Aerosol optical properties and trace gas emissions by PAX and OP-FTIR for laboratory-simulated western US wildfires during FIREX

Abstract

Abstract. Western wildfires have a major impact on air quality in the US. In the fall of 2016, 107 test fires were burned in the large-scale combustion facility at the US Forest Service Missoula Fire Sciences Laboratory as part of the Fire Influence on Regional and Global Environments Experiment (FIREX). Canopy, litter, duff, dead wood, and other fuel components were 15 burned in combinations that represented realistic fuel complexes for several important western US coniferous and chaparral ecosystems including Ponderosa Pine, Douglas Fir, Engelmann Spruce, Lodgepole Pine, Subalpine Fire, Chamise, and Manzanita In addition, dung, Indonesian peat, and individual coniferous ecosystem fuel components were burned stand-alone to investigate the effects of individual components (e.g. "duff") and fuel chemistry on emissions. The smoke emissions were characterized by a large suite of state-of-the-art instruments. In this study we report emission factor (EF, g compound emitted per 20 kg fuel burned) measurements in fresh smoke of a diverse suite of critically-important trace gases measured by open-path Fourier transform infrared spectroscopy (OP-FTIR). We also report aerosol optical properties (absorption EF, single scattering albedo (SSA), and Ångström absorption exponent (AAE)) as well as black carbon (BC) EF measured by photoacoustic extinctiometers (PAX) at 870 and 401 nm. The average trace gas emissions were similar across the coniferous ecosystems tested and most of the variability observed in emissions could be attributed to differences in the consumption of components such as duff and litter, 25 rather than the dominant tree species. Chaparral fuels produced lower EF than mixed coniferous fuels for most trace gases except for NO x and acetylene. A careful comparison with available field measurements of wildfires confirms that several methods can be used to extract data representative of real wildfires from the FIREX lab fire data. This is especially valuable for species not yet measured in the field. For instance, the OP-FTIR data alone show that ammonia (1.65 g kg -1 ), acetic acid (2.44 g kg -1 ), nitrous acid (HONO, 0.61 g kg -1 ) and other trace gases such as glycolaldehyde and formic acid are significant emissions not 30 previously measured for US wildfires. The PAX measurements show that the ratio of brown carbon (BrC) absorption to BC absorption is strongly dependent on modified combustion efficiency (MCE) and that BrC absorption is most dominant for combustion of duff (AAE 7.13) and rotten wood (AAE 4.60): fuels that are consumed in greater amounts during wildfires than prescribed fires. Coupling our lab data with field data suggests that fresh wildfire smoke typically has an EF for BC near 0.1 g kg -1 ), an SSA of ~0.91 and an AAE of ~3.50, with the latter implying that about 86% of the aerosol absorption at 401 nm is due 35 to BrC.

Investigating biomass burning aerosol morphology using a laser imaging nephelometer

“…The time evolution of the MCE, and thereby information about the impact of flaming and smoldering on aerosol optical properties during stages of combustion, was lost due to the mixing of gases and aerosol in the combustion laboratory (Liu et al, 2013b;Pokhrel et al, 2016). The effect of MCE on aerosol optical properties is discussed in other FIREX studies (Liu et al, 2013b;Pokhrel et al, 2016;Selimovic et al, 2017).…”

Non-methane organic gas emissions from biomass burning: identification, quantification, and emission factors from PTR-ToF during the FIREX 2016 laboratory experiment

“…Emission ratios and emission factors (EFs) are based on fire-integrated excess values that benefit from significant signal averaging. Many of the above species have EFs that agree between PTR-ToF and FTIR within 10 % (Selimovic et al, 2017; Table S8). Additionally, it has been shown that the FTIR fire-integrated emission factors derived for hydroxyacetone are in excellent agreement with that reported for real wildfires by Liu et al (2017b;Selimovic et al, 2017).…”

“…Several other types of fuels were also burned, but with fewer replicates, and these included additional pine species (loblolly and Jeffrey pine), bear grass, rice straw, Ceanothus, dung, peat, excelsior, and commercial lumber. Experiments with western fuels included the combustion of both specific components of the fuel, such as canopy, litter, and duff, and more realistic burns that included a mix of components (Selimovic et al, 2017).…”

“…The emission factors (EFs) are in units of gram NMOG emitted per kg of dry fuel burned and are derived from the emission ratios using the carbon mass balance Selimovic et al, 2017):…”

“…This experiment burned a series of natural fuels and characterized the gas-and particle-phase emissions with a range of instrumentation (Selimovic et al, 2017). The aging of these emissions was explored with additional chamber experiments (described elsewhere).…”