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. Reactive nitrogen (Nr, defined as all nitrogen-containingcompounds except for N2 and N2O) is one of the most importantclasses of compounds emitted from wildfire, as Nr impacts bothatmospheric oxidation processes and particle formation chemistry. Inaddition, several Nr compounds can contribute to health impacts fromwildfires. Understanding the impacts of wildfire on the atmosphere requiresa thorough description of Nr emissions. Total reactive nitrogen wasmeasured by catalytic conversion to NO and detection by NO–O3chemiluminescence together with individual Nr species during a seriesof laboratory fires of fuels characteristic of western US wildfires,conducted as part of the FIREX Fire Lab 2016 study. Data from 75 stack fireswere analyzed to examine the systematics of nitrogen emissions. The measuredNr ∕ total-carbon ratios averaged 0.37 % for fuels characteristic ofwestern North America, and these gas-phase emissions were compared with fueland 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: nitricoxide (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 thesemeasured 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 thisanalysis. A number of key species, e.g., HNCO, HCN, and HONO, were confirmed not tocorrelate with only flaming or with only smoldering combustion when usingmodified combustion efficiency, MCE=CO2/(CO+CO2), as arough indicator. However, the systematic variations in the abundance ofthese species relative to other nitrogen-containing species weresuccessfully modeled using positive matrix factorization (PMF). Threedistinct factors were found for the emissions from combined coniferousfuels: a combustion factor (Comb-N) (800–1200 ∘C) with emissionsof the inorganic compounds NO, NO2, and HONO, and a minor contributionfrom organic nitro compounds (R-NO2); a high-temperature pyrolysisfactor (HT-N) (500–800 ∘C) with emissions of HNCO, HCN, andnitriles; and a low-temperature pyrolysis factor (LT-N) (&lt;500 ∘C) with mostly ammonia and NVOCs. The temperature rangesspecified are based on known combustion and pyrolysis chemistryconsiderations. The mix of emissions in the PMF factors from chaparral fuels(manzanita and chamise) had a slightly different composition: the Comb-Nfactor 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; andthe LT-N factor was mostly NH3 with a slight amount of NO contributing.In both cases, the Comb-N factor correlated best with CO2emission, while the HT-N factors from coniferous fuels correlated closelywith 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 aconsequence, CO2 is recommended as a marker for combustion Nremissions, HCN is recommended as a marker for HT-N emissions, and the familyNH3 ∕ particle ammonium is recommended as a marker for LT-N emissions.

The nitrogen budget of laboratory-simulated western US wildfires during the FIREX 2016 Fire Lab study