Refractive Indices of Biomass Burning Aerosols Obtained from African Biomass Fuels Using RDG Approximation
Refractive Indices of Biomass Burning Aerosols Obtained from African Biomass Fuels Using RDG Approximation
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DOI：
10.3390/atmos11010062
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发表时间：
2020-01
期刊：
影响因子：
2.9
通讯作者：
E. Sarpong;Damon M. Smith;R. Pokhrel;M. Fiddler;S. Bililign
中科院分区：
文献类型：
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作者：
E. Sarpong;Damon M. Smith;R. Pokhrel;M. Fiddler;S. Bililign
生物质燃烧（BB）气溶胶对气候强迫有贡献，但这种强迫的程度仍然未知，部分原因是BB气溶胶光学特性的高度不确定性。一个关键的光学参数是折射率，它影响气溶胶的吸收和散射特性。这个量不是直接测量的，而是用RI作为拟合参数，将测量到的散射截面和消光截面拟合到理论模型中得到的。我们使用Rayleigh-Debye-Gans （RDG）近似方法在500-580 nm光谱范围内检索了非洲两种燃料（桉树和橄榄）新排放的BB气溶胶的复合RI。实验测量采用腔衰荡光谱法测量500 - 580nm波长范围内的消光，用比浊法测量尺寸选定的BB气溶胶粒子在450nm、550 nm和700nm三个波长处的散射。燃料在800℃的管式炉中燃烧，这是燃烧的火焰阶段的代表。收集滤光片样品并使用隧道电子显微镜成像，以获得颗粒的形态和大小信息，这些信息用于RDG计算。桉树和橄榄燃料单体的平均半径分别为27.8和31.5 nm。复配RI的分量范围为1.31≤n≤1.56,0.045≤k≤0.468。RI的实部和复部随颗粒迁移率直径的增大而增大。RI的实部较低，虚部高于文献中推荐的由丙烷或混合木材样品燃烧产生的黑碳的值。受控实验室实验得出的依赖燃料的结果可用于气候模拟工作，并限制生物质燃烧的现场测量。
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.