The nitrogen budget of laboratory-simulated western U.S. wildfires during the FIREX 2016 FireLab study.

Total reactive nitrogen (N_r, defined as all nitrogen-containing compounds except for N₂ and N₂O) was measured by catalytic conversion to NO and detection by NO-O₃ chemiluminescence together with individual N_r species during a series of laboratory fires of fuels characteristic of Western U.S. wildfires, conducted as part of the FIREX FireLab 2016 study. Data from 75 stack fires were analyzed to examine the systematics of nitrogen emissions. The N_r/total-carbon ratios measured in the emissions were compared with fuel and ash N/C ratios and mass to estimate that a mean (± std. dev.) of 0.68 (± 0.14) of fuel nitrogen was emitted as N₂ and N₂O. The remaining fraction of N_r was emitted as individual compounds: 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 relative difference between the total reactive nitrogen measurement, N_r, and the sum of measured individual N_r compounds had a mean (± std. dev) of 0.152 (± 0.098). Much of this unaccounted N_r 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 only with flaming or only with smoldering combustion when using modified combustion efficiency (MCE = CO₂/(CO + CO₂)) as a rough indicator. However, the systematic variations of 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, aligning with our understanding of combustion chemistry in different temperature ranges: 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, with the temperature ranges being based on known combustion and pyrolysis chemistry considerations. The mix of emissions in the PMF factors from the chaparral fuels 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 N_r 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. [ABSTRACT FROM AUTHOR]