A new Differential Optical Absorption Spectroscopy instrument to study atmospheric chemistry from a high-altitude unmanned aircraft
A new Differential Optical Absorption Spectroscopy instrument to study atmospheric chemistry from a high-altitude unmanned aircraft
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DOI：
10.5194/amt-10-1017-2017
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发表时间：
2016-11
影响因子：
3.8
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中科院分区：
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热带对流层上层（UT）、热带对流层顶层（TTL）和平流层下层（LS）的大气微量气体观测需要专用的测量平台和仪器。在这里，我们提出了一种新的肢体扫描差分光学吸收光谱（DOAS）仪器，该仪器是为美国宇航局的全球鹰（GH）无人机系统开发的，并在机载热带对流层顶实验（ATTREX）中部署。mini-DOAS系统设计用于在14-18公里高度的非加压和非加热条件下自动运行，收集三个波长窗口的散射太阳光：紫外线（301- 387nm）、可见光（410- 525nm）和近红外（900-1700 nm）。一个望远镜扫描单元允许选择一个视角周围的肢体，以及实时校正飞机俯仰。由于海拔高，太阳参考光谱是用漫射器和直射阳光测量的。DOAS方法允许检索O-3、O-4、NO2和BrO的斜柱密度（SCDs），其相对误差与其他飞机DOAS系统相似。考虑辐射传输的因素表明，基于O-4观测值从观测到的SCD中检索微量气体混合比是DOAS测量中最常用的方法，但对于高海拔观测是不够的。这是由于低空云层的频繁出现，将O-4 SCD的灵敏度转移到低层大气中，使其高度依赖于云层覆盖。通过比较原位和DOAS O-3观测值来限制辐射转移的新开发技术克服了这一问题。广泛的灵敏度计算表明，新的o -3标度技术可以分别以0.5和15 ppt的高精度检索BrO和NO2混合比。因此，在ATTREX期间观察到的BrO和NO2混合比和垂直剖面为UT， TTL和LS中的臭氧和卤素化学提供了新的见解。
Observations of atmospheric trace gases in the tropical upper troposphere (UT), tropical tropopause layer (TTL), and lower stratosphere (LS) require dedicated measurement platforms and instrumentation. Here we present a new limb-scanning Differential Optical Absorption Spectroscopy (DOAS) instrument developed for NASA's Global Hawk (GH) unmanned aerial system and deployed during the Airborne Tropical TRopopause EXperiment (ATTREX). The mini-DOAS system is designed for automatic operation under unpressurized and unheated conditions at 14-18 km altitude, collecting scattered sunlight in three wavelength windows: UV (301-387 nm), visible (410-525 nm), and near infrared (900-1700 nm). A telescope scanning unit allows selection of a viewing angle around the limb, as well as realtime correction of the aircraft pitch. Due to the high altitude, solar reference spectra are measured using diffusors and direct sunlight. The DOAS approach allows retrieval of slant column densities (SCDs) of O-3, O-4, NO2, and BrO with relative errors similar to other aircraft DOAS systems. Radiative transfer considerations show that the retrieval of trace gas mixing ratios from the observed SCD based on O-4 ob-servations, the most common approach for DOAS measurements, is inadequate for high-altitude observations. This is due to the frequent presence of low-altitude clouds, which shift the sensitivity of the O-4 SCD into the lower atmosphere and make it highly dependent on cloud coverage. A newly developed technique that constrains the radiative transfer by comparing in situ and DOAS O-3 observations overcomes this issue. Extensive sensitivity calculations show that the novel O-3-scaling technique allows the retrieval of BrO and NO2 mixing ratios at high accuracies of 0.5 and 15 ppt, respectively. The BrO and NO2 mixing ratios and vertical profiles observed during ATTREX thus provide new insights into ozone and halogen chemistry in the UT, TTL, and LS.