SPIRALE: a multispecies in situ balloonborne instrument with six tunable diode laser spectrometers

The balloonborne SPIRALE (a French acronym for infrared absorption spectroscopy by tunable diode lasers) instrument has been developed for in situ measurements of several tracer and chemically active species in the stratosphere. Laser absorption takes place in an open Herriott multipass cell located under the balloon gondola, with six lead salt diode lasers as light sources. One mirror is located at the extremity of a deployable mast 3.5 m below the gondola, enabling the measurement of very low abundance species throughout a very long absorption path (up to 544 m). Three successful flights have produced concentration measurements of O3, CO, CO2, CH4, N2O, NO2, NO, HNO3, HCl, HOCl, COF2, and H2O2. Fast measurements (every 1.1 s) allow one to obtain a vertical resolution of 5 m for the profiles. A detection limit of a few tens of parts per trillion in volume has been demonstrated. Uncertainties of 3%-5% are estimated for the most abundant species rising to about 30% for the less abundant ones, mainly depending on the laser linewidth and the signal-to-noise ratio.     

Four-laser airborne infrared spectrometer for atmospheric trace gas measurements

We describe the four-laser airborne infrared (FLAIR) instrument, a tunable diode laser absorption spectrometer designed for simultaneous high-sensitivity in situ measurements of four atmospheric trace gases in the troposphere. The FLAIR spectrometer was employed during the large-scale airborne research campaign on tropospheric ozone (TROPOZ II) in 1991 and was used to measure CO, H(2) O(2), HCHO, and NO(2) in the free troposphere where detection limits below 100 parts in 10(12) by volume were achieved.     

Highly sensitive optical waveguide sensor for SO2 and H2S detection in the parts-per-trillion regime using tetraaminophenyl porphyrin

ABSTRACT

Optical waveguide (OWG) sensors present great potential for detecting trace levels of harmful gases because of their high sensitivity and anti-electromagnetic interference. However, OWG-based SO2 and H2S-detecting sensors in the parts-per-trillion (ppt) range are still lacking. We fabricated 5,10,15,20-(tetra-4-aminophenyl) porphyrin (TAPP) thin film-based OWG sensor devices (TAPP-OWG) to detect SO2 and H2S gases, in which TAPP thin film was immobilized over the surface of a potassium ion exchange glass OWG. These sensors successfully measure extremely low concentrations of SO2 and H2S (detection limit?=?1 ppt), providing good repeatability for SO2 (10 ppt) and H2S (10 ppt) gases, with relative standard deviations of 1.67% and 3.68%, respectively. With fast response (t1) and recovery (t2) times for SO2 (t1=4 s, t2=157 s) and H2S (t1=2 s, t2=117 s) at room temperature, TAPP thin film enhances the potential of OWGs for use in high-sensitivity trace-level gas detection.     

Lightweight diode laser spectrometer CHILD (Compact High-altitude iN-situ Laser Diode) for balloonborne measurements of water vapor and methane

A new lightweight near-infrared tunable diode laser spectrometer CHILD (Compact High-altitude In-situ Laser Diode spectrometer) was developed for flights to the stratosphere as an additional in situ sensor on existing balloonborne payloads. Free-air absorption measurements in the near infrared are made with an open-path Herriott cell with new design features. It offers two individual absorption path lengths optimized for CH4 with 74 m (136 pass) and H2O with 36 m (66 pass). New electronic features include a real-time gain control loop that provides an autocalibration function. In flight-ready configuration the instrument mass is approximately 20 kg, including batteries. It successfully measured stratospheric CH4 and H2O profiles on high-altitude balloons on four balloon campaigns (Environmental Satellite validation) between October 2001 and June 2003. On these first flights, in situ spectra were recorded from ground level to 32,000-m altitude with a sensitivity of 0.1 ppm [(parts per million), ground] to 0.4 ppm (32,000 m) for methane and 0.15-0.5 ppm for water.     

Use of a clustering reaction to detect low levels of moisture in bulk oxygen using an atmospheric pressure ionization mass spectrometer

Atmospheric pressure ionization mass spectrometry (APIMS) is being routinely used to quantify trace impurities in bulk gases used in the manufacture of semiconductor devices. APIMS has been successfully applied for the quantification of ppt levels of O(2), H(2)O, CO(2), and CH(4) in Ar, N(2), and He. However, it has not been successfully used to quantify trace impurities in bulk O(2) due to the low ionization potential of O(2). APIMS relies on charge-transfer reaction between the ions of the bulk gas molecules and impurity molecules. Since all the relevant impurity molecules have ionization potentials higher than that of O(2), APIMS has not been used to analyze for impurities in O(2). We report here the detection of sub-ppb levels of H(2)O in O(2) by making use of the clustering reaction between O(2)(+) and H(2)O. The declustering region in an APIMS, which is normally used to break apart unwanted and interfering clusters, has to be carefully adjusted to keep intact the weakly bound cluster O(2)(+)·H(2)O. Our results indicate a statistical detection limit of less than 300 ppt for the detection of H(2)O in O(2).     

Rapid, online quantification of H2S in JP-8 fuel reformate using near-infrared cavity-enhanced laser absorption spectroscopy

One of the key challenges in reforming military fuels for use with fuel cells is their high sulfur content, which can poison the fuel cell anodes. Sulfur-tolerant fuel reformers can convert this sulfur into H(2)S and then use a desulfurizing bed to remove it prior to the fuel cell. In order to optimize and verify this desulfurization process, a gas-phase sulfur analyzer is required to measure H(2)S at low concentrations (<1 ppm(v)) in the presence of other reforming gases (e.g., 25-30% H(2), 10-15% H(2)O, 15% CO, 5% CO(2), 35-40% N(2), and trace amounts of light hydrocarbons). In this work, we utilize near-infrared cavity-enhanced optical absorption spectroscopy (off-axis ICOS) to quantify H(2)S in a JP-8 fuel reformer product stream. The sensor provides rapid (2 s), highly precise (±0.1 ppm(v)) measurements of H(2)S in reformate gases over a wide dynamic range (0-1000 ppm(v)) with a low detection limit (3σ = ±0.09 ppm(v) in 1 s) and minimal cross-interferences from other present species. It simultaneously quantifies CO(2) (±0.2%), CH(4) (±150 ppm(v)), C(2)H(4) (±30 ppm(v)), and H(2)O (±300 ppm(v)) in the reformed gas for a better characterization of the fuel reforming process. Other potential applications of this technology include measurement of coal syngas and H(2)S in natural gas. By including additional near-infrared, distributive feedback diode lasers, the instrument can also be extended to other reformate species, including CO and H(2).     

A field-portable,laser-diode spectrometer for the ultra-sensitive detection of hydrocarbon gases

We have developed a field-portable optical gas sensor for the ultra-sensitive detection of ethane. The system is based on an adaptation of a commercially available system, which uses a cryogenically cooled, lead-salt laser diode at 3.34 μm and a multi-pass astigmatic Herriott sample cell. We have adapted this system to a second derivative wavelength modulation scheme giving a lower detection limit of less than 100 parts per trillion for a one second measurement time. Our custom-designed software controls every aspect of the instrument operation from spectral scanning of the laser diode, to automatic calibration, optical alignment, spectral analysis and complete data logging.     

On-line multicomponent trace-gas analysis with a broadly tunable pulsed difference-frequency laser spectrometer

The design and application of a novel automated room-temperature laser spectrometer are reported. The compact instrument is based on difference-frequency generation in bulk LiNbO(3). The instrument employs a tunable cw external-cavity diode laser (795-825 nm) and a pulsed diode-pumped Nd:YAG laser (1064 nm). The generated mid-IR nanosecond pulses of 50-muW peak power and 6.5-kHz repetition rate, continuously tunable from 3.16 to 3.67 mum, are coupled into a 36-m multipass cell for spectroscopic studies. On-line measurements of methane are performed at concentrations between 200 ppb (parts in 10(9) by mole fraction) and approximately 1%, demonstrating a large dynamic range of 7 orders of magnitude. Furthermore computer-controlled multicomponent analysis of a mixture containing five trace gases and water vapor with an overall response time of 90 s at an averaging time of only approximately 30 s is reported. A minimum detectable absorption coefficient of 1.1 x 10(-7) cm(-1) has been achieved in an averaging time of 60 s, enabling detection limits in the ppb range for many important trace gases, such as CH(4), C(2)H(6), H(2)CO, NO(2), N(2)O, HCl, HBr, CO, and OCS.     

A photothermal interferometer for gas-phase ammonia detection

Detection of gas-phase ammonia is particularly challenging because ambient ammonia concentrations may be less than 1 ppb (molecules of NH(3) per 10(9) molecules of air), ammonia sticks to many materials commonly used to sample air, and particles containing ammonium may interfere with gas-phase measurements. We have built a new and sensitive photothermal interferometer to detect gas-phase ammonia in situ, under typical atmospheric conditions. Ammonia molecules in sampled air absorb infrared radiation from a CO(2) laser at 9.22 μm, with consequent collisional heating, expansion, and refractive index change. This change in refractive index is detected as a phase shift in one arm of a homodyne interferometer. Measurements of vibrational and electrical noise in the interferometer correlate to an instrumental lower limit of detection of 6.6 ppt ammonia in 1 s. The CO(2) laser output is modulated at 1.2 kHz, and the ac signal from the interferometer is measured with a lock-in amplifier. The detector is zeroed by sampling through a H(3)PO(4)-coated denuder tube and is calibrated by dynamic dilution of two permeation tube outputs and by standard addition. Signal gain is insensitive to CO(2) or H(2)O in the sample, and the signal is linear over 5 orders of magnitude. The instrument 2σ precision is 31 ppt when the signal is integrated for 100 s and 250 ppt with a 1-s integration time. The windowless sample cell and inlet is fabricated entirely of glass to minimize sample loss and hysteresis. The instrument response time is demonstrated to be about 1 s.     

Precise Frequency Measurements in Submillimeter and Infrared Region

As a result of frequency stabilization of all the lasers and microwave oscillators in a new multiplication chain consisting of HCN (337 ?m), D2O (84 ?m), CO2/OsO4 (10.53 ?m), CO2 (10.18 ?m), and He-Ne/CH4 (3.39 ?m) lasers signals up to 88 THz have been synthesized with high precision. Owing to phaselocking of HCN and D2O lasers to the primary frequency standard synthesis accuracy of ~1013 up to 30 THz has been achieved for the first time. The frequency of the CO2/OsO4 laser was first measured ?OsO4 = 28 464 676 938.5 ± 1 kHz and the He-Ne/CH4 laser frequency was determined to be ?CH4 = 88 376 181 586 ± 10 kHz.     

Intracavity CO laser photoacoustic trace gas detection: cyclic CH(4), H(2)O and CO(2) emission by cockroaches and scarab beetles

A liquid-nitrogen-cooled CO laser and an intracavity resonant photoacoustic cell are employed to monitor trace gases. The setup was designed to monitor trace gas emissions of biological samples on line. The arrangement offers the possibility to measure gases at the 10(9) by volume (ppbv) level (e.g., CH(4), H(2) O) and to detect rapid changes in trace gas emission. A detection limit of 1 ppbv for CH(4) in N(2) equivalent to a minimal detectable absorption of 3 × 10(-9) cm(-1) can be achieved. Because of the kinetic cooling effect we lowered the detection limit for CH(4) in air is decreased to 10 ppbv. We used the instrument in a first application to measure the CH(4) and H(2) O emission of individual cockroaches and scarab beetles. These emissions could be correlated with CO(2) emissions that were recorded simultaneously with an infrared gas analyzer. Characteristic breathing patterns of the insects could be observed; unexpectedly methane was also found to be released.     

MASERATI: a RocketBorne tunable diode laser absorption spectrometer

The MASERATI (middle-atmosphere spectrometric experiment on rockets for analysis of trace-gas influences) instrument is, to our knowledge, the first rocket-borne tunable diode laser absorption spectrometer that was developed for in situ measurements of trace gases in the middle atmosphere. Infrared absorption spectroscopy with lead salt diode lasers is applied to measure water vapor and carbon dioxide in the altitude range from 50 to 90 km and 120 km, respectively. The laser beams are directed into an open multiple-pass absorption setup (total path length 31.7 m) that is mounted on top of a sounding rocket and that is directly exposed to ambient air. The two species are sampled alternately with a sampling time of 7.37 ms, each corresponding to an altitude resolution of approximately 15 m. Frequency-modulation and lock-in techniques are used to achieve high sensitivity. Tests in the laboratory have shown that the instrument is capable of detecting a very small relative absorbance of 10(-4)-10(-5) when integrating spectra for 1 s. The instrument is designed and qualified to resist the mechanical stress occurring during the start of a sounding rocket and to be operational during the cruising phase of the flight when accelerations are very small. Two almost identical versions of the MASERATI instrument were built and were launched on sounding rockets from the And?ya Rocket Range (69 degrees N) in northern Norway on 12 October 1997 and on 31 January 1998. The good technical performance of the instruments during these flights has demonstrated that MASERATI is indeed a new suitable tool to perform rocket-borne in situ measurements in the upper atmosphere.     

Membrane-based parallel plate denuder for the collection and removal of soluble atmospheric gases

A continuously wetted cellulose acetate membrane-based parallel plate diffusion denuder is described. This is the first membrane-based denuder that has a small enough internal liquid holdup volume to permit reasonably rapid response time (10 --> 90% rise time of approximately 1.2 min for a transient event at a liquid flow rate of 500 microL/min) while permitting quantitative removal of common soluble atmospheric trace gases at flow rates up to 1.7 L/min. The latter attribute permits the use of the device as the first element in a particle sampling and analysis system for the quantitative removal of potentially interfering soluble trace gases. Particle losses in the denuder range from 0.9 to 2.9% over an aerodynamic diameter range of 0.38-3.48 microm, averaging 1.8%. However, only approximately 0.5% of the particles actually appears in the denuder effluent liquid. The relatively compact (300 mm H x 57 mm W x 26 mm D) wet denuder should be attractive in a number of applications. We show excellent agreement for HONO measurements with a conventional larger parallel plate wetted denuder in field measurements.     

Impact of atmospheric clutter on Doppler-limited gas sensors in the submillimeter/terahertz

It is well known that clutter (spectral interference) from atmospheric constituents can be a severe limit for spectroscopic point sensors, especially where high sensitivity and specificity are required. In this paper, we will show for submillimeter/terahertz (SMM/THz) sensors that use cw electronic techniques the clutter limit for the detection of common target gases with absolute specificity (probability of false alarm ? 10?1?) is in the ppt (1 part in 1012) range or lower. This is because the most abundant atmospheric gases are either transparent to SMM/THz radiation (e.g., CO?) or have spectra that are very sparse relative to the 10? Doppler-limited resolution elements available (e.g., H?O). Moreover, the low clutter limit demonstrated for cw electronic systems in the SMM/THz is independent of system size and complexity.     

Application of Balanced Detection to Absorption Measurements of Trace Gases with Room-Temperature, Quasi-cw Quantum-Cascade Lasers

Distributed-feedback quantum-cascade (QC) lasers are expected to form the heart of the next-generation mid-IR laser absorption spectrometers, especially as they are applied to measurements of trace gases in a variety of environments. The incorporation of room-temperature-operable, single-mode QC lasers should result in highly compact and rugged sensors for real-world applications. We report preliminary results on the performance of a laser absorption spectrometer that uses a QC laser operating at room temperature in a quasi-cw mode in conjunction with balanced ratiometric detection. We have demonstrated sensitivities for N(2)O [10 parts in 10(6) volume-mixing ratio for a 1-m path (ppmv-m)] and NO [520 parts in 10(9) volume-mixing ratio for a 1-m path (ppbv-m)] at 5.4 mum. System improvements are described that are expected to result in a 2 orders of magnitude increase in sensitivity.     

Monitoring of ethylene by a pulsed quantum cascade laser

We report on the development and performance of a gas sensor based on a quantum cascade laser operating at a wavelength of approximately 10 microns to measure ethylene (C2H4) concentrations by use of a rotational component of the fundamental nu 7 band. The laser is thermoelectrically cooled and operates in a pulsed mode. The influence of pulse-to-pulse fluctuations is minimized by use of a reference beam and a single detector with time discriminating electronics. Gas absorption is recorded in a 100-m optical path-length astigmatic Herriott cell. With a 10-kHz pulse repetition rate and an 80-s total acquisition time, a noise equivalent sensitivity of 30 parts per billion has been demonstrated. The sensor has been applied to monitor C2H4 in vehicle exhaust as well as in air collected in a high-traffic urban tunnel.     

Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors

A dual-beam detection strategy with automatic balancing is described for ultrasensitive spectroscopy. Absorbances of 2 × 10(-7) Hz(-?) in free-space configurations and 5 × 10(-6) Hz(-?) in fiber-coupled configurations are demonstrated. With the dual-beam technique, atmospherically broadened absorption transitions may be resolved with InGaAsP, AlGaAs, and AlGaInP single-longitudinal-mode diode lasers. Applications to trace measurements of NO(2), O(2), and H(2)O are described by the use of simple, inexpensive laser and detector systems. Small signal gain measurements on optically pumped I(2) with a sensitivity of 10(-5) are also reported.     

Development of a spectrometer using a continuous wave distributed feedback quantum cascade laser operating at room temperature for the simultaneous analysis of N2O and CH4 in the Earth's atmosphere

We report on the development and performance of a gas sensor based on a distributed feedback quantum cascade laser operating in continuous wave at room temperature for simultaneous measurement of nitrous oxide (N(2)O) and methane (CH(4)) concentrations at ground level. The concentrations of the gases are determined by a long path infrared diode laser absorption spectroscopy. The long-term stability of the instrument is evaluated using the Allan variance technique. A preliminary evaluation of the instrument performance is realized by in situ measurements of N(2)O and CH(4) concentrations at ground level during 1 day. The sensor has also been applied to study the time response of N(2)O concentrations to a fertilizer addition in a soil sample and for the comparison between various types of soils.     

Real-time trace-level detection of carbon dioxide and ethylene in car exhaust gases

A direct-absorption spectrometer, based on a pulsed, distributed feedback, quantum cascade laser with a 10.26-microm wavelength and an astigmatic Herriott cell with a 66-m path length, has been developed for high-resolution IR spectroscopy. This spectrometer utilizes the intrapulse method, an example of sweep integration, in which the almost linear wavelength up-chirp obtained from a distributed feedback, quantum cascade laser yields a spectral microwindow of as many as 2.5 wave numbers/cm(-1). Within this spectral microwindow, molecular fingerprints can be monitored and recorded in real time. This system allows both the detection of carbon dioxide and ethylene and the real-time observation of the evolution of these gases in the exhaust by-products from several cars.     

Temperature measurements in a rapid compression machine using mid-infrared H₂O absorption spectroscopy near 7.6 μm

A method for measuring the temporal temperature and number density in a rapid compression machine (RCM) using quantum cascade laser absorption spectroscopy near 7.6?μm is developed and presented in this paper. The ratios of H₂O absorption peaks at 1316.55 cm^-1 and 1316.97 cm^-1 are used for these measurements. In order to isolate the effects of chemical reactions, an inert mixture of argon with 2.87% water vapor is used for the present investigation. The end of compression pressures and temperatures in the RCM measurements are P_C=10, 15, and 20 bar in the range of T_C=1000 to 1200?K. The measured temperature history is compared with that calculated based on the adiabatic core assumption and is found to be within ±5 K. The measured temporal number density of H₂O to an accuracy of 1%, using the absolute absorption of the two rovibrational lines, show that the mixture is highly uniform in temperature. A six-pass, 5.08?cm Herriott cell is used to calibrate the line strengths in air and broadening in an Ar bath gas.