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Spectroscopic measurement of atmospheric trace gases using time-tagged photon detection

使用时间标记光子检测对大气痕量气体进行光谱测量

基本信息

批准号：
NE/I001247/1
负责人：
Jonathan Lapington
金额：
$ 14.83万
依托单位：
University of Leicester
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2011
资助国家：
英国
起止时间：
2011 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FI001247%2F1
关键词：
Spectroscopic measurement atmospheric trace gases

项目摘要

Optical spectroscopy has a rich heritage in the measurement of atmospheric gases and changes to atmospheric composition. Famous early examples include the initial observation of polar stratospheric ozone depletion by measurement of the attenuation of sunlight at characteristic wavelengths due to absorption by the overlying ozone column. Nowadays, global measurements of ozone (and other species) are updated daily from satellite observations using essentially the same spectroscopic principles. There are main two reasons why spectroscopic methods at visible, UV or IR wavelengths are such valuable tools for atmospheric applications. (i) selectivity - within the very complex and sometimes rapidly changing mixture that comprises our atmosphere, it is often possible to target one compound by choosing a particular wavelength that is selectively absorbed by that compound. Alternatively, observations are performed over a sufficiently broad range of wavelengths that distinct features in the compound's absorption spectrum unambiguously identify its presence in the atmospheric sample. (ii) sensitivity - the chemically most important trace gases are typically present at mixing ratios in the range 10-9 to 10-12, thus requiring extremely sensitive detection methods. The observation of an emission signal following excitation of the target compound is a particularly sensitive approach, but can only be applied to a small number of compounds that fluoresce. Absorption methods are much more widely applicable and can often be used to directly quantify the concentration of an absorber without complex calibration procedures. But the light generally needs to travel a long distance through the sample for highly dilute species to be detectable. One way to do this is to fold the light many times inside an optical cavity formed by specialist high reflectivity mirrors. These cavity-based instruments are particularly useful for field observations because they provide in situ gas measurements at a well defined location, an important constraint for very reactive species that are too short-lived to be evenly mixed though the atmosphere. This proposal's aim is to build a new, highly sensitive broadband absorption spectrometer by combining the sensitivity & selectivity advantages of an existing field-tested broadband cavity instrument with innovative detector technology arising from space research. Time-tagged photon imaging offers a unique capability for a multi-wavelength phase-shift version of a technique called 'broadband cavity enhanced absorption spectroscopy' (BBCEAS). The detector tags each detected photon with a three-dimensional x,y,t coordinate which is used to identify whether the photon has passed through the cavity or is a reference signal from the light source (x co-ordinate), and the wavelength of each photon along the other (y) dispersion axis. The time coordinate, t, identifies the photon arrival time. Phase shift and attenuation as a function of wavelength, for both cavity output and the light source, are determined by time-histogramming the imaged spectra. This provides access to a key quantity (the number of times the lights passes back & forth within the cavity) that cannot be measured directly by conventional BBCEAS instruments which consequently require a separate calibration. Our combination of technologies offers an instrument able to be calibrated by a simple procedure not requiring technical input or calibration gases, and which therefore could be automated. Proof of this concept offers the realistic possibility of developing a ubiquitous instrument, with the high performance characteristic of spectroscopic methods, and capable of autonomous operation for remote monitoring of atmospheric pollutants, or even diverse applications such as breath monitoring in healthcare scenarios such as medical research or clinical diagnostics.

光谱学在测量大气气体和大气成分变化方面具有丰富的传统。著名的早期例子包括通过测量由于上层臭氧柱的吸收而导致的特征波长的太阳光衰减来初步观察极地平流层臭氧消耗。如今，臭氧（和其他物种）的全球测量数据每天都在使用基本相同的光谱原理从卫星观测中更新。可见光、紫外或红外波长的光谱方法之所以成为大气应用的宝贵工具，主要有两个原因。(i)选择性-在构成我们大气层的非常复杂而且有时迅速变化的混合物中，通常可以通过选择一种化合物选择性吸收的特定波长来锁定该化合物。或者，在足够宽的波长范围内进行观测，以使化合物吸收光谱中的不同特征明确识别其在大气样本中的存在。(ii)灵敏度-化学上最重要的痕量气体通常以10-9至10-12的混合比存在，因此需要极其灵敏的检测方法。在目标化合物激发后观察发射信号是一种特别灵敏的方法，但只能应用于少数发荧光的化合物。吸收法的适用范围更广，通常可用于直接量化吸收剂的浓度，而无需复杂的校准程序。但是光通常需要穿过样品很长一段距离，才能检测到高度稀释的物质。其中一种方法是在由专业高反射率镜形成的光学腔内多次折叠光线。这些基于腔的仪器对于现场观测特别有用，因为它们在明确定义的位置提供现场气体测量，这对于非常活跃的物种是一个重要的限制，这些物种的寿命太短，无法在大气中均匀混合。该提案的目的是通过将现有经现场测试的宽带腔仪器的灵敏度和选择性优势与空间研究产生的创新探测器技术相结合，建立一种新的高灵敏度宽带吸收光谱仪。时间标记光子成像提供了一种独特的能力，多波长相移版本的技术称为“宽带腔增强吸收光谱”（BBCEAS）。检测器用三维x、y、t坐标标记每个检测到的光子，该坐标用于识别光子是否已穿过腔体或是否是来自光源的参考信号（x坐标），以及每个光子的波长沿着另一个（y）色散轴。时间坐标t标识光子到达时间。相移和衰减作为波长的函数，对于腔输出和光源，通过对成像光谱进行时间直方图来确定。这提供了一个关键的数量（光在腔内来回通过的次数），不能直接由传统的BBCEAS仪器测量，因此需要单独的校准。我们的技术组合提供了一种能够通过简单程序进行校准的仪器，不需要技术输入或校准气体，因此可以自动化。这一概念的证明提供了开发一种无处不在的仪器的现实可能性，该仪器具有光谱方法的高性能特性，并且能够自主操作以远程监测大气污染物，甚至是各种应用，例如医疗保健场景中的呼吸监测，例如医学研究或临床诊断。