A new algorithm for simultaneous ozone and aerosol retrieval from tropospheric DIAL measurements

Aerosols can introduce significant errors in measurements of trace gas concentration profiles with differential absorption lidar (DIAL). Here, a solution to counteract the aerosol effect is introduced, which does not depend on additional measurements and is applicable when absorption at the off-wavelength is not negligible. It is based on a numerical solution of the coupled lidar equations and calculates the trace-gas concentration and the aerosol backscatter ratio simultaneously. Consequently, most of the errors introduced by aerosols to DIAL measurements are inherently corrected and, additionally, the aerosol backscatter profiles at the DIAL wavelengths are obtained. Received: 23 December 2002 / Revised version: 24 March 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-331/288-2178, E-mail: fimmler@awi-bremerhaven.de Present address: AWI Bremerhaven, Am Handelshafen 12, 27570 Bremerhaven, Germany     

New UV tunable solid-state lasers for lidar applications

We present the first lidar/DIAL system based on an Ultra Violet (UV) vibronic laser. A Nd:YAG-pumped Ce:LiSAF laser has been developed for this purpose, providing high pulse energy (5 mJ at 10 Hz), very high slope efficiency (33%), and a tuning range from 284 to 299 nm. Simultaneous measurements of SO₂ and O₃ concentration profiles are presented using this frequency-agile Ce:LiSAF-based lidar system.     

New method of elaboration of the lidar signal

In lidar measurements noise and fluctuations strongly affect the results. The reason is a rapid decrease of the signal-to-noise ratio with an increase of distance. The differential absorption lidar (DIAL) is particularly sensitive to the signal instabilities. In this paper we present a method of the signal acquisition that is suitable for registration of both large light fluxes and single photons. We also present new method of solution of the DIAL equations. Compared to the traditional algorithm used for signal elaboration our procedures are much more stable and they are able to increase the effective range of lidar measurements. Received: 2 February 1999 / Revised version: 30 June 1999 / Published online: 16 September 1999     

Comparison among error calculations in differential absorption lidar measurements

Error determination is a critical issue in differential absorption lidar (DIAL). In this paper, some common error calculations are compared by applying them to a set of artificial data. First, three methods for the evaluation of the signal-to-noise ratio (SNR) of the lidar return have been examined. Then, the effect of the application to DIAL measurements of correction algorithms is investigated. The mean value and the standard deviation of the set of artificial data have been obtained from experimental determinations accomplished with our system. This was made possible by the shot-per-shot recording of the lidar return. The results of this study allows us to recommend a new method for the data analysis used in DIAL experiments: the SNR should be directly measured and all sources of statistical fluctuation have to be taken into account in the calculation of the final error. Otherwise such error could be underestimated, as suggested also by the disagreement of simultaneous measurements carried out by different systems.     

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     

Experimental and theoretical studies related to a dye laser differential lidar system for the determination of atmospheric So2 and NO2 concentrations

Laboratory measurements of the absorption coefficients of NO₂ at wavelengths suitable for the operation of differential absorption lidar (DIAL) systems, at around 446 nm and 489 nm are reported. Such measurements are necessary for the valid determination of absolute levels of concentration of NO₂ in the atmosphere and have been made in order to assess anomalies in existing data. A theoretical assessment of some of the optional systems currently available for the application of DIAL to the measurement of atmospheric concentrations of SO₂ is described.     

Water vapor differential absorption lidar measurements using a diode-pumped all-solid-state laser at 935 nm

A diode-pumped, single-frequency laser system emitting at 935 nm has recently been developed to serve as the transmitter for water vapor differential absorption lidar (DIAL) measurements. This laser uses Nd:YGG (Y₃Ga₅O₁₂) as the active medium and emits radiation directly at 935 nm without the need of additional frequency conversion processes. The system was diode-pumped at 806 nm and was built up in a master-oscillator-power-amplifier configuration. It generates more than 30 mJ of pulse energy at 100 Hz repetition rate with a beam quality (M ²) of better than 1.4. Since water vapor DIAL demands for stringent requirements of the spectral properties those were carefully investigated in the scope of this paper. Single-frequency operation is achieved by injection seeding and active length control of the oscillator cavity. The range of continuously tunable single-frequency radiation extends to ～0.4 nm centered around 935.31 nm. Values of the spectral purity of >99.996% were determined using long-pass absorption measurements in the atmosphere exceeding the requirements by a large margin. Finally, for the first time water vapor DIAL measurements were performed using a Nd:YGG laser. The reported results show much promise of these directly pumped lasers at 935 nm for future spaceborne but also airborne water vapor lidar systems.     

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。     

Combined raman elastic-backscatter LIDAR for vertical profiling of moisture,aerosol extinction,backscatter, and LIDAR ratio

A combined Raman elastic-backscatter lidar has been developed. A XeCl excimer laser is used as the radiation source. Inelastic Raman backscatter signals are spectrally separated from the elastic signal with a filter or grating polychromator. Raman channels can be chosen to register signals from CO₂, O₂, N₂, and H₂O. Algorithms for the calculation of the water-vapor mixing ratio from the Raman signals and the particle extinction and backscatter coefficients from both elastic and inelastic backscatter signals are given. Nighttime measurements of the vertical humidity distribution up to the tropopause and of particle extinction, backscatter, and lidar ratio profiles in the boundary layer, in high-altitude water and ice clouds, and in the stratospheric aerosol layer are presented. Daytime boundary-layer measurements of moisture and particle extinction are made possible by the improved daylight suppression of the grating polychromator. Test measurements of the CO₂ mixing ratio indicate the problems for the Raman lidar technique in monitoring other trace gases than water vapor.     

AML-1车载式大气污染监测激光雷达样机研制

针对环境监测工作的迫切需要,开发研制了我国第一台车载式大气污染监测激光雷达。它具有测量范围大,距离分辨率高、数据可靠、灵敏度高等特点,并通过驱动单元能进行空间三维探测。该雷达系统能监测四种大气污染物,即大气气溶胶、二氧化硫、二氧化氮和臭氧。大气二氧化硫、二氧化氮、臭氧的最高探测灵敏度分别为0．01mg／m^3、0．04mg／m^3、0．01mg／m^3,气溶胶的消光系数探测限为10^-4km^-1,最高探测空间分辨力为7．5m,最大探测距离为3～10km。介绍了其基本原理,关键技术和运行情况。     

Intercomparison of ozone profiles measurements by a differential absorption lidar system and satellite instruments at Buenos Aires, Argentina

A ground-based DIfferential Absorption Lidar (DIAL) system has been implemented at CEILAP (CITEFA-CONICET) laboratory (34°33′S, 58°30′W), located in the Buenos Aires industrial suburbs. The goal is to perform measurements of the stratospheric ozone layer. Systematic measurements of ozone concentration profiles from 18 to 35 km altitude are performed since early 1999. Our measurements are carried out in 5 h in average during the night and in cloudless conditions. The DIAL system allows us to calculate directly the ozone profile from the lidar backscattering radiation since it is a self-calibrating technique. The signal processing takes into account the influence of the temperature profile on the ozone cross section. The temperature data are obtained from the radiosondes measurements performed at Ezeiza International Airport (34°30′S, 58°18′W), 27 km from DIAL station. The evolution of the stratospheric ozone profile is studied for different months. Results are compared with the data obtained by different satellites SAGE II, HALOE and GOME. The comparisons between our DIAL system and the satellite measurements show an agreement better than 20% for 20–35 km altitude range.     

Development of a SNR parameterization scheme for general lidar assessment

We present a general methodology for evaluating the capabilities of a general lidar system encompassing both backscatter (elastic and Raman lidar) and topographic targets. By introducing a well-defined atmospheric reference medium and by individually examining and decomposing the contributions of lidar system parameters including lidar transmitter power, field of view, receiver noise, atmospheric conditions, and sky background on the signal-to-noise ratio, we obtain a simple dimensionless parameterization of the lidar system. Using this parameterization, numerical simulations are carried out to determine achievable lidar performance including operation range, minimum detectable gas concentration, and so on.This revised version was published online in August 2005 with a corrected cover date.     

Detection of small forest fires by lidar

The possibility of detecting small forest fires with the help of a simple and cheap lidar operating at 0.532-μm wavelength up to distances of about 6.5 km is demonstrated. The values of the signal-to-noise ratio (SNR) achieved in the experiments are consistent with theoretical estimations obtained by computational modeling of the lidar detection process, including simulation of the smoke-plume shape and of the laser beam–plume interaction. This model was used to assess the potential of the lidar technique for fire surveillance in large forest areas. In particular, the upper limiting range for effective detection (SNR>5) of small localized fires in dry- and clear-weather conditions is estimated at 7–15 km depending on operation mode, burning rate, and observation geometry. Received: 29 August 2001 / Published online: 29 November 2001     

Preliminary results from the ozone lidar at the University of L''aquila

Summary We describe the ozone lidar operated at the Department of Physics of the University of L'Aquila. Preliminary ozone profiles obtained with the DIAL (differential absorption lidar) technique are also reported and a comparison is made with ECC (electro chemical cell) sonde data obtained at the FISBAT, CNR in Bologna. Although this comparison is of limited value the two sets of data show a good agreement. The influence of the recent detected volcanic aerosols from the eruption of Pinatubo on these measurements is also discussed.     

Laser remote sensing and photoacoustic spectrometry applied in air pollution investigation

We have applied two spectroscopic techniques in order to investigate air pollution dispersion in real and simulated atmosphere: the Titan–Sapphire laser technique based on differential absorption lidar (DIAL), and CO₂ laser photoacoustic spectrometry (LPS). A set of spatial NO₂ measurements was performed over a highway outside a city and in an urban street canyon with extensive traffic. Qualitative agreement was found between the results obtained by DIAL and by physical modeling based on LPS measurements in a wind tunnel.     

Ground-based monitoring of pinatubo aerosols and ozone at L''Aquila, Italy I.—Evidences for ozone depletion during 1991/92 winter months

Summary Possible evidence for stratospheric ozone depletion at northern midlatitudes as a consequence of the Pinatubo aerosol cloud is presented in this work. A ten-month record of aerosol and ozone measurements taken at the lidar station of L'Aquila, Italy, is compared to the 1985–1986 SBUV and SAGE II ozone data. Ozonesonde data collected in the station of Hohenpeissenberg, Germany, and S. Pietro Capofiume, Italy, are used to validate the DIAL measurements corrected for the aerosol presence.     

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     

Lidar Network System for Monitoring the Atmospheric Environment in Jakarta City

A lidar network system consisting of two Mie scattering lidars and one differential absorption lidar was developed to measure the atmospheric environment in Jakarta. The three lidars were installed at three locations in Jakarta to study atmospheric boundary layer structure and transportation of atmospheric pollutants. The Mie scattering lidars employ compact flashlamp pumped Nd:YAG lasers operated at 1064 nm fundamental. They are installed in shelters and directed vertically. One of the Mie lidar has a rotating wedged window for scanning conically to measure wind velocity using a correlation method. The DIAL system employs two Nd:YAG laser-pumped optical parametric oscillators. The DIAL is designed to measure distribution of ozone and SO₂ in the near UV region, and NO₂ in the 450-nm region. The system is installed in a shelter and has a full scanning capability.