Refractive Indices of Biomass Burning Aerosols Obtained from African Biomass Fuels Using RDG Approximation

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.

Three-Dimensional Distribution of Biomass Burning Aerosols from Australian Wildfires Observed by TROPOMI Satellite Observations

“…Thus, we assume the microphysical properties of the latter in the retrieval. BB aerosols are assumed to have a monomodal lognormal size distribution with modal radius of 0.1 µm and a width of 1.43 (taken from AERONET retrieval of the event at Birdsville) and refractive indexes from [52]. All retrievals consider a unique a priori aerosol vertical profile of background concentrations, homogenous up to 9 km of altitude and decreasing above.…”

Impact of combustion conditions on physical and morphological properties of biomass burning aerosol

“…From the climate perspective, estimation of refractive indices (real and imaginary parts) of aerosol is the most important quantity to accurately estimate the aerosol radiative forcing. The optical closure method is the most common way to estimate the refractive indices of aerosol in the atmospheric science community (Sarpong et al 2020;Radney and Zangmeister 2018;Saleh et al 2014Saleh et al , 2016Saleh et al , 2018Saliba et al 2016;Mack et al 2010), where optical properties estimated from the model are manually iterated until measured and estimated values match. For refractive indices calculations, the most commonly used approach is the Mie theory based model (Bohren and Huffman 1983) which treats aerosol particles as homogeneous spheres.…”

“…Particles emitted from BB emissions have a complex morphology. The common technique used to characterize particle morphology is transmission electron microscopy and scanning electron microscopy (Sarpong et al 2020;Girotto et al 2018;Chakrabarty et al 2014;China et al 2013;McMurry 2004, Park et al 2003a). However, none of these techniques can provide continuous online information regarding particle morphology.…”

Optical Characterization of Fresh and Photochemically Aged Aerosols Emitted from Laboratory Siberian Peat Burning

“…Many of these effects are driven by the optical properties of the emitted aerosols; therefore, this has created significant interest in the optical properties of biomass burning emissions and their change during atmospheric processing. Extensive work has been conducted on optical properties, including light absorption, for freshly emitted [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] biomass burning aerosols and on the change of their optical properties during atmospheric processing [22][23][24][25][26][27][28][29][30][31][32][33]. However, work on the optical properties of peat emissions and their changes has been very limited [34].…”