CO(2) DIAL measurements of water vapor

Evaluation of a coherent 2-µm differential absorption lidar for water vapor and radial wind velocity measurements

The performance of a coherent 2-µm differential absorption lidar (DIAL) for simultaneously measuring water vapor (H₂O) and radial wind velocity was evaluated. For measuring H₂O, a wavelength locking technique was applied to the H₂O-DIAL system. The H₂O-DIAL system was evaluated under summer daytime conditions in Tokyo, Japan. H₂O-DIAL measurements were compared with measurements from radiosondes. The H₂O-DIAL-derived volumetric humidity values agreed with the radiosonde-derived values over the range from 11 to 20 g/m³ with a correlation coefficient of 0.81 and a root-mean-square difference of 1.46 g/m³. Comparisons between the H₂O-DIAL and the in-situ surface meteorological sensors demonstrated the simultaneous measurement of H₂O and radial wind velocity.

Long-wave infrared multi-wavelength optical source for standoff detection of chemical warfare agents

We have designed and built a wavelength-tunable optical source for standoff detection of gaseous chemicals by differential absorption spectrometry in the long-wave infrared. It is based on a nanosecond 2 µm single-frequency optical parametric oscillator, whose idler wave is amplified in large aperture Rb:PPKTP crystals. The signal and idler waves are mixed in ZnGeP₂ crystals to produce single-frequency tunable radiation in the 7.5-10.5 µm range. The source was integrated into a direct detection lidar to measure sarin and sulfur mustard inside a closed chamber, in an integrated path configuration with a noncooperative target.

Wavelength selection and measurement error theoretical analysis on ground-based coherent differential absorption lidar using 1.53 µm wavelength for simultaneous vertical profiling of water vapor density and wind speed

A feasibility study of coherent differential absorption lidar is conducted using a 1.53-µm wavelength for simultaneously retrieving the water vapor density and wind speed profiles. We selected the ON and OFF wavelengths to be 1531.383 and 1531.555 nm, respectively, for minimizing the effect of the temperature change in the atmosphere. The systematic measurement error can be reduced to below 5% by stabilizing the ON wavelength from ${-64}$-64 to 102 MHz around the center of the water vapor absorption line. Analysis of the speckle and photon statistics errors reveal that the relative error of the water vapor density is less than 10% at the altitude from 0.1 to 1.7 km with the 100 m range resolution with 10 min data accumulation time. The simultaneous measurement of wind speed and direction can also be achieved by employing a conical scan mechanism.

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.

Wave optics simulation of atmospheric turbulence and reflective speckle effects in CO2 lidar

Laser speckle can influence lidar measurements from a diffuse hard target. Atmospheric optical turbulence will also affect the lidar return signal. We present a numerical simulation that models the propagation of a lidar beam and accounts for both reflective speckle and atmospheric turbulence effects. Our simulation is based on implementing a Huygens-Fresnel approximation to laser propagation. A series of phase screens, with the appropriate atmospheric statistical characteristics, are used to simulate the effect of atmospheric turbulence. A single random phase screen is used to simulate scattering of the entire beam from a rough surface. We compare the output of our numerical model with separate CO(2) lidar measurements of atmospheric turbulence and reflective speckle. We also compare the output of our model with separate analytical predictions for atmospheric turbulence and reflective speckle. Good agreement was found between the model and the experimental data. Good agreement was also found with analytical predictions. Finally, we present results of a simulation of the combined effects on a finite-aperture lidar system that are qualitatively consistent with previous experimental observations of increasing rms noise with increasing turbulence level.

Backscattering measurements of atmospheric aerosols at CO2 laser wavelengths: implications of aerosol spectral structure on differential-absorption lidar retrievals of molecular species

The volume backscattering coefficients of atmospheric aerosol were measured with a tunable CO2 lidar system at various wavelengths in Utah (a desert environment) along a horizontal path a few meters above the ground. In deducing the aerosol backscattering, a deconvolution (to remove the smearing effect of the long CO2 lidar pulse and the lidar limited bandwidth) and a constrained-slope method were employed. The spectral shape beta(lambda) was similar for all the 13 measurements during a 3-day period. A mean aerosol backscattering-wavelength dependence beta(lambda) was computed from the measurements and used to estimate the error Delta(CL) (concentration-path-length product) in differential-absorption lidar measurements for various gases caused by the systematic aerosol differential backscattering and the error that is due to fluctuations in the aerosol backscattering. The water-vapor concentration-path-length product CL and the average concentration C = <CL>/L for a path length L computed from the range-resolved lidar measurements is consistently in good agreement with the water-vapor concentration measured by a meteorological station. However, I was unable to deduce, reliably, the range-resolved water-vapor concentration C(r), which is the derivative of the range-dependent product CL, because of the effect of residual noise caused mainly by errors in the deconvolved lidar measurements.

Ground-based differential absorption lidar for water-vapor and temperature profiling: development and specifications of a high-performance laser transmitter

An all-solid-state laser transmitter for a water-vapor and temperature differential absorption lidar (DIAL) system in the near infrared is introduced. The laser system is based on a master-slave configuration. As the slave laser a Q-switched unidirectional alexandrite ring laser is used, which is injection seeded by the master laser, a cw Ti:sapphire ring laser. It is demonstrated that this laser system has, what is to my knowledge, the highest frequency stability (15 MHz rms), narrowest bandwidth (<40 MHz), and highest spectral purity (>99.99%) of all the laser transmitters developed to date in the near infrared. These specifications fulfill the requirements for water-vapor measurements with an error caused by laser properties of <5% and temperature measurements with an error caused by laser properties of <1 K in the whole troposphere. The specifications are maintained during long-term operation in the field. The single-mode operation of this laser system makes the narrow-band detection of the DIAL backscatter signal possible. Thus the system has the potential to be used for accurate temperature measurements and for simultaneous DIAL and Doppler wind measurements.

Quantitative chemical identification of four gases in remote infrared (9-11 mum) differential absorption lidar experiments

A combined experimental and computational approach utilizing tunable CO(2) lasers and chemometric analysis was employed to detect chemicals and their concentrations in the field under controlled release conditions. We collected absorption spectra for four organic gases in the laboratory by lasing 40 lines of the laser in the 9.3-10.8-mum range. The ability to predict properly the chemicals and their respective concentrations depends on the nature of the target, the atmospheric conditions, and the round-trip distance. In 39 of the 45 field experiments, the identities of the released chemicals were identified correctly without predictions of false positives or false negatives.

Temperature dependence of water vapor absorption coefficients for CO(2) differential absorption lidars

A temperature correction of water vapor differential absorption coefficients for the CO(2) transition line pairs (10R20, 10R18) and (10R20, 10R22) for temperatures between -0.5 °C and 20 °C is computed, with a reference temperature of 27 °C, from medium-range CO(2) lidar field measurements. The empirical temperature correction, X(T), is fitted with the polynomial X(T) = α(0) + α(1) × T + α(2) × T(2). For the transition line pair (10R20, 10R18) the temperature dependence ranges from 1.62%/°C to 3.47%/°C, and the temperature correction for the transition line pair (10R20, 10R22) ranges from 1.32%/°C to 2.43%/°C.

Airborne remote sensing of tropospheric water vapor with a near-infrared differential absorption lidar system

A near-infrared airborne differential absorption lidar (DIAL) system has become operational. Horizontal and vertical water vapor profiles of the troposphere during summer (nighttime) conditions extending from the top of the planetary boundary layer (PBL) up to near the tropopause are investigated. These measurements have been performed in Southern Bavaria, Germany. The system design, the frequency control units, and an estimation of the laser line profile of the narrow-band dye laser are discussed. Effective absorption cross sections in terms of altitude are calculated. Statistical and systematic errors of the water vapor measurements are evaluated as a function of altitude. The effect of a systematic range-dependent error caused by molecular absorption is investigated by comparing the DIAL data with in situ measurements. Typical horizontal resolutions range from 4 km in the lower troposphere to 11 km in the upper troposphere, with vertical resolutions varying from 0.3 to 1 km, respectively. The lower limit of the sensitivity of the water vapor mixing ratio is calculated to be 0.01 g/kg. The total errors of these measurements range between 8% and 25%. A sine-shaped wave structure with a wavelength of 14 km and an amplitude of 20% of its mean value, detected in the lower troposphere, indicates an atmospheric gravity wave field.

Tunable 2.1-,microm Ho lidar for simultaneous range-resolved measurements of atmospheric water vapor and aerosol backscatter profiles

An eye-safe, tunable differential-absorption lidar system has been developed for the range-resolved measurement of aerosol backscatter and water vapor in the atmosphere. The lidar uses a flash-lamp-pumped, qswitched, 10-mJ solid-state Ho:YSGG laser that is continuously tunable over a 20cm(-1) wavelength range near 2.084 microm. Both path-averaged and range-resolved measurements were performed with the Ho differential-absorption lidar system. Preliminary measurements have been made of the temporal variation of atmospheric aerosol backscatter and water-vapor profiles at ranges out to 1 km. These results indicate that the Ho lidar has the potential for the eye-safe remote sensing of atmospheric water vapor and backscatter profiles at longer ranges if suitably enhanced in laser power and laser linewidth.

Water vapor absorption coefficients in the 8-13-microm spectral region: a critical review

Measurements of water vapor absorption coefficients in the thermal IR atmospheric window (8-13 microm) during the past 20 years obtained by a variety of techniques are reviewed for consistency and are compared with computed values based on the AFGL spectral data tapes. The methods of data collection considered were atmospheric long path absorption with a CO(2) laser or a broadband source and filters, a White cell and a CO(2) laser or a broadband source and a spectrometer, and a spectrophone with a CO(2) laser. Advantages and disadvantages of each measurement approach are given as a guide to further research. Continuum absorption has apparently been measured accurately to about the 5-10% level in five of the measurements reported. However, the effect of oxygen broadening has not been fully considered, since most laboratory measurements were made using nitrogen buffering. Oxygen could lead to a small reduction in the adopted value of the water vapor continuum absorption coefficient in air. Also, the temperature dependence does not seem to have been measured well for temperatures <20 degrees C. The rotational and v(2) line absorption coefficients do not appear to have been determined well in this spectral region except at CO(2) laser line frequencies, because the agreement between such measurements and the AFGL spectral data tapes is generally not good.

Airborne and spaceborne lidar measurements of water vapor profiles: a sensitivity analysis

This paper presents an evaluation of the random and systematic error sources associated with differential absorption lidar (DIAL) measurements of tropospheric water vapor (H(2)O) profiles from airborne and spaceborne platforms. The results of this analysis are used in the development and performance evaluation of the Lidar Atmospheric Sensing Experiment (LASE) H(2)O DIAL system presently under development at the NASA Langley Research Center for operation on a high altitude ER-2 (advanced U-2) aircraft. The analysis shows that a <10% H(2)O profile measurement accuracy is possible for the LASE system with a vertical and horizontal resolution of 200 m and 10 km, respectively, at night and 300 m and 20 km during the day. Global measurements of H(2)O profiles from spaceborne DIAL systems can be made to a similar accuracy with a vertical resolution of 500 m and a horizontal resolution of 100 km.

Monte Carlo computer simulations of ground-based and space-based coherent DIAL water vapor profiling

Ground-based and space-based coherent DIAL water vapor measurement performance at the 2.1-microm Ho:YAG wavelength is presented using a Monte Carlo computer simulation. The stochastic simulation allowed improved modeling of lidar system, platform, atmospheric, and data processing parameter effects on performance and better understanding of their interrelationships. Results indicate that accurate water vapor measurements in the lower troposphere are potentially achievable from both ground- and space-based platforms.

Differential absorption lidar signal averaging

This paper presents experimental results using an atmospheric backscatter dual CO(2) laser differential absorption lidar (DIAL). It is shown that DIAL signals can be averaged to obtain an N(-(1/2)) dependence decrease in the standard deviation of the ratio of backscattered returns from two lasers, where N is the number of DIAL signals averaged, and that such a lidar system can make measurements of gas concentrations with a precision of 0.7% in absorptance over 75 m in a short measurement time when the signal strength is high-Factors that eventually limit the rate of improvement in the SNR, such as changes in the ratio of the absorption and/or backscatter at the two laser frequencies and background noise are discussed. In addition, it is noted that DIAL measurements made using hard-target backscatter often show departures from N((1/2)) dependence improvement in the standard deviation, because they are further limited by the combined effects of atmospheric turbulence and speckle, since the relative reproducibility of the speckle pattern on the receiver gives rise to correlations of the lidar signals.