radius of the monomer, ko is the fractal pre-factor [60], and the fractal dimension, D f, describes the dimension of the monomer distribution, which has been reported to be within 1.6–1.9 for soot produced from biomass burning [12,17,60,61]
By retrieving the best fit complex refractive index to fit measured extinction and scattering cross sections, we have modeled the optical properties of Biomass burning (BB) aerosol in the spectral range of 500–580 nm using the RDG approximations
Summary
Among the factors that affect the climate, few are as diverse or as challenging to understand as the impacts of aerosols [1]. Atmosphere 2020, 11, 62 lifetimes ranging from less than one day to a few weeks [2]. Once airborne, they affect the climate both directly, through scattering and absorption of solar radiation, and indirectly, through their impact on cloud properties. Atmospheric aerosol particles have a strong influence on global climate and the Earth’s radiation budget [3]. Changes in the composition of the atmosphere can alter the Earth’s climate, by changing how much infrared radiation from the surface is retained by the atmosphere, or by altering the amount of solar radiation absorbed or redirected back into space [4]
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