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The advent of continuous wave quantum cascade lasers operating at near room temperature has greatly expanded the capability of spectroscopic detection of atmospheric trace gases using infrared absorption at wavelengths from 4 to 12 μm. The high optical power, narrow line width, and high degree of single mode purity result in minimal fractional absorptions of 5x10-6 Hz-1/2 detectable in direct absorption with path lengths up to 210 meters. The Allan plot minima correspond to a fractional absorbance of 1x10-6 or a minimum absorption per unit path length 5x10-11 cm-1 in 50s. This allows trace gas mixing ratio detection limits in the low part-per-trillion (1 ppt = 10-12) range for many trace gases of atmospheric interest. We present ambient measurements of NO2 with detection precision of 10 ppt Hz-1/2. The detection precision for the methane isotopologue 13CH4 is 25 ppt Hz-1/2 which allows direct measurements of ambient ratios of 13CH4/12CH4 with a precision of 0.5‰ in 100 s without pre-concentration. Projections are given for detection limits for other gases including COS, HONO and HCHO as CWRT lasers become available at appropriate wavelengths.

We report the application of quantum cascade (QC) lasers to measurement of atmospheric trace gases in both closed path and open path configurations. The QC laser, a recently available commercial device, is Peltier cooled and pulsed, with emission near 965 cm-1. We use direct absorption with a rapid sweep integration and spectral fits to derive absolute concentrations from tabulated line parameters without calibration. In the closed path configuration, with absorption in a long pathlength multipass cell (210 m, 50 Torr), we examined laser line widths and sensitivity limits. We measured ammonia with a precision of 0.05 nmole/mole (0.05 ppbv) RMS at 1 Hz, limited by detector noise. The laser linewith was 0.007 cm-1 HWHM, based on measurements of ethylene absorption line shapes with a current pulse width of ~14 ns. In the open path configuration, we measured ammonia in the exhaust of automobiles driving through a probe beam. Atmospheric pressure line broadening and turbulence limit the sensitivity, giving a column density noise level at 20 Hz of 1.4 ppm-m. We observed ammonia column densities up to 40 ppm-m in the exhaust plumes. In future systems we will include a CO2 channel, allowing normalization to fuel use rate.

A dual tunable diode laser absorption spectrometer (TDLAS) for continuous field measurement of nitric acid and nitrogen dioxide eddy covariance fluxes is described and preliminary field results are presented. The dual TDLAS simultaneously measures nitric acid (HNO3) and nitrogen dioxide (NO2) by direct absorption spectroscopy over a long path enclosed in an astigmatic Herriott multipass cell. The technique provides sufficient precision and time response (200 ppt RMS in 1 second) needed to record ambient variations and deposition rates by the eddy-covariance method. Real-time fitting of the integrated spectra over multiple absorption features makes the system appropriate for continuous field measurements while retaining the highly selective quality of direct absorption measurements and minimizing potential interferences. This method also produces an absolute, spectroscopic determination of concentration within the multipass cell, eliminating the need for calibrated gas mixtures in the field.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A dual infrared tunable diode laser system (IR-TDL) has been developed for the simultaneous detection of multiple gaseous components in cigarette smoke. The high spectral resolution (0.001 cm -1 ) and rapid time response (20 Hz) of the TDL system are ideal for separating the absorptions from the multitude of gas phase components found in this matrix. The combustion products are sampled into a 0.3 liter, 18 meter multiple pass absorption cell with a flow response time of 0.15 seconds, which provides ample time resolution to observe variations within each 2-second puff. Two independent beam paths allow simultaneous detection in two wavelength regions; the first for ethylene and ammonia and the second for formaldehyde. Rapid scan-sweep integration with direct absorption permits absolute gas concentrations to be determined on-line. A non-linear least squares procedure is used for “fingerprint” fitting of up to four gases with each diode. Results demonstrating the instrument sensitivity and time response, along with potential caveats, for several gaseous components will be presented.

This paper discusses recent advances in our techniques for monitoring atmospheric trace gases using lead salt liquid nitrogen cooled diode lasers. Our approach employs an optical system with all reflective optics. Our closed path systems rely on a proprietary astigmatic multipass cell to achieve long optical path lengths in a low volume sampling cell. We have also developed open path systems which we have used for remote sensing of automobile and aircraft engine exhaust. Our data acquisition method uses rapid frequency sweeping followed by nonlinear least squares analysis of the retrieved spectrum. Recent advances include an emphasis on multi-laser multi-species detection systems, such as simultaneously monitoring the nitric oxide, carbon monoxide and carbon dioxide concentrations in automobile exhaust. Other advances are focused on achieving improved detection sensitivity. In support of this goal, we have demonstrated astigmatic multipass cells with very long optical paths, we have improved the nonlinear least squares spectral fitting routines allowing them to fit complex multi-peak spectra and we have introduced photolytic modulation as a method to discriminate spectra of photolytically active species from background absorption and optical interference fringes. These techniques are being applied to the monitoring of a wide variety of atmospheric molecules including CH4, CO, CO2, N2O, NO, NO2, HONO, HNO3, O3 and HOCl.

Recent requirements for better characterization of the trace species in the exhausts of advanced aircraft engines have resulted in the development, under NASA sponsorship, of a tunable infrared diode laser system capable of making both in situ and extractive sampling measurements. In this paper, we describe the first application of this instrument to nonintrusive, cross-flow measurements in the exhaust of an aircraft engine operating in an altitude test cell. Simultaneous absorption measurements of NO, NO2 and H2O using the combined beams from two lasers were made at a variety of engine and flight conditions. The utility of measuring concentrations of major species such as H2O to convert line of sight observations of trace species into mass fluxes is discussed, as are uncertainties in the measurement resulting from fluctuations in the absorption spectra and from conversion from integrated column density to concentration profiles.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The atmospheric effects of stratospheric aircraft component of the NASA High Speed Research Program will require measurements of trace gas concentrations in the exhausts of high speed civil transport engines. In parallel with the development of these engines by NASA and its industrial partners, a portable infrared tunable diode laser apparatus has been assembled and tested which is capable of both in situ and extractive sampling of combustion gas flows. Infrared diode laser absorption is sensitive, quantitative, and applicable to a wide range of molecular species. In the present apparatus, sensitive detection is achieved by rapid frequency scanning and real-time nonlinear least squares fitting and background subtraction. Sensitivity is further increased for extractive sampling by an advanced design multiple pass cell which gives longer path lengths in smaller volumes. Observations of a laboratory flat flame burner are reported. These observations and spectroscopic models are used to predict detection sensitivities in exhausts and other combustion systems.

We recently have conducted measurements of distributions and fluxes of methane in several small towns in the U.S. These measurements are made with a real-time methane instrument recently developed at Aerodyne Research. This instrument is based on absorption of infrared light from a Zeeman-broadened HeNe laser operating at 3.39 micrometers . A non-uniform magnetic field applied to the laser plasma results in a nearly flat gain profile, which allows piezoelectric tuning of the laser on and off a methane line. Long path (47 m) absorption occurs in a multipass cell. The instrument has a noise level of 5 ppbv (rms, 1 sec.) and a gas sampling time of 6 sec. The instrument is mounted in a van with a suite of other real-time instruments (CO2 and SF6) which can function while moving at roadway speeds. We map the methane distribution in towns, identify the primary methane sources, and measure fluxes with tracer methods. The simultaneous measurement of CO2 with a commercially available NDIR device is useful in identifying some sources of methane. An SF6 instrument (electron capture) allows precise location and measurement of tracer plumes during flux measurement experiments.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Methods to increase the sensitivity of intracavity laser spectroscopy (ILS) have been investigated with application to the measurement of trace gases in the atmosphere. Recent theory has predicted that a longer laser cavity can give an increase in the limiting sensitivity of ILS. Measurements have been performed on a dye laser with two different cavity configurations, with lengths of 54 and 160 cm. No significant difference in the limiting generation time t(g) for these two laser lengths is found. The limiting t(g) of approximately 1000 microsec is achieved at pump powers of 150 to 300 mW, corresponding to 1.5 times the threshold power. When a tuning pellicle is introduced in the cavity, the laser linewidth is halved, the threshold power increases to 220-400 mW and the limiting t(g) decreases to approximately 400 microsec. Other effects at long generation times that will influence the use of ILS for atmospheric measures have been investigated. The spectral noise increases strongly near the limiting t(g), and the spectrum may show structure due to weak interferences in the laser cavity.

An improved real-time methane monitor based on infrared absorption of the 3.39 micron line of a HeNe laser is described. Real time in situ measurement of methane has important applications in stratospheric and tropospheric chemistry, especially when high accuracy measurements can be made rapidly, providing fine spatial-scale information. The methane instrument provides 5 ppb resolution in a 1 sec averaging time. A key feature in this instrument is the use of magnetic (Zeeman) broadening to achieve continuous tunability with constant output power over a range of 0.017/cm. The instruments optical absorption path length is 47 m through sampled air held at 50 torr in a multipass cell of the Herriott (off-axis resonator) type. A microprocessor controls laser frequency and amplitude and collects data with minimal operator attention. The instrument recently has been used to measure methane emissions from a variety of natural and artificial terrestrial sources.