IMPMH, direct tunable diode laser absorption spectroscopy for detection of water vapour under "Optically thick Condition".

Seyeon Hwang, Minyoung Choi, Hongyun So, Kyunghoon Kim, Sun Choi
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Abstract

Water vapour plays a crucial role in atmospheric processes. Hence, monitoring the altitude-related variations in water vapour properties is important to decipher atmospheric processes. Direct tunable diode laser absorption spectroscopy (dTDLAS) measures the concentration and temperature of gas molecules by scanning the rotation-vibration absorption lines using a high-spectral-resolution laser. In this study, we devised an integrated measurement and data processing method (integrative measurement and processing method for hygrometry, IMPMH) to enhance the in-situ airborne measurement capability of dTDLAS. We measured a wide range (240-18,000 ppm) of water vapour concentrations, aiming for atmospheric measurements in a highly water-saturated regime, called the "optically thick condition". For recovering the full absorption spectra, the "integrative area" was defined and a difference factor D, which is the distance between two spectral regions with width corresponding to the half width of half maximum of the Voigt profile, was used to calculate the area. From the data, the low-bound concentration was measured to be 244 ppm. At D = 1.8, the transition concentration to the "optically thick condition" was measured to be 5,800 ppm. By increasing D from 1.8 to 2.8, the measurable upper-bound concentration increased to 17,993 ppm. IMPMH was applied to the measured data to estimate the final absorber density or water vapour concentration. The estimation was well-fitted with the measured detector signal with signal-to-noise ratio (SNR) of ∼ 300 of the residual spectrum, promising its applicability to in-situ airborne measurements. To validate IMPMH, the water vapour concentration range was divided into two regimes: (1) optically thick (5,800 < c < 18,000 ppm) and (2) optically thin (c < 5,800 ppm) conditions. Under the optically thick condition, IMPMH was validated by comparing the results between the short and long-path cells. In the optically thin condition, IMPMH was validated through comparison with the general dTDLAS method. Lastly, long-term stability of the dTDLAS system was validated by measuring 10 different concentrations (240-18,000 ppm) for 1000 s by characterising the precision and SNRs of the residual. The results demonstrate that IMPMH significantly enhances the in-situ airborne measurement capability of dTDLAS under both optically thick and thin conditions. Furthermore, requirements for the implementation of IMPMH in airborne measurement were investigated considering four aspects-sampling, low-pressure measurement, accuracy and precision, and multiplex detection. The results were examined with regard to atmospheric implications.

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IMPMH,直接可调二极管激光吸收光谱,用于在 "光学浓厚条件 "下检测水蒸气。
水蒸气在大气过程中起着至关重要的作用。因此,监测与海拔高度相关的水蒸气特性变化对于破译大气过程非常重要。直接可调谐二极管激光吸收光谱(dTDLAS)通过使用高光谱分辨率激光扫描旋转振动吸收线来测量气体分子的浓度和温度。在这项研究中,我们设计了一种集成测量和数据处理方法(湿度测量的集成测量和处理方法,IMPMH),以增强 dTDLAS 的原位机载测量能力。我们测量的水蒸气浓度范围很广(240-18,000 ppm),目的是在高度水饱和状态下进行大气测量,这种状态被称为 "光厚状态"。为了恢复全吸收光谱,定义了 "积分面积",并使用差分因子 D 计算面积,差分因子 D 是两个光谱区域之间的距离,其宽度与 Voigt 曲线的半最大半宽度相对应。根据数据,测得低束缚浓度为 244 ppm。在 D = 1.8 时,测得过渡到 "光学浓条件 "的浓度为 5,800 ppm。将 D 值从 1.8 增加到 2.8 时,可测量的上限浓度增加到 17,993 ppm。将 IMPMH 应用于测量数据,以估算最终的吸收剂密度或水蒸气浓度。估算结果与测量到的探测器信号拟合良好,残余光谱的信噪比(SNR)为 ∼ 300,有望应用于现场机载测量。为了验证 IMPMH,将水蒸气浓度范围分为两个区域:(1) 光学浓(5,800 °C);(2) 光学淡(5,800 °C)。
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