The nitrogen budget of laboratory-simulated western US wildfires during the FIREX 2016 Fire Lab study
Authors: Roberts James M. , Stockwell Chelsea E. , Yokelson Robert J. , de Gouw Joost , Liu Yong , Selimovic Vanessa , Koss Abigail R. , Sekimoto Kanako , Coggon Matthew M. , Yuan Bin , Zarzana Kyle J. , Brown Steven S. , Santin Cristina , Doerr Stefan H. , Warneke Carsten 
Published: July 2020

Abstract. Reactive nitrogen (Nr, defined as all nitrogen-containing
compounds except for N2 and N2O) is one of the most important
classes of compounds emitted from wildfire, as Nr impacts both
atmospheric oxidation processes and particle formation chemistry. In
addition, several Nr compounds can contribute to health impacts from
wildfires. Understanding the impacts of wildfire on the atmosphere requires
a thorough description of Nr emissions. Total reactive nitrogen was
measured by catalytic conversion to NO and detection by NO–O3
chemiluminescence together with individual Nr species during a series
of laboratory fires of fuels characteristic of western US wildfires,
conducted as part of the FIREX Fire Lab 2016 study. Data from 75 stack fires
were analyzed to examine the systematics of nitrogen emissions. The measured
Nr ∕ total-carbon ratios averaged 0.37 % for fuels characteristic of
western North America, and these gas-phase emissions were compared with fuel
and residue N∕C ratios and mass to estimate that a mean (±SD)
of 0.68 (±0.14) of fuel nitrogen was emitted as N2 and N2O.
The Nr detected as speciated individual compounds included the following: nitric
oxide (NO), nitrogen dioxide (NO2), nitrous acid (HONO), isocyanic acid
(HNCO), hydrogen cyanide (HCN), ammonia (NH3), and 44 nitrogen-containing volatile organic compounds (NVOCs). The sum of these
measured individual Nr compounds averaged 84.8 (±9.8) %
relative to the total Nr, and much of the 15.2 % “unaccounted”
Nr is expected to be particle-bound species, not included in this
analysis. A number of key species, e.g., HNCO, HCN, and HONO, were confirmed not to
correlate with only flaming or with only smoldering combustion when using
modified combustion efficiency, MCE=CO2/(CO+CO2), as a
rough indicator. However, the systematic variations in the abundance of
these species relative to other nitrogen-containing species were
successfully modeled using positive matrix factorization (PMF). Three
distinct factors were found for the emissions from combined coniferous
fuels: a combustion factor (Comb-N) (800–1200 ∘C) with emissions
of the inorganic compounds NO, NO2, and HONO, and a minor contribution
from organic nitro compounds (R-NO2); a high-temperature pyrolysis
factor (HT-N) (500–800 ∘C) with emissions of HNCO, HCN, and
nitriles; and a low-temperature pyrolysis factor (LT-N) (<500 ∘C) with mostly ammonia and NVOCs. The temperature ranges
specified are based on known combustion and pyrolysis chemistry
considerations. The mix of emissions in the PMF factors from chaparral fuels
(manzanita and chamise) had a slightly different composition: the Comb-N
factor was also mostly NO, with small amounts of HNCO, HONO, and NH3;
the HT-N factor was dominated by NO2 and had HONO, HCN, and HNCO; and
the LT-N factor was mostly NH3 with a slight amount of NO contributing.
In both cases, the Comb-N factor correlated best with CO2
emission, while the HT-N factors from coniferous fuels correlated closely
with the high-temperature VOC factors recently reported by Sekimoto et al. (2018), and the LT-N had some correspondence to the LT-VOC factors. As a
consequence, CO2 is recommended as a marker for combustion Nr
emissions, HCN is recommended as a marker for HT-N emissions, and the family
NH3 ∕ particle ammonium is recommended as a marker for LT-N emissions.