Aerosol optical properties and trace gas emissions by PAX and OP-FTIR for laboratory-simulated western US wildfires during FIREX
Authors: Selimovic Vanessa , Yokelson Robert J. , Warneke Carsten , Roberts James M. , de Gouw Joost , Reardon James , Griffith David W. T. 
Published: March 2018

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 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 fir, chamise, and
manzanita. In addition, dung, Indonesian peat, and individual coniferous
ecosystem fuel components were burned 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, grams of compound
emitted per kilogram of fuel burned) measurements in fresh smoke of a diverse
suite of critically important trace gases measured using 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 (PAXs) 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,
rather than the dominant tree species. Chaparral fuels produced lower EFs
than mixed coniferous fuels for most trace gases except for NOx 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 laboratory fire data. This is
especially valuable for species rarely or not yet measured in the field. For
instance, 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 formic
acid (0.36 g kg−1) are significant emissions that were poorly
characterized or not characterized for US wildfires in previous work. 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 laboratory data with field data
suggests that fresh wildfire smoke typically has an EF for BC near
0.2 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 to BrC.