The Chemistry of Fires at the Wildland-Urban Interface (2022) / Chapter Skim
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From page 31... ...
Current understanding of the chemistry of WUI fires and their emissions is largely inferred based on information on wildland and urban fires. Wildland fires are a large source of emissions and have been extensively studied.
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From page 32... ...
: Species emitted into the air, water, soil, or other media, from a process; these are sometimes called releases Enclosure fire: A fire contained within a room or compartment inside a building in which oxygen supply is typically constrained, contrary to open fires; these are sometimes called compartment fires Energy content: The amount of energy contained within a mass of fuel; it can be quantified as the higher heat ing value (or gross calorific value) , defined as the amount of heat released from complete combustion of a dry material when the products are returned to 25°C, or the lower heating value (or net calorific value)
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From page 33... ...
: A complex mixture of solid particles and liquid droplets found in the air Pollutant: A chemical or biological substance that harms water, air, or land quality Plume injection parameters: The initial characteristics of a fire plume, including its injection altitude and multi-phase chemical composition Pyrolysis: The thermal decomposition of a combustible material in the absence of molecular oxygen; the term "pyrolysis" sometimes appears in wildland fire literature to represent oxidative pyrolysis Secondary organic aerosol: Organic particulate matter that is formed in the atmosphere from precursor gases Semi-volatile organic compounds (SVOCs) : Organic compounds that, based on their vapor pressure, tend to evaporate from the particle phase within near-field dilution of plumes; SVOCs are of concern because of their abundance in the indoor environment and their ability to accumulate and persist in the human body, the infrastructure of buildings, and environmental dust Smoldering combustion: Combined processes of thermal decomposition and slow, low-temperature, flameless burning of porous solid biomass fuels; sometimes called glowing combustion Toxic product yield: The maximum possible mass of a combustion product generated during combustion, per unit mass of test specimen consumed (typically expressed in units of grams per gram or kilograms per kilogram)
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From page 34... ...
The unique elemental composition of the materials in the urban environment has a direct impact on the combustion chemistry and emissions described in the other sections of this chapter. Table 3-1 summarizes some of the most common building materials and their fire emissions (Blomqvist et al., 2013; Stec, 2017; Stec and Hull, 2010, 2011; FIGURE 3-1 Examples of the types and quantities of materials that can be found in the home.
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From page 35... ...
. Live foliage in the canopy, dead foliage in the leaf litter, nonwoody vegetation, lichen-moss layers, duff layers, and organic soil may be consumed in wildland fires (Ottmar, 2014; Ottmar and Baker, 2007)
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From page 36... ...
Thus, they can pose risks different from those of WUI fires confined to residential areas. Specific mappings of commercial and industrial structures and their fuel loadings and chemicals of concern are not broadly available.
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From page 37... ...
Wood-based materials are the largest fraction of combustible mass in the examples shown here; wood and engineered wood products can be derived from a variety of species and can exist in many forms. Engineered wood products, such as oriented strand board (OSB)
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From page 38... ...
. The long lifetimes of houses limit the impact of building codes introduced in the last three decades that mandate the use of ignition-/fire-resistant materials to reduce the risk from wildland fires, leading to regulatory initiatives to promote retrofits for existing homes (State of California, 2019)
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From page 39... ...
However, many different types of battery chemistries are used to achieve various performance features, and it is likely that battery compositions and vehicle designs will continue to evolve, along with their overall contribution to emissions from WUI fires.
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In addition, electric vehicles and Li-ion batteries can be the source of hydrogen chloride (HCl) and hydrogen fluoride (HF)
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These non-targeted analyses along with surveys of consumer products and household exposure measurements demonstrate the difficulty of identifying chemicals of concern in the urban environment (Li and Suh, 2019)
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. Additionally, the heat release rate from WUI fires has not been comprehensively evaluated but will likely vary with the differing fuel composition and density in WUI fires compared to wildland fires (Simeoni et al., 2012)
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Changing Materials at the WUI As experience with WUI fires grows, some jurisdictions are changing building codes, which in turn will change the composition of structures and the wildland at the WUI. Table 3-7 describes actions that are currently used or proposed for use in WUI areas.
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From page 44... ...
The design, maintenance, and use of defensible space for fire protection is improved when neighborhoods are developed more densely and are built with stringent fire-resistant building codes; however, clustering may contribute to high fuel loadings. Wildland fire–resistant construction techniques to harden all the homes in a community can be used to minimize risk and protect homes from wildland fires with minimal additional cost (Quarles and Pohl, 2018)
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From page 45... ...
For solid combustibles, all progress through slow and fast decomposition stages to emit volatile, semi-volatile, and particulate combustibles; wildland, urban, and WUI fires can all experience a variety of different types of combustion conditions, and leave residual char and noncombustible materials. Once in the gas phase, combustible materials can further react in flaming combustion and/or in hot regions without flames that support chemical oxidation reactions producing soot precursors, PAHs, and carbon-containing particulates.
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From page 46... ...
Typically, the characteristic reaction times at temperatures below ~500 K are too long to be relevant to the gas phase of the flame and near-field plume but may be important in terms of condensed phase processes (for example, spontaneous heating to ignition; Jones et al., 2015)
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From page 47... ...
. This holds for all homogeneous, gas-phase oxidations of mixtures of RiH species found in wildland and WUI fires.
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From page 48... ...
. In general, in wildland and WUI fires and their hot plumes, where intermediate and high temperatures are present, the gas-phase oxidation of CO to CO2 will be negligibly slow in mixtures of larger-carbon-number hydrocarbons, oxygenated hydrocarbons, and CO, as long as the following is true: ([CO]
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From page 49... ...
The unknown extent of halogen chemistry in WUI fire plumes limits the applicability to WUI fire plumes of empirical results relating emissions to CO concentrations that are based on wildland fire plumes. The Effect of Nitrogen Species Nitrogen species in flames can be produced at relatively low temperatures from the oxidation of organically bound nitrogen species such as nylon, and from the oxidation of plastics (Chaos et al., 2009)
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From page 50... ...
Pyrolysis chemistry and smoldering combustion will also be important in some fire environments, and the nature of these chemistries in WUI fires is expected to be different than pyrolysis and smoldering combustion chemistry in wildland and urban fires for many of the same reasons discussed here. Finding: Both the mass of fuel and the elemental composition of human-made fuel materials are important in assessing the effects of halogens, nitrogen, and other elements that have potential to alter the chemistry and affect the composition of the species found in WUI fire plumes, both in the near field and as the plumes age.
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From page 51... ...
. Different approaches are employed for wildland fires and enclosure fires, but most approaches use empirical relationships between EFs or ERs and some measure of combustion efficiency.
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From page 52... ...
As described in the previous section, the oxidation of CO in WUI fires depends on the temperature history of the fire and the materials combusted in the fire. Historically, the CE and MCE have been used to indicate the contributions of smoldering and flaming combustion in wildfire plumes.
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From page 53... ...
. These types of field programs undertaken for wildland fires demonstrate the types of advances that can be made in developing emissions estimates; however, data for WUI fires similar to the wildland fire data from WE-CAN and FIREX-AQ are
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From page 54... ...
EFs for WUI fires would be expected to contain contributions from halogenated compounds, nitrogen-containing compounds, and other species and would likely need to characterize not only combustion characteristics, but also the relative magnitudes and compositions of urban and wildland fuels consumed by the fires. PM Emissions Particulate matter (PM)
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From page 55... ...
Nonetheless these data show important distinctions in urban fire emissions from wildland fire emissions that should be evaluated for WUI fires. Ventilation conditions and the chemical composition of the fuel likely play an important role in the formation of some pollutants like HCl, HF, SOx, and metals, as these species are emitted only if their precursors are present in the original material consumed in the fire.
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From page 56... ...
56 TABLE 3-8 EFs from Large-Scale Urban Fire Simulations CO NOx/NO HCN HCl PM C 6H 6 CH2O Pb Total PCDD Total PCDF Material (g/kg)
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From page 57... ...
Despite this variability, these test methods allow the collection of data that examine the dependence of EFs for commonly used materials, and mixtures of commonly used materials, on combustion conditions. These data can serve as a starting point for more extensive examination of WUI fire emissions under realistic combustion conditions.
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From page 58... ...
Y Y Post-flashover N N ? Y Y Advantages Widely available; Modification to widely Simple and low cost Designed for smoke Able to replicate high CO currently used for toxicity used standard test; toxicity assessment; ideal yields for under-ventilated assessment in mass measures flammability as for linear or homogeneous flaming transport industries well as toxicity products; correlates with large-scale tests for all conditions Disadvantages Poor interlaboratory No standard equipment Conditions not related Interlaboratory Expensive; not widely used; reproducibility; unable exists; unable to go above to fire stages; does not reproducibility not proven capabilities insufficiently to force under-ventilated 50 kW/m2; toxic product distinguish between for layered materials; demonstrated flaming; fire stage yields do not correlate flaming and non-flaming; separate assessment of unknown; sample probe to large-scale fire tests; limited volume of effluent flammability required can miss toxic plume equivalence ratio only for analysis known after test SOURCE: Stec and Hull, 2010.
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From page 59... ...
Finding: Limited data exist on emission factors and the sources of uncertainty for WUI fires; no studies of single- or multiple-dwelling, full-scale structures, with realistic contents and realistic WUI fire conditions (e.g., strong winds) , have reported emission factors or yields.
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From page 60... ...
and other wildland fire fuels as they generate fire plumes need to be modeled mechanistically and explored experimentally at bench, laboratory, and larger scales. Research need: Experimental and modeling investigations of WUI fire plume compositions should be used to develop emission factors and chemical fingerprints for WUI fire emissions.
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From page 61... ...
2020. "Study of Secondary Organic Aerosol Formation from Chlorine Radical-Initiated Oxidation of Volatile Organic Compounds in a Polluted Atmosphere Using a 3D Chemical Transport Model." Environ mental Science & Technology 54 (21)
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2009. "The Formation, Properties and Impact of Secondary Organic Aerosol: Current and Emerging Issues." Atmospheric Chemistry and Physics 9 (14)
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2019. "Speciated and Total Emission Factors of Particulate Organics from Burning Western US Wildland Fuels and Their Dependence on Combustion Efficiency." Atmospheric Chemistry and Physics 19 (2)
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2018. "Suspect Screening Analysis of Chemicals in Consumer Products." Environmental Science & Technology 52 (5)
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2020. "Wildland Fire Emission Factors in North America: Synthesis of Existing Data, Measurement Needs and Management Applications." International Journal of Wildland Fire 29 (2)
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From page 66... ...
2017. "Secondary Organic Aerosol from Chlorine-Initiated Oxidation of Isoprene." Atmospheric Chemistry and Physics 17.
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From page 67... ...
Exposure Disparities by Socioeconomic Status." Environmental Science and Technology 44 (15)
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From page 69... ...
While national monitoring networks provide data for assessing exposures of routinely monitored air pollutants, such as fine particulate matter, the gas- and particle-phase smoke composition specifically associated with WUI fires is not well understood. WUI smoke composition differs from wildland fire smoke because of direct emissions from the combustion and volatilization of human-made materials and the chemical interaction of these emissions with wildland fire emissions (see Chapter 3)
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From page 70... ...
The possibility that other loss processes dominate in WUI fire plumes (e.g., oxidation by chlorine radical, multiphase chemistry, cloud chemistry) is discussed in the next section, "Atmospheric Transformations." ATMOSPHERIC TRANSFORMATIONS The Downwind Fate of WUI Fire Emissions A wide range of plume studies including studies of wildland fire plumes provide confidence that WUI smoke concentrations decrease with distance downwind through dilution and surface deposition and undergo chemical and phase changes in the atmosphere (Figure 4-1)
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From page 71... ...
in particulate form Primary species with toxic potential: Toxic substance that is emitted directly from the source (e.g., WUI fire) Secondary organic aerosol: Organic particulate matter that is formed in the atmosphere from precursor gases Secondary species with toxic potential: Toxic substances that are formed through atmospheric chemistry (continued)
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