Dual Differential Absorption Lidar for the Measurement of Atmospheric SO(2) of the Order of Parts in 10(9)

Vertical concentration profiles of atmospheric SO(2) of the order of parts in 10(9) (ppb) were measured by a multiwavelength differential absorption lidar system. The error that was due to O(3) and aerosols was successfully reduced by a three-wavelength dual differential absorption lidar (DIAL), and a SO(2) concentration of 1.2 ppb for 2400-3000-m altitude was obtained with 300-m range resolution. The measurement error in dual DIAL was estimated to be <1.1 ppb when several factors were considered. The influence of O(3) on SO(2) measurement error was experimentally evaluated from simultaneous measurements of atmospheric SO(2) and O(3) by two conventional DIAL pairs, each using two wavelengths.     

Ultraviolet DIAL measurements of O3 profiles in regions of spatially inhomogeneous aerosols

The differential absorption lidar (DIAL) technique generally assumes that atmospheric optical scattering is the same at the two laser wavelengths used in the DIAL measurement of a gas concentration profile. Errors can arise in this approach when the wavelengths are significantly separated, and there is a range dependence in the aerosol scattering distribution. This paper discusses the errors introduced by large DIAL wavelength separations and spatial inhomogeneity of aerosols in the atmosphere. A Bernoulli solution for determining the relative distribution of aerosol backscattering in the UV region is presented, and scattering ratio boundary values for these solutions are discussed. The results of this approach are used to derive a backscatter correction to the standard DIAL analysis method. It is shown that for the worst cases of severe range dependence in aerosol backscattering, the residual errors in the corrected DIAL O3 measurements were <10 ppbv for DIAL wavelengths at 286 and 300 nm.     

Side-line tunable laser transmitter for differential absorption lidar measurements of CO2: design and application to atmospheric measurements

A 2 microm wavelength, 90 mJ, 5 Hz pulsed Ho laser is described with wavelength control to precisely tune and lock the wavelength at a desired offset up to 2.9 GHz from the center of a CO(2) absorption line. Once detuned from the line center the laser wavelength is actively locked to keep the wavelength within 1.9 MHz standard deviation about the setpoint. This wavelength control allows optimization of the optical depth for a differential absorption lidar (DIAL) measuring atmospheric CO(2) concentrations. The laser transmitter has been coupled with a coherent heterodyne receiver for measurements of CO(2) concentration using aerosol backscatter; wind and aerosols are also measured with the same lidar and provide useful additional information on atmospheric structure. Range-resolved CO(2) measurements were made with <2.4% standard deviation using 500 m range bins and 6.7 min? (1000 pulse pairs) integration time. Measurement of a horizontal column showed a precision of the CO(2) concentration to <0.7% standard deviation using a 30 min? (4500 pulse pairs) integration time, and comparison with a collocated in situ sensor showed the DIAL to measure the same trend of a diurnal variation and to detect shorter time scale CO(2) perturbations. For vertical column measurements the lidar was setup at the WLEF tall tower site in Wisconsin to provide meteorological profiles and to compare the DIAL measurements with the in situ sensors distributed on the tower up to 396 m height. Assuming the DIAL column measurement extending from 153 m altitude to 1353 m altitude should agree with the tower in situ sensor at 396 m altitude, there was a 7.9 ppm rms difference between the DIAL and the in situ sensor using a 30 min? rolling average on the DIAL measurement.     

Identification and correction of analog-to-digital-converter nonlinearities and their implications for differential absorption lidar measurements

Differential absorption lidar (DIAL) is a powerful remote-sensing technique widely used to probe the spatial and temporal distribution of ozone and other gaseous atmospheric trace constituents. Although conceptually simple, the DIAL technique presents many challenging and often subtle technical difficulties that can limit its useful range and accuracy. One potentially serious source of error for many DIAL experiments is nonlinearity in the analog-to-digital converters used to capture lidar return signals. The impact of digitizer nonlinearity on DIAL measurements is examined, and a simple and inexpensive low-frequency dithering technique that significantly reduces the effects of ADC nonlinearity in DIAL and other applications in which the signal is repetitively averaged is described.     

Future performance of ground-based and airborne water-vapor differential absorption lidar. I. Overview and theory

The performance of a future advanced water-vapor differential absorption lidar (DIAL) system is discussed. It is shown that the system has to be a direct-detection system operating in the rhovarsigmatau band of water vapor in the 940-nm wavelength region. The most important features of the DIAL technique are introduced: its clear-air measurement capability, its flexibility, and its simultaneous high resolution and accuracy. It is demonstrated that such a DIAL system can contribute to atmospheric sciences over a large range of scales and over a large variety of humidity conditions. An extended error analysis is performed, and errors (e.g., speckle noise) are included that previously were not been discussed in detail and that become important for certain system designs and measurement conditions. The applicability of the derived equation is investigated by comparisons with real data. Excellent agreement is found.     

Trace atmospheric SO2 measurement by multiwavelength curve-fitting and wavelength-optimized dual differential absorption lidar

We present a new differential absorption lidar (DIAL) method for atmospheric trace SO2 using multi-wavelength curve fitting. With this method we use five wavelengths around a SO2 absorption peak and obtain SO2 and O3 concentrations by fitting their absorption cross sections to measured DIAL and null results. A SO, concentration of 6 parts in 10(9) (ppb) was obtained for an altitude of 1050 m with 150-m range resolution. In addition, we optimized the wavelengths for dual-DIAL SO2 measurement and demonstrated a high sensitivity of <0.5 ppb with 300-m range resolution. Comparison of these two methods is also presented.     

Experimental comparison of heterodyne and direct detection for pulsed differential absorption CO2 lidar

A pulsed dual-wavelength dual-CO2-laser differential-absorption lidar (DIAL) system has been developed which permits simultaneous heterodyne and direct detection of the same lidar returns. This system has been used to make an experimental comparison of the SNRs and statistical and temporal characteristics of the DIAL returns from several topographic targets. These results were found to be in general agreement with theory and were used to quantify the relative merits of the two detection techniques. The measured parameter values were applied to an analytical treatment to predict system trade-offs for the remote sensing of atmospheric species, with application to both path-averaged and range-resolved measurements.     

Analytical differentiation of the differential-absorption-lidar data distorted by noise

A method of analytical differentiation is developed for processing differential absorption lidar (DIAL) data. The method is based on simple analytical transformation of the DIAL on and off signal ratio. The derivatives consequently are found for either individual data points or local zones of the measurement range. The method makes possible the separation of local zones of interest and the separate investigation of these. The smoothing level is established by the selected value of the exponent in a transformation formula rather than by the selection of the resolution range. The method does not require the calculation of local signal increments. This reduces significantly the high-frequency noise in the measured concentration. The method is general and can be used for different experimental data, including inelastic (Raman) lidar data. The processing technique is practical and does not require a determination of the solution for a large set of algebraic equations. It is based on the simple repetition of the same type of calculations with different constants. The method can easily be implemented for practical computations.     

Improved speckle statistics in coherent differential absorption lidar with in-fiber wavelength multiplexing

Remote detection of gaseous pollutants and other atmospheric constituents can be achieved with differential absorption lidar (DIAL) methods. The technique relies on the transmission of two or more laser wavelengths and exploits absorption features in the target gas by measuring the ratio of their detected powers to determine gas concentration. A common mode of operation is when the transmitter and receiver are collocated, and the absorption is measured over a return trip by a randomly scattering topographic target. Hence, in coherent DIAL, speckle fluctuation leads to a large uncertainty in the detected powers unless the signal is averaged over multiple correlation times, i.e., over many independent speckles. We examine a continuous-wave coherent DIAL system in which the laser wavelengths are transmitted and received by the same single-mode optical fibers. This ensures that the two wavelengths share a common spatial mode, which, for certain transmitter and target parameters, enables highly correlated speckle fluctuations to be readily achieved in practice. For a DIAL system, this gives the potential for improved accuracy in a given observation time. A theoretical analysis quantifies this benefit as a function of the degree of correlation between the two time series (which depends on wavelength separation and target depth). The results are compared with both a numerical simulation and a laboratory-based experiment.     

Laser prf considerations in differential absorption lidar applications

Laser pulse repetition frequency (prf) is a major factor in determining the performance of differential absorption lidar (DIAL) systems. The effects of laser prf on concentration measurement accuracy and maximum usable range are quantified. Heterodyne-detection (coherent) and direct-detection (noncoherent) as well as path-averaged and range-resolved DIAL systems are analyzed.     

Complementary study of differential absorption lidar optimization in direct and heterodyne detections

A detailed study using both analytical and numerical calculations of direct and heterodyne differential absorption lidar (DIAL) techniques is conducted to complement previous studies. The DIAL measurement errors depend on key experimental parameters, some of which can be adjusted to minimize the statistical error. Accordingly, the pertinent criteria on optical thickness, the number of photons emitted at the on and off wavelengths, are discussed to reduce the relative error on the total column content or range-resolved measurements that rely on either hard target or atmospheric backscatter returns. In direct detection, the optimal optical thickness decreases from 1.3 to 0.8 when the background increases while the on-line-to-off-line optimal energy ratio decreases from 3.6 to 2.7. In heterodyne detection, the minimum error is obtained for an optical thickness of 1.2 and an energy ratio of 4.3.     

Error reduction in laser remote sensing: combined effects of cross correlation and signal averaging

A systematic analysis is presented of the extent to which the accuracy of a differential-absorption lidar (DIAL) measurement may be improved by using the combined effects of signal averaging and temporal cross correlation. Previous studies which considered these effects separately are extended by incorporating both effects into a single analytical framework. In addition, experimental results involving lidar returns from a diffusely reflecting target using a dual-CO2 laser DIAL system with both heterodyne and direct detection are presented. These results are shown to be in good agreement with the theoretical analysis and help establish the limits of accuracy achievable under various experimental conditions.     

NASA multipurpose airborne DIAL system and measurements of ozone and aerosol profiles

An airborne differential absorption lidar (DIAL) system has been developed for the remote measurement of gas and aerosol profiles in the troposphere and lower stratosphere. The multipurpose DIAL system can operate from 280 to 1064 nm for measurements of ozone, sulfur dioxide, nitrogen dioxide, water vapor, temperature,pressure, and aerosol backscattering. The laser transmitter consists of two narrow linewidth Nd: YAG pumped dye lasers with automatic wavelength control. The DIAL wavelengths are transmitted with a 00-,usec temporal separation to reduce receiver system complexity. A coaxial receiver system is used to collect and optically separate the DIAL and aerosol lidar returns. Photomultiplier tubes detect the backscattered laser returns after optical filtering, and the analog signals from three tubes are digitized and stored on high-speed magnetic tape. Real-time gas concentration profiles or aerosol backscatter distributions are calculated and displayed for experiment control. Operational parameters for the airborne DIAL system are presented for measurements of ozone, water vapor, and aerosols in the 290-, 720-, and 600-nm wavelength regions, respectively. The first ozone profile measurements from an aircraft using the DIAL technique are discussed in this paper. Comparisons between DIAL and in situ ozone measurements show agreement to within +/-5 ppbv in the lower troposphere. Lidar aerosol data obtained simultaneously with DIAL ozone measurements are presented for a flight over Virginia and the Chesapeake Bay. DIAL system performance for profiling ozone in a tropopause folding experiment is evaluated, and the applications of the DIAL system to regional and global-scale tropospheric investigations are discussed.     

Laser remote sensing of atmospheric ammonia using a CO2 lidar system

A CO2 differential-absorption lidar system has been used for the remote sensing of ammonia in the atmosphere. For CO2 lidar returns backscattered from topographic targets at ranges up to 2.7 km, the path-averaged sensitivity of the DIAL system was 5 ppb of NH3 . Concentrations of atmospheric ammonia were found to vary during the day from undetectable levels (<5 ppb) to as high as 20 ppb, depending on temperature and humidity conditions.     

Airborne differential absorption lidar system for measurements of atmospheric water vapor and aerosols

An airborne differential absorption lidar (DIAL) system has been developed at the NASA Langley Research Center for remote measurements of atmospheric water vapor (H(2)O) and aerosols. A solid-state alexandrite laser with a 1-pm linewidth and > 99.85% spectral purity was used as the on-line transmitter. Solid-state avalanche photodiode detector technology has replaced photomultiplier tubes in the receiver system, providing an average increase by a factor of 1.5-2.5 in the signal-to-noise ratio of the H(2)O measurement. By incorporating advanced diagnostic and data-acquisition instrumentation into other subsystems, we achieved additional improvements in system operational reliability and measurement accuracy. Laboratory spectroscopic measurements of H(2)O absorption-line parameters were perfo med to reduce the uncertainties in our knowledge of the absorption cross sections. Line-center H(2)O absorption cross sections were determined, with errors of 3-6%, for more than 120 lines in the 720-nm region. Flight tests of the system were conducted during 1989-1991 on the NASA Wallops Flight Facility Electra aircraft, and extensive intercomparison measurements were performed with dew-point hygrometers and H(2)O radiosondes. The H(2)O distributions measured with the DIAL system differed by ≤ 10% from the profiles determined with the in situ probes in a variety of atmospheric conditions.     

Preliminary measurements with an automated compact differential absorption lidar for the profiling of water vapor

The design and preliminary tests of an automated differential absorption lidar (DIAL) that profiles water vapor in the lower troposphere are presented. The instrument, named CODI (for compact DIAL), has been developed to be eye safe, low cost, weatherproof, and portable. The lidar design and its unattended operation are described. Nighttime intercomparisons with in situ sensors and a radiosonde are shown. Desired improvements to the lidar, including a more powerful laser, are also discussed.     

Intrapulse temporal and wavelength shifts of a high-power 2.1-µm Ho:YAG laser and their potential influence on atmospheric lidar measurements

A high-power, flash-lamp-pumped, Q-switched Ho:YAG laser has been developed to produce up to 150 mJ in a 100-ns Q-switched pulse. The Ho laser was initially used in a direct detection lidar-differential absorption lidar (DIAL) system to measure vertical density profiles of aerosols and water vapor in the atmosphere. It was found, however, that the Ho laser operated simultaneously on two closely spaced spectral emission wavelengths (2.090 and 2.097 μm) and that the distribution of energy between the two wavelengths could change significantly on time scales of several seconds to minutes. Such intrapulse temporal and wavelength shifts were found to alter the atmospheric lidar return significantly because one of the laser lines coincided with a water vapor absorption line in the atmosphere. This laser spectral output problem was overcome by the use of intracavity étalons that controlled the laser spectral-temporal characteristics but reduced the laser output energy to approximately 75 mJ/pulse in a 100-ns pulse length. These results are important as they serve to point out the difficulties of developing and using a high-power 2.1- μm Ho laser for atmospheric lidar when high-resolution spectral and temporal characteristics can significantly influence the lidar return and be misinterpreted as resulting from atmospheric signals.     

Wide-range sounding of free-tropospheric water vapor with a differential-absorption lidar (DIAL) at a high-altitude station

A differential absorption lidar (DIAL) system has been developed for the measurement of water vapor throughout the free troposphere [3 to 12 km above sea level (asl.)] with high vertical resolution varied from 50 m next to the ground to 300 m above an altitude of 10 km. The system was installed at the Schneefernerhaus high-altitude research station (2675 m asl., Zugspitze, Germany). The DIAL system is based on a tunable single-mode laser system with a high pulse energy of currently 250 mJ and a repetition rate of 20 s(-1). For lidar operation with energies typically between 100 mJ and 150 mJ and an integration time of 1000 s (10000 laser shots for both DIAL wavelengths) a vertical range of at least 10 km has been demonstrated even under dry conditions and during daytime, while daytime measurements up to 12 km have been possible under humid conditions. The system was intercompared with radiosondes, which suggests an agreement within 5% in a major part of the operating range. Further improvements are planned in the upper troposphere to approach the accuracy requirements needed in climate research.     

二氧化氮差分吸收激光雷达拉曼激光源的优化研究

Nd:YAG三倍频激光(355nm)抽运高压CH4,获得其第一级斯托克斯光S1(395.60nm),作为NO2差分吸收激光雷达(NO2-DIAL)的激光源λon.通过增大激光脉冲宽度、改变拉曼池耦合透镜的焦距和气体压强等条件,获得了满足实际使用要求的转化效率高、稳定性好的395.60nm波长拉曼激光输出.该实验结果已应用于差分吸收激光雷达的NO2探测.     

Modeling the performance of direct-detection Doppler lidar systems including cloud and solar background variability

Previous modeling of the performance of spaceborne direct-detection Doppler lidar systems assumed extremely idealized atmospheric models. Here we develop a technique for modeling the performance of these systems in a more realistic atmosphere, based on actual airborne lidar observations. The resulting atmospheric model contains cloud and aerosol variability that is absent in other simulations of spaceborne Doppler lidar instruments. To produce a realistic simulation of daytime performance, we include solar radiance values that are based on actual measurements and are allowed to vary as the viewing scene changes. Simulations are performed for two types of direct-detection Doppler lidar system: the double-edge and the multichannel techniques. Both systems were optimized to measure winds from Rayleigh backscatter at 355 nm. Simulations show that the measurement uncertainty during daytime is degraded by only approximately 10-20% compared with nighttime performance, provided that a proper solar filter is included in the instrument design.