Long-path Absorption Measurements Research Articles

Monitoring Atmospheric Atomic Mercury by Optical Techniques

Mercury is a serious neurotoxic agent, and the control and monitoring of emissions are important. Optical spectroscopy is a powerful technique for measurement of mercury, which in the atmosphere predominantly appears in atomic form. The mercury resonance line close to 254 nm can be utilized in long-path absorption measurements of average concentrations or in light detection and ranging (lidar) studies, where range-resolved concentration values can be obtained. In addition, point monitors often use optical detection, frequently after pre-concentration, as well as for mercury compounds in conjunction with denuders, which transfer the compounds into atomic mercury. The present review discusses mercury measurement methods with respect to merits and sensitivity to interference. The main focus is on remote-sensing techniques, and many examples from industrial and mining monitoring are given. Further, mercury emissions related to the extraction of geothermal energy are discussed. Finally, an example from archaeology—the Qin tomb in Xi’an—is presented. Advanced measurement techniques can help in shaping an environment largely free from mercury contamination. Further, the aspect of mercury being an important geophysical tracer gas can also be exploited.

MULTICHARME: a modified Chernin-type multi-pass cell designed for IR and THz long-path absorption measurements in the CHARME atmospheric simulation chamber

Abstract. We have developed MULTICHARME, a modified Chernin-type multi-pass cell especially designed for IR and THz long-path absorption measurements in the CHamber for Atmospheric Reactivity and Metrology of the Environment (CHARME). By measuring the output power using a near-IR diode-laser and a THz amplified multiplication chain, we have established that the effective reflectivity of MULTICHARME is better than 94 % over approximately three decades of frequency. Absorption measurements of N2O have been performed by probing highly excited rovibrational transitions in the near-IR and ground state rotational transitions at submillimeter wavelengths. In each case the linearity of the absorbance with the path lengths was verified. Finally, we demonstrate that THz spectroscopy is able to study the isotopic composition of greenhouse polar gases such as N2O and to absolutely quantify stable (N2O) and reactive (O3) species at trace levels. At low pressure the ozone concentration was continuously monitored and its decay characterized. The deduced ozone lifetime of 3.4 ± 0.1 h is shorter compared with previous measurements performed in CHARME at atmospheric pressure. For the first time, the ability of THz rotational spectroscopy to monitor, with a very high degree of selectivity, stable and reactive polar compounds at trace level in an atmospheric simulation chamber is demonstrated. However, the sensitivity of the THz monitoring needs to be improved to reach atmospheric trace levels. For this purpose, it is necessary to fully understand the origin of the observed baseline variations caused by the complex multiple standing waves present in MULTICHARME.

Optical Design of a Space Retroreflector Using a Hybrid Search

Retroreflectors on satellites are used as targets for laser ranging and laser long-path absorption measurements of atmospheric trace species. To compensate for the velocity aberration, slight tuning of the design is necessary. The curvatures of mirrors, the angles between mirrors, and the direction of the retroreflector on the satellite must be tuned to optimize the optical efficiency of the retroreflector. An enumerative scheme is not applicable when the number of parameters is large. We proposed earlier a technique using a genetic algorithm for the optical design of a space retroreflector.1) In this paper, we have improved the method introducing a hybrid technique that is a combination of a genetic algorithm and simulated annealing.

Optical monitoring of volcanic sulphur dioxide emissions—comparison between four different remote-sensing spectroscopic techniques

The emissions of sulphur dioxide from the Italian volcanoes Mt. Etna and Stromboli were studied in ship-borne underpasses of their plumes. Four different optical spectroscopy techniques were used and inter-compared. All techniques utilise the absorption signature of the gas in the wavelength region of around 300 nm. A differential absorption lidar was employed in active gas concentration assessment. In parallel, a differential optical absorption spectroscopy system (DOAS) provided spectrally resolved absorption spectra. In one configuration the DOAS used a vertically looking telescope and the absorption of the sky-light was studied, while a different DOAS implementation utilised the sun disc as the light source in slant-angle, long-path absorption measurements. Parallel measurements with the customary correlation spectroscopy method were also performed. Path length Monte Carlo simulations of the down-welling radiation through the volcanic plume at different sun altitude and azimuth angles have been performed taking into account also the effects of other geometric parameters as the plume height and extension. The results are discussed with special emphasis on systematic effects due to scattering.

Optical design of a hollow cube-corner retroreflector for a geosynchronous satellite

The optical characteristics of a single-element hollow cube-corner retroreflector for a geosynchronous satellite were numerically evaluated for laser ranging and laser long-path absorption measurements of atmospheric species. An optical design with spherical surfaces and tuned dihedral angles was considered to compensate for velocity aberrations. The parameters for the retroreflector were optimized with genetic algorithms for different retroreflector sizes and wavelengths (500 nm and 1, 3, and 10 microm). We found that 20-cm retroreflectors are sufficient for realistic measurements when the laser wavelength is 500 nm or 1 microm. However, a larger retroreflector is necessary to overcome the detector noise level at 3 and 10 microm.

Optical design of space retroreflector using genetic algorithm

Hollow retroreflectors installed on satellites are used as targets for satellite laser ranging and earth-satellite laser long-path absorption measurements. The optical characteristics of a space retroreflector are determined by several parameters, including mirror curvatures, dihedral angles, and the direction of the retroreflector on the satellite. In this paper, we propose using a genetic algorithm to design space retroreflectors. We applied this method for design of a retroreflector 50 cm in diameter. We found that the optimum design of space retroreflectors having high optical efficiency is obtained in a short time by using the genetic algorithm. © 1999 Scripta Technica, Electron Comm Jpn Pt 2, 81(12): 10–16, 1998

Laser transmitter/receiver system for earth-satellite-earth long-path absorption measurements of atmospheric trace species using the retroreflector in space

A laser transmitter/receiver system using a retroreflector in space (RIS) to experiment with earth-satellite-earth laser long-path absorption measurements of atmospheric trace species is described. The system employs two single-longitudinal-mode pulsed CO2 lasers to measure the atmospheric spectrum and a second-harmonic Nd:YAG laser for active satellite tracking using the image of the RIS reflection. Details of the laser system and the initial experiment are reported.

Heterodyne Spectroscopy Using Broadband Short-Pulse Laser for Long-Path Absorption Measurement of Atmospheric Trace Species

A concept of heterodyne spectroscopy for absorption measurements which employs a broadband short-pulse laser is presented. The absorption line structure of the sample in the spectral width of a short-pulse laser is measured with broadband heterodyne detection. The absorption spectrum can be measured, in principle, with a single laser shot, and the method is potentially useful for laser long-path absorption measurements of atmospheric trace species. Application to an airborne laser long-path absorption system for measuring CH4 is discussed.

Fast scanning laser DOAS ? A very promising technique for monitoring OH and other tropospheric trace gases

A very promising technique for time resolved local OH measurements is presented which makes use of differential optical absorption spectroscopy (DOAS). The light source is a rapidly tuned, narrowband, frequency-doubled, and power-stabilized dye laser. Due to fast scanning and power stabilization the fluctuations of the atmosphere and the light source are minimized; thus a detection limit better than 10−5 can be achieved for atmospheric long-path absorption measurements. In a first test near Frankfurt a.M. tropospheric OH was observed with concentrations ranging from 8×105 OH cm−3 to 2×106 OH cm−3. The absorption path-length was 800 m and the acquisition time 5 min. In a second generation system the performance of the apparatus was highly improved. A multiple reflection system with a special design for tropospheric open path measurements was constructed. Local OH-measurements in a compartment of only some meters with a well known chemistry are now feasible. Furthermore, a self-test technique was installed for verifying the results and to improve the reliability of tropospheric data. Further application of the experimental device will be its use in monitoring of trace gases absorbing in the ultraviolet, like SO2, and in the visible, such as NO2, to gain a more complex data set for the use with model calculations and to realize a multicomponent device.

Microprocessor-controlled laser tracker for atmospheric sensing

An optical tracking system comprising a visible HeNe laser, an imaging detector, and a microprocessor-controlled mirror, has been designed to track a moving retroreflector located up to 500 m away from an atmospheric instrument and simultaneously direct spectrally tunable infrared laser radiation to the retroreflector for double-ended, long-path absorption measurements of atmospheric species. The tracker has been tested during the recent flight of a balloon-borne tunable diode laser absorption spectrometer which monitors the concentrations of stratospheric species within a volume defined by a 0.14-m-diameter retroreflector lowered 500 m below the instrument gondola.

Two-wavelength and power-balanced oscillation of a CO2 laser for application to differential absorption measurements

A new technique of alternate two-wavelength oscillation of a CO2 laser is discussed for application to various differential absorption spectroscopic measurements. Power-balanced, two-adjacent branch oscillation using a single CO2 laser was achieved by modulating the angle of a mirror inside the laser cavity and adjusting automatically the cavity length. The two-wavelength modulation frequency was extended up to about 1.2 kHz. Line tuning and power modulation characteristics were studied. The laser was used in long-path differential absorption measurements of ethylene air pollution molecules to demonstrate the capability of this power-balanced, two-wavelength oscillation method. The minimum detectable absorption was nearly 1×10−3 in a short-path cell experiment and 3×10−3 in a long-path experiment.

Remote Fourier Transform Infrared Air Pollution Studies

A commercial Fourier transform interferometer system has been installed in a van and used to make long-path absorption and single-ended emission measurements of gaseous pollutant concentrations at a variety of pollutant sources. The system covers the infrared spectral region from 650 to 6000 cm-1 at a maximum resolution of 0.06 cm-1. In the absorption mode, many gaseous pollutants can be detected at parts per billion levels over a one kilo-meter path. In the emission mode, warm gases exiting industrial stacks can be detected at parts per million levels. The temperature of the stack gases can be determined directly from the spectra. This paper describes the interferometer system and results of recent measurements of jet engine, brick kiln, gypsum pond, and industrial stack emissions.

Remote measurement of gaseous pollutant concentrations using a mobile Fourier transform interferometer system

A commercial Fourier transform interferometer system with telescopic optics has been installed in a van and used to make long-path absorption and single-ended emission measurements of gaseous pollutant concentrations at a number of geographical locations. The system covers the IR spectra region from 650 cm(-1) to 6000 cm(-1) at a maximum resolution of 0.06 cm(-1). For many pollutants, concentrations in the 1-10-ppb range can be detected over a 1-km path length. To date, measurements have been made in the absorption mode across fertilizer plant gypsum ponds, an oil refinery, and jet engine plumes; industrial stacks, waste gas flares, and jet engine plumes have been studied in the emission mode.

Laboratory absorption strengths for the H2 /4,0/ and /3,0/ S/1/ lines

view Abstract Citations (11) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Laboratory absorption strengths for the H2 (4,0) and (3,0) S(1) lines. Trauger, J. T. ; Mickelson, M. E. ; Larson, L. E. Abstract Long-path absorption measurements of the (3,0) and (4,0) S(1) quadrupole lines of molecular hydrogen have been made, yielding an accurate experimental line strength for the (4,0) line and confirming previous estimates for the (3,0) line. The H2 (4,0) S(1) line is significantly weaker than theoretical predictions, a result of importance to our understanding of the atmospheres of the outer planets. Publication: The Astrophysical Journal Pub Date: November 1978 DOI: 10.1086/182816 Bibcode: 1978ApJ...225L.157T Keywords: Absorption Spectra; Gas Giant Planets; Hydrogen; Line Spectra; Molecular Absorption; Planetary Atmospheres; Space Environment Simulation; Space Laboratories; Astrophysics; Laboratory Spectra:Molecular Hydrogen; Molecular Hydrogen:Planetary Atmospheres full text sources ADS |