Atmos. Chem. Phys., 18, 9263–9281, 2018 https://doi.org/10.5194/acp-18-9263-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. High- and low-temperature pyrolysis profiles describe volatile organic compound emissions from western US wildfire fuels Kanako Sekimoto1,2,3,*, Abigail R. Koss1,2,4,a,*, Jessica B. Gilman1, Vanessa Selimovic5, Matthew M. Coggon1,2, Kyle J. Zarzana1,2, Bin Yuan1,2,6, Brian M. Lerner1,2,b, Steven S. Brown1,4, Carsten Warneke1,2, Robert J. Yokelson5, James M. Roberts1, and Joost de Gouw1,2,4 1NOAA Earth System Research Laboratory (ESRL), Chemical Sciences Division, Boulder, CO 80305, USA 2Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309, USA 3Graduate School of Nanobioscience, Yokohama City University, Yokohama, Kanagawa 236-0027, Japan 4Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO 80302, USA 5Department of Chemistry, University of Montana, Missoula, MT 59812, USA 6Institute for Environment and Climate Research, Jinan University, Guangzhou, China anow at: Department of Civil & Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02142, USA bnow at: Aerodyne Research, Inc., Billerica, MA 01821, USA *These authors contributed equally to this work. Correspondence: Kanako Sekimoto ([email protected]) Received: 19 January 2018 – Discussion started: 19 February 2018 Revised: 28 May 2018 – Accepted: 18 June 2018 – Published: 3 July 2018 Abstract. Biomass burning is a large source of volatile or- mers at high and low temperatures. These pyrolysis pro- ganic compounds (VOCs) and many other trace species to the cesses are thought to be the main source of VOC emis- atmosphere, which can act as precursors to secondary pol- sions. “High-temperature” and “low-temperature” pyrolysis lutants such as ozone and fine particles. Measurements per- processes do not correspond exactly to the commonly used formed with a proton-transfer-reaction time-of-flight mass “flaming” and “smoldering” categories as described by mod- spectrometer during the FIREX 2016 laboratory intensive ified combustion efficiency (MCE). The average atmospheric were analyzed with positive matrix factorization (PMF), in properties (e.g., OH reactivity, volatility, etc) of the high- and order to understand the instantaneous variability in VOC low-temperature profiles are significantly different. We also emissions from biomass burning, and to simplify the descrip- found that the two VOC profiles can describe previously re- tion of these types of emissions. Despite the complexity and ported VOC data for laboratory and field burns. variability of emissions, we found that a solution including just two emission profiles, which are mass spectral repre- sentations of the relative abundances of emitted VOCs, ex- plained on average 85 % of the VOC emissions across var- 1 Introduction ious fuels representative of the western US (including var- ious coniferous and chaparral fuels). In addition, the pro- Biomass burning is a large source of volatile organic com- files were remarkably similar across almost all of the fuel pounds (VOCs) and other trace species to the atmosphere. types tested. For example, the correlation coefficient r2 of Reactions involving these VOCs produce ozone and fine par- each profile between ponderosa pine (coniferous tree) and ticles, which are important air pollutants and radiative forc- manzanita (chaparral) is higher than 0.84. The compositional ing agents (Alvarado et al., 2009, 2015; Yokelson et al., differences between the two VOC profiles appear to be re- 2009; Jaffe et al., 2012). Some VOCs from fires also have lated to differences in pyrolysis processes of fuel biopoly- direct health effects (Naeher et al., 2007; Roberts et al., 2011). Biomass burning occurs in wildfires, controlled burns Published by Copernicus Publications on behalf of the European Geosciences Union. 9264 K. Sekimoto et al.: High- and low-temperature pyrolysis profiles of wildland and agricultural fuels, and in residential wood of gas-chromatographic preseparation experiments, literature stoves and industrial processes. Given the variety of fuels and review, time-series analysis, and comparison to other instru- burning conditions, it is unsurprising that the VOC compo- ments (Koss et al., 2018). Approximately 90 % of the instru- sition of biomass burning emissions varies greatly between ment signal could be attributed to identified VOCs. different fire states, locations, and studies. Therefore, it is The aims of this work are to understand the variation in important to understand VOC emissions from biomass burn- gas-phase emissions both over the course of a fire and on a ing in detail and develop a predictive capability that explains fire-integrated basis. Ultimately, this improved understand- some of the variability in VOC emissions. ing of emissions variability could be used to simplify predic- Multiple complex processes take place in biomass burn- tions of the emission of secondary organic aerosol (SOA) and ing, including (i) distillation with release of water vapor and ozone precursors. To do this, the VOCs observed by PTR- terpenes, (ii) pyrolysis of solid biomass giving off flammable ToF-MS in stack burns were analyzed using positive matrix gases, (iii) flaming combustion, and (iv) nonflaming pro- factorization (PMF). We show that much of the observed cesses loosely lumped with smoldering combustion such as variability in VOCs can be explained by only two factors, and glowing (gasification) of biomass (Yokelson et al., 1996, that these two factors are qualitatively related to the tempera- 1997; Collard and Blin, 2014; Liu et al., 2016). The main ture of the pyrolysis processes, which are the main sources of source of VOC emissions is pyrolysis of the polymers that the VOC emissions from biomass burning. Based on this re- form biomass such as cellulose, hemicellulose, and lignin. sult, the two factors are named as a high-temperature pyrol- The temperature of the reaction and the physical charac- ysis factor and a low-temperature pyrolysis factor. The two teristics of the biopolymer control which pyrolysis mecha- factors are compared between fuels. Importantly, the high- nism (e.g., depolymerization, fragmentation, or aromatiza- temperature factor is quantitatively similar between different tion) is the main source of emitted VOCs (Yokelson et al., fuels, and the same is true for the low-temperature factor. 1996, 1997; Collard and Blin, 2014; Liu et al., 2016). In a The VOCs present in each factor are discussed in terms of given fire, the processes (i)–(iv) occur simultaneously, but composition, reactivity with OH, and propensity to form sec- the relative importance of each process and temperature can ondary organic aerosol. The relative importance of high- and change with time, which relates to the variability in inte- low-temperature pyrolysis factors is quantified for each fuel grated VOC emissions between different fires. This variabil- and discussed with respect to physical properties of the fuel ity is often parameterized as a function of modified combus- and the burn dynamics. We also investigate how well VOC tion efficiency (MCE D 1CO2/(1COC1CO2)) (Yokelson et emissions in biomass burning can be modeled by the two al., 1996). CO2 and CO are representative gases emitted from PMF emission profiles through comparisons with previously the flaming and smoldering combustion processes, respec- reported data from laboratory burns and wildfires. Finally, tively, and are measured in most biomass burning studies. emissions of some specific compounds are discussed. MCE is generally higher in flaming combustion (> 0.9) and lower in smoldering combustion (< 0.9) (Akagi et al., 2011). The National Oceanic and Atmospheric Administration 2 Methods (NOAA) led the Fire Influence on Regional and Global En- vironments Experiment (FIREX) 2016 laboratory intensive 2.1 VOC measurements by PTR-ToF-MS conducted at the US Forest Service Fire Sciences Laboratory in Missoula, Montana, to study emissions of trace gases and Fire emissions were measured during the FIREX 2016 inten- aerosol from wildfires. Emissions from various fuels repre- sive at the Fire Sciences Laboratory in Missoula, Montana. sentative of the western US were sampled under controlled The facility consists of a large combustion chamber and has conditions by extensive instrumentation (https://www.esrl. been described in detail previously (Christian et al., 2003, noaa.gov/csd/projects/firex/firelab/instruments.html, last ac- 2004; Burling et al., 2010). cess: 19 January 2018). Experiments included so-called stack VOC measurements were performed using several instru- burns, in which emissions from an evolving burn were en- ments, including a PTR-ToF-MS. This instrument employed trained into a large-diameter stack and sampled by vari- a high-resolution ToF mass analyzer (Aerodyne Research ous instruments. VOCs were measured by several instru- Inc, MA, USA; Tofwerk AG, Thun, Switzerland) and mea- ments, including a PTR-ToF-MS (proton-transfer-reaction sured with a time resolution of 2 Hz. VOCs and some in- time-of-flight mass spectrometer) which captured gas-phase organic compounds were ionized by proton transfer from C emissions with a fast time response during stack burns. H3O reagent ions. We include the inorganic compounds in The measurements show variability in VOC composition as the discussion of VOCs. Species with a proton affinity higher the fire shifts between