Identification of Gas-phase Pyrolysis Products in a Prescribed Fire : 1 Seminal Detections Using Infrared Spectroscopy for Naphthalene , 2 Methyl Nitrite , Allene , Acrolein and Acetaldehyde * * 3 4

**This manuscript was prepared in part by a federal government employee as a part of his official duties and is therefore in the public domain and not subject to copyright. Identification of Gas-phase Pyrolysis Products in a Prescribed Fire: 1 Seminal Detections Using Infrared Spectroscopy for Naphthalene, 2 Methyl Nitrite, Allene, Acrolein and Acetaldehyde** 3 4 Nicole K. Scharko1, Ashley M. Oeck1, Russell G. Tonkyn1, Stephen P. Baker2, 5 Emily N. Lincoln2, Joey Chong3, Bonni M. Corcoran3, Gloria M. Burke3, David R. Weise3, 6 Tanya L. Myers1, Catherine A. Banach1, and Timothy J. Johnson1* 7 8 1Pacific Northwest National Laboratories, Richland, WA, USA 9 2USDA Forest Service, Rocky Mountain Research Station, Missoula, MT, USA 10 3USDA Forest Service, Pacific Southwest Research Station, Riverside, CA, USA 11 12


INTRODUCTION
Wildland fire releases significant quantities of trace gases into the environment (Crutzen et al., gases can significantly influence atmospheric chemistry (Crutzen et al., 1990). In some parts of 29 the world, wildfires are becoming more prevalent as well as increasing in impact (Miller et al.,30 2009; Turetsky et al., 2011). In many areas, however, prescribed burning is used as a preventive 31 tool to reduce hazardous fuel buildups in an effort to reduce or eliminate the risk of such wildfires 32 (Fernandes et al., 2003). Understanding the products associated with the burning of biomass has 33 received considerable attention since the emissions can markedly impact the atmosphere. Fourier 34 transform infrared (FTIR) spectroscopy is one technique that has been extensively used to identify determine if such species' signatures are also found sequestered in the IR spectra associated with 52 wildland fire, and are thus amenable to IR detection. A second goal of the present study, whose 53 biomass burning results are mostly detailed in a separate manuscript, is to better understand 54 pyrolysis. Every wildland fire consists of two processes: thermal decomposition (pyrolysis) of 55 solid wildland fuels into gases, tars, and char followed by combustion (oxidation) of the pyrolysis 56 products resulting in flame gases and particulate matter in the smoke. Description and 57 measurement (by any means) of the pyrolysis products adjacent to the flames of a wildland fire 58 has seldom been performed. Non-intrusive measurement of the (pyrolysis) gases in the near-flame 59 environment is desirable from both a scientific and safety perspective. 60 The major gas-phase compounds emitted from wildland fires are H2O, CO2, CO and CH4 (Ward 61 et al., 1991), all of which are easily identified and quantified via FTIR spectroscopy. Lightweight 62 hydrocarbons, oxygenated hydrocarbons, nitrogen and sulfur species are all minor products that 63 are also generated during burns (Talbot et al., 1988;Lobert et al., 1991;Yokelson et al., 1996). A 64 host of more complex gases which can condense to form tar are also produced by pyrolysis of of compounds not found in Table 1. As a partial guide of species for which to investigate, we

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In this study, we have chosen to examine field fire spectra for species that can be detected and 78 quantified via IR spectroscopy both to add to the list of compounds, but also to improve the 79 characterization (and ultimately the detection limits) of the other species listed in Table 1. That is   80 to say, fire IR spectra are very complex and contain many overlapping peaks; the success of the   emitted at the base of the flames before ignition were collected using an extractive probe and stored 99 in 3-liter Summa canisters. This approach was performed to selectively collect pyrolysis gases 100 prior to the onset of combustion. Details regarding the site description and sampling apparatus will 101 be provided in a separate paper.

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Gases were analyzed in the laboratory (on the same day or the day following the fire) using an 8-104 meter multipass (White) cell (Bruker Optics, A136/2-L) mounted in the sample compartment of a 105 Bruker Tensor 37 FTIR. Ten canisters were returned from the field to the laboratory and in turn 106 connected to the gas cell via 3/8" stainless steel tubing. The tubing and gas cell were both heated 107 to 70°C to prevent analyte adhesion to the inner surfaces. The White cell (White, 1942) was 108 equipped with a pressure gauge and temperature probe, both of which were located on the gas 109 outlet port; the thermocouple wire temperature probe extended into the White cell volume in order  The White cell contained the analyte smoke for the sample spectrum measurement, but was filled 124 with ultra-high purity nitrogen gas for the reference spectrum measurement (Johnson et al., 2013).

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The FTIR interferometer, detector and sample compartments were purged with dry air from a dry-126 air generator. The Tensor 37 was equipped with a globar source, a KBr beamsplitter and a 127 broadband liquid nitrogen cooled mercury cadmium telluride (MCT) detector, providing spectral 128 coverage from 7,500 to 500 cm -1 . The spectral resolution was 0.6 cm -1 and a 2 mm Jacquinot 129 aperture was used. The acquisition mode was set to double-sided, forward-backward. For the 130 Fourier transform, the data were apodized with a Blackman-Harris 3-Term function using a zerofill 131 factor of 4, and phase corrected via the Mertz (Mertz, 1967)

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The program used for quantitative spectral analysis was MALT5 (Griffith, 2016), which uses both

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As mentioned in section 2.2, the spectral resolution was set to 0.6 cm -1 , which is the highest 159 resolution obtainable with this instrument. There are many benefits, but also a few disadvantages 160 to using higher resolution (Herget et al., 1979). Most importantly, the higher resolution allows one  deresolved spectra at b) 1 cm -1 , c) 2 cm -1 , and d) 4 cm -1 . With the reference spectra for the original 172 0.6 cm -1 measurement and the 1 cm -1 deresolved spectrum (Figure 2a and b), the absorption lines 173 for C2H2 and naphthalene overlap, but the 782 cm -1 feature from naphthalene is still slightly visible 174 in the original spectra. The naphthalene peak appears clearly in the residuals when it is not included 175 in the fitting process, but is removed from the residual when naphthalene is included in the fit 176 (discussed further below). As the resolution is reduced (Figures 2c and 2d), however, the features 177 broaden and the distinction of the naphthalene peak from C2H2 and other minor components (i.e.

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CO2, HCN, H2O, spectra not shown) is compromised. The specificity between the compounds is 179 lost and the confidence in the identification/quantification of the target species, particularly for the 180 weaker absorbers, diminishes as the resolution decreases. The well-known benefits of using a 181 lower resolution are that spectra can be acquired more quickly with an improved signal-to-noise 182 ratio. For the present measurements, 0.6 cm -1 was deemed an appropriate resolution. with and without naphthalene included in the fit for the a) original spectrum collected at 0.6 cm -1 and the 186 deresolved spectra at b) 1 cm -1 , c) 2 cm -1 , and d) 4 cm -1 . The reference spectra for CO2, HCN and H2O are 187 not shown (HCN was not included in fit when the resolution was 4 cm -1 ; for resolutions 1, 2 and 4 cm -1 , 188 H2O was not included in the fit when naphthalene was removed from the fit). Spectra are offset for clarity. 189 complex features arising from numerous chemicals. That is to say, the residual is not due to just 197 random instrumental noise, but instead, due to spectral features that can arise in the spectra, e.g.

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imperfectly subtracted features from strong absorbers or unidentified absorbers. For that reason, 199 we report signal-to-residual, not signal-to-noise detection limits. The detection limits for each 200 compound in this study were thus derived using a value of three times the root-mean-square (RMS) 201 value of the residual calculated over the corresponding frequency range (e.g. 800-760 cm -1 was 202 used for naphthalene). The peak-to-peak noise is more sensitive to fluctuations in the fit with levels 203 typically 4 to 5× the RMS noise (Griffith et al., 2006). For the present data, however, the peak-to-204 peak values ranged from 5 to 10× the RMS noise, thus suggesting the peak-peak values tend to

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When modeling the burning process (Byram, 1959), complete combustion of 1 kg dry wood 215 produces 1.82 kg CO2 and 0.32 kg H2O for a total mass of products of 2.14 kg. Incomplete     Table 3 presents the range of measured mixing ratios for naphthalene along with averaged 266 detection limits for the 10 measurements collected during the prescribed burns as well as for the 267 other four reported compounds. In the measurements, naphthalene's mixing ratios ranged from 1.4 268 to 19.9 ppm, and the averaged RMS-derived detection limit was 1.6 ± 0.5 ppm; different detection 269 limits were observed for each spectrum. One of the measurements had a mixing ratio of 2.9 ppm, 270 yet its RMS-derived detection limit was 3.7 ppm, and is thus below the estimated detection limit   Naphthalene emitted from prescribed burns is thus clearly detectable using IR spectroscopy. The

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We also note that methyl nitrite is an oxidizing agent and is used as a rocket propellant. It is thus 302 plausible that the methyl nitrite detected in the present study was not a product of the fire, but  With regards to the IR spectra, methyl nitrite exists in equilibrium as a mixture of two conformers-307 cis and trans; at room temperature (25°C) it is estimated as 58% cis and 42% trans (Bodenbinder 308 et al., 1994). We were able to use the same band associated with both conformers, namely the ν8 309 band, which is at 841.1 cm -1 for the cis conformer and at 812.4 cm -1 for the trans conformer 310 (Ghosh et al., 1981). The ν8 mode is associated with the N-O stretch and is very strong for both 311 conformers (Ghosh et al., 1981). We note that methyl nitrite also has very strong bands at 627.8 312 cm -1 (cis) for ν9 ONO bending, as well as at 1620.1 cm -1 (cis) and 1677.4 cm -1 (trans) due to the 313 ν3 N=O stretch (Ghosh et al., 1981). These bands, however, are of lesser utility for IR detection:

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The ν9 peak is masked by CO2 bending mode lines, and the ν3 peak is obfuscated by the H2O 315 bending mode lines. The spectral region used for evaluation was 865-775 cm -1 , which contains the ν8 band for both the 317 cis and trans conformers (Ghosh et al., 1981). Figure 4 shows the experimental spectrum from 318 the prescribed burn, along with the scaled reference spectra for the two major compounds used in The mixing ratio and RMS-derived detection limit for methyl nitrite for the displayed experimental 334 spectrum in Figure 4 are 21.0 ppm and 1.4 ppm, respectively. The range for the mixing ratios and 335 the averaged detection limits for methyl nitrite are summarized in Table 3. Methyl nitrite was 336 detected with confidence in 9 of the 10 measurements; only one of the measurements was below 337 the RMS-derived detection limit. 338 We report the detection via IR spectroscopy of methyl nitrite in wildland fire emissions not only 339 because it is novel, but also because of its influential role in atmospheric chemistry: Methyl nitrite  propyne (CH2=C=CH2 ↔ CH3-C≡CH) will take place via a unimolecular reaction faster than the 353 decomposition reaction (Lifshitz et al., 1975). Additionally, these same authors investigated the 354 pyrolysis of allene and propyne and observed that C2H4 was generated from allene while CH4 and 355 C2H2 were mainly formed from propyne (Lifshitz et al., 1976). Unfortunately, the strongest IR band for propyne (near 634 cm -1 ) is obscured by CO2 bending mode lines. Due to the interferences 357 we cannot with confidence identify propyne in the measurements; we can, however, detect allene.

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In the mid-IR, allene has several strong rotational-vibrational lines near 845 cm -1 associated with 359 the sub-bands of the perpendicular band ν10, which is due to CH2 rocking (Lord et al., 1952).

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Additionally, allene has a moderately strong band at 1958.6 cm -1 due to the ν6 C-C stretching 361 (Lord et al., 1952). However, the ν6 band is not useful for detection due to interference from the 362 H2O bending mode lines. with the scaled reference spectrum for allene. For clarity, the spectral contributions for C2H2, CO2, HCN, 366 naphthalene, C2H4, methyl nitrite, and H2O are not shown. All spectra are at 0.6 cm -1 resolution and have 367 been offset for clarity. The calculated mixing ratio of allene in this measured spectrum is 37.8 ± 0.6 ppm 368 (values obtained from MALT5 software, and error represents standard error). 369 370 Figure 5 shows the measured absorbance spectrum, scaled allene reference spectrum and the 371 associated residual with and without allene included in the fit. The absorption lines associated 372 with allene are clearly seen in the resulting spectrum when allene is not included in the fit (green stretch (Hamada et al., 1985) at 1724.1 cm -1 , but this band is heavily overlapped by water lines.

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There is also the ν16 band (Hamada et al., 1985) at 958.8 cm -1 , but this feature overlaps with 403 multiple other strongly absorbing compounds, such as C2H4. We have therefore focused acrolein's 404 analysis using the ν10 band (C-C stretch) (Hamada et al., 1985) at 1157.7 cm -1 .  components, such as acrolein, C2H2 and H2O, were also included in the fit, but their reference 427 spectra are not displayed in Figure 7. The spectral profile of acetaldehyde with its P and R branches 428 of ν3 is easily discernable even before deconvolution of the measured spectrum. Similar to 429 acrolein, all of the mixing ratios for acetaldehyde were above the RMS-derived detection limit.     (3.6) Allene (Propadiene)** 1.05 (0.24) n/a 0.1 (0.1) 8.73 (0.28) n/a n/a n/a *GC-FID is gas chromatography with flame ionization detector

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Gas-phase compounds with appreciable band intensities and appreciable concentrations can be 471 both identified and quantified using IR spectroscopy. We have used such spectral information for 472 seminal IR detection of five compounds generated during prescribed forest fire burns. Deriving 473 the mixing ratios from the congested spectra obtained from wildland smoke samples is more