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. Western wildfires have a major impact on air quality in the US. In the fallof 2016, 107 test fires were burned in the large-scale combustion facility atthe US Forest Service Missoula Fire Sciences Laboratory as part of the FireInfluence on Regional and Global Environments Experiment (FIREX). Canopy,litter, duff, dead wood, and other fuel components were burned incombinations that represented realistic fuel complexes for several importantwestern US coniferous and chaparral ecosystems including ponderosa pine,Douglas fir, Engelmann spruce, lodgepole pine, subalpine fir, chamise, andmanzanita. In addition, dung, Indonesian peat, and individual coniferousecosystem fuel components were burned alone to investigate the effects ofindividual components (e.g., “duff”) and fuel chemistry on emissions. Thesmoke emissions were characterized by a large suite of state-of-the-artinstruments. In this study we report emission factor (EF, grams of compoundemitted per kilogram of fuel burned) measurements in fresh smoke of a diversesuite of critically important trace gases measured using open-path Fouriertransform infrared spectroscopy (OP-FTIR). We also report aerosol opticalproperties (absorption EF; single-scattering albedo, SSA; andAngstrom absorption exponent, AAE) as well as black carbon (BC) EFmeasured by photoacoustic extinctiometers (PAXs) at 870 and 401 nm. Theaverage trace gas emissions were similar across the coniferous ecosystemstested and most of the variability observed in emissions could be attributedto differences in the consumption of components such as duff and litter,rather than the dominant tree species. Chaparral fuels produced lower EFsthan mixed coniferous fuels for most trace gases except for NO x andacetylene. A careful comparison with available field measurements ofwildfires confirms that several methods can be used to extract datarepresentative of real wildfires from the FIREX laboratory fire data. This isespecially valuable for species rarely or not yet measured in the field. Forinstance, the OP-FTIR data alone show that ammonia (1.62 g kg −1) ,acetic acid (2.41 g kg −1) , nitrous acid (HONO, 0.61 g kg −1) ,and other trace gases such as glycolaldehyde (0.90 g kg −1) and formicacid (0.36 g kg −1) are significant emissions that were poorlycharacterized or not characterized for US wildfires in previous work. The PAXmeasurements show that the ratio of brown carbon (BrC) absorption to BCabsorption is strongly dependent on modified combustion efficiency (MCE) andthat BrC absorption is most dominant for combustion of duff (AAE 7.13) androtten wood (AAE 4.60): fuels that are consumed in greater amounts duringwildfires than prescribed fires. Coupling our laboratory data with field datasuggests that fresh wildfire smoke typically has an EF for BC near0.2 g kg −1 , an SSA of ∼  0.91, and an AAE of ∼  3.50, withthe latter implying that about 86 % of the aerosol absorption at 401 nmis due to BrC.

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