First Measurements of the Nitrogen Isotopic Composition of NO_x from Biomass Burning

The nitrogen isotopic composition (δ¹⁵N) of NO_x (NO + NO₂) was measured during the fourth Fire Lab at Missoula Experiment (FLAME-4). The δ¹⁵N-NO_x produced by burning a variety of biomass types ranged from -7 to +12‰ (vs air N₂). In the laboratory experiments, two types of emissions were sampled: "stack" fires where the emissions were measured within a few seconds of production from the fire and "chamber" fires where the emissions were held in a room for 1-2 h and sampled continuously. For both types of emissions sampled, the primary control on δ¹⁵N-NO_x is the δ¹⁵N of the biomass burned (δ¹⁵N-biomass), although differences were found for δ¹⁵N-NO_x between the two types of fires. For the stack emissions, δ¹⁵N-NO_x = 0.41 × δ¹⁵N-biomass +1.0 (R² = 0.83, p-value <0.001) and for the chamber fires, δ¹⁵N-NO_x = 0.98 × δ¹⁵N-biomass +1.7 (R² = 0.94, p-value <0.001). While a large range of δ¹⁵N-NO_x values were observed, the strong relationship between δ¹⁵N-NO_x and δ¹⁵N-biomass suggests that in any given environment, the δ¹⁵N-NO_x can be predicted.