Airborne laser infrared absorption spectrometer (ALIAS-II) for in situ atmospheric measurements of N2O, CH4, CO, HCl, and NO2 from balloon or remotely piloted aircraft platforms

The Airborne Laser Infrared Absorption Spectrometer II (ALIAS-II) is a lightweight, high-resolution (0.0003-cm(-1)), scanning, mid-infrared absorption spectrometer based on cooled (80 K) lead-salt tunable diode laser sources. It is designed to make in situ measurements in the lower and middle stratosphere on either a balloon platform or high-altitude remotely piloted aircraft. Chemical species that can be measured precisely include long-lived tracers N(2)O and CH(4), the shorter-lived tracer CO, and chemically active species HCl and NO(2). Advances in electronic instrumentation developed for ALIAS-I, with the experience of more than 250 flights on board NASA's ER-2 aircraft, have been implemented in ALIAS-II. The two-channel spectrometer features an open cradle, multipass absorption cell to ensure minimal contamination from inlet and surfaces. Time resolution of the instrument is 相似文献   

Atmospheric CH4 and H2O monitoring with near-infrared InGaAs laser diodes by the SDLA, a balloonborne spectrometer for tropospheric and stratospheric in situ measurements

The Spectromètre à Diodes Laser Accordables (SDLA), a balloonborne spectrometer devoted to the in situ measurement of CH(4) and H(2)O in the atmosphere that uses commercial distributed-feedback InGaAs laser diodes in combination with differential absorption spectroscopy, is described. Absorption spectra of CH(4) (in the 1.653-mum region) and H(2)O (in the 1.393-mum region) are simultaneously sampled at 1-s intervals by coupling with optical fibers of two near-infrared laser diodes to a Herriott multipass cell open to the atmosphere. Spectra of methane and water vapor in an altitude range of ~1 to ~31 km recorded during the recent balloon flights of the SDLA are presented. Mixing ratios with a precision error ranging from 5% to 10% are retrieved from the atmospheric spectra by a nonlinear least-squares fit to the spectral line shape in conjunction with in situ simultaneous pressure and temperature measurements.     

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.     

Spectroscopic investigation of methylated amines by a cavity-ringdown-based spectrometer

High-resolution absorption spectra of gas-phase monomethylamine (MMA, CH(3)NH(2)) and dimethylamine [DMA, (CH(3))(2)NH] in the region of the first overtone of the NH stretch vibration are reported. Measurements were performed with a near-infrared laser spectrometer based on the cavity-ringdown (CRD) detection technique. The minimum detectable absorption coefficient for the CRD detection setup is alpha(min)=1.55 x 10(-8) cm(-1) (for SNR = 1). This corresponds to detection limits of 350 parts in 10(9) (ppb) for MMA and 1.6 parts in 10(6) (ppm) for DMA in synthetic gas mixtures under interference-free conditions, or 10 ppm and 60 ppm for MMA and DMA, respectively, in the case of gas mixtures such as exhaled human breath containing H(2)O, CO(2), and other absorbing gases in this range.     

Shot-noise-limited dual-beam detector for atmospheric trace-gas monitoring with near-infrared diode lasers

A dual-beam detector is used to measure atmospheric trace species by differential absorption spectroscopy with commercial near-infrared InGaAs laser diodes. It is implemented on the Spectromètre à Diodes Laser Accordables, a balloonborne tunable diode laser spectrometer devoted to the in situ monitoring of CH(4) and H(2)O. The dual-beam detector is made of simple analogical subtractor circuits combined with InGaAs photodiodes. The detection strategy consists in taking the balanced analogical difference between the reference and the sample signals detected at the input and the output of an open optical multipass cell to apply the full dynamic range of the measurements (16 digits) to the weak molecular absorption information. The obtained sensitivity approaches the shot-noise limit. With a 56-m optical cell, the detection limit obtained when the spectra is recorded within 8 ms is ~10(-4) (expressed in absorbance units). The design and performances of both a simple substractor and an upgraded feedback substractor circuit are discussed with regard to atmospheric in situ CH(4) absorption spectra measured in the 1.653-mum region. Mixing ratios are obtained from the absorption spectra by application of a nonlinear least-squares fit to the full molecular line shape in conjunction with in situP and T measurements.     

Synthesis of germanium oxide nanoparticles in low-pressure premixed flames

Germanium oxide (GeOx) nanoparticles in the size range from 1 nm to 5 nm were synthesized in a low-pressure premixed H2/O2/Ar flat flame of 30 mbar. The premixed flame was doped with different amounts of tetramethyl germanium (Ge(CH3)4) ranging from 250 ppm to 2000 ppm. The influence of process parameters such as the amount of oxygen in the reaction gas, the condensation and reaction time, standoff, and precursor concentration with respect to growth of germanium oxide particles were investigated. The particles formed were analyzed in situ according to their mass and charge with a particle mass spectrometer. The specific surface area was determined ex situ by the BET method. The crystal form and chemical composition of produced nanopowders were characterized by EDX analysis and X-ray diffraction measurements.     

Differential optical absorption spectroscopy instrument for stratospheric balloonborne trace-gas studies

A newly developed UV-visible instrument for differential optical absorption spectroscopic measurements of atmospheric trace gases from balloon platforms is described. Direct solar light at daytime in the near-ultraviolet (320.6-422.6-nm) and the visible (417.6-670.7-nm) spectral ranges can be simultaneously analyzed for the atmospheric column abundances or profiles of O(3), NO(2), NO(3), BrO, OClO, O(4), H(2)O, and possibly other species (HNO(2), IO, CH(2)O). Compared with previously used balloonborne UV-visible spectrometers, the instrument has the superior properties of low mass (42 kg), low power consumption (30 W), decreased spectral drift that is caused by temperature and pressure changes, low detector dark current, and low spectrometer stray light. The three last-named characteristics are achieved by enclosure of the entire spectrometer in a pressurized and thermostated container and by inclusion of separately thermostated photodiode array detectors. The optical setup is simplified to reduce its weight. The spectral stray light is reduced by suppression of the higher-order and zero-order grating reflections by use of light traps and in the UV by addition of a dispersive prism preanalyzer. The major instrumental design characteristics and the instrumental performance as tested in the laboratory and during several stratospheric balloon flights are 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.     

Wavelength modulated cavity enhanced absorption spectroscopy of water in the 1.37 microm region

We describe the performance of a high-sensitivity wavelength modulated cavity enhanced infrared tunable diode laser absorption spectrometer for the detection of water vapor in the 1.37 mum region. The spectrometer can measure a fractional absorption of approximately 10(-5) for an absorption path length of a few kilometers. The instrument's sensitivity is more than sufficient to detect water isotopomers (H(2)(16)O, H(2)(18)O, HDO) at Martian atmospheric concentrations. The instrument is amenable to miniaturization, so a future compact, multiple-species version of the spectrometer will be highly suitable for in situ planetary exploration.     

Multipass optical absorption spectroscopy: a fast-scanning laser spectrometer for the in situ determination of atmospheric trace-gas components, in particular OH

The optical design of an absorption spectrometer for in situ measurements of atmospheric trace gases is reported. The light source is a rapidly tuned and power-stabilized dye-ring laser, which is frequency doubled by an intracavity BBO crystal. The second harmonic and the fundamental are used simultaneously for measurement of OH, SO(2), CH(2)O, and naphthalene in the UV and of NO(2) in the visible. The 1.2-km absorption path is folded within a 6-m White-cell-type multiple-reflection system with an open-path setup. The absorption sensitivity of the spectrometer is better than 1 part in 10(-5) under tropospheric conditions (integration time 1 min., signal-to-noise ratio 1).     

In situ measurements of H2O from a stratospheric balloon by diode laser direct-differential absorption spectroscopy at 1.39 microm

A distributed-feedback InGaAs laser diode emitting near 1.393 mum is used in conjunction with an optical multipass cell that is open to the atmosphere to yield ambient water-vapor measurements by infrared absorption spectroscopy. To obtain the high dynamic range for the measurements that is required for continuous water-vapor monitoring in the upper troposphere and the lower stratosphere, we used a simple circuit that combined differential and direct detection. Furthermore, the laser emission wavelength was tuned to balance the steep decrease in H(2)O concentration with altitude by sweeping molecular transitions of stronger line strengths. The technique was implemented by use of the Spectromètre à Diodes Laser Accordables (SDLA), a tunable diode laser spectrometer operated from a stratospheric balloon. Absorption spectra of H(2)O in the 5-30-km altitude range obtained at 1-s intervals during recent balloon flights are reported. Water-vapor mixing ratios were retrieved from the absorption spectra by a fit to the full molecular line shape in conjunction with in situ pressure and temperature measurements, with a precision error ranging from 5% to 10%.     

Correcting for methane interferences on δ2H and δ18O measurements in pore water using H2Oliquid-H2Ovapor equilibration laser spectroscopy

Cavity ring-down spectroscopy (CRDS) is a new and evolving technology that shows great promise for isotopic δ(18)O and δ(2)H analyses of pore water from equilibrated headspace H(2)O vapor from environmental and geologic cores. We show that naturally occurring levels of CH(4) can seriously interfere with CRDS spectra, leading to erroneous δ(18)O and δ(2)H results for water. We created a new CRDS correction algorithm to account for CH(4) concentrations typically observed in subsurface and anaerobic environments, such as ground waters or lake bottom sediments. We subsequently applied the correction method to a series of geologic cores that contain CH(4). The correction overcomes the spectral interference and provides accurate pore water δ(18)O and δ(2)H values with acceptable precision levels as well as accurate concentrations of CH(4).     

Mobile Fourier-transform infrared spectroscopy monitoring of air pollution

Fourier transform infrared spectroscopy is an efficient technique for the detection and quantification of molecules in gas mixtures. Measurement results from a mobile laboratory for ambient air analysis and for remote sensing of plume emission with the commercially available K300 spectrometer are reported. CO, CO(2), NO, NO(2), N(2)O, NH(3), CH(4), SO(2), H(2)O, HCl, and HCHO concentrations have been determined with good agreement with in situ results. The on-line multicomponent analysis software is based on line-by-line retrieval and least-squares fitting procedures, including the effects of multiple aerosol scattering and cloud and rain influences.     

Algorithm for the retrieval of columnar water vapor from hyperspectral remotely sensed data

A new algorithm for the retrieval of columnar water vapor content is presented. The proposed procedure computes the area of the H2O absorption centered about 940 nm to allow its integrated columnar abundance as well as its density at ground level to be assessed. The procedure utilizes the HITRAN 2000 database as the source of H2O cross-section spectra. Experimental results were derived from radiometrically calibrated hyperspectral images collected by the Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) sensor over the Cuprite mining district in Nevada. Numerical simulations based on the MODTRAN 4 radiative transfer code were also employed for investigating the algorithm's performance. An additional empirical H2O retrieval procedure was tested by use of data gathered by the VIRS-200 imaging spectrometer.     

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).     

Potential of far-ultraviolet absorption spectroscopy as a highly sensitive quantitative and qualitative analysis method for aqueous solutions, part I: determination of hydrogen chloride in aqueous solutions

This paper reports the usefulness of far-ultraviolet (FUV) absorption spectroscopy in highly sensitive quantitative and qualitative analysis of aqueous solutions. We propose a totally new idea for the utilization of FUV spectroscopy in pure water and aqueous solution analyses. We use an absorption band near 170 nm due to an n --> sigma* transition of water. The intensity of the foot of this band, which can be observed in the 190-210 nm region by use of an ordinary ultraviolet-visible (UV-Vis) spectrometer, is very sensitive to changes in hydration and hydrogen bonds of water. To demonstrate the potential of FUV spectroscopy in analytical chemistry, we undertook three kinds of experiments. The first one is concerned with the discrimination of eight kinds of commercial natural mineral water. The eight kinds of mineral water can be discriminated straightforwardly from the spectral patterns in the 190-250 nm region without any spectral pretreatment or spectral analysis such as multivariate analysis. The second experiment is the determination of hydrogen chloride (HCl) in aqueous solutions. FUV spectra of aqueous solutions of HCl over a concentration of 0-20 ppm were measured. A calibration model for predicting the concentration of HCl in the aqueous solutions was developed based on the absorbance at 193 nm. This method does not require any spectral pretreatment or multivariate analysis. The correlation coefficient and standard error of prediction of the calibration model developed are 0.9987 and 0.18 ppm, respectively. The detection limit of the proposal method for the determination of HCl in aqueous solutions was estimated to be 0.5 ppm (13.7 microM). The determination of HCl was also tried for natural mineral water to which HCl solutions with the concentrations of 2, 4, 6, 8, 12, 16, and 20 ppm were artificially added. The third study was the determination of ammonia (NH3) and hydrogen peroxide (H2O2) in aqueous solutions containing both NH3 and H2O2. It has been found that the present method is also useful for the determination of the two-component system.     

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.     

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.     

Infrared spectroscopy of tropospheric trace gases: combined analysis of horizontal and vertical column abundances

Vibration-rotation absorptions in high-resolution Fourier-transform infrared spectra from a 246-m horizontal path were used to derive local concentrations of trace gases at the Alpine observatory at the Zugspitze summit, Germany (2964 m above sea level). The analysis was performed by using the line-by-line nonlinear least-squares spectral fitting software, sfit, based on the 1992 hitran line parameter compilation. (hitran is a high-resolution transmission molecular absorption database.) A comparison to in situ measurements shows an agreement of better than 4.3% for the species CO, CO(2), and CH(4). Using the same spectrometer and analysis software, we obtained the vertical column density of N(2)O together with an adjusted vertical volume mixing ratio distribution. This translates to a local N(2)O concentration at the altitude of Zugspitze that agrees with the horizontal path-derived value to within 1%.     

Surface modification using a commercial triple quadrupole mass spectrometer

We report instrumental modifications to a commercial mass spectrometer that allow surface modification experiments to be performed using low-energy (electronvolt range) mass-selected ion beams. The design of the detector housing allows placement of the surface on the ion optical axis and some distance beyond the off-axis detector. Manipulation of the potentials applied to the final lens, detector housing, conversion dynode, and electron multiplier allow the ions to pass through the detector housing and impinge upon the surface without loss of the normal mode of detector operation. Ex situ analysis of the modified surface is performed using a home-built multisector mass spectrometer. The ability to modify organic thin films is demonstrated by a number of soft landing and surface modification experiments including (i) soft landing of (CH3)2SiNCS+ ions formed from trimethylsilyl isothiocyanate upon a fluorinated self-assembled monolayer (F-SAM) surface, (ii) soft landing and dissociative soft landing of the pseudomolecular cation of triphenylpyrylium tetrafluoroborate, viz. the triphenylpyrylium cation, upon an F-SAM surface, (iii) dissociative soft landing of 35ClCH2(CH3)2SiOSi(CH3)2+ formed from 1,3-bis(chloromethyl)disiloxane upon an F-SAM surface, (iv) surface passivation by reaction of the trimethylsilyl cation, Si(CH3)3+, with a hydroxyl-terminated self-assembled monolayer (OH-SAM), and (v) transhalogenation by reaction of CCl3+ (m/z 119) with an F-SAM surface.