Aerosol Backscatter Measurements Using a Compact CO2 Lidar Sensor

The subject of this paper is an estimation of possibility of gas analysis by differential absorption lidars (DIAL) based on gas lasers of the middle IR spectrum range. The potential of lidar systems based on CO2 laser with radiation frequency converter for ground and onboard sensing of atmospheric water vapor and carbonic oxide concentration profiles is analyzed. Possibilities of NO and NO2 emission detection in ground atmospheric layers using converted frequencies of CO and CO2 laser radiation in onboard DIAL are discussed. Absorption lines for methane and ammonia sensing by lidar system based on tunable TEA CO2 laser with frequency converter has chosen. The results of an estimation of methane leakage detection from pipelines by onboard lidar are submitted. Applicability of the DF laser in onboard DIAL for a control of atmospheric gases is reported.

: Recent improvements in mechanical and optical instrumentation precision have occurred. Therefore, the removal of meteorologically caused systematic errors is now a major consideration in stellar position accuracy and in the repeatability of arrangement in positions taken during nightly observations. The two problems singled out in this report are absolute water vapor content and temperature structure measurements. The study revealed that (1) estimates of water vapor distribution with an accuracy of approximately 20% would routinely correct position errors of 0.01 of angle and (2) that knowledge of the temperature structure with an accuracy of 0.5K would remove the systematic errors caused by isopycnic tilts, which range from 0.01 up to 0.1 of angle at extreme zenith viewing angles. The study showed that a remote-sensing solution to the measurement problems of the U.S. Naval Observatory in Washington, DC, is provided by LIDAR (Light Detection and Ranging). Two major LIDAR systems that fulfill the Observatory's needs are based on Raman and DIAL (Differential Absorption Lidar) techniques. A study and review of experimental system using the DIAL and Raman techniques indicate tht a DIAL system is the recommended approach for simultaneous measurement of water vapor density and temperature. A DIAL system should be constructed to obtain ground-based vertical water vapor and temperature measurements. Further, alexandrite should be used as the lasing medium for the LIDAR system.

The LIDAR (LIght Detection And Ranging) technique has developed into one of the practical high performance techniques for conducting air quality and meteorological measurements. DIfferential Absorption Lidar (DIAL) is used for measuring trace gases, including pollutants like ozone at low concentrations. Multiple wavelengths backscatter and depolarization measurements give information about particles in the atmosphere, and single wavelength lidar can be used for meteorological measurements like wind velocity and temperature. Range resolved in situ data obtained by lidar can play a significant role in our understanding of the air quality in the planetary boundary layer, particularly when applied in conjunction with air quality models. This is because DIAL can be used to measure concentrations in three dimensions in real time with a spatial resolution that corresponds well to that used in the model calculations. Thus one obtains an advantage when comparing with point measurements at or near ground level which are often perturbed by local emissions. A summary of the lidar principle is presented here, followed by different examples of vertical ozone profiles and time series obtained with a new optical layout of the EPFL-LPAS DIAL system using dual telescope detection. These data were obtained during the summer 96 field campaign in the Geneva area. Results are then compared with the mesoscale Eulerian model calculations performed in our laboratory. The overall results provide new insight into air pollution dynamics in the Geneva area and calculations are under way, using the model adjusted by the measurements, to optimize air pollution abatement strategies under certain atmospheric conditions in this part of Switzerland.

Absorption resonances in the ν2 band of D2O in near coincidence with CO2 laser lines are reported and assigned. Optical pumping induces strong laser emission at new submillimeter wavelengths, confirming the assignments. Acousto−optic modulation is used to tune the R (22) line of the CO2 laser into coincidence with the 000,533−010,422 D2O transition. Its center frequency is 1079.8628 cm−1; the linewidth (HWHM) due to collisional broadening is 13 MHz/Torr. D2S and HDS transitions in near coincidence with CO2 laser lines are also reported.

The Stark effect in ammonia has been theoretically and experimentally analyzed using lead salt tunable diode laser absorption spectroscopy and CO2 laser absorption spectroscopy of several absorption lines around 1050 cm−1 applied to an all-optical sensor for measuring of electric field strength. Measurements of the Stark splitting effect of theaR(5,K) ammonia lines forK=1–5 as well as for the sR(3,K) lines forK=0–3 have been made at Doppler broadening pressures and for several different electric field strengths. Theoretical electric field dependent energy levels have been evaluated by diagonalization of a 6×6 energy matrix constructed using both electric field independent and dependent terms. From the theoretical analysis the resolution can be predicted and optimized both in the Doppler broadened and in the pressure broadened regimes. The predicted resolution is 0.5% at an electric field strength of 20 kV/cm. The theoretical calculations and the experimental data recorded with the tunable diode laser system were compared with independent measurements made with a CO2 laser system. The agreement between experimentally recorded and theoretically calculated spectra is good which indicates that the theoretical model is satisfactory for our purposes. The contribution from the normally forbidden ssR(5, 3) ammonia line to the absorption at theP(12) CO2 laser line in the 9 μm band is briefly discussed.

: The report describes laboratory water vapor absorption measurements at CO laser frequencies. A companion report (AD-760 140) which is being issued at the same time includes a more extensive discussion of the experimental methods which are common to the measurements in this report.

Experimental coincidences between CO laser lines and seventeen NO lines have been observed. Three important parameters, the separation between each laser line and the nearest NO line (Deltanu), the pressure broadening parameter (gammaL degrees ), and the line strength ((Supsilon,J)), have been determined for each laser wavelength using the best match between experimental absorption data and theoretical calculations. These parameters are compared, where possible, with previously published results. The measured data are important for pollution monitoring systems employing tunable lasers in a direct absorption approach.

Atmospheric measurements of nitric acid are relevant both in the troposphere, where it is a precursor of inorganic free radicals as well as of nitrates and aerosols1, and in the stratosphere2, where it is a principal repository of odd nitrogen3,4. Single frequency laser detection offers the possibility of distinguishing nitric acid from other atmospheric species if the effective extinction coefficient for a given laser line is known; such information on more than one line heightens the discrimination. Recently, the constants of the Hamiltonian for this band have been published5, so that the transition frequencies and relative line strengths may be calculated. This information has been combined with transmittance data at four different CO laser frequencies which are coincident with the v2 band of nitric acid such that the extinction coefficient is determined for each laser frequency. In addition, the b-component of the transition moment for the band is found at each CO laser line used; these values are consistent among themselves and overlap the earlier determination of Goldman et al. 6 within the quoted uncertainties.

One potential approach to monitoring atmospheric pollutants is the use of differential absorption lidar (DIAL). In this technique two laser pulses are transmitted: one laser pulse on a resonant absorption of the species being monitored and the other off resonance. The CO2 TEA laser has been successfully used to obtain path-averaged concentration measurements using a retroreflector or a topographical target1. The usefulness of this technique would be greatly increased by using a distributed scatterer such as the naturally occurring atmospheric aerosols. A CO2 TEA laser can provide sufficient peak power to perform this task; however data collection is limited by the low PRF's (1-100Hz) available. A possible alternative is a low pressure Q-switched CO2 laser in conjunction with a heterodyne receiver such as is currently being used for existing CO2 laser radar systems2. Heterodyne detection lowers the required transmitter power while the high PRF (20-50kHz) of a Q-switched laser reduces the data collection time. Such a system designed for DIAL operation could provide range-resolved blanket area coverage with high up-date rates (~1 per min.).

An ozone (O(3)) gas sensor with a sensitivity of parts per 10(9) (ppb) level and a high level of selectivity based on the resonant photoacoustic effect was developed using an electronically modulated cw CO(2) laser beam. Quite different from the standard chopper modulation of a laser beam, here the laser source was electronically modulated to overcome the inherent problem of frequency instability associated with chopper modulation. With electronic modulation, in conjunction with the fast Fourier transform (FFT) of transient signals, we were able to improve significantly the sensitivity of the photoacoustic (PA) system for the detection of O(3). In addition to the improved sensitivity, our method proved that the FFT of a laser modulated PA signal could suppress the noise signal generated by spurious window diffused absorption, which in the case of most commonly used lock-in techniques is rather unavoidable. The dependence of the PA signal on various experimental parameters such as buffer gas, laser power, modulation frequency, and trace gas concentration was investigated. In the case of buffer gas, argon proved to be more suitable than nitrogen and helium in terms of enhancing the sensitivity of the system. The limits of detection of O(3) using the 9 P(14) CO(2) laser line in our PA system are 5 parts per 10(9) by volume (ppbv) and 14 ppbv with electronic and standard chopper modulation, respectively. This detection limit of O(3) is quite applicable for detection of safe levels of O(3), at ground level.

By using an acoustooptic modulator we extend the 300 MHz tunability of a waveguide CO2 laser to 480 MHz. The CD3OH was optically pumped by the 10R(32), 10R(34), and 10R(36) CO2 laser lines, and 17 new FIR laser lines were discovered. The Stark effect on previously known FIR laser lines was investigated, and some tentative FIR laser lines assignments are suggested.

A two-wavelength CO2 laser designed for atmospheric pollution measurements has been used for determining the absorption coefficients of thirteen gas pollutants at CO2 laser lines between 9.2 μm and 10.8 μm. All measurements were done with the pollutant at a low pressure in the presence of 1 atm of air. Sensitivities of about 5 ppb for a 1-km path have been obtained for ethylene.

Laser magnetic resonance spectroscopy of the 2-0 overtone band of ClO at 6.0 μm

Hyperfine and Λ-doubling parameters for the v = 1 state of NO from infrared-radiofrequency double resonance

Atmospheric CO2 plays an important role in controlling climate change and its effect on the carbon cycle. However, detailed information on the dynamics of CO2 vertical mixing remains lacking, which hinders the accurate understanding of certain key features of the carbon cycle. Differential absorption lidar (DIAL) is a promising technology for CO2 detection due to its characteristics of high precision, high time resolution, and high spatial resolution. Ground-based CO2-DIAL can provide the continuous observations of the vertical profile of CO2 concentration, which can be highly significant to gaining deeper insights into the rectification effect of CO2, the ratio of respiration photosynthesis, and the CO2 dome in urban areas. A set of ground-based CO2-DIAL systems were developed by our team and highly accurate long-term laboratory experiments were conducted. Nonetheless, the performance suffered from low signal-to-noise ratio (SNR) in field explorations because of decreasing aerosol concentrations with increasing altitude and surrounding interference according to the results of our experiments in Wuhan and Huainan. The concentration of atmospheric CO2 is derived from the difference of signals between on-line and off-line wavelengths; thus, low SNR will cause the superimposition of the final inversion error. In such a situation, an efficient and accurate denoising algorithm is critical for a ground-based CO2-DIAL system, particularly in field experiments. In this study, a method based on lifting wavelet transform (LWT) for CO2-DIAL signal denoising was proposed. This method, which is an improvement of the traditional wavelet transform, can select different predictive and update functions according to the characteristics of lidar signals, thereby making it suitable for the signal denoising of CO2-DIAL. Experiment analyses were conducted to evaluate the denoising effect of LWT. For comparison, ensemble empirical mode decomposition denoising was also performed on the same lidar signal. In addition, this study calculated the coefficient of variation (CV) at the same altitude among multiple original signals within 10 min and then performed the same calculation on the denoised signal. Finally, high-quality signal of ground-based CO2-DIAL was obtained using the LWT denoising method. The differential absorption optical depths of the denoised signals obtained via LWT were calculated, and the profile distribution information of CO2 concentration was acquired during field detection by using our developed CO2-DIAL systems.