Letter pubs.acs.org/ac
Photoacoustic Analyzer for the Artifact-Free Parallel Detection of Soot and NO2 in Engine Exhaust Christoph Haisch* and Reinhard Niessner Chair for Analytical Chemistry and Institute of Hydrochemistry, Technische Universität München, Marchioninistrasse 17, D-81377 Munich, Germany S Supporting Information *
ABSTRACT: Soot particles and NO2 are among the most hazardous emissions from diesel combustion engines. Currently, no analytical system exists which allows for the simultaneous, time-resolved online analysis of these two components. Furthermore, state-of-the-art NO2 analyzers for exhaust gas require particle filtration prior to the analysis, which may induce artifacts and measurement errors. We present a photoacoustic instrument which overcomes these drawbacks. The sensitivity of the instrument (LODNO2 = 0.3 ppm, LODsoot = 0.54 μg m−3, limit of detection/quantification) as well as the temporal resolutions are dictated by the needs of typical automotive applications. Also required for this specific application, we developed PA cells which can be heated to 80 °C, while the microphones maintain a temperature of 45 °C. Setup and specific parameters of the instrument are discussed, and the results of typical engine tests are compared to reference analytical instrumentation. NO2 findings and errors in the NO/NO2 ratio.9,10 Still, all commercial analyzers for NO2 in exhaust gas require particle filtration and thus are prone to artifact formation. PA spectroscopy is based on the absorption of a modulated light by the analyte and the subsequent conversion into heat. In gases, the local modulated gas heating leads to modulated expansion, which is detected as a sound wave.11 Fundamentals of gas phase PA spectroscopy are described, e.g., by Sigrist et al.12,13 Several groups employed PA for aerosol analysis.14−17 As mentioned above, we presented a PA soot sensor dedicated for exhaust gas monitoring.6 NO2 detection by PA spectroscopy is described in the literature by some authors, either employing CO2 laser systems18 or, in the visible spectral range, by frequency doubled, pulsed, or modulated Nd:YAG lasers.19 To the best of our knowledge, no system dedicated for exhaust gas monitoring is described in the literature apart from our own PA soot sensor. Although PA based parallel or sequential detection of several gas components is discussed by many authors,20−24 the specific combination of aerosol and NO2 detection was, to our knowledge, not tested up until now. While most activities in PA-based gas-phase analysis are focused on extreme sensitivity, the scope of the presented work was set on the applicability under the harsh conditions of routine engine testing.
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oot and NOx are among the most hazardous compounds emitted by diesel combustion engines, and thus, both are limited by legislation, for instance by the European Euro 5 and 6 standard.1 In particular, NO2 is highly toxic,2 but it is required for in-use diesel particulate filter (DPF) regeneration.3 Reduction of NO2 can result in higher soot emissions and vice versa, thus requiring parallel optimization of both components.4 The soot emission mass can be quantified by filter sampling and following thermo-gravimetric analysis.5 State of the art for online soot mass monitoring in exhaust gas is a photoacoustic (PA) sensor manufactured by the AVL List GmbH (Graz, Austria), which is a commercial realization of a prototype system developed by us earlier.6 NO2 in exhaust gas is generally monitored by chemiluminescence detection (CLD). However, it is known that this kind of detection is prone to artifacts due to its cross sensitivity to other oxidized nitrogen compounds such as peroxyacetyl nitrate and nitric acid.7 Alternatively, sensitive detection of NO2 and many other relevant emission gases is possible by Fourier transforminfrared spectroscopy detection.8 These instruments are highly versatile, yet cost-intensive, and do not allow for soot monitoring. Consequently, we present a two-channel, PA based instrument which exploits the advantages of photoacoustics in exhaust gas monitoring for the detection of both, soot and NO2. It is essentially an upgrading of our PA soot sensor mentioned above. Beyond the second detection channel, it features high-temperature PA cells, which allow for a direct analysis of hot exhaust gases without dilution. Another major advantage of the new instrument is its ability to directly analyze the NO2 concentration in the gas without filtration of the soot prior to the NO2 analysis. The interaction of the NO2 with the soot particles deposited on a filter surface can lead to reduced © 2012 American Chemical Society
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INSTRUMENTAL SETUP We developed a two-channel PA system with two separate PA cells in one heated cell block, employing only one single laser Received: July 5, 2012 Accepted: August 8, 2012 Published: August 8, 2012 7292
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the second PA cell, which is primarily intended for the detection of NO2. Behind the cell, the optical intensity is monitored via a photodiode (BPX65 Silicon PIN Photodiode, OSRAM, Germany), covered by an optical diffusor (DG05-220, Thorlabs, Germany). PA Cells. The core component of the instrument is a brass block, comprising the two PA cells. The size of this cell block is 60 × 100 × 120 mm3, with the two PA cells and their optical axes being aligned along one of the long sides. The acoustical resonators are combinations of classical longitudinal resonators with Helmholtz resonators. Similar to an earlier design,27 this arrangement allows for heated PA cells, while the microphone temperature can be kept at 45 °C, which guarantees longtime stability of the temperature-sensitive microphone. The microphones are electret microphones (EK 3029, Knowles, GB) mounted in a cylindrical cell at the end of a stainless steel tube (outer diameter Da = 3 mm, inner diameter Di = 2 mm, length 20 mm) which is connected to the middle of the resonator tube. The longitudinal quartz resonator tube has the following size, length 40 mm, Da = 6 mm, and Di = 4 mm. The Q-factor of the first longitudinal mode (001) we used is 13.3. A low Qfactor was chosen in order to achieve higher stability under varying gas conditions like compositions and temperature. Gas in- and outlets are shaped as rings of 0.2 mm width around the optical axis, positioned in the pressure nodes of the standing acoustical wave (see Figure 1), thus minimizing the influence of the acoustic flow noise on the acoustic signal. The cell is heated by a 200 W heat cartridge situated in the center of the cell block. By use of a fuzzy-logic thermocontroller (UR3274, Wachendorff, Germany), a cell temperature of 80 ± 0.7 °C is maintained during the measurements. Cell and sample gas temperature, gas pressure, and humidity are monitored on several test positions in the gas flow system. The sample gas is lead through a heated inlet tube to the cells and split up in two parallel flows, one for each cell. Behind each cell, the gas is filtered by a HEPA-filter (high efficiency particulate air filter, DIF-BN-30, datatronic GmbH, Germany) and guided through a combined flow meter and a needle valve (KD1, Kobold, Germany). A gas flow of 3 L min−1 through each of the PA cells is maintained by two separate pumps (NMP 850 KNDC 12 V, KNF Neuberger, Germany). The outlet flows are recombined to be fed into an exhaust system, in the case of toxic gases. Data Recording. Data recording is carried out via a 4 channel, 24-bit dynamic signal acquisition card (NI PCI-4474, National Instruments) in combination with a three-channel software-emulated lock-in detector (Labview 2010, National Instruments). Each PA channel is read-out via one of the lockin channels; the third channel is employed to monitor the excitation light intensity of the 532 nm light beam.
source for both components. The overall system, which we call TwinPAS, fits into a 19 in. rack (height 85 cm) on wheels, containing the instrument itself and the control computer along with its peripherals. The system is mobile and robust, allowing for routine application at engine test stands. The PA instrument is assembled into an in-house tailored 45 cm × 47 cm × 36 cm (w × l × h) housing, which contains the optical system, the PA cells, the gas handling system, and the control electronics (see Figure S-1 in the Supporting Information). Optical Setup. The presented instrument (see Figure 1) is a advancement of the PA soot sensor presented earlier;6 thus, it
Figure 1. Schematic representation of the system.
employs the same type of laser diode for soot detection. This laser features a maximum emission power of 2.1 W at an emission wavelength of 806 nm. As shown earlier, this wavelength is well suitable for soot detection without interferences to gaseous exhaust components.6 The light is guided through an optical fiber (Di = 200 μm) to a four-rod optical bench. The light is collimated by an f = 20 nm lens through one of the two PA cells. Behind the cell, which is meant for the analysis of the soot concentration, and a 90° mirror, the light beam is focused by another plano-convex lens (f = 12 mm) with a wide-band antireflective coating into a DPM (diode pumped microchip crystal) module (CASIX DPM1102, Laser Components, Germany). A DPM is a nonlinear crystal combination, consisting of a Nd:YVO4 laser crystal in direct optical contact to a KTP (potassium titanyl phosphate) crystal. The laser crystal is pumped by the laser diode radiation and emits at a wavelength of 1064 nm, which is frequency-doubled to 532 nm by the KTP. The size of this DPM is 1.5 mm × 2.5 mm × 2.5 mm. NO2 is among the few gases which feature a significant optical absorption at a wide range is the visible optical spectrum.25 Although the peak absorption of NO2 is around 400 nm, sensitive detection is possible at 532 nm.26 The main advantage of this dual use of one single laser source for both components is its inherent simplicity and the low involved costs for the second component. To generate the PA signal, the laser is modulated with a frequency of 4800 Hz, 50% duty cycle, which results in a mean laser power of the diode laser of 1.0 W. Conversion efficiency of the crystal combination is 33%, resulting in a laser power of 330 mW at 532 nm, again modulated with the identical modulation frequency. Maximum power conversion is achieved at a crystal temperature of 35.1 °C, which is maintained by a thermoelectric cooling. Behind another 90° mirror, which folds the optical path antiparallel to the 806 nm laser beam, the 532 nm beam is focused by a f = 80 mm plano-convex lens through
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RESULTS AND DISCUSSION Quantitative Analysis. Three different approaches were employed for the calibration of the system for NO2. The first one is based on standard test gases with concentrations of 500 ppm (Air Liquide, Germany), 1010 ppm, and 2022 ppm (Westphalen AG, Germany), each with dry nitrogen as the carrier gas. Concentrations between about 5 ppm and the maximum concentration of 2000 ppm were produced by dilution with nitrogen or air. Dilution ratios were controlled via needle valves and rotameters, and flow values were additionally measured by a Gilibrator-2 Air Flow Calibrator (Sensidyne, Canada). The second approach is based on an in-house made 7293
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temporal response of the three systems differ, sections of these data sets with step concentration gradients were removed from the data. In a next step, two least-squares fits, based on eqs 1 and 2, were carried out, matching the measured signals of the two PA cells (532 and 806 nm) to the concentrations revealed by the reference analysis, with the calibration values asoot/NO2, each for the two wavelengths, as variable values. The mathematical calibration procedures were carried out in Matlab 2010 (The Mathworks, Inc.). The results of this calibration procedure are depicted in Figure S-2 in the Supporting Information. It has to be mentioned that cross sensitivity of the two channels does not only include the fact that both analytes absorb more or less light of both wavelengths. The specific optical arrangement also means that high soot concentrations, absorbing light in the first (806 nm) PA cell, result in a lower light intensity in the second (532 nm) PA cell. This effect is likewise corrected for by the above calibration procedure. For low concentrations near the respective detection limits, these calibrations were verified by the permeation tube system for NO2 (up to 10 ppm) and diesel soot behind an exhaust gas particle filtration system (up to 25 μg m−3). Standard deviations of the background for both analytes were measured over 30 s, analyzed on a 0.2 s time base. For NO2 it is 0.10 ppm, resulting in an LOD = 0.3 ppm (3sB) and a LOQ = 0.6 ppm (6sB). For the same analytical conditions, an LOD = 0.54 μg m−3 and LOQ = 1.08 μg m−3 (6sB) were determined for soot analysis. It was found that pressure variations of up to ±50 mPa can be corrected for based on the assumption of an ideal gas. No significant influence of other gases, such as CO, CO2, NO, NH3, H2O, H2, and butane on the measured concentrations of soot and NO2 was detected. The replacement of air as the carrier gas by N2 did not influence the quantitative analysis. Time Resolution. The temporal resolution of the system can be limited by several factors. One is the data acquisition rate of the software lock-in amplifier, which is set to a time base of 5 Hz. However, the limiting factor for the time resolution is not the electronic data rate but the gas exchange rate of the PA cells. It can be influenced by the gas flow rate through the cells, which can be raised to 3 L min−1 per cell. For higher flow rates, the background noise increases by more than 1 order of magnitude, thus limiting the sensitivity. This background noise might be attributed to the flow noise of the gas through the cell. As it can be appreciated in Figure 2, the rise time of the raw signal on response to a soot concentration rising from 0 to 1.0 g m−3 is 0.8 s (10−90%). Although the theoretical rise time, calculated from a PA cell volume of 2.2 × 10−5 m−3 and a flow rate of 3 L min−1, is 0.44 s, we consider this value of 0.8 s the maximum rise time possible, limited by internal mixing in the cell. However, the rise time found for NO2, measured for an increase of the NO2 concentration from 0 to 126 ppm, was found to be 1.2 s under identical conditions (see Figure S-3 in the Supporting Information). This reduced time response can be attributed to chemical adsorption of the NO2 to the tube and cell surfaces.9 These measurements were carried out with very short sampling tubes (∼10 cm). We found that for longer, more practical tubings, the effects are worse, since the chemical interaction takes place mainly in the sampling tubes and not in the PA cells. Real-World Applications. A typical application of automotive test equipment is the analysis of transient driving cycles, which are commonly applied for the testing and optimization of engines and exhaust systems. As an example,
permeation tube system with liquid NO2 enclosed in a special 5 mL glass container, sealed off by a PTFE membrane of 1 mm thickness and 2 mm diameter.28 The tube is kept inside a glass flow tube, which is flushed by a constant flow of air at 30 °C for months. Thus the permeation rate keeps constant and the absolute mass permeating from the tube can be determined directly by weighing of the tube. Limited by the permeation rate, the permeation tube is suitable only for low NO2 concentrations, but as a primary standard, it is well suitable for a precise calibration. We employed the permeation system for the calibration at low concentrations up to 10 ppm and to establish the detection limit of the TwinPAS system. Calibration for soot analysis was carried out using soot emitted by a diesel engine, with a concentration determined by a commercial PA soot sensor (Micro Soot Sensor 483, AVL List GmbH, Austria), which again is calibrated by filter sampling of soot and thermo-gravimetric analysis. Details on the detection of soot by PA spectroscopy, such as the size dependency,29 are discussed elsewhere.6,30 Although interferences to other particles cannot be excluded completely, the soot concentration measured by the PA-based Micro Soot Sensor is generally accepted for soot analysis in exhaust gas.31 As the mass specific absorption is different for different kinds of soot, calibration has to be carried out with real diesel soot.32 Quantitative analysis of both components without prior filtration of the sample gas implies a special calibration procedure for the two channels (532 and 806 nm) of the system. While NO2 does not feature a significant optical absorption at 806 nm, the black soot particles strongly absorb at both wavelengths. In the most general way, it can be assumed that both channels (532 and 806 nm) are sensitive to both analytes to a certain degree. Then, the two signals, S532 and S806, can be described in the following way: 532 532 S532 = C NO2·aNO + Csoot·asoot 2
(1)
806 806 S806 = Csoot·asoot + C NO2·a NO 2
(2)
with Csoot/NO2 being the concentrations of the two analytes soot and NO2, respectively, while the values asoot/NO2 are the calibration values for the respective analytes at the wavelengths indicated in the upper indices. For a full calibration, four values have to be determined, which theoretically requires measurements of only four calibration data points. The most straightforward approach would be the analysis of three samples, one containing only NO2 and no soot and the other one vice versa, while the other two points could be deduced from a sample containing neither soot nor NO2, which would suffice to reveal two calibration points. In practice, this four-point calibration results in rather imprecise results, especially for a low concentrations of one component and very high concentrations of the other. High and low in this context means the upper limit of the calibration range (high) and close to the LOD (low), respectively. For more stable calibrations, parallel analyses of the both components were carried out by the TwinPAS and by stateof-the-art analytical instrumentation. For NO2, a FT-IR instrument was employed (SESAM i60, AVL List GmbH, Austria), while soot was analyzed by the Micro Soot Sensor mentioned above. This parallel analysis was carried out during a 600 s dynamic driving cycle, which is employed for engine testing. It covers a large range of concentrations for both analytes (NO2, ∼5−900 ppm; soot, ∼4−1800 μg m−3). As the 7294
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reproducibly at the given positions at many repeated runs of the same driving cycles, while the effect could not be reproduced in alternative driving cycles or by singular engine settings. A reason for the observation could not be found, and due to the lack of a third independent analytical method, it was not possible to ascertain which of the two instruments showed the correct value. It could be suspected that the reason for the deviation is an incomplete mathematical correction of the cross-correlation of the two PA channels. In this case, a high soot concentration would result in an artifact for the NO2 analysis. However, this explanation can be ruled out as the soot concentration at the two critical positions is low (see Figure S-4 in the Supporting Information), and much higher soot concentrations in other positions of the test cycle do not manifest as artifacts in the NO2 analysis. Over the testing period of about 1 year, with a total of about 20 full test days at real engine test stands and many more days of testing with different artificial exhaust gas mixtures, including several long-time tests, no degradation of the instrument’s sensitivity was observed. In particular, the DPM crystal did not exhibit any indication of degradation, nor did the microphones lose sensitivity, as it might be expected due to the temperature gradient from the PA cell (80 °C) to the cooled microphones (45 °C). We did not observe indications for selective condensation of exhaust gas constituents, in particular water, on the cool microphone, which could reduce the microphone performance.
Figure 2. Rise time of the raw signals of both PA cells in response to a soot concentration of 1.0 g m−3.
soot and NO2 concentrations measured by the new instrument and the corresponding reference analysis are compared during a “US06 Supplemental Federal Test Procedure”. This driving cycle simulates aggressive, high speed, and/or high acceleration driving behavior, rapid speed fluctuations, and driving behavior following startup. It was chosen for representation here due to the high dynamics of the concentration changes. For the soot analysis, no significant deviation between the new system and the reference was detected (see Figure S-4 in Supporting Information). This observation is not surprising, since the commercial reference system, which is the current standard method, is based on the first PA soot sensor developed by the authors.6 The temporal resolution of both instruments is essentially the same. Comparing the results for NO2 (see Figure 3), it becomes obvious that our new
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CONCLUSIONS AND OUTLOOK
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ASSOCIATED CONTENT
To our knowledge, the new combined PA system is the only analytical tool which is optimized for a simultaneous detection of the two crucial components, soot and NO2, in exhaust gas. The direct optical analysis and the lack of filtration prior to the NO2 analysis help to avoid artifact formation. We believe that, similar to its predecessor, the Photoacoustic Soot Sensor,6 the new instrument, can be a valuable tool for combustion engine and exhaust gas after-treatment developers. The detection limits we summarize here are inferior to the one found for some other PA instruments, yet sufficient for exhaust gas applications. The other parameters, such as operating temperature, dynamic range, and temporal resolution, are optimized for the needs of such routine applications. Future work will be devoted to some questions of practical use: higher operation temperatures of more than 110 °C are desirable to avoid condensation of hydrocarbons present in exhaust gas. A possible alternative to the heat-sensitive microphones may be a Quarz tuning fork detection as proposed by Kosterev et al.33 Other tubing materials will be tested to reduce chemical interaction between the NO2 and the tubes, which hopefully will result in even better temporal resolution. Obviously, the implementation of further measuring channels for additional gaseous components is of great interest, in particular for components which need to be detected with high sensitivity at the subppm level and high temporal resolution.
Figure 3. Comparison of the NO2 concentrations measured by the new instrument and the corresponding reference analysis.
instrument has a temporal resolution slightly lower than the one of the FT-IR employed as reference (see for instance t = 470 s and t = 500 s). However, at two ranges, (t = 240 s and t = 290 s) significant deviations between our instrument’s results and the reference are observed, which cannot be explained by the lower temporal resolution. These deviations are observed
* Supporting Information S
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. 7295
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(27) Beck, A. H.; Bozoki, Z.; Niessner, R. Anal. Chem. 2000, 72, 2171−2176. (28) Nelson, O. G. Controlled Test Atmospheres: Principles and Techniques; Ann Arbor Science Publishers: Ann Arbor, MI, 1971; p 260. (29) Krämer, L.; Bozoki, Z.; Niessner, R. Anal. Sci. 2001, 17, S563− S566. (30) Petzold, A.; Niessner, R. Mikrochim. Acta 1995, 117, 215−237. (31) Kirchen, P.; Obrecht, P.; Boulouchos, K.; Bertola, A. J. Eng. Gas Turbines Power 2010, 132, 112804. (32) Schnaiter, M.; Horvath, H.; Mohler, O.; Naumann, H. K.; Saathoff, H.; Schock, W. O. J. Aerosol Sci. 2003, 34, 1421−1444. (33) Kosterev, A. A.; Bakhirkin, A. Y.; Curl, F. R.; Tittel, K. F. Opt. Lett. 2002, 27, 1902−1904.
AUTHOR INFORMATION
Corresponding Author
*Phone: +49 89 2180 7824. Fax: +49 89 2180 99 7824. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The support by A. Heubuch and J. Wolf during many hours of testing and calibration is gratefully acknowledged. We thank the team of the Institute of Internal Combustion Engines of the TUM (Prof. G. Wachtmeister) for the possibility to test the instrument at their engine test stand. We are indebted to R. Hoppe and S. Wiesemann of our mechanical workshop for the excellent manufacturing of the instrument.
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REFERENCES
(1) Myung, L. C.; Park, S. Int. J. Automot. Technol. 2012, 13, 9−22. (2) Lenner, M. Atmos. Environ. 1987, 21, 37−43. (3) Heeb, V. N.; Schmid, P.; Kohler, M.; Gujer, E.; Zennegg, M.; Wenger, D.; Wichser, A.; Ulrich, A.; Gfeller, U.; Honegger, P.; Zeyer, K.; Emmenegger, L.; Petermann, L. J.; Czerwinski, J.; Mosimann, T.; Kasper, M.; Mayer, A. Environ. Sci. Technol. 2010, 44, 1078−1084. (4) Johnson, V. T. Int. J. Engine Res. 2009, 10, 275−285. (5) Muller, O. J.; Su, S. D.; Jentoft, E. R.; Wild, U.; Schlogl, R. Environ. Sci. Technol. 2006, 40, 1231−1236. (6) Beck, A. H.; Niessner, R.; Haisch, C. Anal. Bioanal. Chem. 2003, 375, 1136−1143. (7) Steinbacher, M.; Zellweger, C.; Schwarzenbach, B.; Bugmann, S.; Buchmann, B.; Ordonez, C.; Prevot, H. A. S.; Hueglin, C. J. Geophys. Res. Atmos. 2007, 112, D11307. (8) Yamada, H.; Misawa, K.; Suzuki, D.; Tanaka, K.; Matsumoto, J.; Fujii, M.; Tanaka, K. Proc. Combust. Inst. 2011, 33, 2895−2902. (9) Tighe, J. C.; Twigg, V. M.; Hayhurst, N. A.; Dennis, S. J. Ind. Eng. Chem. Res. 2011, 50, 10480−10492. (10) Tighe, J. C.; Twigg, V. M.; Hayhurst, N. A.; Dennis, S. J. Combust. Flame 2012, 159, 77−90. (11) Haisch, C. Measure. Sci. Technol. 2012, 23, 012001. (12) Sigrist, W. M.; Fischer, C. J. Phys. IV 2005, 125, 619−625. (13) Sigrist, W. M.; Bartlome, R.; Marinov, D.; Rey, M. J.; Vogler, E. D.; Wachter, H. Appl. Phys. B: Laser Opt. 2008, 90, 289−300. (14) Petzold, A.; Niessner, R. Sens. Actuators, B 1993, 14, 640−641. (15) Arnott, P. W.; Moosmuller, H.; Rogers, F. C.; Jin, F. T.; Bruch, R. Atmos. Environ. 1999, 33, 2845−2852. (16) Moosmüller, H.; Chakrabarty, K. R.; Arnott, P. W. J. Quant. Spectrosc. Radiat. Transfer 2009, 110, 844−878. (17) Ajtai, T.; Filep, A.; Schnaiter, M.; Linke, C.; Vragel, M.; Bozoki, Z.; Szabo, G.; Leisner, T. J. Aerosol Sci. 2010, 41, 1020−1029. (18) Schramm, D. U.; Sthel, M. S.; da Silva, M. G.; Carneiro, L. O.; Junior, A. J. S.; Souza, A. P.; Vargas, H. Infrared Phys. Technol. 2003, 44, 263−269. (19) Slezak, V.; Santiago, G.; Peuriot, L. A. Opt. Lasers Eng. 2003, 40, 33−41. (20) Berrou, A.; Raybaut, M.; Godard, A.; Lefebvre, M. Appl. Phys. B 2010, 98, 217−230. (21) Bijnen, C. F. G.; Harren, M. F. J.; Hackstein, P. J. H.; Reuss, J. Appl. Opt. 1996, 35, 5357−5368. (22) Bohren, A.; Sigrist, W. M. Infrared Phys. Technol. 1997, 38, 423− 435. (23) Miklos, A.; da Silva, M. G.; Hess, P. J. Phys. IV 2005, 125, 3−5. (24) Sthel, S. M.; Schramm, U. D.; Faria, T. R.; Castro, P. M. P.; Carneiro, O. L.; Ribeiro, S. W.; Vargas, H. J. Phys. IV 2005, 125, 881− 883. (25) Slezak, V. Appl. Phys. B 2001, 73, 751−755. (26) Mihalcea, M. R.; Baer, S. D.; Hanson, K. R. Appl. Opt. 1996, 35, 4059−4064. 7296
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