Extending differential optical absorption spectroscopy for limb measurements in the UV
Extending differential optical absorption spectroscopy for limb measurements in the UV
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DOI：
10.5194/amt-3-631-2010
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发表时间：
2010-05
影响因子：
3.8
通讯作者：
J. Puķı̄te;Sven Kühl;T. Deutschmann;U. Platt;T. Wagner
中科院分区：
文献类型：
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作者：
J. Puķı̄te;Sven Kühl;T. Deutschmann;U. Platt;T. Wagner
摘要。通过散射光测量来确定地球大气成分的紫外/可见吸收光谱方法通常基于比尔-朗伯定律，例如微分光学吸收光谱（DOAS）。虽然比尔-朗伯定律仅对单一光路严格有效，但对于单一波长的散射光观测，如果吸收较弱，光学深度与任何吸收剂浓度之间的关系也可以近似为线性。如果光路分布近似不随波长变化，那么也可以假定光深与截面和吸收剂浓度的乘积之间呈线性关系。这些假设被广泛用于散射光观测的DOAS应用。对于散射光的中等和强吸收（例如沿非常长的光路，如在翼形几何），光学深度和吸收剂浓度之间的关系不再是线性的。此外，对于较宽的波长间隔，在不同波长下传播的光路的差异变得很重要，特别是在紫外线中，散射的可能性随着波长的减少而强烈增加。然而，DOAS方法可以通过所谓的空气质量因子修正（或扩展）DOAS和加权函数修正DOAS扩展到中到强吸收和更宽波长间隔的情况。这些方法考虑了斜柱密度（SCDs）的波长依赖性，但也需要对空气质量因子或辐射传输模型的加权函数的先验知识。我们描述了一种方法，该方法将从DOAS （SCDs）获得的拟合结果视为波长和垂直光学深度的函数，并将该函数扩展为两个量的泰勒级数。然后将泰勒系数作为DOAS分析中的附加拟合参数加以应用。因此，拟合窗口内SCD的可变性是由检索本身决定的。这种新方法提供了SCD的描述，其准确性取决于泰勒展开的顺序，并且独立于所考虑的吸收剂的任何假设或先验知识。在模拟和测量的紫外光谱（332-357 nm）的案例研究中，我们证明了该方法在从SCIAMACHY分支观测中检索BrO垂直剖面方面的改进。与标准DOAS方法相比，该方法对臭氧SCDs的模拟结果更接近真实剖面。对于实测光谱，特别是对于强臭氧吸收的情况，与验证值的一致性也得到了显著提高。虽然本文的重点是改进SCIAMACHY肢体测量的BrO剖面检索，但这种新方法可以应用于更广泛的DOAS检索。
Abstract. Methods of UV/VIS absorption spectroscopy to determine the constituents in the Earth's atmosphere from measurements of scattered light are often based on the Beer-Lambert law, like e.g. Differential Optical Absorption Spectroscopy (DOAS). While the Beer-Lambert law is strictly valid for a single light path only, the relation between the optical depth and the concentration of any absorber can be approximated as linear also for scattered light observations at a single wavelength if the absorption is weak. If the light path distribution is approximated not to vary with wavelength, also linearity between the optical depth and the product of the cross-section and the concentration of an absorber can be assumed. These assumptions are widely made for DOAS applications for scattered light observations. For medium and strong absorption of scattered light (e.g. along very long light-paths like in limb geometry) the relation between the optical depth and the concentration of an absorber is no longer linear. In addition, for broad wavelength intervals the differences in the travelled light-paths at different wavelengths become important, especially in the UV, where the probability for scattering increases strongly with decreasing wavelength. However, the DOAS method can be extended to cases with medium to strong absorptions and for broader wavelength intervals by the so called air mass factor modified (or extended) DOAS and the weighting function modified DOAS. These approaches take into account the wavelength dependency of the slant column densities (SCDs), but also require a priori knowledge for the air mass factor or the weighting function from radiative transfer modelling. We describe an approach that considers the fitting results obtained from DOAS, the SCDs, as a function of wavelength and vertical optical depth and expands this function into a Taylor series of both quantities. The Taylor coefficients are then applied as additional fitting parameters in the DOAS analysis. Thus the variability of the SCD in the fit window is determined by the retrieval itself. This new approach provides a description of the SCD the exactness of which depends on the order of the Taylor expansion, and is independent from any assumptions or a priori knowledge of the considered absorbers. In case studies of simulated and measured spectra in the UV range (332–357 nm), we demonstrate the improvement by this approach for the retrieval of vertical profiles of BrO from the SCIAMACHY limb observations. The results for BrO obtained from the simulated spectra are closer to the true profiles, when applying the new method for the SCDs of ozone, than when the standard DOAS approach is used. For the measured spectra the agreement with validation measurements is also improved significantly, especially for cases with strong ozone absorption. While the focus of this article is on the improvement of the BrO profile retrieval from the SCIAMACHY limb measurements, the novel approach may be applied to a wide range of DOAS retrievals.