Pollutant emission monitoring using QC laser-based mid-ir sensors

Transcription

1 Header for SPIE use Pollutant emission monitoring using QC laser-based mid-ir sensors David M. Sonnenfroh a, Mark G. Allen a, W. Terry Rawlins a Claire Gmachl b, Federico Capasso b, Albert Hutchinson b, Deborah Sivco b, and A.Y. Cho b a Physical Sciences Inc., 2 New England Business Center, Andover, MA 181 b Bell Laboratories, Lucent Technologies, 7 Mountain Ave., Murray Hill, NJ 7974 Copyright 21 Society of Photo-Optical Instrumentation Engineers This paper was published in Water, Ground and Air Pollution Monitoring and Remediation, Tuan Vo-Dinh, Robert L. Spellicy, Editors, Proceedings of SPIE Vol (21), and is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.

2 Pollutant emission monitoring using QC laser-based mid-ir sensors David M. Sonnenfroh a, Mark G. Allen *a, W. Terry Rawlins a Claire Gmachl b, Federico Capasso b, Albert Hutchinson b, Deborah Sivco b, and A.Y. Cho b a Physical Sciences Inc., 2 New England Business Center, Andover, MA 181 b Bell Laboratories, Lucent Technologies, 7 Mountain Ave., Murray Hill, NJ 7974 ABSTRACT Recent advances in mid-ir semiconductor laser technology based on intersubband transitions in InGaAs quantum wells promise a dramatic impact on tunable diode laser-based sensors for trace gases. This paper reports recent progress toward this realization of room-temperature laser-based sensors for combustion-generated pollutants such as NO x and SO x. Laboratory measurements of SO 2 at 8.6 µm are presented with detection limits on the order of 1 ppm. Extensions of these approaches for higher sensitivity measurements in exhaust gas conditions are described, as well as measurements of SO 3. Keywords: Quantum cascade laser, diode laser, pollutant monitoring, combustion sensor 1. INTRODUCTION Modern jet aircraft engines produce oxidized sulfur species, such as SO 2, SO 3, and H 2 SO 4, which may contribute to the formation of aerosols and clouds in the atmosphere. 1 The reaction mechanisms and rates for conversion of combustiongenerated SO 2 to SO 3 and thence to H 2 SO 4 in jet aircraft exhaust are the subjects of intense scrutiny in the assessment of the atmospheric effects of aviation, and can be resolved by quantitative determinations of SO 2 and SO 3 yields in the exhaust streams of test engines at high-altitude cruise conditions. Fuel-bound sulfur is present in jet fuels as impurities and as lubricity agents. During the combustion process in the engine, this sulfur is essentially completely oxidized to form SO 2, which is exhausted into the atmosphere. Some of the SO 2 is further oxidized over a long time scale by atmospheric species to form H 2 SO 4, which condenses to form small aerosol particles. In addition, some of the SO 2 is promptly oxidized in the combustor exhaust stream to produce SO 3, which is in turn rapidly converted into H 2 SO 4 through reaction with H 2 O in the exhaust. The H 2 SO 4 produced in the exhaust stream can deposit on combustion-generated soot particles to produce hydrophilic condensation nuclei, 2,3 promoting the formation of contrails and persistent cirrus clouds in the wake of the aircraft. These clouds, and their evolutionary development in highly traveled aircraft corridors, can have significant impacts on the radiative heating and cooling budgets in the local upper troposphere and lower stratosphere, with highly uncertain effects on regional climate and on mass transfer across the tropopause. 1 Hence aircraft-generated high-altitude clouds, resulting from SO x chemistry in the exhaust, may affect global climate and stratospheric ozone dynamics. Because of these potentially serious long-term atmospheric effects, Federal and industrial research and development of advanced gas turbine engine concepts are pursuing means of confirming and quantifying the linkages between fuel-bound sulfur, combustion-generated SO 2, SO 3, and H 2 SO 4, and the formation of condensation nuclei. A key element of this R&D is the determination of the kinetics, concentrations, and chemical fates of SO 2 and SO 3 in combustor exhaust streams. This requires highly sensitive (ppmv level) and quantitative detection of SO 2 and SO 3 at elevated temperatures (3 to 7 K), reduced pressures (.2 to.4 atm), and elevated H 2 O and CO 2 mole fractions (.1 to.15) characteristic of combustor exhaust conditions. A summary of the temperature and pressure history through the exhaust turbine and the resultant equilibrium SO x concentration history is shown in Figure 1. Note that thermodynamic equilibrium favors SO 3 formation; however it is well established that almost all of the emitted sulfur is in the form of SO 2 due to the slow kinetic rate of conversion of SO 2 to SO 3 under these conditions. * Correspondence: allen@psicorp.com; Telephone: ; Fax:

3 Figure 1. Calculated equilibrium SO x from a P-T cycle analysis of a commercial service jet aircraft engine operating with Jet-A fuel having.5% sulfur content and at an overall equivalence ratio of.37. These data were provided by Dr. Quang-Viet Nguyen of NASA/GRC, May SO X MONITORING TECHNIQUES While techniques to detect these species have been devised in several research laboratories, their implementation requires considerable effort and specialized skills in physical chemistry, optics, and spectroscopy, so these methods are not readily available to the combustion engineering community. Previous researchers investigating the kinetics of SO 2 and SO 3 have devised means of detecting these species using electronic transitions in the ultraviolet region of the spectrum. SO 2 can be detected by absorption or laser-induced fluorescence in its electronic bands near 3 nm. In addition, SO 3 has absorption bands in the vacuum ultraviolet (145 to 16 nm), and has been detected by laser photodissociation to excite fluorescence from excited SO 2 near 3 nm. 4 Detection of SO 3 by this method requires a fluorine excimer laser at 157 nm, and is impractical in systems containing substantial concentrations of oxygen since O 2 absorbs strongly at the wavelengths required to photodissociate the SO 3. The SO 2 electronic band is accessible to pulsed tunable dye lasers, and can also be reached by frequency summing of two high-power near-ir solid state lasers in a non-linear optical crystal. The infrared portion of the spectrum offers an alternative approach for probing trace-level gas-phase species, principally by laser absorption in fundamental and overtone vibration-rotation bands. However, the infrared absorption approach has historically found less application at high sensitivities than ultraviolet techniques owing to the inherent sensitivity limitations of conventional absorption measurements and to the paucity of adequate infrared light sources. Over the past 15 years, semiconductor laser absorption spectrometers with the highest sensitivities have operated primarily in the mid-infrared, where the fundamental vibrational transitions occur. These transitions have the largest infrared transition strengths for a given molecule, since they arise from single-quantum change in the vibrational quantum number. To access this wavelength region, laser absorption spectrometers have been developed using lead salt diode lasers. 5 Although these devices have achieved high sensitivities and have operated on airplanes, balloons, and combustion facilities, there are several aspects of lead salt lasers that result in a complex instrument design or inhibit routine use. Lead salt lasers are hygroscopic, are highly sensitive to electrostatic discharge, exhibit performance degradation due to temperature cycling, and produce relatively low output power (.1 to 1 mw). More importantly, these lasers do not operate at room temperature but require cryogenic cooling. In addition, these lasers are only available in Fabry-Perot designs, with the result that they operate multimode and can require a monochromator to select a single longitudinal mode. They are available in few wavelengths from limited vendors because of low demand. They find their best use in highly specialized, fundamental research applications.

4 3. QUANTUM CASCADE LASERS The recent emergence of quantum cascade lasers enables the next major advancement in trace species detection: access to the strong, mid-infrared fundamental vibration-rotation transitions with a room temperature, single-mode, tunable diode laser. QC lasers achieve gain via the transitions of electrons between two subbands in the conduction band of a coupled quantum well structure. 6,7 Inversion is created by engineering the lifetimes of the states involved. The paired electron-injection and active-well regions are replicated many times over (cascaded) to increase output power. Since the transitions occur entirely within the conduction band of the material, the output wavelength is determined by the thickness of the active region and is independent of the band gap. QC lasers can be fabricated at any wavelength from ~ 4.5 to 17 µm using AlInAs/InGaAs lattices on InP. The QC laser design overcomes two main disadvantages of lead salt diode lasers. First, they can be operated at room temperature, and second, DFB versions can be fabricated to operate on a single longitudinal mode. By accessing the stronger bands in the mid-infrared fingerprint region, a QC laser absorption spectrometer requires a considerably shorter path than a near-ir device of comparable sensitivity. These advantages result in an overall sensor system that will be smaller and less complex than existing lead salt or near-ir sensors. Thus, the advent of QC lasers enables a new generation of laserbased sensors which achieve the sensitivities of lead salt laser sensors and incorporate the robustness and ease of operation of near-ir diode laser sensors. Several groups have recently reported applications of QC lasers to trace gas sensing. Sensitive absorption spectroscopy using frequency modulation (FM) detection and a room temperature, pulsed DFB QC laser has been reported. 8 Sensitive absorption spectroscopy has also been demonstrated with cryogenically cooled, cw QC lasers using either FM detection 9 or photoacoustic detection techniques. 1 Detection of isotopic composition has also been demonstrated using cryogenically cooled, cw QC lasers. 11 Our work has focused on quasi-cw, room temperature operation of the laser source with high sensitivity detection achieved using the balanced ratiometric technique. We have demonstrated the operation of a breadboard QC laser system to detect N 2 O and NO near 5.4 µm, 12 and are currently developing a field sensor for measurements of formaldehyde in the troposphere. Our general approach is to combine QC laser light sources with multipass absorption cell and noise reduction techniques previously developed for our near-ir diode laser sensors, to achieve sub-ppmv sensitivities for infrared-active species. While QC lasers can be operated cw at cryogenic conditions, we take the approach of pulsed operation at thermoelectrically cooled conditions near room temperature. The noise reduction techniques include (1) reduction of thermal noise through use of liquid-nitrogen-cooled field of view, and (2) dual-beam, balanced ratiometric detection (BRD). The BRD system is an analog circuit which ratios the signal and reference beams to cancel common mode laser intensity fluctuations. 13 Here the BRD is interfaced with the pulsed laser source by operating the BRD circuit at a bandwidth which is low enough to average the laser pulses into a "quasi-cw" signal, but high enough to reject common mode laser noise. 4. IR SPECTROSCOPY OF SO 2 AND SO 3 We have examined the quantitative infrared spectroscopy of SO 2 and SO 3 using data bases available in the literature. A detailed, line-by-line compilation of the vibration-rotation transitions of SO 2 is given in the HITRAN96 data base on atmospheric infrared transitions. The HITRAN code and data base was developed by the Air Force Research Laboratory to provide model simulation capability for high-resolution infrared absorption spectra of atmospheric species as functions of temperature, pressure, concentration, and path length. We have performed spectral modeling simulations using the commercial version of HITRAN marketed by Ontar Corp. The modeling calculations simulated the conditions of our roomtemperature FTIR and QC laser measurements, as well as the conditions of high-temperature combustor exhaust measurements at both atmospheric and reduced pressures. The high-temperature calculations also included simulations of the absorption spectra of H 2 O and CO 2, which are the most important sources of spectral interference with SO 2 and SO 3 detection in combustor exhaust streams. The high-temperature simulations of H 2 O and CO 2 absorption spectra used the HITEMP spectroscopic data base for these species; this data base includes hot-band transitions which are applicable to hightemperature systems up to ~ 1 K. Unfortunately, there is no high-temperature analogue of this data base for SO 2, so the scaling of SO 2 transitions and line strengths to combustor exhaust temperatures is not well validated. The experimental SO 2 spectra were obtained at room temperature with a commercial FTIR spectrometer (Midac Corp.) interfaced to a multipass absorption cell. The cell was connected to a gas supply manifold and a vacuum system allowing operation from vacuum to 1 atm. The multipass optical system within the cell was set up to give 2 passes, resulting in a total path length of 4.89 m. Matched pairs of background (cell evacuated) and sample (cell containing gas sample) spectra were obtained at.5 cm -1 spectral resolution by co-adding 32 interferograms for each case. The background and sample

5 spectra were ratioed to give quantitative spectra of absorbance, ln(i o /I), versus transition frequency in cm -1. The measurements were performed over the pressure range.1 to.4 atm for a commercial gas mixture of 51.6 ppmv SO 2 diluted in N 2. As illustrated by the experimental spectra in Figure 2, SO 2 has two vibrational bands in the 7 to 9 µm region: the strong asymmetric stretch (ν 3 ) band centered near 136 cm -1 (7.35 µm) and the weaker symmetric stretch band (ν 1 ) centered near 115 cm -1 (8.7 µm). The 8.6 µm QC laser measurements performed for this effort probe the latter, ν 1 band; however, transitions in the ν 3 band will achieve the highest possible sensitivity. The most recently reported integrated band intensities at 296 K (including hot band contributions) are.38 x 1-17 and 2.97 x 1-17 cm -1 /(molecule cm -2 ) for the ν 1 and ν 3 bands, respectively. 15 Figure 2. FTIR spectra of 52 ppm SO 2, balance N 2 at 295 K in a 4.89 m path. To check for spectral interferences by air at room temperature, we filled the FTIR absorption cell to atmospheric pressure with room air at ~ 2% relative humidity. The resulting absorbance spectrum is shown in Figure 3, overlain with the SO 2 spectrum. All the transitions in the room air spectrum are due to H 2 O, as confirmed by HITRAN calculations. At room temperature, there are no interfering H 2 O transitions in the SO 2 (ν 1 ) band, confirming that the laboratory QC laser measurements at 8.6 µm did not require a purge flow in the beam path. However, there are several H 2 O features in the region of the SO 2 (ν 3 ) band, as this region is near the edge of the strong H 2 O(ν 2 ) band centered near 6.2 µm. H 2 O interference in this region is a key factor in the design of a system for high-temperature measurements, as described below. The infrared spectrum of gas-phase SO 3 consists of a ν 3 band centered at cm -1 (7.18 µm), 14 and ν 2 and ν 4 (bending) bands near 5 cm -1 (2 µm). 15 SO 3 is a planar symmetric (D 3h ) molecule, and thus the symmetric stretch ν 1 is infraredinactive. Only the ν 3 band is accessible to a QC laser. Detailed spectroscopic measurements of this band have been reported by Henfrey and Thrush. 14 A portion of their published spectrum, obtained at.1 cm -1 resolution for 1 Torr of SO 3 in a 15 cm gas cell, is shown in Figure 4. The authors report detailed spectroscopic constants which can be used to compute the line-by-line transition frequencies, however no line strength data are available. The peak absorbances shown in Figure 4 are consistent with line strengths comparable in magnitude to those for SO 2 (ν 3 ). An extensive computer literature search indicates that there are no open-literature data on the line strengths or high-temperature spectroscopy of this band.

6 Room Air Absorbance (ln Io/I) SO Frequency, cm( -1 ) Figure 3. Comparison of FTIR spectra of SO 2 and room air, showing spectral overlap of H 2 O absorption features. Figure 4. High resolution spectrum of the center of the SO 3 (ν 3 ) band. 5. QUANTUM CASCADE LASER MEASUREMENTS Both distributed feedback (DFB) and non-dfb Quantum Cascade (QC) laser devices operating near 8.6 µm were available to support this effort. The DFB lasers of carrier D237CC2 were partially characterized at Lucent for pulsed operation near room temperature. A plot of the temperature tuning for two representative QC lasers is presented in Figure 5. This data was obtained for pulsed operation at 82.4 khz with 6 ns pulse width. The current tuning rate is approximately 1-2 cm -1 /ma (3 MHz/mA). Example L-I and V-I curves are presented for the laser in Figure 6. As can be seen from the figure, these QC lasers have a current threshold of approximately 2.5 to 3.5 A near room temperature and have a characteristic emission rolloff at higher currents. In between, the L-I curve is linear. These devices require a relatively high compliance voltage (~6 V) compared to other semiconductor diode lasers. For zero bias, the devices have an effective resistance of approximately 2 ohm, which decreases to 1.6 ohm above threshold.

7 1165 w = T cm -1 D237CC "top" Wavenumber (cm -1 ) 1161 "bottom" 1159 w = T cm Temperature (K) Figure 5. Example QC laser temperature tuning data for two devices (top and bottom). Figure 6. Example L-I and V-I curves for the same two devices as in Figure 5. The laser setup and drive current configuration for these experiments is the same as described in Reference 12. Absorption measurements were typically made by pumping the laser with a 5 khz injection current pulse train consisting of 65 ns pulses for an overall duty cycle of.3%. This waveform was superimposed upon a low-amplitude ramp waveform with a typical repetition frequency of 1 Hz. The collimated laser beam exits the mount and is split into a signal and a reference beam using an ar-coated Ge beamsplitter. The signal beam propagates through a.5 m electropolished, stainless steel absorption cell having ar-coated, 3' wedged Ge windows. After exiting the cell, the beam is refocused onto an LN2-cooled photovoltaic HgCdTe detector by a 1 in. diameter, ar-coated Ge f/1 lens. Similarly, the reference beam is refocused onto a second HgCdTe detector using another Ge lens. Provision was made to purge the beam paths. The detectors were 1 mm diameter devices. Each detector was fitted with a 6 deg cold field-of-view. This helps minimize the background current arising from collection of ambient radiation.

8 Outputs from the detectors were sent to a high gain current preamplifier and then into gated integrators. The gated integrator signals were sampled, recorded and integrated to produce a real time history of the number density of the species under investigation. This dual beam, conventional detection method allowed us to investigate spectral absorbances down to ~.5%. Higher detection sensitivity used BRD-based detection methods will be pursued in future work. We monitored the compound absorption feature that arises from the partial overlap of six lines of the ν 1 band that occur near cm -1 (8.62 µm). These transitions are listed in Table 1. A predicted absorption spectrum near 116 cm -1, for 1 ppmv of SO 2 at 1 Torr, is shown in Figure 7. Additional modeling with HITRAN96 at lower pressures indicated that strong absorption spectra of individual transitions can be produced in the Doppler-broadened limit, using 1 to 1 Torr of undiluted SO 2 over the available.5 m path. It is important to note that the band spectrum for SO 2 is highly congested and overlapped, even under low pressure, Doppler-broadened conditions. Thus, we do not sample a true zero-absorption baseline, but rather the differential absorption produced by the compound, overlapping features. Table 1. Individual Transition in the ν 1 Band of SO 2 Contributing to the Observed Absorption Feature Frequency [cm -1 ] Line Strength [cm -1 /(molecule*cm -2 ] Q Q x x x x x x Target Feature Absorption Frequency (cm -1 ) Figure 7. Computed spectrum of SO 2 absorption feature near 116 cm -1, for 1 ppmv SO 2 /air at 1 Torr. An example experimental absorption spectrum is presented in Figure 8. This spectrum was acquired for.5 Torr (1.6 x 1 16 cm -3 ) of SO 2 at 296 K, and is an average of ten single sweeps (.1 s averaging). The thinner line is a running average of ten data points and improves the signal-to-noise ratio. The peak absorption is.6 and the signal-to-noise ratio (SNR) is ~3, which implies a minimum detectable peak absorption of 2 x 1-3 for 2.7 x 1 14 cm -3 SO 2 and a 1 m path. By extrapolating these results to the typical detection limits of BRD or FM-based spectrometers and accounting for increased collisional broadening near atmospheric pressure, we estimate a detection sensitivity at room temperature of about.1 ppm.

9 Peak abs =.6 SNR = 3 Smoothed 1 Sweeps.4 Absorption Frequency [au] Figure 8. Example QC laser absorption spectrum of the compound SO 2 feature near cm -1. The spectrum was obtained with.5 Torr pure SO 2 at room temperature in a 5 cm cell. 6. SIMULATIONS OF EXHAUST GAS MEASUREMENTS The extension of the present room-temperature results will require QC lasers operating at different wavelengths so as to optimize the sensitivity of the targeted lines as well as ensure that they are free from interferences by major exhaust gas species such as CO 2 and H 2 O. In order to determine optimum wavelengths for SO 2 and SO 3, we used extensive spectral simulations by HITRAN and HITEMP to predict high-resolution spectral absorbances for a variety of combustion exhaust scenarios from.1 to 1 atm. A nominal set of conditions used here and representative of many exhaust gas conditions in flight are P~.2 atm, T~ 7 K, Χ CO2 ~.1, and Χ H2O ~.1. The predicted spectra were evaluated in terms of absorbance per meter of path length for 1 ppmv of SO 2. In evaluating the detection limits in terms of concentration and path length, we assumed a minimum detectable absorbance of 1 x 1-5. This value is projected based on our breadboard QC laser measurements at 5.4 and 8.6 µm with and without the BRD noisecanceling circuit, and assuming modest improvements in the pulse generation electronics and thermal noise reduction as discussed in Reference 12 and presently under development in our laboratory. The absorbance computations used Beer's Law, and thus neglected the possible laser line broadening effects due to thermal frequency chirp. 12 The computations also used the pressure broadening coefficient for SO 2 in air contained in the HITRAN data base and thus neglect possible additional broadening due to collisions with H 2 O and CO 2. Despite these limitations, the HITRAN/HITEMP spectral modeling computations provide the best estimates within the known state of the art for high-temperature, high-resolution infrared absorption by SO 2, H 2 O, and CO 2. Absorbance spectra for SO 2 (ν 3 ), H 2 O, and CO 2 in the 132 to 139 cm -1 region (7.2 to 7.6 µm), for P = 1 atm, T = 7 K, Χ H2O =.2, Χ CO2 =.1, Χ SO2 = 1 ppmv, were simulated. The SO 2 absorbances generally exceed those of CO 2, and are well above the expected absorbance detection limit of ~ 1-5. However, they are much lower than those of H 2 O. Most importantly, at 1 atm the pressure-broadened wings of the H 2 O transitions fill in the gaps between the transitions, so that the H 2 O absorbance is a near-continuum which is always at least 1 times greater than that of SO 2 at 1 ppmv. Higher resolution simulations of selected "windows" in the H 2 O spectrum indicate that spectral discrimination of SO 2 features under these conditions would be extremely difficult, and would require sophisticated data processing and spectral fitting procedures to identify 1 to 1 ppmv of SO 2.

10 In contrast, there is much less H 2 O interference in the ν 1 band, as shown in Figure 9 for the same 1 atm conditions. This is consistent with our experimental observations at room temperature. Although the SO 2 absorbances for 1 ppmv-m are near the expected detection limit of ~ 1-5, the features of SO 2 are much easier to discriminate from the H 2 O background in the spectral "windows". Thus at 1 atm and elevated temperature, SO 2 is likely to be more easily detected in the weaker ν 1 band than in the ν 3 band H 2 O SO NO CO Frequency (cm -1 ) Figure 9. Computed atmospheric pressure exhaust gas absorption spectra near SO 2 (ν 1 ) band. 1-7 For ν 3 band transitions at reduced pressure, the peak SO 2 absorbance for 1 ppmv-m is reduced, but still exceeds 1-5. More importantly, the collision-broadened wings of the H 2 O lines are greatly diminished, opening several spectral windows where SO 2 features can be recognized. Since reduced pressure conditions are more relevant to engine exit conditions at altitude, we focus on those conditions for more detailed calculations in the SO 2 (ν 3 ) band. To simulate the conditions of engine exhaust measurements at high-altitude cruise conditions, we performed a series of high-resolution calculations throughout the 132 to 139 cm -1 region for P =.2 atm, T = 7 K, Χ H2O =.1, Χ CO2 =.1, Χ SO2 = 1 ppmv. These calculations led to the identification of three suitable spectral regions for detection of SO 2. These are illustrated in Figures 1 through 12. Here we show the individual contributions from H 2 O, CO 2, and SO 2, and the sum of the three. In each of these spectral windows, a scan of the laser frequency will give quantitative identification of H 2 O and SO 2 ; in addition, spectral scans near 1343 cm -1 will give CO 2 determinations as well. Simultaneous determinations of SO 2 along with H 2 O and/or CO 2 will permit direct determination of the emission index of SO 2 with a single sensor. The 1332 cm -1 region offers the strongest SO 2 transitions, however the 1343 cm -1 region has better isolation of the SO 2 lines. The predicted SO 2 peak absorbances in these windows for 1 ppmv-m are well above the expected detection limit of ~ 1-5, indicating that sub-ppmv levels of SO 2 will likely be detectable with a 1 m path length. Spectral simulations of the H 2 O and CO 2 absorbances near the SO 3 band center are shown in Figure 13 for the.2 atm exhaust conditions. The H 2 O baseline absorbance is somewhat higher in this region, (2 to 3) x 1-4. Based on the peak absorbances of SO 3 at room temperature for this region, we estimate peak SO 3 absorbances for 1 ppmv-m to be about 7 x 1-5 as indicated by the bar on the graph. The SO 3 structure should be readily detectable amidst the H 2 O features near cm -1. Thus it appears that SO 3 can also be detected to ~ 1 ppmv or less with a 1 m path length. As with SO 2, it is likely that spectral scans of the laser will permit simultaneous determinations of H 2 O and/or CO 2, thus yielding the emission index of SO 3. There may well be additional spectral windows for detection of SO 3, however the region to lower frequency contains increasing contributions from SO 2 (ν 3 ), while the region to higher frequency contains increasing contributions from H 2 O(ν 2 ). Conclusive determination of the detection windows for SO 3 awaits direct spectral measurements at representative temperatures and pressures.

11 .3.25 SN (m -1 ).2.15 SUM.1.5 H 2 O SO Frequency, (cm -1 ) Figure 1. Computed exhaust gas spectra at.2 atm near 1332 cm SN (m -1 ).3.25 SUM CO 2 H 2 O SO Frequency (cm -1 ) Figure 11. Computed exhaust gas spectra at.2 atm near 1343 cm -1. The principal conclusions from the spectral modeling exercise for.2 atm and 7 K are as follows: (1) SO 2 is detectable at or below 1 ppmv-m in the ν 3 band; we have identified three spectral windows at , , and cm -1 (7.51, 7.45, and 7.39 µm). QC lasers can be manufactured at any of these wavelengths. (2) SO 3 is likely detectable at 1 ppmv-m near cm -1 (7.18 µm). Other spectral windows may also be available, but experimental measurements of transition strengths and frequencies are required to evaluate them. (3) Spectral scanning of the laser frequency will permit simultaneous determinations of H 2 O and/or CO 2, leading to direct determinations of SO 2 and SO 3 emission indices.

12 SN (m -1 ) H 2 O CO 2 SO 2 SUM Frequency (cm -1) Figure 12. Computed exhaust gas spectra at.2 atm near 1354 cm SN (m -1 ) SUM H 2 O SO 3 CO 2 SO 3 P Q Frequency (cm -1 ) Figure 13. Computed exhaust gas spectra at.2 atm near 1392 cm SUMMARY We have used a combination of laboratory experimental measurements and spectral modeling computations to evaluate and quantify the feasibility of high-sensitivity detection of SO 2 and SO 3 in combustor exhaust streams, using room-temperature quantum cascade laser absorption at specified operating wavelengths near 7 µm. Due to severe interference from collisionally broadened H 2 O lines at 1 atm, it appears that the QC laser absorption method is best suited for the subatmospheric conditions of high-altitude simulation test facilities when applied to the trace levels found in aircraft emission exhaust streams. Laboratory measurements of QC laser absorption by SO 2 at 8.6 µm demonstrate a breadboard QC laser detection system and point to likely detection limits that can be achieved for a fully developed prototype sensor system. Based on straightforward

13 projections of design improvements in laser intensity on detector, pulse generation electronics, heat-sinking of the laser chip, reduction of detector thermal noise, and quasi-cw balanced ratiometric rejection of common mode noise, we conservatively estimate a noise-equivalent absorbance detection limit of ~ 1 x 1-5. The required improvements in the laser and detector operating conditions are now under way and will be completed within the next 6 to 9 months. Spectral modeling simulations of high-resolution infrared absorption spectra for high-altitude test chamber conditions (.2 atm, 7 K) indicate a few discrete spectral windows between 7. and 7.5 µm where SO 2 and SO 3 absorption features can be observed with minimal interference from H 2 O and CO 2 in the exhaust stream. From comparisons of the predicted peak absorbances and the expected detection limit given above, it is likely that both SO 2 and SO 3 can be detected at or below 1 ppmv, with a path length of 1 m, using a multipass optical system. ACKNOWLEDGMENTS The research at PSI was sponsored by the NASA Glenn Research Center under contract NAS3-44, Dr. M.J. Rabinowitz, Contract Monitor. The authors would like to thank Dr. Quang-Viet Nguyen of NASA for providing the exhaust gas composition simulations shown in Figure 1. The research at Bell Laboratories is sponsored in part by DARPA/ARO under contract DAAG55-98-C-5. REFERENCES 1. Friedl, R.R., Ed., Atmospheric Effects of Subsonic Aircraft: Interim Assessment Report of the Advanced Subsonic Technology Program, NASA Reference Publication 14, Wyslouzil, B.E., K.L. Carleton, D.M. Sonnenfroh, W.T. Rawlins, and S. Arnold, "Observation of Hydration of Single, Modified Carbon Aerosols," Geophys. Res. Lett. 21, p. 217, Rawlins, W.T., S.G. Kang, D.M. Sonnenfroh, K.L. Carleton, and B.E. Wyslouzil, "Activation of Carbon Aerosol by Deposition of Sulfuric Acid," Biomass Burning and Global Change, J.S. Levine, Ed., MIT Press, Cambridge, MA, 1996, Vol. 1, pp Suto, M., C. Ye, R.S. Ram, and L.C. Lee, "SO 2 Fluorescence from Vacuum Ultraviolet Dissociative Excitation of SO 3," J. Phys. Chem. 91, p. 3262, T.A. Berkoff, J. Wormhoudt, and R.C. Miake-Lye, "Measurement of SO 2 and SO 3 Using a Tunable Diode Laser System," SPIE Proceedings 3534, p. 686, Faist, J., F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, and A.Y. Cho, Science 264, p. 553, Gmachl, C., J. Faist, J.N. Baillargeon, F. Capasso, C. Sartori, D.L. Sivco, S.N.G. Chu, and A.Y. Cho, "Complex- Coupled Quantum Cascade Distributed-Feedback Laser," IEEE Photonics Tech. Letts. 9, p. 19, K. Namjou, S. Cai, E.A. Whittaker, J. Faist, C. Gmachl, F. Capasso, D.L. Sivco, and A.Y. Cho, "Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser," Opt. Letts. 23, pp , S.W. Sharpe, J.F. Kelly, J.S. Hartman, C. Gmachl, F. Capasso, D.L. Sivco, J.N. Baillargeon, and A.Y. Cho, "High resolution (Doppler limited) spectroscopy using quantum-cascade distributed-feedback lasers," Opt. Letts. 23, pp , B.A. Paldus, T.G. Spence, R.N. Zare, J. Oomens, F.J.M. Harren, D.H. Parker, C. Gmachl, F. Capasso, D.L. Sivco, J.N. Baillargeon, A.L. Hutchinson, and A.Y. Cho, "Photoacoustic spectroscopy using quantum-cascade lasers," Opt. Letts. 24, pp , A.A. Kosterev, R.F.Curl, F.K. Tittel, C. Gmachl, F. Capasso, D.L. Sivco, J.N. Baillargeon, A.L. Hutchinson, and A.Y. Cho, "Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser," Opt. Letts. 24, pp , Sonnenfroh, D.M., Wetjen, E.W., Miller, M.F., Allen, M.G., Gmachl, C., Capasso, F., Hutchinson, A.L., Sivco, D.L., Baillargeon, J.N., and Cho, A.Y., "Mid-IR Gas Sensors Based on Quasi-CW, Room-Temperature Quantum Cascade Lasers," Paper No , 38th AIAA Aerospace Sciences Meeting, January, 2. A revised version, "Application of Balanced Detection to Absorption Measurements of Trace Gases with Room-Temperature, Quasi-CW QC Lasers," accepted for publication in Applied Optics, September, Allen, M.G., K.L. Carleton, S.J. Davis, W.J. Kessler, C.E. Otis, D.A. Palombo, and D.M. Sonnenfroh, "Ultra-Sensitive Dual-Beam Absorption and Gain Spectroscopy: Applications for Near-IR and Visible Diode Laser Sensors," Appl. Opt. 34, p. 324, Henfrey, N.S., and B.A. Thrush, "The ν 3 Band of SO 3 at High Resolution," Chem. Phys. Lett. 12, p. 135, Ortigoso, J., R. Escribano, and A.G. Maki, "The ν 2 and ν 4 IR Bands of SO 3," J. Mol. Spectrosc. 138, p. 62, 1989.