The current level of technical development has led to increased number of anthropogenic hazards including emissions from industrial and agricultural enterprises, terrorist threats using chemical and organic components etc. An important factor in ensuring safety is the timely detection, identification, and localization of sources of danger. In this regard, remote methods, including laser ones, become more and more important allowing detecting and identifying danger in real time and at a distance from its source. The paper presents a calculation of the laser radiation absorption by water vapor for the wavelengths of CO2-laser generation lines. It is shown that absorption caused by H2O, C2H4 and NH3 at some laser generation lines can significantly affects the results of remote detection of organic components depending on the concentration of these gases in the atmosphere. These results can be useful for specialists that develop instruments and methods for remote detection of organic compounds in the atmosphere.
Substances with different absorption spectra have different backscatter spectra. If the range of sounding wavelengths is narrow, but includes evident absorption bands of an aerosol substance, then this substance can be detected from the backscattering spectrum. This is a ground for the use of the differential scattering (DICS) technique for the detection of known organic compounds in natural and anthropogenic aerosol. CO2 lasers with the wavelength tuning range 9–11 μm, which includes the absorption bands of many organic substances, are apparently the most suitable for DICS implementation. It should be borne in mind that when the imaginary part of the refractive index changes (for example, when an absorbing substance is added to a water drop), the real part of the refractive index is also changes. For large particles (r ≥ 10 μm), even small changes in the refractive index significantly affect the backscattering efficiency. The accuracy of the literature data is insufficient for preliminary (a priori) calculation of aerosol backscatter coefficients. Therefore, the development of DICS requires extensive field measurements and their complex mathematical processing with the use of machine learning algorithms. This research sets out the task of systematization of the backscattering radiation by atmospheric aerosol with different organic substances and finding out the wave lengths where the backscatter signal is higher than the selective and continuum atmospheric molecular absorption.
The nature of the pure water vapor continuum absorption in the rotational absorption band is investigated. The earlier retrieved continuum spectra at 296 and 326 K in the spectral region of 70–700 cm–1 are considered1 . The contribution of the absorption by water dimers is estimated at the level of no more than 50%. Two modern continuum models–water dimers and intermediate line wings–are jointly adopted to the new continuum spectra. It is shown that the model used allows describing the observed continuum spectra mainly in the central part of the rotational absorption band.
This paper presents results of the study of the water vapour continuum absorption within 8800 and 10600 cm-1 absorption bands. The continuum spectra recorded at increased temperatures (398, 431, and 471 K) and pressures are discussed on the basis of water dimer model1 supplemented by a contribution of intermediate line wings according to the model2 . The values of semiempirical parameters that characterize contribution and effective spectral width of the intermediate line wings are derived from fitting to the experimental continuum absorption spectra. A preliminary analysis of the temperature and pressure dependences of the fitted parameter is performed. It is shown that a better agreement between the simulated and experimental continuum spectra can be reached both within centre and in close wings of the absorption bands when intermediate line wings are accounted according to the model2 .
In this paper we investigate the water vapour continuum absorption retrieved from experimental high-resolution Fourier-Transform-Spectroscopy spectra, obtained earlier from high-temperature and pressure pure water vapour measuremets. The retrieved spectra of the water vapour continuum, related to the least-studied today near-IR absorption bands at 8800 and 10600 cm-1, are described on the basis of the water dimer hypothesis of the continuum. It is shown that water dimers should cause, at least, half of the water vapour continuum absorption in the studied spectral region. The temperature dependence of the effective dimerization constant is derived.
Potentialities of detection of minor gases characterizing the biosystem-atmosphere gas exchange, such as C2H4, N2O,
NH3, H2O2, using a gas-analyzer based on waveguide CO2 laser with discrete frequency tuning are studied numerically.
The measured signal is recorded over the whole laser frequency scan for 9-μm or 10-μm transitions, i.e. for 25-30
generation lines. An effect of interference of the main gaseous atmospheric components H2O and CO2 on the minor gas
detection limit and the uncertainty in the retrieved concentration is analyzed. Absorption coefficients of atmospheric
gases at CO2 laser lines are calculated using the HITRAN-2004 spectral database and the CKD water vapor continuum
model. Aerosol effect is not taken into account.
The development of the techniques for spaceborne detecting gas anomalies in the lower atmosphere is very important. The results of the Kioto protocol, an extended use of hydrocarbon raw material, the occurrence of new chemical emissions call for further realization of global control over gaseous contaminations in the atmosphere. A spaceborne location of sensors is very promising for solving this problem. In this case the light automatic satellites, oriented to a limited area of application, are the most promising. As a rule, for such satellites the orbits at 500 km altitude and more are selected. This altitude and small mass of a satellite impose severe requirements on the efficiency of the techniques and apparatus realizations. Taking account of the last-named fact, the paper describes the use of the differential absorption method with a reflection from the Earth's surface for global monitoring of gaseous contaminations. The experiments were performed to assess lidar detection of ground anomalies of hydrocarbons in the 3-5 micrometers transmittance window. It is shown that, as applied to a spaceborne platform MKA- 200, this technique provides for localization of the background concentration excess of gases of hydrocarbon cycle with an error from 15% to 25%.
The present work is aimed at quantitative estimation of the capabilities of remote detection of aerosol and gas leakages from distances 0.5 - 15 km with an airborne DF-laser lidar, placed onboard an aircraft or a helicopter and operating with a topographic target, by numerical modeling. The requirements for guiding the development of this lidar system are also formulated.
An algebraic approximation for a line halfwidth at the half- maximum ((delta) ) was obtained from the calculated values of (delta) , which depends on the collision broadening and narrowing parameters and the Doppler halfwidth. Both the hard and soft collision models describing the effect of velocity-changing collisions were approximated. The mean discrepancy between the approximated and exact values of (delta) does not exceed 0.1 divided by 0.15%. A simple criterion was developed which allows one to judge about the correspondence of the experimental dependence of (delta) on buffer gas pressure to hard or soft collision model.
The characteristics of a helicopter-borne lidar based on tunable TEA CO2 laser and its third harmonic designed for remote detecting the methane and ammonia leakages from pipe-lines are analyzed numerically. The spectral range near 3 micrometers is shown to be most promising for remote sensing of methane emissions. Parameters of radiation of the tunable pulse-periodic mini-TEA CO2 laser and generators of harmonics to be utilized in the helicopter-borne differential absorption lidar are estimated. Emissions of different gases intensity are analyzed for possible detectability at a distance up to 1 km. The use of the third harmonic of the TEA CO2 laser allows methane emissions from a pipe-line to be detected and measured with mean measurement error from 10 to 15% for methane concentrations varying from the background level to the explosion-hazardous one. The optimal pair and possibilities of the ammonia remote sensing on the base of the first harmonic of TEA CO2 laser was determined as well.
The retrieval technique for sensing the columnar content of CO2 via differential solar transmission measurements in and out absorption bands is presented. The calibration procedure based on the line-by-line method is described. The k-distribution method is used for acceleration calculation. The multiwavelength Sun-photometer has six channels for measurements in and out the H2O, CO2, CH4 absorption bands. The interferometer filters are used for spectral selection with resolution (Delta) (lambda) /(lambda) approximately equals 0.01 - 0.02. The wavelengths are centered at 0.87, 0.94, 1.06, 2.06, 2.18, 2.32 micrometers. Errors of the columnar content retrieval connected with the parametrization of atmospheric transmittance and uncertainties of the spectroscopy information are analyzed. The correction of signal on the basis of the measurements of aerosol optical depth is described.
The dialog package SAGDAM (sounding of atmospheric gases by differential absorption method) is intended for simulation of the potentialities of the DAS method for sounding water vapor and ozone on the vertical and slant directions with ground-based, airborne, and spaceborne lidars. Moreover, the package enables one to model the ground-based, airborne, and spaceborne systems for sounding the integral gas content by a long-path differential absorption method. The spectral range is near UV, visible, and infrared spectral ranges. The program calculates: a) vertical profiles of the absorption coefficients and transmittance of the gas under study and the foreign gases, as well as the transmittance of aerosol and Rayleigh atmosphere; b) spatially resolute error including random error and systematic error. The program enables one to determine the optimal pairs of wavelengths for sounding a gas in the specified altitude range and the minimum-detectable concentration of the gas under study (for the long-path method).
KEYWORDS: Absorption, Gases, Atmospheric modeling, Systems modeling, Computer programming, Interference (communication), Signal processing, Signal attenuation, Laser systems engineering, Transmittance
The dialog program LPM (long-path method) is intended for imitative modeling of the concentration gas analysis (H2O, CO2, O3, NH3, C2H4) with the path double-wave gas analyzer, equipped with two tunable CO2-lasers. Modeling is designed for four laser systems using the isotopes of CO2. While modeling the program provides: 1) the calculation of transmittance for a sounding path at wavelengths 'on-line' and 'off-line'; 2) the calculation of atmospheric signals at wavelengths 'on-line' and 'off-line'; 3) the distortion of a signal with a random number for imitation of measuring noise; 4) the calculation of gas concentration from the inverse problem while imitating the sounding of a single gas; 5) the calculation of concentration of several gases (up to five gases) by using the simultaneous signal processing for several pairs of wavelengths (imitation of sounding of several gases). In addition, the program determines: i) optimal pairs of wavelengths; ii) the gas concentration reconstruction error from the sounding data, including the measuring error and the systematic error.
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