Smoke aerosol chemistry and aging of Siberian biomass burning emissions in a large aerosol chamber

Abstract

Vegetation open fires constitute a significant source of particulate pollutants on a global scale and play an important role in both atmospheric chemistry and climate change. To better understand the emission and aging characteristics of smoke aerosols, we performed small-scale fire experiments using the Large Aerosol Chamber (LAC, 1800 m³) with a focus on biomass burning from Siberian boreal coniferous forests. A series of burn experiments were conducted with typical Siberian biomass (pine and debris), simulating separately different combustion conditions, namely, flaming, smoldering and mixed phase.

Following smoke emission and dispersion in the combustion chamber, we investigated aging of aerosols under dark conditions. Here, we present experimental data on emission factors of total, elemental and organic carbon, as well as individual organic compounds, such as anhydrosugars, phenolic and dicarboxylic acids. We found that total carbon accounts for up to 80% of the fine mode (PM_2.5) smoke aerosol. Higher PM_2.5 emission factors were observed in the smoldering compared to flaming phase and in pine compared to debris smoldering phase. For low-temperature combustion, organic carbon (OC) contributed to more than 90% of total carbon, whereas elemental carbon (EC) dominated the aerosol composition in flaming burns with a 60–70% contribution to the total carbon mass. For all smoldering burns, levoglucosan (LG), a cellulose decomposition product, was the most abundant organic species (average LG/OC = 0.26 for pine smoldering), followed by its isomer mannosan or dehydroabietic acid (DA), an important constituent of conifer resin (DA/OC = 0.033). A levoglucosan-to-mannosan ratio of about 3 was observed, which is consistent with ratios reported for coniferous biomass and more generally softwood.

The rates of aerosol removal for OC and individual organic compounds were investigated during aging in the chamber in terms of mass concentration loss rates over time under dark conditions and compared to the loss rate of EC. The latter is used as an inert tracer for estimating aerosol mechanical deposition and wall losses of the otherwise chemically conserved aerosol species. The OC/EC ratio increased with smoke aging for the flaming phase, suggesting a production/partitioning of organic compounds after emission. On the other hand, for smoldering burns OC/EC ratios decreased further with aging due to additional sinks of OC, other than those related to deposition and wall losses alone, such as evaporation of semi-volatile compounds. The chemical fingerprints of the major PM components of fresh and aged smoke found in this study are proposed to be used for the assessment of contributions from Siberian biomass burning to atmospheric pollution in source apportionment studies like those using molecular marker approaches.

Introduction

Biomass burning is known to be a major contributor to the global budget of aerosols (Amiridis et al., 2012; Bond et al., 2013; Kasischke and Penner, 2004). It releases into the atmosphere particulate species such as black carbon (BC) and organic carbon (OC), and it is considered an important contributor to radiative forcing of climate. Arctic climate is especially BC-sensitive because of the impact of its deposition on snow, accelerating the icecap melting and changing the surface albedo (AMAP, 2012). The direct and indirect impacts of BC aerosols induce the highest uncertainty amongst climate active species with respect to temperature changes, radiative forcing, and cloud formation (Boucher et al., 2013; Koch et al., 2009; Popovicheva, 2010).

Siberia is one of the world's major boreal forest fire areas (Damoah et al., 2004; Lee et al., 2005; Stocks et al., 1998). Smoke pollution plumes from Siberian forest fires can be transported over hundreds of kilometers, often approaching the Arctic coast at approximately 70 °N (Paris et al., 2009), as well as southwards towards the Mediterranean sea (Diapouli et al., 2014), and have been identified as a major source of climate-relevant species emitted at northern latitudes (Lavoué et al., 2000). Significant contribution to springtime Arctic aerosols from fires in Russia was demonstrated when biomass burning (BB) plumes increased the atmospheric burden of BC and OC 2.6 times above the Arctic Haze background (Warneke et al., 2009). Long-range transport of Siberian BB has significantly influenced the chemical composition of carbonaceous aerosols globally: in the western North Pacific rim (Agarwal et al., 2010) and North East Greenland (Nguyen et al., 2013).

Physico-chemical characteristics of BB aerosols are highly variable. Much of the variance in the particle emissions and chemical composition is due to the phase of burning (that is, flaming versus smoldering) and the ecosystem characteristics (type of biomass, fuel moisture, soil properties, temperature, etc.) (Reid et al., 2005). Slow and low-temperature smoldering fires (flameless form of combustion) yield a substantially higher conversion of a fuel to toxic compounds than does flaming combustion (Alves et al., 2016; McKenzie et al., 1995; Morawska and (Jim) Zhang, 2002). Such fires produce brown carbon (BrC) that consists largely of weakly light-absorbing resinous organic carbon aerosols (Popovicheva et al., 2017). Increasing the oxygen supply leads to higher temperatures and flaming combustion, which produces particulate matter (PM) with a lower OC and higher BC content, the latter being a strongly light-absorbing component. Regional smoke plumes result from numerous mixed phase fires where the relative amount of flaming versus smoldering burns is poorly characterized. Due to the mixing of these two different types of thermal decomposition, the climatic impact of biomass burning remains rather uncertain.

Despite concerns about the climatic impact and environmental hazards related to millions of tons of PM emitted by wildfires, observations of aerosol chemical composition and BC in Siberia have been reported for a limited number of sites (Kozlov et al., 2008; Kuokka et al., 2007; Paris et al., 2009). During forest fire episodes, most of the aerosol mass appears to consist of organic particulate matter with high concentrations of BB markers such as levoglucosan, oxalate and potassium (Kuokka et al., 2007). A few measurement campaigns during experimental fires were undertaken to address sub-Arctic boreal forest fire emissions (Samsonov et al., 2012). According to their findings, the mass of total emitted PM represented about 1–7% of consumed biomass, and consisted to 77% of organic substances, 8% of elemental carbon (EC), and 6% of inorganic species.

The OC fraction of biomass burning aerosols is dominated by carbohydrates (Reid et al., 2005). Among them, some monosaccharides, which are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates, have been identified as molecular markers of biomass burning (Puxbaum et al., 2007; Simoneit et al., 1999; Ward et al., 2006). This is the case of anhydrosugars (levoglucosan and its stereoisomers mannosan and galactosan) which are present in combustion products from biomass containing cellulose and hemicellulose and which tend to be adsorbed to aerosols because of their low volatility (Kirchgeorg et al., 2014; Simoneit et al., 1999). Other compounds, such as phenolic acids, which are lignin breakdown products, have also been used as proxy for biomass burning (Alves et al., 2010). Moreover, dehydroabietic acid is a product of the thermal decomposition of diterpenoids, which are important biomarker constituents of conifer resins (Oros and Simoneit, 2001b). Simoneit et al. (1999) reported that the burning of Douglas fir, Lodgepole pine, Ponderosa pine, Sitka spruce and Western white pine emit more dehydroabietic acid than levoglucosan. Thus, this resin acid has been proposed as a potential molecular marker specifically for combustion of Gymnosperm (conifers) (Alves et al., 2016; Simoneit et al., 1999, 2000). Aliphatic diacids, (from C₂ to C₁₀), unsaturated diacids, ω-oxocarboxylic acids and α-dicarbonyls are also products of biomass burning (Kawamura et al., 2013; Kawamura and Bikkina, 2016). While these acids have been measured in ambient air in many different places (including some areas dominated by certain emission sources), there have been few source emission (e.g., chamber) measurements. Therefore, it is of interest to quantify these species under controlled combustion conditions. Many trace elements, mostly alkali earth metals (carbonates), dominate the inorganic fraction in fly ash with large impact of soil evolving by intensive fires (Kavouras et al., 2012; Popovicheva et al., 2016a). Water-soluble ions (potassium, halides, sulfates) are also produced by decomposition of biomass and subsequent condensation in the smoke plume.

Following emission, smoke particles are subject to physico-chemical processes in the plume that can alter their properties. Dynamic changes in particle concentration and size may increase aerosol mass loadings in the smoke plume, and heterogeneous reactions in humid conditions produce more water-soluble inorganic salts (Li et al., 2015). Additionally, secondary organic aerosol (SOA) formation in the smoke plume can occur, while it becomes significant especially when solar radiation can induce photochemical reactions (Popovicheva et al., 2014; Reid et al., 2005; Tiitta et al., 2016). These changes in the particle properties can alter the direct and indirect effects of BB, impacting single scattering albedo and hygroscopicity.

In view of the difficulty of accounting for all the variability in particle emissions from natural biomass fires, combustion chamber experiments have been performed allowing for measurement of fire-integrated smoke properties from a single fuel type under conditions simulating the entire burning process, i.e., from ignition to smoldering through flaming. In the Fire Lab at Missoula Experiments (FLAME), a series of open biomass burning experiments were performed to derive emission factors of PM for 33 different plant materials that are frequently consumed by wildfires in the U.S (Carrico et al., 2010; Engling et al., 2006b; Hosseini et al., 2010; Levin et al., 2010; McMeeking et al., 2009). A detailed chemical characterization of particle emissions from the combustion of European conifer species, savannah grass, African hardwood, and German and Indonesian peat has also been performed by Iinuma et al. (2007). Such studies were lacking for typical Siberian closed-canopy coniferous forest biomass, despite the frequency and intensity of forest fires in this region and the environmental hazards related to the impact on Arctic climate.

In that context and to fill the gaps in available data on particulate emissions and smoke chemistry of fires specific to the Siberian boreal region, typical Siberian biomass (pine and debris) were burned in the Large Aerosol Chamber (LAC) of the Institute of Atmospheric Optics Russian Academy of Sciences (Tomsk, Siberia). The originality of our approach resides in the fact that experiments were conducted under controlled combustion conditions to simulate separately flaming, smoldering and mixed burning phases. A comprehensive optical and microstructural analysis showed that the temperature regime of combustion plays a key role in the formation and time evolution of smoke (Popovicheva et al., 2015). Smoldering fires produced particles with high OC while flaming fires produced a high EC content and soot agglomerates. Individual particle analysis of smoke microstructure was used to apportion the particles into major characteristic groups: Soot and Organic, which accounted for about 90% and 60% of total particle numbers emitted from the flaming and smoldering fires, respectively. Extended individual particle analyses also revealed the hygroscopicity of smoke aerosol (Popovicheva et al., 2016b).

Studies in the LAC, as well as in the other combustion chambers mentioned above, were performed in dark conditions. Simulating sun-light source in the combustion chamber has the advantage of enabling the study of photochemical aging. In a few smog chamber studies, this was done by exposing the diluted emission plume to UV light (Grieshop et al., 2009; Hennigan et al., 2010, 2011; Hoffmann et al., 2010). Still, simulations of small-scale fires under controlled combustion conditions in the dark provide us with fundamental knowledge about the processes, smoke characteristics and aging in the atmosphere at night time (Li et al., 2015).

Here, we continue studies in the Large Aerosol Chamber and focus on the comprehensive characterization of chemical composition of smoke aerosols under well-controlled combustion conditions. Total carbon, OC, EC, and selected organic molecular markers including dicarboxylic acids, oxoacids, α-dicarbonyls and related compounds in PM₁₀ and PM_2.5 samples were measured in flaming, smoldering and mixed phases. Emission ratios (related to OC) as well as emission factors (related to fuel consumed) from small-scale fires of Siberian forest pine wood and debris are determined. Furthermore, aging of smoke particles following the emission is investigated in order to assess the transformation of the aerosol chemical properties under dark conditions.