Raman lidar for the study of liquid water and water vapor in the troposphere

A Raman lidar system based on a tripled Nd:YAG laser is used for profiling of water vapor and liquid water in the troposphere. The Raman signals from water in the gas and liquid state are separated by interference filters and their relative intensities are studied for different atmospheric conditions. For clean weather or immediately after the rain the Raman signal from liquid water inside PBL is about one order of magnitude lower than the signal from water vapor. But during cloud measurements both Raman signals become comparable and the results of water vapor measurements must be corrected for the interference of liquid water Raman scattering. The obtained results are used for the estimation of liquid water content in the atmosphere. Received: 4 October 1999 / Revised version: 18 February 2000 / Published online: 11 May 2000     

Convenient method for calibrating system constant of scanning water vapor Raman lidar

<正>Lower tropospheric water vapor measurements are performed at nighttime using the mobile atmosphere monitoring lidar-2(AML-2) which is operated by the Anhui Institute of Optics and Fine Mechanics.In this lidar system,a 354.7-nm light from a Nd:YAG laser is used as stimulating source,whose Raman shifted center wavelengths are at 386.7 and 407.5 nm for nitrogen and water vapor,respectively.We present a novel and convenient method for determining the Raman lidar calibration constant according to the scanning performance of this lidar.We are likewise able to realize the measurement of water vapor profile in the low troposphere.The error induced by the uncertainty of calibrated constants is within 7% for the Raman lidar system.Experimental results from two months of study indicate that the method of calibrating the lidar system constant is feasible,and the Raman lidar performance is stable and reliable.     

Depolarization properties of cirrus clouds from polarization lidar measurements over Hefei in spring

A new polarization lidar has been developed for detecting depolarization characteristics of aerosol and cirrus over Hefei (31.90°N, 117.16°E), China. The fundamental principle of polarization lidar is briefly introduced.     

Measurement of tropospheric CO2 and aerosol extinction profiles with Raman lidar

A prototype Raman lidar was designed for monitoring tropospheric CO2 profile and other scientific investigatious.The third harmonic of Nd:YAG laser (354.7-nm wavelength) was used as stimulated light source to provide nighttime measurements.Filter with high rejection ratio performance was used to extract CO2 Raman signals from Rayleigh-Mie scattering signals effectively.To improve the real time monitoring function,a two-channel signal collection system was designed to collect CO2 and N2 Raman scattering signals simultaneously. The N2 Raman scattering signals were used to retrieve aerosol extinction coefficient.Typical features of CO2 concentration profile and aerosol extinction coefficient in Herei were presented.The mixing ratio of atmospheric CO2 in Hefei can reach about 360-400 ppmv.     

青藏高原对流层气溶胶的激光雷达监测与研究

运用激光雷达方程和Fernald方法,对西藏那曲地区和北京地区对流层气溶胶的微脉冲激光雷达(MPL)探测数据进行时空反演和比较,结果表明:气溶胶散射比廓线有着较为相似的结构分布,分层锯齿结构非常明显,主要包括贴地层、气溶胶混合层和气溶胶对流层。那曲测站上空气溶胶散射比在无云条件下最大值基本上保持在2.0左右;在测站上空均存在密度较大且较厚的积云;夏季的混合层或残留层(浅蓝色部分)高度抬高;夏季对流层低层积云的云量和积云出现的概率较冬季要少。     

水汽探测拉曼激光雷达的新型光谱分光系统设计与分析

提出并设计了一套新型的大气水汽和气溶胶探测用紫外域拉曼激光雷达系统, 以二向色镜和超窄带滤光片构成高效率拉曼光谱分光系统, 实现激光雷达大气回波信号中米-瑞利散射信号、 氮气和水汽的振动拉曼散射信号的精细分离和高效率提取. 利用美国标准大气的分子散射模型和实测的大气米散射信号模型, 对分光系统的米-瑞利散射信号的抑制率、大气水汽测量的信噪比和误差进行数值仿真设计. 搭建实验系统对西安地区夜间的大气水汽进行实验观测, 并利用有云天气下实测的激光雷达回波信号, 反演获得大气后向散射比和水汽混合比的相关特性, 验证了该拉曼光谱分光系统对米-瑞利信号的抑制率达到10^-7以上量级. 理论和实验结果表明, 设计的新型拉曼光谱分光系统可以在大气后向散射比为17时, 实现水汽探测误差小于15%, 满足拉曼激光雷达系统对大气水汽的高效率探测. 关键词： 拉曼激光雷达 水汽混合比 大气后向散射比     

Raman and fluorescence lidar measurements of aircraft engine emissions

Laser induced Raman and fluorescent measurements were made in the exhaust of a gas turbine engine with a new field portable instrument devised specifically for gas turbine exhaust measurements. The gas turbine exhaust was analyzed by conventional instruments for CO, CO₂, NO, NO_x, total hydrocarbons, smoke and temperature, and these data were used as a ‘calibration’ standard for the evaluation of the laser Raman instrument. Results thus far indicate good correlations for CO₂, O₂, smoke, hydrocarbons and temperature. The instrument was not sensitive enough for NO detection but the data analysis indicates that 100 ppm may be detectable with instrument improvements. CO analysis was not attempted, but it is expected that CO could be detected with further research. NO₂ (or NO_x) was not attempted because theoretical and experimental laboratory analysis indicated severe interference with CO₂. The conclusion was that laser Raman shows a good potential for aircraft gas turbine emission analysis.     

Tunable ultraviolet optical parametric oscillator for differential absorption lidar measurements of tropospheric ozone

A pulsed optical parametric oscillator (OPO) with intracavity sum frequency mixing was developed generating energies of up to 16 mJ in the 281–293 nm wavelength range. Both OPO process and sum frequency mixing are pumped by the harmonics of a single, medium-sized Nd:YAG laser. The system is characterized by a high overall efficiency (∼4% conversion from 1064 nm to the UV), a very compact set-up and stable and reliable operation. This system was successfully employed to measure tropospheric ozone using the differential absorption lidar (DIAL) technique and shows much promise as a lidar transmitter in airborne case studies as well as in unattended lidar systems for long-term monitoring. An unattended ozone profiling system could already be successfully realized. Received: 1 April 2002 / Revised version: 30 May 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-8153/28-1271, E-mail: Andreas.Fix@dlr.de     

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.     

Remote sensing of atmospheric gases with optical correlation spectroscopy and lidar: first experimental results on water vapor profile measurements

In this paper, the first experimental demonstration of the optical correlation spectroscopy lidar (OCS-lidar) is proposed. It is a new active remote sensing methodology to measure range-resolved atmospheric gas concentrations, based on broadband laser spectroscopy and light amplitude modulation. As a first step, a numerical study is performed for OCS-lidar measurements to optimize the accuracy of the range-resolved gas concentration measurement. Then, we demonstrate the ability of the OCS-lidar methodology to monitor the water vapor in the planetary boundary layer using the 4ν 720-nm absorption band. In addition to this first experimental proof, two different experimental configurations are proposed. The amplitude modulation, related to the optical correlation spectroscopy, is operated either at the emission with an active amplitude modulator before the backscattering process, or with passive optical filters on the laser backscattered light. For both configurations, range-resolved gas concentration measurements, achieved with a micro-pulse ground-based OCS-lidar, are presented. An extended discussion presents the mixing-ratio accuracy, which reaches ±1,000 ppm at a 2,000-m range for a range resolution of 200 m. The differences between the two experimental configurations are also discussed.     

In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer

12 molecules cm^-3 Hz^-1/2 (signal-to-noise ratio 3). This is equivalent to a mixing ratio of 0.3 ppmv at average midlatitude tropopause conditions or a mixing ratio of 0.6 ppmv under boundary-layer conditions. The corresponding minimum measurable absorbance is 10^-5 Hz^-1/2. The laser hygrometer was field-demonstrated aboard the NASA P3B research aircraft, during a series of flights spanning several weeks in the summer of 1997. During this demonstration, the laser hygrometer was intercompared with two optical chilled mirror hygrometers. In general, the laser hygrometer performed well; however, under some conditions, it reported water vapor number densities 20% greater than the cryogenic frost-point hygrometer. This difference is currently under study. Received: 31 March 1998/Revised version: 3 June 1998     

Depth-resolved temperature measurements of water using the?Brillouin lidar technique

Coupling between the atmosphere and the ocean takes place predominantly in the upper ocean mixed layer. Knowledge of the temperature profile in this region is therefore of particular interest. In this paper we report a successful measurement of a temperature distribution in a water tube based on Brillouin scattering with a fiber amplifier as the light source. It represents an important step towards the practical implementation of a lidar system for field use. Such a system could provide cost-effective on-line data over an extended region of the ocean and has potential impact to studies concerning climate, oceanography, weather forecasting and hurricane movements.     

Spectrum characteristics of multichannel water Raman lidar signals and principal component analysis

Using a multichannel full Raman lidar, water Raman signals composed of 32 different Raman wavelengths were obtained, and the time and cloud-height dependences of each spectrum on clear and cloudy days were determined. Although the Raman signals of winter cloud have a higher intensity at short wavelengths than those of summer cloud, these two types of clouds have large overlapping regions in wavelength space. However, in eigenvector space, winter and summer clouds are located at different points and have no overlapping regions. We have suggested that, as a cloud Raman lidar, a multiple-wavelength Raman lidar must be used, and the principal component analysis (PCA) method should be applied to determine the relative amount of each water phase.     

用激光雷达探测合肥高空钠层的变化

 介绍了自行设计的钠共振荧光激光雷达系统,并对钠层密度、钠层柱密度、中心高度及均方根宽度进行了计算和分析。计算结果表明:从2005年12月到2006年3月合肥上空钠层变化显著,钠层柱密度从12月平均值5.4×10⁹cm^-2变化到3月平均值2.4×10⁹cm^-2,下降了50%以上。均方根宽度也下降,中心高度略有波动。合肥钠层的变化趋势与中纬度其它地区具有可比性。     

Evaluation of dual differential absorption lidar based on Raman-shifted Nd:YAG or KrF laser for tropospheric ozone measurements

Based on Raman-shifted Nd:YAG or KrF laser, a method of three-wavelength Dual DIfferential Absorption Lidar (DIAL) for tropospheric ozone measurements is proposed. A theoretical analysis and numerical simulations of the measurement error have been performed. The results show that this method can reduce the error in ozone measurements caused by the aerosol layer in the troposphere by a factor of ten. The proposed method is also shown to be insensitive to aerosol optical properties, and therefore, one does not need to know the wavelength dependence of aerosol scattering. The dual-DIAL with 277.1, 291.8, 313.2 nm radiation based on a Raman-shifted KrF laser can be used both during day- and night-time. The dual-DIAL with 289.0, 299.1, 316.1 nm radiation based on Raman-shifted Nd:YAG laser can only be used during night-time.     

合肥上空大气二氧化碳Raman激光雷达探测研究

Raman激光雷达是用于大气成分探测与特性研究的有效工具.介绍了中科院安徽光学精密机械研究所自行研制的一台用于测量低对流层大气CO2时空分布的Raman激光雷达系统,并进行了一系列观测实验和对比分析.系统选用波长355nm的紫外激光作为光源,利用光子计数卡双通道采集大气中N2和CO2的Raman后向散射信号与Li-7500型H2O/CO2分析仪进行对比标定,通过反演获得了大气CO2水平与垂直方向时空分布廓线,并且获得了合肥地区大气边界层CO2的夜变化趋势.结果表明,大气CO2在空间的分布相对均匀,Raman激光雷达与CO2分析仪变化趋势一致性较好,能够对大气CO2时空分布进行有效、连续的观测.     

The airborne multi-wavelength water vapor differential absorption lidar WALES: system design and performance

A high-performance airborne water vapor differential absorption lidar has been developed during the past years. This system uses a four-wavelength/three-absorption line measurement scheme in the 935 nm H₂O absorption band to cover the whole troposphere and lower stratosphere simultaneously. Additional high spectral resolution aerosol and depolarization channels allow precise aerosol characterization. This system is intended to demonstrate a future space-borne instrument. For the first time, it realizes an output power of up to 12 W at a high wall-plug efficiency using diode-pumped solid-state lasers and nonlinear conversion techniques. Special attention was given to a rugged optical layout. This paper describes the system layout and technical realization. Key performance parameters are given for the different subsystems.     

Development of a mobile Raman-Mie lidar system for all time water vapor and aerosol detection

A turn-key Raman-Mie lidar system for water vapor and aerosol detection is described, which is placed in a boxcar and can be operated both at night and daytime. The operation of the lidar is very simple, especially, it does not need readjustment after road transit. The effective detection range of the system can reach tropopause for aerosol, and the improved system can obtain water vapor profile in boundary layers during daytime. Some experimental results and validations are presented.     

Frequency-stabilized cavity ring-down spectrometer for high-sensitivity measurements of water vapor concentration

We present a portable spectrometer that uses the frequency-stabilized cavity ring-down spectroscopy technique capable of high-precision measurements of trace water vapor concentration. Measuring one of the strongest rovibrational transitions in the ν₁+ν₃ water vapor combination band near ˜ν=7181.156 cm^-1, we compare spectroscopic and thermodynamic determinations of trace water vapor in N₂, and find systematic differences attributable to water vapor background effects and/or uncertainties in line intensities. We also compare the frequency-stabilized ring-down method with other cavity ring-down approaches that are based on unstabilized probe lasers and unstabilized ring-down cavities. We show that for the determination of water vapor concentration, the frequency-stabilized cavity ring-down method has the minimum measurement uncertainty of these techniques. The minimum noise-equivalent absorption coefficient of the spectrometer was 1.2×10^-10 cm^-1 Hz^-1/2, which further corresponds to a minimum detectable water vapor mole fraction equal to 0.7×10^-9 for an absorption spectrum of 10 minutes duration. PACS 33.20.-t; 33.70.Jg; 33.70.Fd; 42.62.Fi