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The ICAD (iterative cavity-enhanced DOAS) method

机译：ICAD（迭代腔增强DOAS）方法

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Cavity-enhanced differential optical absorption spectroscopy (CE-DOAS or BB-CEAS DOAS) allows us to make in situ measurements while maintaining the kilometre-long light paths required by DOAS. This technique has been successfully used for several years to measure in situ atmospheric trace gases. A property of optical cavities is that in the presence of strong absorbers or scatterers the light path is reduced, in contrast to classical long-path DOAS measurements where the light path is fixed. Typical CE-DOAS or BB-CEAS evaluation schemes correct this effect using the measured total light intensity attenuation. This makes them sensitive to any variations in the light intensity not arising from the trace gas absorption. That means an important DOAS advantage, to be independent of total light intensity, is actually lost. In order to cope with this problem, the instrument setup would require a thorough stabilisation of the light source and a very rigid mechanical setup, which would make instrumentation more complex and error prone. We present a new approach to cavity-enhanced (CE) DOAS based on an iterative algorithm (ICAD) which actually models the light path reduction from the derived absorbers in the optical resonator. It allows a sensitive and robust data analysis that does not depend on the total light intensity, allowing a simpler and more compact instrument setup. The algorithm is discussed and simulated measurements demonstrate its sensitivity and robustness. Furthermore, a new ICAD NO2 instrument is presented. It takes advantage of the advanced data evaluation to build a compact (50 cm cavity) and lightweight instrument (10 kg) with low power consumption (25 W) for sensitive measurements of NO2 with a detection limit of 0.02 ppbv at an averaging time of 7 min. The instrument is characterised with a NO2 calibration source and good long-term stability is demonstrated in a comparison with a commercial chemiluminescence detector. As a new application of ICAD we show measurements on an automobile platform to investigate the two-dimensional NO2 distribution in an urban area. The instrument is so robust that even strong vibrations do not lead to any measurement problems.
机译：腔增强的差分光学吸收光谱（CE-DOA或BB-CEAS DOAS）允许我们在原位测量中进行，同时保持DOAS所需的公里长的光路。该技术已成功地使用了几年的时间来衡量原位大气痕量气体。光学腔的性质是，在存在强的吸收剂或散射体的存在中，光路减小，与光路固定的经典的长路径DOA测量相反。典型的CE-DOA或BB-CEAS评估计划使用测量的总光强度衰减来纠正此效果。这使得它们对不受痕量气体吸收不产生的光强度的任何变化敏感。这意味着重要的DOAS优势，与总光强度无关，实际上丢失。为了应对这个问题，仪器设置需要彻底稳定光源和非常刚性的机械设置，这将使仪器更加复杂并且容易出错。我们基于迭代算法（ICAD）来提高腔增强型（CE）DOA的新方法，该算法实际上模拟了光学谐振器中的衍生吸收器的光路减小。它允许敏感和强大的数据分析，不依赖于总光强度，允许更简单和更紧凑的仪器设置。讨论该算法并模拟测量展示了其灵敏度和鲁棒性。此外，提出了一种新的ICAD NO2仪器。它利用了先进的数据评估，以构建具有低功耗（25 W）的紧凑型（50厘米腔）和轻量级仪器（<10千克），用于NO2的敏感测量，在平均时间下的检测限为0.02 ppbv 7分钟。仪器的特征在于NO2校准源，并在与商业化学发光探测器进行比较中对良好的长期稳定性进行说明。作为ICAD的新应用，我们在汽车平台上显示了测量，以研究市区的二维No2分配。该仪器非常强大，即使强烈的振动也不会导致任何测量问题。

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