Gas-Phase Pyrolysis Products Emitted by Prescribed Fires in Pine Forests with a Shrub Understory in the Southeastern United States

Abstract

16In this study we capture and identify pyrolysis gases from prescribed burns conducted in pine 17 forests with a shrub understory using a manual extraction device. The device selectively sampled 18 emissions ahead of the flame front, minimizing collection of oxidized gases, with the captured 19 gases analyzed in the laboratory using infrared absorption spectroscopy. Results show that 20 emission ratios (ER) relative to CO for ethene, and acetylene were significantly greater than 21 previous fire studies, suggesting that the sample device was able to collect gases prior to ignition. 22Further evidence that ignition had not begun was corroborated by novel infrared detections of 23 several species, in particular naphthalene. With regards to oxygenated species, several aldehydes 24 (acrolein, furaldehyde, acetaldehyde, formaldehyde) and the carboxylic acids (formic, acetic) were 25 all observed; results show that ERs for acetaldehyde were noticeably greater while ERs for 26 formaldehyde and acetic acid were lower compared to other studies. The acetylene-to-furan ratio 27 also suggests that high temperature pyrolysis was the dominant process generating the collected 28 gases. This hypothesis is further supported by the presence of HCN and the absence of NH3. 29Atmos. Chem. Phys. Discuss., https://doi.levels of isoprene and 1,3-butadiene in the smoke from a living tree when compared to dead stumps 77 under the same conditions. Emission characteristics obtained during such campaigns can be 78 especially useful for the implementation or verification of effective burning techniques to 79 minimize the gases released during prescribed burns. 81However, few investigations have studied the pre-ignition or pyrolysis gases emitted prior to the 82 flaming combustion stage. Most prescribed burn studies have focused only on the flaming and 83 smoldering stages. The hotter flaming stage is characterized by more oxidized products and a 84 higher modified combustion efficiency (MCE) (Ward and Hao, 1991), which is defined as:The cooler smoldering phase with lower MCE values (typically ranging from 0.65-0.85) 86(Urbanski, 2013) displays more non-oxidized or less-oxidized species. The present study differs 87 from these earlier works in that we exclusively attempt to investigate pyrolysis, which is the first 88 step in the burning process (Collard et al., 2014), in particular we investigate the gas phase 89 pyrolysis species generated during prescribed burns. Primary mechanisms associated with 90 pyrolysis of biomass are char formation, depolymerization and species fragmentation. Volatile 91 products are generated and, if unstable, can continue to undergo secondary (non-combustion) 92 reactions such as cracking or recombination (Collard and Blin, 2014). Pyrolytic reactions produce 93 fuel gases that, if sufficient in quantity and in the presence of oxygen, will maintain the flame via 94 combustion pathways (Ward and Hardy, 1991; Di Blasi, 1993). Thus, the primary objectives of 95 the present study are a) to detect pyrolysis gases in pr...

Emissions of Reactive Nitrogen From Western U.S. Wildfires During Summer 2018

“…Fixed N in burning biomass is initially pyrolyzed as a handful of small N‐containing compounds: NH 3 , HCN, HNCO, and CH 3 CN and minor amounts of other NVOCs (Glarborg et al., 2018; Lobert & Warnatz, 1993; Roberts et al., 2020). Immediately following this decomposition of fuel N and subsequent volatilization, and still within the flame, some of these species are quickly oxidized by rapid radical chemistry to form the remainder of the species we consider to be primarily emitted by fires: N 2 , NO, NO 2 , N 2 O, and HONO (Hansson et al., 2004; Ren & Zhao, 2012, 2013a, 2013b; Scharko et al., 2019; Sekimoto et al., 2018). Prior to the FIREX Fire Lab experiments in 2016, laboratory and field studies found strong relationships between the observed forms of N r emissions and the dominant combustion conditions, with more NO x and HONO emissions during flaming combustion and more NH 3 and HCN emissions during smoldering combustion (Burling et al., 2010; Goode et al., 1999; McMeeking et al., 2009; Roberts et al., 2010; Yokelson et al., 1996, 1997).…”

Daytime Oxidized Reactive Nitrogen Partitioning in Western U.S. Wildfire Smoke Plumes

“…Small nitrogen‐containing molecules (e.g., hydrogen cyanide (HCN), ammonia (NH 3 ), and isocyanic acid (HNCO)) are typically emitted during pyrolysis (Glarborg et al., 2018; Hansson et al., 2004; Roberts et al., 2020). Radical chemistry within flames converts a fraction of these species to more oxidized forms including N 2 , nitrous oxide (N 2 O), nitrogen oxide (NO), nitrogen dioxide (NO 2 ), and nitrous acid (HONO) (Ren & Zhao, 2012; Scharko et al., 2019). The most abundant emitted reactive N species include nitrogen oxides (NO x = NO + NO 2 ), NH 3 , nitrous acid (HONO), HCN, and acetonitrile (CH 3 CN) (e.g., Akagi et al., 2011; Andreae, 2019; Andreae & Merlet, 2001).…”