Quantitative HNO detection behind shock waves

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Proceedings of the Combustion Institute 000 (2016) 1–9 www.elsevier.com/locate/proci

Quantitative HNO detection behind shock waves Nancy Faßheber, Marvin C. Schmidt, Gernot Friedrichs∗ Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Kiel 24118, Germany Received 30 November 2015; accepted 27 May 2016 Available online xxx

Abstract Knowledge about the high temperature kinetics of HNO, which represents a key species in NOx flame chemistry, is scarce. Using the very sensitive absorption based frequency modulation (FM) spectroscopy, HNO has been measured behind shock waves for the first time. The UV photolysis of glyoxal/NO mixtures at λ = 193 nm served as an HNO source with stable HNO concentration plateaus generated via the fast initial formation of HCO radicals followed by the reaction HCO + NO → CO + HNO. FM line shapes of three selected transitions of the A˜ 1 A (100) − X˜ 1 A (000) band of HNO have been recorded and their pressure broadening coefficient has been determined. By analyzing HNO and HCO concentration-time profiles measured under very similar reaction conditions it was possible to confirm an HCO/HNO formation mechanism and to determine the HNO absorption cross section. The expression log [σc (HNO, p = 0 bar, 295 K < T < 1800 K)/cm2 mol−1 ] = 5.86 – 1.62 × 10−3 × T /K + 2.74 × 10−7 × T 2 /K2 with an uncertainty of ± 40% in σ c is recommended for the R R3 (4) rotational line at ν˜ = 16173.86 cm−1 (618.2816 nm). The capability to perform quantitative HNO measurements behind shock wave make possible direct rate constant studies of bimolecular HNO reactions in future experiments. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: HNO detection; NOx formation; Shock tube measurements; High-temperature kinetics; Frequency modulation (FM) spectroscopy

1. Introduction HNO (nitrosyl hydride, azanone, nitroxyl) is an important flame intermediate closely linked to NOx pollutant formation. Many HNO reactions with other flame intermediates such as H, OH, and O directly form NO [1]. Depending on the combustion conditions, most of these reactions can also proceed in the opposite direction and in this case ∗

Corresponding author. Tel.: +49 4318807742. E-mail address: [email protected] (G. Friedrichs).

decrease the amount of NO formed in flames. For example, the reaction H + NO + (M)  HNO + (M) contributes significantly to NO reduction under oxy-fuel conditions [2] and by reburning using CO/H2 mixtures as reducing agents [3]. Detailed modeling of HNO chemistry remains difficult because most HNO reactions are poorly investigated experimentally. In general, intermediate concentrations of HNO in flames are low and high temperature absorption cross sections are small. HNO measurements at high temperatures have only been reported by Lozovsky et al. [4–6]. They used the sensitive intracavity laser absorption spectroscopy

http://dx.doi.org/10.1016/j.proci.2016.05.035 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.

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(ICLAS) to monitor HNO spectra in low-pressure hydrocarbon flames doped with nitrogen oxides. Earlier, based on combined HNO and HCO measurements, Cheskis et al. [7] reported a low room temperature HNO absorption cross section value of (2.0 ± 1.0) × 104 cm2 /mol. Rate expressions for many HNO reactions implemented into combustion mechanisms often rely on experiments at low temperatures or on theoretical estimates. Early experimental rate constant data from flame measurements are available [8–10] for the two NO forming hydrogen abstraction reactions HNO + H → H2 + NO

(1)

and HNO + OH → H2 O + NO. The results of these three studies differ by a factor of about 2.6 for reaction (1) and 8.3 for the reaction HNO + OH. More recent ab initio calculations [11,12] provided significantly higher reaction rates for HNO + H. For the reaction HNO + O, most combustion mechanisms still rely on a rate expression measured at temperatures below 473 K [13], although more recent theoretical calculations for high temperatures are available [14]. For the reaction HNO + O2 → HO2 + NO only one experimental expression is available in the literature for temperatures between 296 and 421 K [15]. More data exist for the reaction HNO (+ M)  H + NO (+ M) where three recent experimental studies are in reasonable agreement up to temperatures of T = 1170 K [16–18]. Finally, the reactions HNO + NO [19] and NO2 [20] have been implemented in many mechanisms using mostly estimated rate expressions. In this work we present the first HNO absorption measurements behind shock waves. The determined HNO high-temperature absorption cross section forms the basis for much-needed direct kinetic measurements of bimolecular HNO reactions at combustion relevant temperatures. 2. Experimental 2.1. Shock tube and slow flow cell setup High temperature measurements were carried out in a shock tube with inner diameter of 81 mm, which has been described in detail elsewhere [21]. It was operated with hydrogen/nitrogen mixtures as driver gas using aluminum foils as diaphragms. The experimental conditions were calculated from the pre-shock conditions together with the shock wave velocity. A frozen-chemistry code has been applied taking into account shock wave damping. Room temperature experiments were performed in a 45 cm long slow flow cell. An ArF excimer laser was used for 193 nm photolysis of glyoxal/NO mixtures. The detection and the UV laser beams were collinearly overlapped in front and behind the shock tube/flow cell. For

the shock tube experiments the photolysis beam, which was triggered with a delay of about 20 μs with respect to the reflected shock wave arrival, was slightly focused by a 1 m lens (about 4 mm effective beam diameter corresponding to a laser energy fluence of about 60 mJ/(cm2 pulse)). For the room temperature measurements the area of the UV beam was reduced by a telescope to about 1 cm2 , hence in both cases the excimer laser beam diameter was larger than the diameter of the detection laser (about 1 mm) to minimize diffusional losses. The slight focus of the photolysis beam in the shock tube experiments provided for a uniform photolysis yield despite significant light attenuation by glyoxal absorption [21]. Glyoxal has been prepared by dehydration of its trimer dihydrate ((CHO)2 )3 × 2H2 O by 3 molar equivalents of phosphoric anhydrate (P2 O5 ) [21,22]. NO was purified by several freeze-pumpthaw cycles. Storage gas mixtures of about 2% glyoxal and 3% NO in Ar were prepared in a gas mixing system using the partial pressure method. Further mixing and dilution was accomplished by a flow system with mass flow controllers. 2.2. FM spectroscopy HNO and HCO were detected by means of frequency modulation (FM) spectroscopy, a sensitive absorption based detection method, which has been successfully used for shock tube detection of NH2 , 1 CH2 , and HCO in the past [23–25]. Reported minimum detectable absorbances are below 1 × 10−4 at a time resolution of about 1 μs, hence comparable to the sensitivities reported for novel cavity-enhanced absorption spectroscopy approaches [26,27]. A summary of the theoretical background of the FM method including its experimental implementation for quantitative timeresolved measurements can be found in the review by Friedrichs [28]. The setup used in this work was similar to the one described by Friedrichs et al. [25]. Briefly, the detection light was generated by a cw ring dye-laser (Coherent 899) pumped by a solid state Nd:YVO4 laser. The wavelength was measured interferometrically by a wavemeter with an accuracy of ν˜ ± 0.008 cm−1 [29]. The laser beam was modulated at a frequency of 1 GHz by a resonant electro-optic modulator, the resulting frequency modulated spectrum with a modulation index of M ≈ 1.4 was analyzed by a scanning etalon. The laser beam was focused through the shock tube windows into an optical fiber and was detected by a fast silicon photodiode. The resulting demodulated FM signal IFM is proportional to the concentration [c] of the absorbing species according to I0  f σc [c]l × G. (1) 2 σ c is the narrow-bandwidth line center absorption cross section and l the absorption path length. The IFM =

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electronic gain factor G = 184 ± 18 of the used FM setup was determined experimentally [28]. The FM factor f was calculated from line shape data and was about  f = 0.8 for HNO and  f = 0.3 for HCO under the experimental conditions of this work. The lower value for HCO resulted from the broader line shape of the Q(6)P(1) absorption line of the A˜ 2 A − X˜ 2 A (090 0 ← 001 0) transition at λ = 614.752 nm. Whereas the FM detection scheme for HCO has been well established in previous work, HNO has been detected for the first time. From previous room temperature measurements, HNO spectroscopy is well known [30–32]. In the high-temperature studies of Lozovsky et al. [4,6], HNO has been detected on (100 ← 000) and (011 ← 000) transitions of the A˜ 1 A X˜ 1 A band at wavelengths around 618 nm and 643 nm, respectively. Although it was found that the observed absorption lines in the (011 ← 000) band are about three times more intense, (100 ← 000) transitions have been selected for this work. This band is spectrally close to the detection wavelength used for HCO detection and hence allowed us to switch in-between HNO and HCO detection without extensive readjustment of the laser system. Alternating HNO and HCO detection was necessary to ensure comparable reaction and photolysis conditions for successive HNO and HCO experiments.

3

Table 1 Experimental pressure broadening coefficients for selected HNO absorption lines. Line ν˜center / assignment cm−1 RQ RR RR RR

0 (16) 3 (3) 3 (3) 3 (4)

Taverage / paverage / ν˜ p / K mbar (GHz/bar)

16002.36 1250 16171.99 970 16171.99 298 16173.86 770

1140 880 100 570

1.0 1.2 3.3 1.5

chemistry including further HNO reactions have been adopted from GRI-Mech 3.0 [36]. 3. Results and discussion Shock tube and room temperature measurements of HNO and HCO concentration-time profiles at similar reaction conditions (temperature, pressure, mixture composition, and photolysis energy) have been performed. Different reaction mixtures with initial [NO]/[glyoxal] ratios from 0.18 to 0.87 were used. Additionally, some photolysis experiments with pure glyoxal/argon mixtures have been performed in order to validate the assumed glyoxal/HCO submechanism. All measured HCO profiles could be nicely reproduced using the initial H/HCO yield as the only adjustable parameter. 3.1. HNO detection and pressure broadening coefficient

2.3. HNO source HCO generation from λ = 193 nm photolysis of glyoxal/NO mixtures and the subsequent reaction HCO + NO served as HNO source according to the reaction sequence (CHO )2 + hν → (H, HCO, CO, H2 , CH2 O),

(2)

H + (CHO )2 → HCO + CO + H2 ,

(3)

HCO + NO → HNO + CO.

(4)

To model the overall HNO yield, all relevant glyoxal and HCO reactions [22,33,34], the initial [H]0 /[HCO]0 ratios from the photolysis reaction (2) (about three at high temperatures and about two at room temperature [21]), as well as the HCO absorption cross section [25] have been adopted from our previous work. The HCO and glyoxal submechanisms have recently been merged and tested for their reliability [35]. For reaction (4), rate constant data have been taken from the recent shock tube study of Dammeier et al. [34]. As the obtained HNO concentration is directly linked to the HCO yield through reaction (4), the overall HNO yield could be determined by quantitative measurements of HCO profiles resulting from the photolysis of glyoxal and glyoxal/NO mixtures. Table 2 lists the most important reactions and their kinetic parameters as used in this work. Additional background

For quantitative HNO detection, the exact line position and the value of the pressure broadening coefficient needs to be determined experimentally. For this purpose, glyoxal/NO photolysis experiments with the detection laser incrementally scanned over three selected absorption lines have been performed. The room temperature HNO absorption spectra measured by Pearson et al. [32] together with simulations of the high temperature absorption spectrum using the program PGOPHER [40] were used as a starting point. Corresponding FM line shapes, which reflect the wavelength dependence of the FM factor and hence also depend on the applied modulation index, were calculated with home-written software. Figure 1 illustrates the measured FM data points (symbols) in comparison with the simulated normalized absorption bands (dashed curves) and FM line shapes (solid curves). The three FM profiles (experimental conditions see Table 1) could be well reproduced by adjusting the pressure broadening coefficient that entered into the calculation of the underlying Voigt line profiles. In agreement with the PGOPHER simulations, absorptions have been found to be similar for the two R R3 (J) lines but two times higher for the R Q0 (16) line at T ≈ 1000 K. Assuming that the pressure broadening is similar for all three lines, the measurements at T = 298 K,

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Fig. 1. Scaled experimental high temperature FM line shape data of HNO (symbols) in comparison with simulated FM (solid curves, M = 1.4) and normalized absorption spectra (dashed curves) for three absorption lines of the A˜ 1 A (100) − X˜ 1 A (000) transition (experimental conditions see Table 1). Table 2 Important reactions for modeling the HCO/HNO yield from the photolysis of glyoxal/NO mixtures. Parameters for the Arrhenius expression k = A exp(−Ea /(RT )) are given in units of mol, cm, s and kJ. No.

Reaction

A

1 2 3 4

HNO + H → H2 + NO (CHO)2 + hν → (H, HCO, CO, H2 , CH2 O) (CHO)2 + H → H2 + CO + HCO HCO + NO → HNO + CO

1.8 × 1013

5 6 7 8 9

HCO + H → H2 + CO HCO + M → H + CO + M HCO + HCO → CH2 O + CO NO2 + H → NO + OH (CHO)2 + OH → H2 O + CO + HCO

10 a

HNO diffusion

Ea

5.4 7.1 8.1 1.1 4.0 2.7 9.0 1.3 6.4 1 7

× × × × × × × × × × ×

1013 1012 1012 1014 1013 1013 1013 1013 1012 103 102

4.2 18

65

Ref.

T range

[37] [21] [21] [34] [34] [33] [25] [33] [38] [21] [39] This work This worka

298–2000 K 295–1107 K 770–1300 K 295 K 295–2100 K 835–1230 K 295-21 K 195–2000 K 700–1150 K 298 K 750–1275 K 298 K

Estimated based on the high temperature value assuming kdiffusion ∝T1.5 /(p d), d is the diameter of the photolysis beam.

770 K, 970 K, and 1250 K yielded a temperature dependent pressure broadening coefficient of ν p = 3.3 × (T /298 K)−0.84 GHz/bar. 3.2. HNO formation at high temperatures For quantitative HNO modeling it is necessary to know the initial [HCO]0 concentrations arising from the glyoxal photolysis. Ideally, simultaneous measurements of HCO and HNO profiles would have been performed to directly correlate HCO and HNO yield. However, as either HCO or HNO could be detected with our setup, all experiments have been performed at least twice under very similar reaction conditions by successively detecting HCO and HNO. In Fig. 2a and b typical experimental and numerically modeled HCO and HNO concentration-time profiles are displayed. Based on

the reaction mechanism outlined above, the HCO concentration-time profiles could be very well simulated without any adjustment. Only the initial [HCO]0 concentrations were varied to fit the maximum of the measured HCO profile. More or less constant HNO concentration plateaus have been observed toward longer reaction times showing that HNO is a stable species under the reaction conditions applied in this work. Note that HNO is a closed-shell species. We attribute the slow HNO decay to minor diffusional losses of HNO out of the photolysis volume rather than a real HNO loss reaction. A first-order rate constant of kdiff ≈ 103 s−1 was consistent with the observed HNO losses at longer reaction times. The sensitivity analysis of the HNO experiment illustrated in Fig. 2c reveals the five most important reactions for HNO formation and consumption. Essentially, the

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Fig. 2. (a) HCO concentration-time profile measured after glyoxal photolysis in comparison with the simulation for HCO and H. (b) Experimental and two simulated HNO concentration-time profiles assuming k1 from Tsang and Herron [37] (solid curve) and Nguyen et al. [12] (dotted curve) at similar reaction conditions. (c) Corresponding HNO sensitivity analysis for k1 taken from [37].

HNO profile is only sensitive to reactions from the glyoxal/HCO submechanism. Their rate constant values cannot be changed without deteriorating the simulations of the HCO profiles. However, to asses the potentially high uncertainty that exists for the HNO chemistry, the rate constants of all HNO reactions, like HNO + M/H/OH/O/O2 /NO, were varied by a factor of up to 100. Only the assumed rate constant for reaction (1), HNO + H, turned out to change the modeled HNO profile by more than 1%. Available literature data for k1 at high temperatures differ by two orders of magnitude [9,11,12,37]. Using the recommended value of Tsang and Herron [37] the reaction is barely sensitive under the experimental conditions of this work. However, using the 5.4 times higher rate constant from the recent theoretical estimate by Nguyen et al. [12], the simulation of the experiment shown in Fig. 2b would

yield a 20% lower value for the HNO plateau (dotted curve) and hence a 20% higher value for the HNO absorption cross section. Knowing the absolute values of the HNO plateau concentrations from the consistent modeling of HCO and HNO profiles, next the HNO absorption cross section could be determined from Eq. (1). The experimental conditions of 22 shock tube experiments and the determined narrow bandwidth absorption cross section values for the R R3 (4) transition at ν˜ = 16173.86 cm−1 are listed in Table 3 and are plotted as function of temperature in Fig. 3 (open circles). Between 721 K < T < 1133 K the Napierian cross section can be approximated by the linearized expression log (σc (p = 0)/(cm2 mol−1 )) = 5.45 − 8.8 × 10−4 × (T /K).

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Table 3 Experimental conditions and results for HNO line center absorption cross section σ c . All values refer to the R R3 (4) transition at ν˜ = 16173.86 cm−1 and have been corrected to Doppler limited conditions (i.e., p = 0 bar). p / mbar

x((CHO)2 ) / ppm

Room temperature 50 9767 50 9990 95 10400 100 9850 101 10080 102 9970 104 15400 T /K

p / mbar

Reflected shock wave 721 495 765 554 771 570 785 591 808 625 837 660 863 596 898 755 908 770 916 781 920 790 938 819 948 835 983 891 1043 1360 1046 1373 1049 1371 1055 1032 1065 1049 1117 942 1118 1542 1133 1170

x(NO) / ppm

x(O2 ) / ppm

x(NO2 ) / ppm

x(HCO) / ppm

σc × 10−5 / cm2 mol−1

4010 4570 6530 8220 4160 4120 6655

6900 1500 6095 2000 640 2790 3400

30 70 15 80 20 15 10

16 20 19 19 12 13 22

2.4 2.2 2.0 2.5 2.3 2.1 2.1

x0 (CHO)2 / ppm

x(NO) / ppm

x(HCO) / ppm

σc × 10−4 1 / cm2 mol−

8915 7995 9990 9990 9990 7735 10645 7880 7735 8080 7710 7690 7975 7880 11505 11640 11505 10810 10270 9040 11500 10270

7780 4370 8670 8670 8670 4425 6750 4600 4425 1454 5525 6620 5090 4600 7180 7260 7180 5825 8050 6990 7175 8050

100 125 120 120 130 150 120 120 150 130 160 160 130 120 90 90 90 100 110 150 90 110

7.4 6.9 5.8 5.6 5.0 4.2 5.1 5.4 3.9 4.0 3.7 4.8 3.9 4.9 3.1 4.2 3.7 4.0 2.9 2.9 2.8 2.6

Fig. 3. Temperature dependence of the HNO absorption cross section of the R R3 (4) absorption line.

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Fig. 4. (a) Experimental concentration-time profiles of HCO and (b) HNO under the same reaction conditions (room temperature, p = 50 mbar, x((CHO)2 ) = 9990 ppm, x(O2 ) = 1500 ppm) together with simulated profiles. Dashed and dotted curves represent simulations without oxygen and hence without NO2 and a simulation neglecting reaction channel (4b) (see text), respectively.

Based on simulations, the cumulated uncertainties of the rate constants of the HNO formation mechanism (±20%), the gain factor (±10%), and the statistical scatter of the data (±15%) together with a 30% uncertainty in the HCO absorption cross section account for a reasonable overall error estimate of about 40%. 3.3. HNO formation at room temperature FM measurements of HNO at room temperature have been performed to verify the absolute value and the temperature dependence of the HNO absorption cross section determined from the shock tube experiments. The experimental conditions of 7 experiments are summarized in Table 3. Mixtures of about 1% glyoxal and 4010 to 8220 ppm NO were used at pressures of 50 and about 100 mbar. O2 has been added to the reaction mixtures on purpose to capture H atoms from glyoxal photolysis by the formation of NO2 during the mixing of the reactants according to the equilibrium 2NO + O2  2NO2 and the fast consecutive reaction NO2 + H → NO + OH (8). In Fig. 4a and b a typical HCO and HNO concentration-time pro-

file is shown together with the best numerical fits. Simulations without O2 and hence without NO2 are shown as dashed curves as well. As expected, the overall HNO yield as well as the HCO concentration at longer reaction time would have been significantly lower without the addition of O2 . It is known from theoretical work of Xu et al. [41] that at room temperature a second reaction channel for the HNO forming reaction (4) needs to be considered. HCO + NO → HNO + CO

(4a)

HCO + NO → HC(O)NO

(4b)

For reaction channel (4b), to simplify matters, the formation of a stable complex was assumed that did not further contribute to the overall observed HNO yield. Starting from the theoretically predicted branching fraction φ4a = 0.75, φ 4a has been fine-tuned in the simulations by carefully fitting the shape of the HNO concentration-time profiles. As it is illustrated in Fig. 4b by the strongly diverging dotted curve obtained by setting φ 4a = 1, the assumed branching ratio is also crucial for the overall absolute HNO concentration. As both the shape

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and absolute concentrations of the HCO profiles from photolysis experiments without the addition of NO/O2 could be well reproduced (not shown in Fig. 4), the HCO and HNO profiles from the experiments with NO/O2 addition were analyzed by a combined fitting procedure using the initial NO2 mole fraction, the branching ratio φ 4a , and the HNO absorption cross section as the only adjustable parameters. In very good agreement with the value reported by Xu et al., a branching ratio of φ4a = 0.70 for the HNO forming channel was found to be fully consistent with our experiments. Overall, an averaged room temperature absorption cross section of σ c ( p = 0 bar) = (2.2 ± 0.9 ) × 105 cm2 /mol has been determined. The estimated cumulative uncertainty of ±40% mainly reflects the uncertainties of the assumed initial mole fractions of HCO, H, and NO2 as well as of the assumed branching ratio φ 4a . 3.4. HNO absorption cross section Figure 3 compares the experimental cross section values (scaled to p = 0 bar) for the R R3 (4) absorption line from the room temperature (squares) and high temperature experiments (circles) with a theoretical prediction of the expected temperature dependence (dashed curve) taking into account state population and line shape effects: σc (T ) Q(Tref ) g(νc , T ) = × σc (Tref ) Q(T ) g(νc , Tref )    E 1 1 × exp − − kB T Tref Here, E/hc = 180.5 cm−1 is the term value of the lower state of the probed transition, Q the rovibrational partition function of HNO, and g(ν c , T) the Doppler line shape function value at line center. Molecular constants were taken from Jacox and Milligan [31] and Pearson et al. [32]. Overall, the temperature dependence of the experiments is well described by the scaled theoretical curve showing that the kinetic modeling of the room and high temperature experiments is consistent. Therefore, an extrapolation of the absorption cross section to flame temperatures is reliable. Over the temperature range 295K < T < 1800 K the Doppler limited narrow-bandwidth Napierian absorption cross section at line center can be approximated by the expression log[σc (HNO, p = 0 bar )/cm2 mol−1 ] = 5.86 − 1.62 × 10−3 × (T /K) +2.74 × 10−7 × (T 2 /K2 ) For reference, the temperature dependent cross section at p = 1 bar is shown in Fig. 3 as well (dotted curve). At a typical flame temperature of T = 1750 K, with a 30% lower value for σ c , the effect of pressure broadening is significant.

To our knowledge the only other value for an HNO absorption cross section was given by Cheskis et al. [7]. They estimated the room temperature absorption cross section for the R R3 (6) rotational line of the (100 ← 000) vibronic band, which exhibits roughly the same intensity as the R R3 (4) line measured in this work. Similar to this work, they used the flash photolysis of acetaldehyde/NO mixtures followed by the reaction (5) as an HNO source. Based on a known value of the HCO absorption cross section and on the assumption that the formed HCO is completely converted into HNO, they reported a cross section value of (2.0 ± 1.0) × 104 cm2 /mol. For a comparison with our data, their value needs to be corrected by a factor of about 1.4 to account for the neglected channel (4b), a factor of 1.4 for pressure broadening and a factor of about 2.7 to account for the lower spectral resolution of the ICLAS detection scheme. The resulting cross section values of about σc = (1.1 ± 0.5) × 105 cm2 /mol compares well with the value of σc = (2.2 ± 0.9) × 105 cm2 /mol obtained in this work. Interestingly, based on their cross section value, Lozowsky et al. [6] measured absolute HNO concentrations in the pre-flame zone of a methane/NO flame at T = 600 K, which turned out to be a factor of about 10–15 higher than the kinetic model prediction. Using our cross section value, scaled to the experimental conditions of Lozowsky et al., would still yield a factor of 3–5 too low modeled HNO peak mole fractions for the pre-flame zone. This still obvious discrepancy clearly shows that the current understanding of HNO flame chemistry remains unsatisfying. With the experimental setup and HNO detection scheme presented in this work, direct hightemperature measurements of still poorly investigated bimolecular HNO reactions such as HNO + H/O/OH/O2 /NO/NO2 become possible. In principle, in future experiments the current detection limit of about 3.5 × 10−10 mol/cm3 at T = 1000 K and p = 1000 mbar can be further improved by selecting transitions in the (011 ← 000) vibronic band at wavelength around 643 nm in combination with higher experimental modulation strengths attainable with the latest generation of resonant electrooptic modulators. Acknowledgment We acknowledge the German Science Foundation (DFG-FR 1529/4) for financial support. References [1] N. Lamoureux, P. Desgroux, A.E. Bakali, J.F. Pauwels, Combust. Flame 157 (10) (2010) 1929–1941. [2] B. Wang, L. Sun, S. Su, J. Xiang, S. Hu, H. Fei, Appli. Energy 92 (2012) 361–368.

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Please cite this article as: N. Faßheber et al., Quantitative HNO detection behind shock waves, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.05.035