Abstract. Total reactive nitrogen (Nr, 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 Nr 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 Nr/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 Nr 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, Nr, and the sum of measured individual Nr compounds had a mean (± std. dev) of 0.152 (± 0.098). Much of this 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 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) (;
