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Using aircraft measurements to determine the refractive index of Saharan dust during the DODO Experiments

机译:在DODO实验中使用飞机测量值确定撒哈拉尘埃的折射率

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摘要

Much uncertainty in the value of the imaginary part of the refractive indexof mineral dust contributes to uncertainty in the radiative effect ofmineral dust in the atmosphere. A synthesis of optical, chemical andphysical in-situ aircraft measurements from the DODO experiments duringFebruary and August 2006 are used to calculate the refractive index mineraldust encountered over West Africa. Radiative transfer modeling andmeasurements of broadband shortwave irradiance at a range of altitudes areused to test and validate these calculations for a specific dust event on 23August 2006 over Mauritania. Two techniques are used to determine therefractive index: firstly a method combining measurements of scattering,absorption, size distributions and Mie code simulations, and secondly amethod using composition measured on filter samples to apportion the contentof internally mixed quartz, calcite and iron oxide-clay aggregates, wherethe iron oxide is represented by either hematite or goethite and clay byeither illite or kaolinite. The imaginary part of the refractive index at550 nm () is found to range between 0.0001 i to 0.0046 i, andwhere filter samples are available, agreement between methods is founddepending on mineral combination assumed. The refractive indices are alsofound to agree well with AERONET data where comparisons are possible. is found to vary with dust source, which is investigatedwith the NAME model for each case. The relationship between both sizedistribution and on the accumulation mode single scatteringalbedo at 550 nm (ω) are examined and size distributionis found to have no correlation to ω, while shows a strong linear relationship with ω.Radiative transfer modeling was performed with differentmodels (Mie-derived refractive indices, but also filter sampling compositionassuming both internal and external mixing). Our calculations indicate thatMie-derived values of and the externally mixed dust wherethe iron oxide-clay aggregate corresponds to the goethite-kaolinitecombination result in the best agreement with irradiance measurements. Theradiative effect of the dust is found to be very sensitive to the mineralcombination (and hence refractive index) assumed, and to whether the dust isassumed to be internally or externally mixed.
机译:矿物粉尘的折射率的虚部的值的很多不确定性导致了大气中矿物粉尘的辐射效应的不确定性。根据DODO实验于2006年2月至2006年8月进行的光学,化学和物理原位飞机测量结果的综合,用于计算西非上空遇到的矿物尘埃的折射率。毛利塔尼亚2006年8月23日特定尘埃事件的辐射转移模型和宽带短波辐照度的测量值用于测试和验证这些计算结果。两种技术可用于确定折光率:首先是结合散射,吸收,尺寸分布和Mie代码模拟的测量方法,其次是使用在过滤器样品上测量的成分来分摊内部混合石英,方解石和氧化铁-粘土骨料的含量的方法。 ,其中氧化铁以赤铁矿或针铁矿为代表,粘土以伊利石或高岭石为代表。发现在550 nm()处的折射率的虚部在0.0001 i至0.0046 i之间,并且在可获得过滤器样品的情况下,方法之间的一致性取决于假定的矿物组合。还发现了折射率与可以比较的AERONET数据非常吻合。被发现会随灰尘源而变化,这将通过NAME模型针对每种情况进行调查。研究了550 nm(ω)处的尺寸分布与累积模式单散射反照率之间的关系,发现尺寸分布与ω不相关,而与ω则显示出很强的线性关系。对不同模型进行了辐射转移建模(Mie-得出折射率,但同时过滤采样成分(包括内部和外部混合)。我们的计算表明,氧化铁-粘土聚集体对应于针铁矿-高岭石组合的Mie值和外部混合粉尘与辐照度测量结果最佳吻合。发现粉尘的辐射效应对假定的矿物成分(以及因此的折射率)以及假定粉尘是内部混合还是外部混合非常敏感。

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