We review our recent results in development of high-precision laser spectroscopic instrumentation using midinfrared quantum cascade lasers (QCLs). Some of these instruments have been directed at measurements of atmospheric trace gases where a fractional precision of 10−3 or better of ambient concentration may be required. Such high precision is needed in measurements of fluxes of stable atmospheric gases and measurements of isotopic ratios. Instruments that are based on thermoelectrically cooled midinfrared QCLs and thermoelectrically cooled detectors have been demonstrated that meet the requirements of high-precision atmospheric measurements, without the need for cryogens. We also describe the design of and results from a new dual QCL instrument with a 200-m path-length absorption cell. This instrument has demonstrated 1-s noise of 32 ppt for formaldehyde (HCHO) and 9 ppt for carbonyl sulfide (OCS).
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 present an overview of the dual QC laser spectrometer developed at Aerodyne Research and various examples of its application for atmospheric trace gas detection. The instrument incorporates two pulsed QC lasers, a compact 76-m (or 56-m) multipass absorption cell, a dual HgCdTe detector, and a sophisticated signal generation, data acquisition and processing system. Recent findings and hardware innovations are highlighted. Our results show that the precision and minimal detectable absorbance obtainable with pulsed QC lasers are comparable to those achieved with cryogenically cooled CW Pb-salt lasers in spite of the broader laser linewidths inherent to pulsed operation. This is demonstrated through in situ measurements of several trace gases, including methane, nitrous oxide, carbon monoxide, formaldehyde, formic acid, nitrous acid and ethylene. Recent measurements of HCHO and HCOOH on board a NOAA aircraft are presented. The precision, stability and intrinsic accuracy of the instrument were assessed through inter-comparisons measuring CH4 and CO. These measurements were made either comparing two QC lasers sweeping over different transitions or comparing the dual QCL spectrometer and a standard instrument (NDIR CO). The absorbance precision achieved is typically 2x10-5 Hz-1/2. For long-lived species, such as CH4 and N2O, this implies 1-Hz fractional precisions of 0.1% or better, which fulfill the requirement for meaningful measurements from aircraft platforms. Spectroscopically derived mixing ratios are accurate within 5% or better. The spectrometer is equipped to perform automatic, periodic calibrations with zero and span gases whenever higher accuracy is required.
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.
On-road remote sensors can measure the emissions of motor vehicles under real-world conditions. The most sensitive and versatile remote sensor reported to date is based on Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS). This study applied this TILDAS remote sensor to the measurement of the emissions from heavy-duty diesel trucks (HDDTs). The remote sensor could operate with an optical pathlength of 88 m, or more than 5 times that of competing instruments. Remote sensing of NO2 emissions was demonstrated for the first time. Good agreement was obtained when comparing the TILDAS measurements with the on- board measurements of an instrumented HDDT. The distribution of NO emissions from HDDT was found not to be skewed. HDDTs are estimated to contribute about 3/5 of the on-road NOx emission inventory. These emissions are underestimated by a factor of 2.2 in the latest EPA inventory.
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 nonlinear 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.
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.
IR absorption using tunable diode laser spectroscopy provides a sensitive and quantitative detection method for laboratory kinetic studies of atmospheric trace gases. Improvements in multipass cell design, real time signal processing, and computer controlled data acquisition and analysis have extended the applicability of the technique. We have developed several optical systems using off-axis resonator mirror designs which maximize path length while minimizing both the sample volume and the interference fringes inherent in conventional 'White' cells. Computerized signal processing using rapid scan (300 kHz), sweep integration with 100 percent duty cycle allows substantial noise reduction while retaining the advantages of using direct absorption for absolute absorbance measurements and simultaneous detection of multiple species. Peak heights and areas are determined by curve fitting using nonlinear least square methods. We have applied these techniques to measurements of: (1) heterogeneous uptake chemistry of atmospheric trace gases (HCl, H2O2, and N2O5) on aqueous and sulfuric acid droplets; (2) vapor pressure measurements of nitric acid and water over prototypical stratospheric aerosol (nitric acid trihydrate) surfaces; and (3) discharge flow tube kinetic studies of the HO2 radical using isotopic labeling for product channel and mechanistic analysis. Results from each of these areas demonstrate the versatility of TDL absorption spectroscopy for atmospheric chemistry applications.
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