大尺度区域水汽浓度激光检测方法的研究

为了实现大尺度区域下大气中水汽浓度的高灵敏度、高精确性、快速响应检测,与遥感反演的数据进行校正,采用了TDLAS直接吸收技术结合开放式监测的方法。选择水汽分子在1.27 μm附近的单根吸收谱线为目标谱线,设计了大尺度区域水汽激光检测系统。结合多次反射池验证了系统性能,40 m光程下极限灵敏度为14.803 mmol·mol-1。利用本系统在中国科学院禹城综合试验站完成了1 420 m光程下的连续外场实验,系统工作稳定,并与同场地涡度相关观测系统中的气体分析仪LI-7500的测量数据进行了对比,数据一致性较好。为在复杂野外非均匀下垫面的水汽浓度变化的监测提供新方法。     

Thermal model of pulsed laser ablation: back flux contribution

This paper proposes improvement of the boundary conditions for the thermal model of laser-induced solid heating and ablation. The refinement of the model takes into account the back flux from the vapor cloud to the evaporation surface. For a wide range of evaporation rates, the value of the back flux was determined by direct Monte Carlo simulation of vapor cloud expansion. The obtained back flux values are substituted to the thermal model. The improved model has been shown to be in good agreement with the experimental results on graphite as an example. PACS 52.38.Mf; 79.20.Ds; 81.15.Fg     

The N2-broadened water vapor absorption line shape and infrared continuum absorption—I. Theoretical development

Attenuation of infrared radiation in the troposphere is dominated by water vapor absorption. Most past work on the spectroscopy of water vapor has been on the analysis of the rotational and vibrational bands. As a result, a thorough listing of line positions, line strengths, and half-widths exists today. Because of the weak continuum absorption in the water vapor windows centered at 10 and 4 μm, efforts to measure and model the continuum have not been as succesful as efforts in the analysis of the bands. Yet, a precise understanding of continuum absorption is important for long path energy transmission. For this reason, the study of the water vapor windows has had a long and speculative history.The purpose of this study is to demonstrate the importance of far wing phenomena in characterizing H₂O continuum absorption. A total line shape for water vapor-nitrogen interactions valid under tropospheric conditions is derived. The complexity of such a model is substantial and many approximations are necessary to obtain a tractable theory. This effort represents one of the initial attempts at a far wing line shape solution of continuum absorption. Application of the model to existing experimental data is presented in the following article.     

Infrared- and millimeter-wavelength continuum absorption in the atmospheric windows: Measurements and models

Attenuation of electromagnetic waves by atmospheric gases in an important consideration in a variety of radar and electro-optical applications. Molecular absorption by strong bands of H₂O and CO₂ defines the atmospheric window regions. The window regions are not totally transparent and feature weak line absorption and continuum absorption. The experimental character of continuum absorption by water vapor in atmospheric window regions at millimeter wavelengths and 10, 4 and 2.2 μm and nitrogen at 4 μm is surveyed. This includes recent measurements at the The Johns Hopkins University Applied Physics Laboratory on H₂O at 2.2 μm and the nitrogen collision-induced band at 4.3 μm. Also, some of the concepts and models used to characterize these phenomena will be reviewed. The search for a unified theory on the water vapor continuum has been elusive, yet the frequency and pressure dependence is consistent with current far-wing theories. The temperature dependence is not totally understood. A model describing the temperature dependence of the integrated intensity of the collision-induced nitrogen vibrational band is emphasized.     

1.517 μm附近水汽分子谱线加宽系数的腔衰荡光谱测量

 以超低膨胀系数微晶玻璃为腔体,以分布反馈式激光器为光源,建立了一套等噪声测量灵敏度为3.2×10^-9 cm^-1的调腔式连续波腔衰荡光谱系统。应用该系统对1.517 μm附近(6 586.5~6 596.5 cm^-1范围内)的7条水汽吸收谱线进行了实验研究,测得了这些谱线在氮气、空气环境下的谱线加宽系数。根据测得的结果,得到了此波段水汽的空气加宽系数与氮气加宽系数之比为0.896 9±0.068 7(3倍标准误差范围内),并就测得水汽的谱线加宽系数与HITRAN2004数据库数据进行了比较。     

基于六角形和球形冰晶模型的卷云辐射特征研究

卷云中冰晶粒子的单次光散射计算是卷云辐射传输及云微物理参数反演的重要基础。近年,利用高观测频率的静止气象卫星数据来反演水云和卷云的光学和微物理参数,进而计算地表光通量的研究倍受重视。然而,很多研究中卷云的冰晶用球形模型来模拟。由于不同形状和尺度大小的冰晶对电磁波的散射特征的不同,导致不同冰晶模型计算的卷云环境下卫星观测的辐射值及地表光通量的不同。利用不同尺度大小和电磁波波长的球形和六角形冰晶的单次散射数据,结合RSTAR辐射传输模式来定量分析了卷云环境下不同形状的冰晶模型对计算卫星观测的辐射和地表光通量中的影响。结果显示利用不同形状的冰晶模块来计算的卫星观测的辐射,地表向下辐射通量明显不同。波长在0.4～1.0 μm之间的大气窗口部分的光谱辐射通量的差距最大。总辐射通量受云粒子形状的影响显著。研究证实了正确选择冰晶模型对卫星反演卷云微物理和光学参数的反演及计算地表光通量的重要性。该结果对于云微物理参数的反演及地表向下辐射通量的模拟具有参考价值。     

基于中红外差频激光测量水汽分子同位素丰度

稳定同位素比值的测量在地质学、气象学和地球科学的研究中具有重要的应用价值。水汽同位素丰度的测量对理解与干旱相关的同温层大气科学具有重要的意义。水汽分子在2.7 μm附近具有较强的吸收,适宜于高灵敏度光谱的测量。文章报道了利用差频技术(Difference frequency generation)和准相位匹配技术(Quasi-phase matching),将调谐范围在750～840 nm之间的连续可调谐钛宝石激光器和单频连续的Nd∶YAG激光器,耦合到周期性极化铌酸锂非线性光学晶体中,产生2.5～4 μm波段的中红外可调谐激光。选择周期为20 μm的PPLN晶体,产生2.7 μm附近的中红外差频激光,利用差频产生的中红外激光光源,具有窄线宽、宽调谐等优点。结合光程为100 m的Herriott型多通吸收池,采用直接吸收光谱方法测量了实验室大气中的水汽分子同位素,得到了同位素比值R及17O,18O,D的丰度值δ,实验所测R值与国际标准具有很好的一致性。     

1.315μm区域高分辨力水汽吸收光谱研究

水汽吸收特性的研究对评估激光通讯和激光传输等具有重要的意义,因此水汽吸收光谱的研究在理论和工程方面都有重要应用。用工作在室温下,窄线宽、宽调谐范围分布反馈(DFB)二极管激光器作光源,结合光程可调、最长光程为1097m、可控温的改进型千米级怀特(White)池,精确地测量了水汽在1．315μm附近的高分辨力、高灵敏度吸收光谱,得到了水汽分子1．315μm附近31条主要吸收线的位置、谱线强度、压力自加宽系数等参量,测量结果与HITRAN数据库中的值很好地一致。实验中使用先进的电子技术和计算机控制技术,大大提高了实验的效率,确保了实验结果的可靠性。     

卷云短波反射特性的模拟计算研究

利用通用大气辐射传输(CART)软件模拟计算了0.4～2.5μm波段卷云大气反射率,分析了卷云大气的反射率随波长、光学厚度、有效尺度、卷云高度和地表类型变化情况,并模拟计算了0.55,1.38,2.75μm波段卷云大气反射率间关系。结果表明,可见光到近红外波段,卷云大气反射率随卷云光学厚度的增大而增大。可见光波段,卷云大气反射率随卷云粒子有效尺度变化很小;近红外波段,卷云大气的反射率随卷云粒子有效尺度增大而减小;近红外大气强吸收波段,卷云大气的反射率随卷云高度的增大而增大。大气窗口区卷云大气的反射率随地表类型的变化有显著的变化。通过0.55μm和2.75μm波段,1.38μm和2.75μm波段的卷云大气反射率间关系可以反演卷云光学厚度和有效尺度。     

Radiative transfer due to atmospheric water vapor: Global considerations of the earth's energy balance

A simple analytical formulation is presented for describing radiative transfer due to atmospheric water vapor. The radiative model is then applied to a global energy balance for earth, and the net infrared flux to space is expressed in terms of the mean surface temperature and atmospheric lapse rate. Water vapor and clouds are assumed to be the only sources of infrared opacity. When compared with empirical information, and for a global mean surface temperature of 288 K, the radiative model indicates a cloud top altitude for a single effective cloud of 6·8 km. Alternatively, when applied to a more realistic three-cloud formulation, the model predicts a comparable value of 6·5 km for an average cloud top altitude.With respect to changes in mean surface temperature, again comparing with empirical results, a discussion relating to the model suggests that the cloud top altitude decreases with decreasing surface temperature, which results in the surface temperature being roughly twice as sensitive to changes in factors such as planetary albedo than for the conventional assumption of a fixed cloud top altitude. Implications of this are discussed with respect to possible albedo changes due to atmospheric particulate matter as well as cloudiness as a climate feedback mechanism.     

Radiation transfer in absorbing-scattering liquids—I. Radiance and flux measurements

Radiation measurements have been made in highly absorbing and scattering aqueous suspensions for which interface reflectivities, as well as suspension properties, can significantly influence results. Measurements were made for the spectral band from 0.40 to 0.68 μm, using probes which determined vertical and directional distributions of the radiance, as well as the vertical distribution of the downwelling flux. Agreement between the separate measurements has established confidence in the experimental procedures, and the data have been used to delineate the effect of the suspension opacity, albedo and bottom reflectivity on the radiation field.     

Effects of oceanic turbidity and index of refraction of hydrosols on the flux of solar radiation in the atmosphere-ocean system

The effects of the concentration and complex index of refraction of hydrosols on the flux of solar radiation in the atmosphere and ocean have been studied by solving the equation of radiative transfer in model atmosphere-ocean systems. Scattering and absorption by aerosols, hydrosols, air and water molecules, as well as reflection and refraction at the ocean surface, are taken into account by applying the matrix method to the combined atmosphere-ocean system. Nine values of the index of refraction and four values of the concentration are assumed for the hydrosols. The flux of the solar radiation at the top of the atmosphere, just above and below the ocean surface, and at several depths in the ocean is calculated in the spectral interval between 0.3 and 0.8 μm. The flux of solar radiation in the atmosphere and ocean is significantly affected by the oceanic turbidity, as well as by the complex index of refraction of hydrosols.     

国外差分吸收激光雷达探测大气水汽廓线的研究进展

水汽含量是大气最基本的物理参量之一,大气水汽垂直分布结构对于大气过程的研究十分有意义。差分吸收激光雷达可以昼夜获取高精度、高距离分辨率的大气水汽垂直分布廓线,是最有潜力的探测手段。国际上已经发展出几种类型的差分吸收激光雷达,对它们的发展路径做一梳理,理清发展脉络,具有有益的参考价值。其中,稍早时期水汽差分吸收激光雷达工作在4ν振动吸收带720～730 nm频域,以Alexandrite为主流的激光器或者Nd∶YAG/ruby固体激光器泵浦的染料激光器作为发射光源,光电倍增管仍然可以在这个波段担任探测器,代表性的仪器是法国的机载LEANDRE Ⅱ。此后发展的820 nm波段的水汽差分吸收激光雷达,以钛宝石激光器或钛宝石光放大器为发射机,以硅的雪崩二极管作为探测器,紧跟前置放大和数据的AD采集器,如德国Hohenheim大学的车载扫描激光雷达,可以获得对流层300～4 000 m之间水汽两维或三维分布结构;德国Institutfür Meteorologie und Klimaforschung所建立的差分吸收激光雷达可以探测3～12 km高度之间大气的水汽垂直分布。720和820 nm波段水汽吸收截面较小,更适合于地基或车载的对流层水汽廓线探测。而水汽3ν振动谱935 nm区域吸收截面较大,是为了空间探测大气对流层上、平流层下相对干燥区域的水汽分布而准备的,且可以安排多个探测波长,和一个参考波长,它们对水汽的吸收截面大小呈梯度分布,以应对空间对地观测时不同高度大气水汽浓度的差别。基于种子注入的光参量振荡器或Nd∶YGG全固态激光器的935 nm差分吸收激光雷达,以德国Deutsches Zentrumfür Luft- und Raumfahrt的研究最为成功,推动了欧洲空间局立项发展空间水汽差分吸收激光雷达WALES（Water Vapour Lidar Experiment in Space）,测量从地球表面到平流层下、高垂直分辨率和高精度水汽浓度分布。机载多波长水汽差分吸收激光雷达1999年建立起来,担当空间WALES任务的模拟器,2006年完成了机载飞行试验。以823～830 nm分布布拉格反射半导体激光器和半导体光放大器为核心、采用雪崩二极管盖格光子计数技术的微脉冲差分吸收激光雷达,是差分吸收激光雷达面向商业化、可普及的方向迈出的重要一步,目前已经发展到第四代产品。发射机激光工作波长的长期稳定十分重要而棘手,以窄带连续波种子激光注入脉冲激光器的谐振腔锁定其的腔长,种子激光的波长以水汽的多通道光吸收池为参照标准,或以高精度波长计为误差获取手段,通过负反馈进行主动稳频;其次,需要仔细考虑大气对激光的后向散射光谱线型,显然Rayleigh后向散射光的多普勒展宽与水汽吸收光谱线宽度可以比拟,所以其吸收截面σ_on和σ_off必需加以修正;水汽的空间垂直分布梯度大,因此差分吸收激光雷达应该实行分通道探测。     

The absorption spectrum of water in the 1.25 μm transparency window (7408–7920 cm)

The absorption spectrum of water vapor in “natural” isotopic abundance has been recorded between 7408 and 7920 cm⁻¹ by high sensitivity cw-cavity ring down spectroscopy (CW-CRDS). This region covers the low energy part of the 1.25 μm transparency window and corresponds to weak water absorption of the first hexad of interacting vibrational bands. The achieved sensitivity – on the order of α_min ∼ 2 × 10⁻¹⁰ cm⁻¹ – has allowed one to newly measure 2028 weak transitions with intensities down to 2 × 10⁻²⁹ cm/molecule at 296 K i.e. more than two orders of magnitude lower than previous observations. Three hundred and fourty-one new and corrected energy levels belonging to 26 vibrational states of H₂¹⁶O, H₂¹⁸O, H₂¹⁷O, and HD¹⁶O could be determined from the vibration–rotation analysis based on variational calculations by Schwenke and Partridge. The previous investigations in the studied region by Fourier Transform Spectroscopy and existing databases have been critically evaluated. The most complete list for water in the region is provided as Supplementary Material.     

Calibration of the spectral sensitivity of instruments for the near infrared region

We present an analysis of methods for calibration of the spectral sensitivity of instruments in the near IR region of the spectrum (0.90–2.05 μm), using as an example recording of the luminescence spectra of PbS semiconductor quantum dots using a diffraction monochromator and an InGaAs photodiode as the detector. We show that when high-sensitivity detectors are employed for calibration using the emission spectrum of an ideal black body, the problem of attenuation of the radiation flux is still important. Instead of neutral density glass and mesh light filters for attenuation of the radiation, we propose using UFS ultraviolet optical glasses (together with PS purple glasses), the maximum optical density of which is within the region of maximum spectral sensitivity of InGaAs photodiodes. We give examples of spectral calibration, taking into account instrumental characteristics and the effect of absorption by water vapor in the air, and also corrections of the luminescence spectra of quantum dots.     

Infrared absorption in dense sodium vapor

The absorption spectra of a dense resonance medium were experimentally studied for the example of thermally heated dense sodium vapor. Several mechanisms that might cause substantial absorption and enhanced intensity of emission in the IR spectral region, λ τ; 0.9 μm, were considered. For the first time, a detailed study of the structure of the absorption spectra of sodium vapor in the specified wavelength range was performed to determine the influence of the kind and pressure of the buffer gas. It was found that buffer gas characteristics had a substantial effect on the absorption coefficient of vapor. The presence of the molecular component (dimers and trimers) in sodium vapor could not explain the experimental dependences of absorption in the infrared region. Possible influence of microparticles formed in condensation of convective sodium vapor flows in heated cells on the optical properties of vapor was considered. Microparticles could contribute to the observed absorption, but were incapable of explaining the substantial intensity of vapor radiation reported earlier. Possible many-particle effects on the absorption in the far spectral line wing were discussed. For the first time, the method of molecular dynamics was used to show for the example of the distribution function of ionic microfields in a dense plasma that such effects were in principle capable of substantially raising the profile of the line and increasing absorption in the region of large detunings from the resonance compared with the simple quasi-static model in the nearest-neighbor approximation.     

海雾的遥感光学辐射特性

为实现海雾的卫星自动监测,利用大量AVHRR3/NOAA17卫星观测数据,比较不同目标物(海雾区、不同云系及晴空下垫面)在可见光、近红外波段的反射辐射特性差异,统计分析了海雾的遥感光学辐射特性.结果表明,海雾区三通道反射率满足RCh1RCh3a>RCb2,甚至出现RCh3a>RCh1>RCh2不同于其它目标物(RCh1>RCh2>RCh3a.运用Streamer辐射传输模式模拟了海雾、不同云系在卫星高度处的三通道反射率特性,从理论上进一步验证了海雾所具有的光学辐射特性,同时指出,Ch3a的反射率对粒子粒径的响应明显,即粒子粒径越小,Ch3a的反射率越高.通过米氏散射理论对这一现象进行原因分析.     

First-time lidar measurement of water vapor flux in a volcanic plume

The CO₂ laser-based lidar ATLAS has been used to study the Stromboli volcano plume. ATLAS measured water vapor concentration in cross-sections of the plume and wind speed at the crater. Water vapor concentration and wind speed were retrieved by differential absorption lidar and correlation technique, respectively. Lidar returns were obtained up to a range of 3 km. The spatial resolution was 15 m and the temporal resolution was 20 s. By combining these measurements, the water vapor flux in the Stromboli volcano plume was found. To our knowledge, it is the first time that lidar retrieves water vapor concentrations in a volcanic plume.     

Self- and N2-broadened spectra of water vapor between 7.5 and 14.5 μm

Self- and N₂-broadened absorption spectra of water vapor have been measured in the infrared between 7.5 and 14.5 μm at 294, 311, 333 and 339 K. The so-called continuum aspect of the observed absorption coefficients is interpreted in terms of existing theoretical models.     

Multiple scattering of light by water cloud droplets with external and internal mixing of black carbon aerosols

The mixture of water cloud droplets with black carbon impurities is modeled by external and internal mixing models.The internal mixing model is modeled with a two-layered sphere(water cloud droplets containing black carbon(BC) inclusions),and the single scattering and absorption characteristics are calculated at the visible wavelength of 0.55 μm by using the Lorenz-Mie theory.The external mixing model is developed assuming that the same amount of BC particles are mixed with the water droplets externally.The multiple scattering characteristics are computed by using the Monte Carlo method.The results show that when the size of the BC aerosol is small,the reflection intensity of the internal mixing model is bigger than that of the external mixing model.However,if the size of the BC aerosol is big,the absorption of the internal mixing model will be larger than that of the external mixing model.