Coherent differential absorption lidar measurements of CO2

A differential absorption lidar has been built to measure CO2 concentration in the atmosphere. The transmitter is a pulsed single-frequency Ho:Tm:YLF laser at a 2.05-microm wavelength. A coherent heterodyne receiver was used to achieve sensitive detection, with the additional capability for wind profiling by a Doppler technique. Signal processing includes an algorithm for power measurement of a heterodyne signal. Results show a precision of the CO2 concentration measurement of 1%-2% 1sigma standard deviation over column lengths ranging from 1.2 to 2.8 km by an average of 1000 pulse pairs. A preliminary assessment of instrument sensitivity was made with an 8-h-long measurement set, along with correlative measurements with an in situ sensor, to determine that a CO2 trend could be detected.     

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.     

Effect of speckle on lidar pulse-pair ratio statistics

The ratio of temporally adjacent lidar pulse returns is commonly used in differential absorption lidar (DIAL) to reduce correlated noise. These pulses typically are generated at different wavelengths with the assumption that the dominant noise is common to both. This is not the case when the mean number of laser speckle integrated per pulse by the lidar receiver is small (namely, less than 10 speckles at each wavelength). In this case a large increase in the standard deviation of the ratio data results. We demonstrate this effect both theoretically and experimentally. The theoretical value for the expected standard deviation of the pulse-pair ratio data compares well with the measured values that used a dual CO(2) laser-based lidar with a hard target. Pulse averaging statistics of the pulse-pair data obey the expected varsigma(1)/ radicalN reduction in the standard deviation, varsigma(N), for N-pulse averages. We consider the ratio before average, average before ratio, and log of the ratio before average methods for noise reduction in the lidar equation. The implications of our results are discussed in the context of dual-laser versus single-laser lidar configurations.     

Rotational vibrational-rotational Raman differential absorption lidar for atmospheric ozone measurements: methodology and experiment

A single-laser Raman differential absorption lidar (DIAL) for ozone measurements in clouds is proposed. An injection-locked XeCl excimer laser serves as the radiation source. The ozone molecule number density is calculated from the differential absorption of the anti-Stokes rotational Raman return signals from molecular nitrogen and oxygen as the on-resonance wavelength and the vibrational-rotational Raman backscattering from molecular nitrogen or oxygen as the off-resonance wavelength. Model calculations show that the main advantage of the new rotational vibrational-rotational (RVR) Raman DIAL over conventional Raman DIAL is a 70-85% reduction in the wavelength-dependent effects of cloud-particle scattering on the measured ozone concentration; furthermore the complexity of the apparatus is reduced substantially. We describe a RVR Raman DIAL setup that uses a narrow-band interference-filter polychromator as the lidar receiver. Single-laser ozone measurements in the troposphere and lower stratosphere are presented, and it is shown that on further improvement of the receiver performance, ozone measurements in clouds are attainable with the filter-polychromator approach.     

Error analysis of DIAL measurements of ozone by a Shuttle excimer lidar

An error analysis of DIAL (differential absorption lidar) measurements of stratospheric ozone from the Space Shuttle is discussed. A transmitter system consisting of a KrF excimer laser pumping gas cells of H2 or D2 producing output wavelengths in the near UV is shown to be useful for the measurement of ozone in a 15-50-km altitude range.     

Sensitivity analysis of differential absorption lidar measurements in the mid-infrared region

The availability of new laser sources that are tunable in the IR spectral region opens new perspectives for differential absorption lidar (DIAL) measurements. A region of particular interest is located in the near IR, where some of the atmospheric pollutants have absorption lines that permit monitoring of emissions from industrial plants and in urban areas. In DIAL measurements, the absorption lines for the species to be measured must be carefully chosen to prevent interference from other molecules, to minimize the dependence of the absorption cross section on temperature, and to optimize the measurements with respect to the optical depth. We analyze the influence of these factors and discuss a set of criteria for selecting the best pairs of wavelengths (lambda(on) and lambda(off)) to be used in DIAL measurements of several molecular species (HCl, CO, CO(2), NO(2), CH(4), H(2)O, and O(2)). Moreover, a sensitivity study has been carried out for selected lines in three different regimes: clean air, urban polluted air, and emission from an incinerator stack.     

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.     

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.     

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.     

Frequency-agile bandpass filter for direct detection lidar receivers

We discuss the development of a frequency-agile receiver for CO(2) laser-based differential absorption lidar (DIAL) systems. The receiver is based on the insertion of a low-order tunable etalon into the detector field of view. The incorporation of the etalon into the receiver reduces system noise by decreasing the instantaneous spectral bandwidth of the IR detector to a narrow wavelength range centered on the transmitted CO(2) laser line, thereby improving the overall D* of the detection system. A consideration of overall lidar system performance results in a projected factor of a 2-7 reduction in detector system noise, depending on the characteristics of the environment being probed. These improvements can play a key role in extending the ability of DIAL systems to monitor chemical releases from long standoff distances.     

An a posteriori method based on photo-acoustic cell information to correct for lidar transmitter spectral shift: application to atmospheric CO(2) differential absorption lidar measurements

An a posteriori corrective method based on photo-acoustic cell (PAC) information is proposed to correct for laser transmitter spectral shift during atmospheric CO(2) measurements by 2 microm heterodyne differential absorption lidar (HDIAL) technique. The method for using the PAC signal to retrieve the actual atmospheric CO(2) absorption is presented in detail. This issue is tackled using a weighting function. The performance of the proposed corrective method is discussed and the various sources of error associated with the PAC signal are investigated. For 300 shots averaged and a frequency shift (from the CO(2) absorption line center) lower than the CO(2) absorption line half-width, the relative error on HDIAL CO(2) mixing ratio measurements is lower than 1.3%. The corrective method is validated in absolute value by comparison between HDIAL and in situ sensor measurements of CO(2).     

Edge technique Doppler lidar wind measurements with high vertical resolution

We have developed a Doppler lidar system using the edge technique and have made atmospheric lidar wind measurements. Line-of-sight wind profiles with a vertical resolution of 22 m have a standard deviation of 0.40 m /s for a ten-shot average. Day and night lidar measurements of the vector wind have been made for altitudes from 200 to 2000 m. We validated the lidar measurements by comparing them with independent rawinsonde and pilot balloon measurements of wind speed and direction. Good agreement was obtained. The instrumental noise for these data is 0.11 m /s for a 500-shot average, which is in good agreement with the observed minimum value of the standard deviation for the atmospheric measurements. The average standard deviation over 30 mins varied from 1.16 to 0.25 m /s for day and night, respectively. High spatial and temporal resolution lidar profiles of line-of-sight winds clearly show wind shear and turbulent features at the 1 -2-m /s level with a high signal-to-noise ratio and demonstrate the potential of the edge-technique lidar for studying turbulent processes and atmospheric dynamics.     

Effect of the differential geometric form factor on differential absorption lidar measurements with topographical targets

When the integrated differential absorption lidar (DIAL) technique is used to perform concentration measurements over long distances, the alignment between on and off laser beams becomes important. Here, through analysis of alignment error and of the corresponding differential geometric form factor, the effect of misalignment between off and on lines on performance of integrated concentration measurements by a coaxial DIAL system is considered.     

Rocketborne Rayleigh lidar for in situ measurements of neutral atmospheric density

We describe the design of a small Rayleigh scattering lidar for launch on a sounding rocket as well as the first, to our knowledge, in situ measurements of neutral number density performed with a rocketborne lidar in the mesosphere. The aim of the experiment is to study the dynamics of the neutral atmosphere with emphasis on turbulent structures and gravity waves. The altitude resolution of the density profile is better than 10 m. The uncertainty is 0.3% below 55 km and better than 1% to an altitude of 65 km. The lidar technique meets the requirement of measurement of total molecular density outside the shock front surrounding the supersonic payload, which is necessary for precision measurements of neutral atmospheric density. We have compared different component technologies and design approaches and show performance calculations for two electro-optical systems. The first system has laser and detector components that were available in 1993, the second has new solutions that became available in 1995. The second system has a signal-to-noise ratio that is five times higher than the first and employs a pulsed high-power laser diode array as the transmitter and a large-area avalanche photodiode as the receiver.     

Ground-based differential absorption lidar for water-vapor profiling: assessment of accuracy, resolution, and meteorological applications

The accuracy and the resolution of water-vapor measurements by use of the ground-based differential absorption lidar (DIAL) system of the Max-Planck-Institute (MPI) are determined. A theoretical analysis, intercomparisons with radiosondes, and measurements in high-altitude clouds allow the conclusion that, with the MPI DIAL system, water-vapor measurements with a systematic error of <5% in the whole troposphere can be performed. Special emphasis is laid on the outstanding daytime and nighttime performance of the DIAL system in the lower troposphere. With a time resolution of 1 min the statistical error varies between 0.05 g/m(3) in the near range using 75 m and-depending on the meteorological conditions-approximately 0.25 g/m(3) at 2 km using 150-m vertical resolution. When the eddy correlation method is applied, this accuracy and resolution are sufficient to determine water-vapor flux profiles in the convective boundary layer with a statistical error of <10% in each data point to approximately 1700 m. The results have contributed to the fact that the DIAL method has finally won recognition as an excellent tool for tropospheric research, in particular for boundary layer research and as a calibration standard for radiosondes and satellites.     

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.     

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.     

Aerosol-profile measurements in the lower troposphere with four-wavelength bistatic argon-ion lidar

The scattering properties of aerosols over a tropical urban station, Pune, India, (18° 32' N, 73° 51' E, 559 m above mean sea level), are studied with a bistatic, multiwavelength, continuous-wave, argon-ion lidar. The scattered-intensity profiles (up to 1000 m above ground level) measured at four wavelengths (0.4765, 0.4880, 0.4965, and 0.5145 μm) of the laser during November 1987-March 1990 revealed certain spectral dependence, in conformity with the Mie theory of aerosol particles. Methods for retrieving the bulk as well as the height variation of aerosol-size distribution from the inversion of angular distribution of scattered-light-intensity measurements from a constant altitude and scattered-intensity verticalprofile measurements at different wavelengths are explained. Results obtained from these approaches are presented and compared with results reported by other investigators. The deviations in the results are discussed in relation to the assumptions involved and the terrain-atmospheric conditions at the experimental station. It is found that the aerosol-size-distribution parameter is altitude dependent beside its dependence on refractive index and wavelength of incident radiation. The results of the study suggest that the information content from bistatic, multiwavelength laser scattering measurements is useful for inferring aerosol-size distribution.     

Two-micrometer heterodyne differential absorption lidar measurements of the atmospheric CO2 mixing ratio in the boundary layer

A 2 microm heterodyne differential absorption lidar (HDIAL) has been operated at the Inst?tut Pierre Simon Laplace, Laboratoire de Météorologie Dynamique (Paris) to monitor the CO(2) mixing ratio in absolute value at high accuracy in the atmospheric boundary layer. Horizontal measurements at increasing range are made to retrieve the optical depth. The experimental setup takes advantage of a heterodyne lidar developed for wind velocity measurements. A control unit based on a photoacoustic cell filled with CO(2) is tested to correct afterward for ON-line frequency drift. The HDIAL results are validated using in situ routine measurements. The Doppler capability is used to follow the change in wind direction in the Paris suburbs.     

Three-wavelength dual differential absorption lidar method for stratospheric ozone measurements in the presence of volcanic aerosols

We present a three-wavelength dual differential absorption lidar (dual-DIAL) method with which one can obtain an accurate stratospheric ozone profile in the presence of volcanic aerosols. Results of theoretical analysis and comparisons with conventional DIAL and backscatter correction methods show that the three-wavelength dual-DIAL method greatly reduces the effect of volcanic aerosol on stratospheric ozone measurements, and system errors that are due to aerosols are kept at a low level. In addition this method is almost completely insensitive to wavelength dependence of aerosol backscatter, its spatial change, and spatial inhomogeneity of aerosol loading. Therefore, one does not need to know detailed information about these aerosol properties, and accurate stratospheric ozone profiles can be obtained directly from lidar return signals. An example of the experimental result indicates that the proposed method is effective.