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Nitrous acid (HONO) is important as a significant source of hydroxyl radical (OH) in the troposphere and as a potent indoor air pollutant. It is thought to be generated in both environments via heterogeneous reactions involving nitrogen dioxide (NO2). In order to enable fast-response HONO detection suitable for eddy-covariance flux measurements and to provide a direct method that avoids interferences associated with derivatization, we have developed a 2-channel tunable infrared laser differential absorption spectrometer (TILDAS) capable of simultaneous high-frequency measurements of HONO and NO2. Beams from two mid-infrared continuous-wave mode quantum cascade lasers (cw-QCLs) traverse separate 210 m paths through a multi-pass astigmatic sampling cell at reduced pressure for the direct detection of HONO (1660 cm−1) and NO2 (1604 cm−1). The resulting one-second detection limits (S/N=3) are 300 and 30 ppt (pmol/mol) for HONO and NO2, respectively. Our HONO quantification is based on revised line-strengths and peak positions for cis-HONO in the 6-micron spectral region that were derived from laboratory measurements. An essential component of ambient HONO measurements is the inlet system and we demonstrate that heated surfaces and reduced pressure minimize sampling artifacts.

We present a new approach to the measurement of stable isotopic ratios of carbon dioxide using a near-room-temperature pulsed quantum cascade laser and a spectral ratio method based upon dual multiple pass absorption cells. The spectral ratio method improves precision and accuracy by reducing sensitivity to variations in the laser tuning rate, power and line width. The laser is scanned across three spectral lines (near 2310 cm-1) quantifying three CO2 isotopologues: 12C16O2, 13C16O2 and 12C16O18O. Isotopic ratios are determined simultaneously with a precision of 0.2δ for each ratio with a one-second measurement. Signal averaging for 400 s improves the precision to better than 0.03δ for both isotopic ratios (13 R and 18 R). Long-term accuracy of 0.2 to 0.3δ is demonstrated with replicate measurements of the same sample over a one-month period. The fast time response of this instrument is suitable for eddy flux measurements.

A quantum cascade laser operating near room temperature with thermoelectric (TE) cooling has been used in both continuous-wave (cw) mode (-9 °C) and pulsed mode (+45 °C) to detect atmospheric nitric oxide using spectral lines at 1900.07 cm-1 (5.3 μm). The totally non-cryogenic spectrometer integrates the laser with a 69-m astigmatic multi-pass cell and a TE-cooled infrared detector to enable operation for extended time periods without operator attention. The pattern of reflections on the astigmatic cell mirrors has been designed to minimize optical interference fringes, which are substantially greater with cw mode than with pulsed operation. The detection method uses direct absorption with rapid- scan sweep integration to achieve sub-second time response. Detection precision for NO in air of 0.5 parts in 109 Hz-1/2 (1σ) is obtained in pulsed mode with an Allan variance minimum corresponding to 0.1 parts in 109 after 30-s averaging time. The precision in cw mode improves to 0.1 parts in 109 Hz-1/2 and 0.03 parts in 109 after 30-s averaging, corresponding to an absorbance per unit path length of 2×10-10 cm-1. The advantages and disadvantages of cw compared to pulsed operation are discussed.

Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detection of trace gases in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than 1 nmole/mole (

Context Abstract: Nitrous oxide (NO2) is a potent greenhouse gas whose atmospheric budget is poorly constrained. One known atmospheric source is the formation of N2O on three-way motor vehicle catalytic converters followed by emission with the exhaust. Previous estimates of the magnitude of this N2O source have varied widely. Two methods employing tunable infrared lasers to measure N2O/CO2 ratios from a large number of on-road motor vehicles have been developed. Both methods add support to lower estimates of N2O emissions from the US motor vehicle fleet, although significant uncertainty remains. Main Abstract: Two tunable infrared laser differential absorption spectroscopy (TILDAS) techniques have been used to measure the N2O emission levels of on-road motor vehicle exhausts. Cross road, open path laser measurements were used to assess N2O emissions from 1361 California catalyst equipped vehicles in November, 1996 yielding an emission ratio of (8.8±2.8)×10−5 N2O/CO2. A van mounted TILDAS sampling system making on-road N2O measurements in mixed traffic in June, 1998 in Manchester, New Hampshire yielded a mean N2O/CO2 ratio of (12.8±0.3)×10−5, based on correlated N2O and CO2 concentration peaks attributed to motor vehicle exhaust plumes. The correlation of N2O emissions with vehicle type, model year and NO emissions are presented for the California data set. It is found that the N2O emission distribution is highly skewed, with more than 50% of the emissions being contributed by 10% of the vehicles. Comparison of our results with those from four European tunnel studies reveals a wide range of derived N2O emission indices, with the most recent studies (including this study) finding lower values.

O, and NO2 emissions have been measured with this instrument. The system is also capable of measuring CO, NH3, H2CO, CH3OH, and other small molecules in vehicle exhaust.

A multipass absorption cell, based on an astigmatic variant of the off-axis resonator (Herriott) configuration, has been designed to obtain long path lengths in small volumes. Rotation of the mirror axes is used to obtain an effective adjustability in the two mirror radii. This allows one to compensate for errors in mirror radii that are encountered in manufacture, thereby generating the desired reentrant patterns with less-precise mirrors. Acombination of mirror rotation and separation changes can be used to reach a variety of reentrant patterns and path lengths with a fixed set of astigmatic mirrors. The accessible patterns can be determined from trajectories, as a function of rotation and separation, through a general map of reentrant solutions. Desirable patterns for long-path spectroscopy can be chosen on the basis of path length, distance of the closest beam spot from the coupling hole, and tilt insensitivity. We describe the mathematics and analysis methods for the astigmatic cell with mirror rotation and then describe the design and test of prototype cells with this concept. Two cell designs are presented, a cell with 100-m path length in a volume of 3 L and a cell with 36-m path length in a volume of 0.3 L. Tests of low-volume absorption cells that use mirror rotation, designed for fast-flow atmospheric sampling, show the validity and the usefulness of the techniques that we have developed.

Breath analysis is a powerful noninvasive technique for the diagnosis and monitoring of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD). Nitric oxide (NO) and carbon monoxide (CO) are markers of airway inflammation and can indicate the extent of respiratory diseases. We have developed a compact fast response laser system for analysis of multiple gases by infrared absorption. The instrument uses room temperature quantum cascade lasers to simultaneously measure NO, CO, carbon dioxide (CO(2)) and nitrous oxide (N(2)O) in exhaled breath. Four breath flow rates are employed to explore their exchange dynamics in the lungs and airways. We obtain 1-s detection precisions of 0.5-0.8 parts-per-billion (ppb) for NO, CO, and N(2)O with an instrument response time of less than 1 s. The breath analysis system has been demonstrated in a preliminary study of volunteers. It is currently deployed in a trial clinical study.

Applied Physics B, 92 (3), ISSN:0946-2171, ISSN:1432-0649, ISSN:0721-7269, ISSN:0340-3793

We have developed a compact instrument for sensitive, rapid and continuous measurement of trace gases in air, with the focus here on ammonia. This instrument takes advantage of recent technology in quantum cascade (QC) lasers and infrared detectors, which allows high sensitivity to ammonia, 0.2 ppb (0.2times10-9) of NH3 in air in 1 s without cryogenic liquids. One may substitute a QC laser operating at a different wavelength to measure other gases. The instrument operates continuously, requiring little operator attention, and web-based remote access is provided for instrument control, calibration and data retrieval. The instrument design includes a thermoelectrically (TE) cooled pulsed QC laser operating at 967 cm-1, a low volume (0.5 l) multi-pass cell offering 76 meter absorption path length and a TE cooled detector. Integrated software for laser control and data analysis using direct absorption provides quantitative trace gas measurements without calibration gases. The instrument may be applied to field measurements of gases of environmental concern.

Summary form only given. Environmental applications of mid-IR TILDAS instruments including: the measurement of greenhouse gas fluxes from natural and agricultural landscapes; the mapping of urban pollutants to identify and quantify emission sources; and the measurement of automobile exhaust emissions from on-road vehicles are presented and discussed.

A research program involving two applications of tunable infrared laser differential absorption spectroscopy (TILDAS) with multipass, long-path absorption cells to the detection of explosives contamination in soils is reported. In the first application, sensitive, specific real-time species concentration measurements by TILDAS have led to new understanding of the processes involved in explosives detection by the heating of contaminated soils and the quantification of the resulting pyrolysis gases. In the second, we present results of our calculations of the properties of astigmatic off-axis resonator absorption cells, which show that useful TILDAS path lengths can be achieved inside a cone penetrometer probe.